Paul Gilster's Blog, page 74

A Deep Dive into Tidal Lock

Mention red dwarf habitable zones and tidal lock invariably comes up. If a planet is close enough to a dim red star to maintain temperatures suitable for life, wouldn’t it keep one face turned toward it in perpetuity? But tidal lock, as Ashley Baldwin explains in the essay below, is more complex than we sometimes realize. And while there are ways to produce temperate climate models for such planets, tidal lock itself is a factor in not just M-dwarfs, but K- and even G-class stars like the Sun. Flip a few starting conditions and Earth itself might have been in tidal lock. The indefatigable Dr. Baldwin keeps a close eye on the latest exoplanet research, somehow balancing his astronomical scholarship with a career as consultant psychiatrist at the 5 Boroughs Partnership NHS Trust (Warrington, UK). Read on to learn a great deal about where current thinking stands on a subject critical to the question of red dwarf habitability.

by Ashley Baldwin

“Tidal locking”, “captured rotation” or “spin-orbit locking” etc occurs in most recognised guise when an orbiting astronomical body (be it a moon, planet or even a star) always presents the same face towards the object it is orbiting. In this instance, the orbit of the “satellite” body can be referred to as “synchronous”, whereby the tidally locked body takes as long to rotate around its own axis as to orbit its partner. This occurs due to the primary body’s gravity flexing the orbiting body into an elongated “prolate” shape. This in turn is then exposed to varying gravitational interaction with the central body.

Figure 1: Tidal stresses and tidal locking

As the “orbiter” rotates, its now elongated axis falls out of line with the central mass, which consequently perturbs it as it rotates across its orbit. It thus becomes subject to gravitationally induced torques that can act as a brake — through energy exchange and dissipation, the latter via friction-induced heat loss in the perturbed orbiting body. As M dwarf habitable zones are closer to their central star and their gravitational influence thus greater, it’s easy to see how this dissipated heat can contribute substantially to an exoplanet’s overall energy flux and can even affect its habitability potential – possibly tipping it into a runaway greenhouse scenario. (Kopparapu 2013).

Over millions of years (or more) this process can lead to “orbital synchronisation”. This arises when the orbiting body reaches a state where there is no longer any net exchange of rotation during the course of a completed orbit (Barnes 2010). Leaving a tidal locking state would only be possible with the addition of energy to the system. This might occur should some other massive object (such as a planet, or a star in, say, a binary system) break the equilibrium. If the masses of the two bodies (for instance Pluto & Charon) are similar, they can become tidally locked to each other.

Not all tidal locking involves synchronisation. “Super-synchronisation” occurs where an orbiting body becomes tidally locked to its parent body but rotates at a fixed but quicker rate. A topical example of this is the erstwhile “geosynchronous transfer orbit” (GTO). We see this on launcher specs all the time: “Payload to GTO”. This orbit is external to geosynchronous orbit, where many satellites start their operational lives, but allows for pre-orbital insertion inclination changes — economically expending less propellant prior to final insertion. Alternatively, such orbits can be used as dumping grounds for non-functioning satellites or related debris, so-called “geo-graveyard belts” (Luu 1998). Simulations suggest many exoplanets could exist in variants of such orbital types.

Gravitational interaction with a central star leads to progressive rotational slowing of a smaller planetary body like Mercury via energy exchange and heat dissipation. This is due to subtle but important tidal force variations across the orbiting body (remembering that gravity is inversely proportional to the square of the distance between any two bodies — thus “gravitational gradients” exist across solid bodies, leading to bulges). However, if the initial planetary orbit is significantly eccentric, this effect varies substantially across the orbital period (especially at periapsis — the point of strongest gravitational interaction) and can instead result in a spin-orbit resonance. In Mercury’s case, this is 3:2 (three rotations per two orbits) but other ratios can occur from 2:1 through 5:2 (Mahoney 2013). It’s worth noting that this effect is most pronounced for closer-in planets where the gravitational effects are greatest, so the effect should be even more relevant for the tightly packed exoplanetary architectures (e.g. TRAPPIST-1) that seem to be prevalent.

In extreme cases where the orbiting body’s orbit is nearly circular AND has a minimal or zero axial tilt — such as with the Moon — then the same hemisphere (libration allowing) faces the primary mass.

That said, for simplicity we will now assume that a smaller mass body (exoplanet) is orbiting a very much more massive body (star) — this is the focus of this review, with an unavoidable nod towards habitability.

For reasons of brevity and also pertaining to the exoplanet subject matter of recent posts, we will limit ourselves to the specific case of terrestrial exoplanets and their orbits around smaller main sequence stars.

The time to tidal locking can even be described by the adapted equation :

Tlock ≈ wa6 (0.4 m*R2) / 3 Gmp2 kR5 (Goldreich, Goldreich & Soter 1966); (Peale 1977); (Gladman 1996); (Greenberg 2009)

Where Tlock is “time to tidal locking”, w and k are constants which can be ignored for simplicity, m* is mass of the star, mp is mass of the planet, R is the exoplanet radius and “G” is Newton’s all important gravitational constant.

Tlock is substantially lengthened by “a” — increasing planetary semi-major axis (to the sixth power!). Tidal locking time is also increased by 0.4 X m* in this equation. However it is important to remember the context and just how massive a star, indeed ANY star, is — even an M dwarf star — many times, orders of magnitude even, more massive than a planet. A star thus plays the major role in the tidal locking of its attendant planets.

The gravitational constant G ensures that increasing stellar mass will substantially decrease Tlock. All other things being equal, increasing stellar mass is a major factor in reducing time to tidal locking.

Figure 2: Stellar mass & type versus semi-major axis orange / red graph with superimposed Tsync for 0.1,1 and 10 gigayear times for an Earth mass planet. (Penz 2005)

The concept of synchronisation is relatively new, dating back to Stephen Dole’s seminal Habitable Planets for Man at the beginning of the space age in the early 1960s. The concept was purely theoretical, with somewhat arbitrary parameters at this point, but it implied that tidal lock would be a major impediment to the human-friendly “habitable” exoplanets Dole had in mind for his book. It was here that tidally locked orbits and planets in M-dwarf systems were first linked, in a negative way that to some extent still exists today (before we even get to coronal mass ejections, EUV and stellar flares et al !) Atmospheric collapse due to freezing out on the side of the planet facing away from the star is not the least of these problems.

It was only in 1993 that Kasting et al employed sophisticated 1-D climate modelling as part of describing what constituted habitable planets. Habitable planets essentially now meant planets with conditions that could sustain liquid water on their surfaces. This is a rather lower bar than that set by Dole thirty years earlier, but far more applicable and still a pillar of exoplanet science today. More importantly, Kasting’s team also simulated star/planet gravitational interaction.

They did this by utilising the “Equilibrium Tide” model (ET). Refined variants of this have now become THE staple of all subsequent related studies, as it too has “evolved”. The model essentially assumes that the gravitational force of the tide-raiser (star) produces an elongated shape in the perturbed body (exoplanet) and that its long axis is slightly misaligned with respect to an imaginary line connecting the two centres of mass.

The misalignment is crucial and is due to the dissipating processes within the “deformed” exoplanet, leading to evolution of the orbit and spin angular moments. From this, various equations can be created which map out the orbital and rotational evolutionary history of exoplanets over time (see above). ET was originally derived from the Earth/Moon system by Darwin in 1880 before refinement by Pearle in 1977. Iterations vary in subtle but significant ways and are used as the basis for increasingly sophisticated simulations as computing power increases. Barnes 2017 has carried out a detailed review of synchronising and ET modelling (see below).

Kasting et al showed synchronisation of putative exoplanets orbiting in the habitable zones of M-dwarfs, stars with a mass of up to 0.42 Msun, within 4.5 billion years. They introduced the now familiar term “tidal locking radius”. Though a big step forward, this had the unfortunate consequence of continuing to propagate a pessimistic view of habitable exoplanets orbiting such stars. Importantly, stellar mass was still viewed as the major if not sole cause of synchronisation. The graph below (from Yang et al 2014), though based on sophisticated modelling, still captures this type of thinking. Here various habitable zone model ranges are superimposed on a graph of relative stellar insolation (and star type) versus semi-major axis examples of known exoplanets, adding realistic perspective. You will note also that for a 0.42 Msun star, with a temperature around 3500 K, the 1-D inner habitable range is very close to the value attributed to recently discovered TOI 700d — mid-80s percent.

Figure 3: Temperature of star versus stellar flux graph with superimposed coloured star classes and dashed gray “tidal locking radius” line.

The effects of other factors — such as starting orbital eccentricity (already encountered above with Mercury), baseline rotation rate, the presence of companion bodies (Greenberg, Corriea 2013) thermal tides arising from atmospheres (Leconte et al 2015), and stellar and planetary interiors (Driscoll & Barnes 2015), orbital tilt (Barnes 2017) — were not considered. As can be seen, it has only been over the last five years or so that these things have been added to simulations. Indeed, the results of these studies very much alter the whole tidal locking paradigm with particular relevance to habitable zones, which despite refinement (Kopparapu 2013, Selsis 2007) have only changed slightly, a big compliment to Kasting’s work in 1993.

Taken altogether, habitable zone planets of M,K and G stars all have the potential to become tidally locked. Not just M dwarfs — though their potential remains very much the greatest and especially for < 0.1 Msun stars such as TRAPPIST-1. Even the Earth, had its starting rotation been greater than just three days, according to Barnes 2017, might have become synchronous.

For the sake of brevity, this review has largely focused on stellar mass as a major driver in exoplanetary synchronisation. As can be seen above, as knowledge in this area progresses, other processes come into account. It is also becoming increasingly difficult to tease these out from drivers of exoplanetary habitability. So to this end we must look in more detail at some of the factors named above.

The planet Venus is unusual in many ways, but one in particular stands out: its retrograde and slow rotation rate that is longer than its orbital period. Why? What makes Venus different? One factor is that it is a rocky planet with a substantial atmosphere (92 bar at its surface). We all know about the infamous runaway greenhouse effect this drives, making Venus the hottest planet in the Solar System despite being further from the Sun than (spin/orbit resonant) Mercury. However, does this atmosphere have any other effects?

On Earth, the day/night cycle leads to variations in heat distribution in the atmosphere. It is known that the hottest time of day on Earth does not occur when the Sun is at its zenith and thus nearest to the Earth, but rather several hours later. This is because of thermal inertia. There is a delay between solar heating and thermal response, leading to mass redistribution. As the atmosphere and the Earth’s surface are generally well linked via friction, this will give rise to non-negligible thermal torques.

These torques are akin to the torques arising from the Sun’s uneven gravitational interaction with the Earth described above, though not as potent. On the Earth with its extended 1 AU orbit, they are largely inconsequential, but for 0.3 AU nearer Venus, they become significant. Depending on their direction, they can either slow up OR speed up planetary rotation, but either way they help to resist synchronisation. Over time, torques arising in Venus have acted to slow down its rotation, so much so that it has reversed to the retrograde pattern we see today.

So if this is true of Venus, how about exoplanets? Can these atmospheric torques resist or at least delay synchronisation and tidal locking in vulnerable areas around a star? This has been extensively modelled by Leconte et al 2015 and the answer was a resounding yes, especially for smaller, less luminous stars with close-in habitable zones, and not just for exoplanets with 90 bar atmospheres, either. Even 1 bar Earth-like atmospheres could help resist synchronisation for the habitable zones in stars of 0.5 Mearth – 0.7 Mearth.

Ten bar atmospheres were simulated and shown to resist synchronisation even for habitable zone planets orbiting 0.3 Mearth stars (mid-M dwarfs). These are the high bar “maximum greenhouse” CO2 atmospheres that are postulated to occur in the outer regions of stellar habitable zones. But there are limits. Venus’ 92 bar atmosphere is ironically so thick that most of the incident sunlight that isn’t reflected back into space is either absorbed or scattered before it can reach the planetary surface and exert the driving effect of thermal torques (Leconte et al 2015).

Figure 4: Red arrow synchronous rotation / blue arrow asynchronous rotation graph (Leconte 2015).

Orbital synchronisation and exoplanet habitability remains a contentious theoretical field that is subject to continual debate and constant change. Modern Global Climate Modelling (GCM) has become a sophisticated sub-science. Using an earlier iteration of GCM, Yang et al showed in 2013 that synchronised M-dwarf habitable zone planets would form thick cloud banks above their sub-stellar point. This would then reflect much of the incident stellar flux, thus reducing the energy reaching the surface. In turn, this would reduce the overall energy reaching the planet and so reduce global temperatures. The net effect in theory is to extend the stellar habitable zone inwards. However, the same author collaborated with Wolf and Kopparapu in 2016 to apply an updated 3-D model to the same problem. This showed that a sub-stellar cloud bank could not form, or would form and then move, a result effectively rebutting the 2013 findings and moving the habitable zone back to its original pre 2013 starting point. Expect more of this !

So, all things considered, just how easy is it for an exoplanet to become tidally locked and just how easy can habitable zone planets become tidally locked ? Barnes 2017 attempted to address just this question for exoplanets in circular orbits. He applied two well recognised refined variants (CPL left, CTL right in the graphic below) of the ET to two model populations of exoplanets orbiting differing stellar masses, and ran thousands of giga-year simulations for each (think of the computing power and time!) One population had a starting orbital period of 8 hours and an orbital tilt of 60°. The other had a starting period of ten days and a tilt of 0°. This produced the four outcomes illustrated below. The superimposed grey shading represents the latest habitable zones (Kopparapu 2013) iteration, with the dark grey representing the “conservative” and the light the “optimistic”.

Figure 5: “Four in one” black and white stellar mass vs semi-major axis / superimposed greyscale habzone graphs.

These results are indicative and significantly different from the status quo, which is that tidal locking is only something that applies to exoplanets orbiting in close to M dwarf and smaller K dwarf stars. For one thing, even this older paradigm implies that at least some “Goldilocks” stars are not quite as homely as expected (more Kasting than Dole). The Barnes work hints at potential overlap of the habitable zone for potentially a large fraction of K-class and even many G-class stars, driven by factors beyond simple stellar mass. Clearly planets with a slow initial rotation rate and low orbital tilt are at greater risk, as may prove the case. Opposed to this are non-synchronising factors such as, inter alia, higher baseline orbital eccentricities and the close proximity of other orbiting bodies (moons, planets …thinking TRAPPIST-1 and binary stars/brown dwarfs, as with the recently described Gliese 229Ac system).

What this also shows is the inextricable link between orbital features and planet habitability. No more so demonstrated than by Kepler, and likely even more so with its greater number of short orbital period planets, with any potential habitable zone planetary candidates lying within just tenths or less of an AU from their parent star. This is very much in the “red arrow” synchronous zone in the Leconte graphic above.

There are now over 4000 known exoplanets. The current focus is on their “characterisation” and this is largely about atmospheres and biosignatures. However, it is obvious that we need to know far more about their evolving and historical orbital properties. This is a part of a process of determining habitable planets/zones, which are about so much more than stellar mass.

Most of the exoplanets discovered already by Kepler et al orbit close in to their stars, including those few in the potential tidal lock habitable zone. Ongoing Doppler photometry and TESS will identify thousands more such exoplanets, many of which will be even closer to their latest star given TESS’ shorter 27 day observation runs. TOI 700d and Gliese 229Ac are just for starters. Hopefully the search for habitability will expand to encompass the unavoidable connexion with planetary orbital features.

Know the star to know the planet, but know the orbit to know them both.

Figure 6: Stellar effects/planetary properties/planetary systems (Meadows and Barnes 2018)

References

Barnes,R. Formation and evolution of exoplanets. John Wiley & Sons, p248, 2010

Barnes, R. Tidal locking of habitable exoplanets. Celestial mechanics and dynamical astronomy Vol 129, Issue 4, pp 509-536, Dec 2017

Darwin, G H. On the secular changes in the elements of the orbit of a satellite revolving about a tidally distorted planet. Royal Society of London Philosophical Transactions, Series I, 171:713-891 ; 1880.

Dole, S H. Habitable Planets for Man. 1964

Goldreich, P. Final spin rates of planets and satellites. Astronomical Journal, 71, 1966

Goldreich, P., Soter, A., Q in the solar system. Icarus 5, 375-389, 1966

Gladman, B et al. Synchronous locking of tidally evolving satellites. Icarus 133 (1) 166-192, 1996

Greenberg, R. Frequency dependence of tidal Q, The Astrophysical Journal, 698, L42-45, 2009

Kasting, J. F. Habitable zones around main sequence stars. Icarus,101 d 108-128 Jan 1993

Kopparapu, R K et al. Habitable zones around main sequence stars: New Estimates. The Astrophysical Journal, 765;131, March 2013

Kopparapu R K, Wolf E, Yang et al. The inner edge of the habitable zone for synchronously rotating planets around low-mass stars using general circulation models. The Astrophysical Journal Volume 819, Number 1, March 2016

Luu, K. Effects of perturbations on space debris in super-synchronous storage orbits. Air Force Research Laboratory Technical Reports, 1998

Mahoney,T J. Mercury. Springer Science & Business Media, 2013

Meadows V S, Barnes R K. Factors affecting exoplanet habitability. In Handbook of Exoplanets P57, 2018

Peale, S J. Rotation histories of natural satellites. Burns, J A, Editor, IAU Colloquium 28; Planetary Satellites, p 87-111, 1977

Penz,T et al. Constraints for the evolution of habitable planets: Implications for the search of life in the Universe: Evolution of Habitable planets, 2005

Yang, J et al. Stabilising cloud feedback dramatically expands the habitable zone of tidally locked planets. The Astrophysical Journal Letters: 771:L45, July 2013

Yang, J et al. Strong dependence of the inner edge of the habitable zone on planetary rotation rate. The Astrophysical Journal Letters: 787:1, April 2014

A Possible Proxima Centauri c

While we continue to labor over the question of planets around Alpha Centauri A and B, Proxima Centauri — that tiny red dwarf with an unusually interesting planet in the habitable zone — remains a robust source of new work. It’s surely going to be an early target for whatever interstellar probes we eventually send, and is the presumptive first destination of Breakthrough Starshot. Now we have news of a possible second planet here, though well outside the habitable zone. Nonetheless, Proxima Centauri c, if it is there, commands the attention.

A new paper offers the results of continuing analysis of the radial velocity dataset that led to the discovery of Proxima b, work that reflects the labors of Mario Damasso and Fabio Del Sordo, who re-analyzed these data using an alternative treatment of stellar noise in 2017. Damasso and Del Sordo now present new evidence, working with, among others, Proxima Centauri b discoverer Guillem Anglada-Escudé, and incorporating astrometric data from the Gaia mission’s Data Release 2 (DR2). The result of the new analysis is a possible planet with an orbital period of 5.2 years and a minimum mass of 5.8 ± 1.9 times the mass of the Earth.

Image: This is Figure 5 from the paper. Caption: Outcomes of the combined analysis of the astrometric and RV datasets. Left: True mass of Proxima c versus the sine of the orbital inclination, as obtained from the astrometric simulations. The black line is the simulated exact solution, the blue dots represent the values derived from the Gaia astrometry alone, while the red dots are the values derived by combining the Gaia astrometry with the radial velocities. Right: Fractional deviation of the true mass (defined as the difference between the simulated and retrieved masses for Proxima c divided by the simulated value) versus sine of the orbital inclination. Credit: Damasso et al.

Remember that when dealing with radial velocity results, we can only draw conclusions on the minimum mass in question, as we don’t know how the system is inclined around the star. The researchers find that by analyzing the photometric data and spectroscopic results, they cannot explain the planetary signal through stellar activity, but they also argue that a good deal of follow-up work is needed through a variety of means. The paper notes, for example, that Proxima was observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in 2017, with an unknown source detected at 1.6 AU. Is this evidence for Proxima c?

It’s quite an interesting question, and one that involves more than a new planet:

ALMA imaging could corroborate the existence of Proxima c if the secondary 1.3-mm source is confirmed: In this sense, ALMA follow-up observations will be essential. In (28), the possible existence of a cold dust belt at ∼30 AU, with inclination of 45°, is also mentioned. If Proxima c orbits on the same plane, its real mass would be mc = 8.2 M⊕

Image: Artist’s impression of dust belts around Proxima Centauri. Discovered in data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the cold dust appears to be in a region between one to four times as far from Proxima Centauri as the Earth is from the Sun. The data also hint at the presence of an even cooler outer dust belt and may indicate the presence of an elaborate planetary system. These structures are similar to the much larger belts in the Solar System and are also expected to be made from particles of rock and ice that failed to form planets. Such belts may also prove useful in helping us investigate the presence of a possible second planet around this star. Credit: ESO / M. Kornmesser.

But Gaia astrometry is also crucial, for there is some evidence of an anomaly in Proxima’s tangential velocity that, if confirmed, would be compatible with the existence of a planet with a mass in the 10 to 20 Earth range, and a distance between 1 and 2 AU. Further work with Gaia data is clearly in the cards:

Given the target brightness and the expected minimum size of the astrometric signature…, Gaia alone should clearly detect the astrometric signal of the candidate planet at the end of the 5-year nominal mission, all the more so in case of a true inclination angle significantly less than 90°. Proxima is one of the very few stars in the Sun’s backyard for which Gaia alone might be sensitive to an intermediate separation planetary companion in the super-Earth mass regime.

A final consideration is that while the flux contrast between the hypothetical Proxima c and the parent star (depending on albedo, among other things) is beyond the capabilities of our current direct imaging technologies, the apparent separation of planet and star should be accessible to future high-contrast imaging instruments, perhaps the European Extremely Large Telescope, which the paper mentions along with other ground- and space-based instruments. So we have what the authors describe as ‘a very challenging target,’ but one with huge interest for astronomers continuing to characterize this closest of all stellar systems.

It seems premature to get too far into a discussion of how Proxima c formed, since we have yet to confirm it. However, the authors make the case that if it is there, this planet would challenge us to explain how it formed so far beyond the snowline, where super-Earths could take advantage of the accumulation of ices. Perhaps the protoplanetary disk here was warmer than we’ve assumed. In any case, the apparent circularity of the orbit and the absence of more massive planets closer in makes migration from the inner system unlikely. And I think we should leave formation issues there while we await new work, especially the authors note, from ALMA.

The Damasso paper reanalyzing the Proxima Centauri radial velocity data in 2017 is “Proxima Centauri reloaded: Unravelling the stellar noise in radial velocities,” Astronomy & Astrophysics 599, A126 (2017) (abstract/ preprint). The new Damasso et al. paper is “A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU,’ Science Advances Vol. 6, No. 3 (15 January 2020). Full text.

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New Planets from Old Data

We rightly celebrate exoplanet discoveries from dedicated space missions like TESS (Transiting Exoplanet Survey Satellite), watching the work go from initial concept to first light in space and early results. But let’s not forget the growing usefulness of older data, tapped and analyzed in new ways to reveal hidden gems. Thus recent work out of the Carnegie Institution for Science, where Fabo Feng and Paul Butler have mined the archives of the Ultraviolet and Visual Echelle Spectrograph survey of 33 nearby red dwarf stars, a project operational from 2000 to 2007.

The duo have uncovered five newly discovered exoplanets and eight more candidates, all found orbiting nearby red dwarf stars. Two of these are conceivably in the habitable zone, putting nearby stars GJ180 and GJ229A into position as potential targets for next-generation instruments. Both of these stars host super-Earths (7.5 and 7.9 times the mass of Earth), with orbital periods of 106 and 122 days respectively. Like the other planets unveiled in the discovery paper in The Astrophysical Journal Supplement Series, these worlds were all found using radial velocity methods, uniquely powerful when deployed on low-mass red dwarfs.

Temperate super-Earths are interesting in their own right, but one of these has a particular claim to our attention, as lead author Feng explains:

“Many planets that orbit red dwarfs in the habitable zone are tidally locked, meaning that the period at which they spin around their axes is the same as the period at which they orbit their host star. This is similar to how our Moon is tidally locked to Earth, meaning that we only ever see one side of it from here. As a result, these exoplanets [have] a very cold permanent night on one side and very hot permanent day on the other—not good for habitability. GJ180d is the nearest temperate super-Earth to us that is not tidally locked to its star, which probably boosts its likelihood of being able to host and sustain life.”

But GJ229Ac is also intriguing, a possibly temperate super-Earth in a system where the host star has a brown dwarf companion. That object, GJ229B, was one of the first brown dwarfs to be imaged, making this system an interesting testbed for planet formation models. We also have a Neptune-class planet orbiting GJ433 well out of the habitable zone, far enough from its star that the authors see it as a realistic candidate for future direct imaging. The planet is the coldest Neptune-like world we’ve yet found around another star, and also the nearest to Earth.

Image: Artist’s concept of GJ229Ac, the nearest temperate super-Earth to us that is in a system in which the host star has a brown dwarf companion. Credit: Robin Dienel, courtesy of the Carnegie Institution for Science.

Efforts like this don’t stop with a single dataset, but rely on multiple follow-ups to increase the fidelity of the data. Thus Feng and Butler used the Planet Finder Spectrograph at Las Campanas (Chile), ESO’s HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher) at La Silla, and HIRES (High Resolution Echelle Spectrometer) at the Keck Observatory in a combination that, in Butler’s words, “increases the number of observations and the time baseline, and minimizes instrumental biases.”

I think Feng and Butler are right to emphasize the utility of the UVES data. From the paper:

The most important observation in a precision velocity data set is the first observation because observers cannot go back in time. For most of the stars in the UVES M Dwarf program, these are the first observations taken with state-of-the-art precision. This data set is all the more remarkable for focusing on some [of] the nearest stars, and the stars most likely to harbor detectable potentially habitable planets. These observations will continue to be important in finding and constraining planets around these stars for decades to come. We do not expect this to be the final word on this remarkable data set. We look forward to future researchers reanalyzing this data set with a superior Doppler reduction package, and producing the surprises that emerge from better measurement precision.

All of which emphasizes how creative we are learning to be with the data that in recent decades have been cascading in quantity, quality and importance. Nice work by the UVES M Dwarf team. As the paper goes on to say: “Starting back in the infancy of precision velocity measurements, they boldly went straight to the heart of the most interesting and challenging problem, finding potentially habitable planets around the nearest stars.”

And kudos to Feng and Butler for dedicating their paper to Carnegie astronomer and system manager Sandy Keiser, who died suddenly in 2017 during the analysis of the data from which these results emerged, but not before she produced work critical to this paper.

The paper is Butler et al., “A Reanalysis of the UVES M Dwarf Planet Search Program,” Astronomical Journal Vol. 158, No. 6 (2 December 2019). Abstract.

A Satellite for Eurybates

3548 Eurybates is a Jupiter trojan, one of the family of objects that have moved within the Lagrange points around Jupiter for billions of years (the term is libration, meaning these asteroids actually oscillate around the Lagrange points). Consider them trapped objects, of consequence because they have so much to tell us about the early Solar System. The Lucy mission aims to visit both populations (the ‘Greeks’ and the ‘Trojans’) at Jupiter’s L4 and L5 Lagrangians when it heads for Jupiter following launch in 2021.

Image: During the course of its mission, Lucy will fly by six Jupiter Trojans. This time-lapsed animation shows the movements of the inner planets (Mercury, brown; Venus, white; Earth, blue; Mars, red), Jupiter (orange), and the two Trojan swarms (green) during the course of the Lucy mission. Credit: Astronomical Institute of CAS/Petr Scheirich (used with permission).

Right now the focus is on Eurybates as mission planning continues, for we’ve just learned thanks to the Hubble instrument’s Wide Field Camera 3 that this asteroid has a moon, an object more than 6,000 times fainter than Eurybates itself. According to mission principal investigator Hal Levison (Southwest Research Institute), that implies a diameter of less than 1 kilometer. The tiny moon will be among the smallest asteroids visited by Lucy, which is intended to perform flybys of six trojans, as well as a main belt asteroid along the way.

Thomas Statler is a Lucy program scientist at NASA headquarters in Washington:

“There are only a handful of known Trojan asteroids with satellites, and the presence of a satellite is particularly interesting for Eurybates. It’s the largest member of the only confirmed Trojan collisional family – roughly 100 asteroids all traceable to, and probably fragments from, the same collision.”

It took three tries with Hubble to confirm the satellite’s existence, a tricky job given the object’s faintness and unknown orbit around the much brighter Eurybates. Now the task is to figure out when it will become visible again, for Eurybates won’t be observable until well clear of the Sun, which won’t happen until June. No major changes to the existing flight planning are needed to incorporate the small moon into the mission, but refining its orbit around the asteroid will help scientists schedule the best observing time during the Eurybates encounter.

Image: This diagram illustrates Lucy’s orbital path. The spacecraft’s path (green) is shown in a frame of reference where Jupiter remains stationary, giving the trajectory its pretzel-like shape. After launch in October 2021, Lucy has two close Earth flybys before encountering its Trojan targets. In the L4 cloud Lucy will fly by (3548) Eurybates (white), (15094) Polymele (pink), (11351) Leucus (red), and (21900) Orus (red) from 2027-2028. After diving past Earth again Lucy will visit the L5 cloud and encounter the (617) Patroclus-Menoetius binary (pink) in 2033. As a bonus, in 2025 on the way to the L4, Lucy flies by a small Main Belt asteroid, (52246) Donaldjohanson (white), named for the discoverer of the Lucy fossil. After flying by the Patroclus-Menoetius binary in 2033, Lucy will continue cycling between the two Trojan clouds every six years. Credit: Southwest Research Institute.

We already have one binary to look forward to, for Patroclus, in the L5 cloud, has a small satellite called Menoetius. Note how stuffed with interesting things these Lagrangian points seem to be. We have D-type asteroids like Patroclus, which likely have water ice in the interior, as well as C- and P- class asteroids, the latter darker and bearing more similarities to Kuiper Belt objects than main belt asteroids. All are thought to be rich in volatiles. Our explorations here should offer insights into primordial planet-building materials in the early Solar System.

Orange Dwarfs: ‘Goldilocks’ Stars for Life?

Our Sun is a G2V type star, or to use less formidable parlance, a yellow dwarf. It was inevitable that as we began considering planets around other stars (well before the first of these were discovered), we would imagine solar-class stars as the best place to look for life, but attention has swung to other possibilities in recent years, especially toward red dwarfs, which comprise a high percentage of all the stars in the galaxy. Now it seems that the problems of M-dwarfs are causing a reconsideration of the class in between, the K-class orange dwarfs.

Alpha Centauri B is such a star, although its proximity to Centauri A may raise problems in planet formation that we have yet to observe. Fortunately, our long-distance exploration of the Centauri stars is well underway, and we should have new information about what orbits the two primary stars here within a few short years. If we were to find a habitable zone rocky world around Centauri B, one thing that makes it interesting is the longevity of such stars.

Unlike our Sun, which is about halfway through its 10 billion year lifetime, orange dwarfs can live for tens of billions of years, offering abundant opportunity for life’s growth and evolution. While not as ubiquitous as M-dwarfs, K-dwarfs appear to be about three times more numerous than G-dwarfs like the Sun. These percentages are always being adjusted, of course, but I’ve seen estimates of G-dwarfs between 3 and 8 percent of the stellar population. A higher population of K-dwarfs, though, gives us plenty of search space for planets possibly bearing life.

How likely are the various kinds of stars to produce habitable conditions around them? M-dwarfs give us fantastically longer lifetimes (into the trillions of years), but at the recent meeting of the American Astronomical Society in Hawaii, Edward Guinan and Scott Engle (Villanova University) described the extreme levels of UV and X-ray radiation through flares and coronal mass ejections that planets in the habitable zones of these stars can receive, with the real possibility of atmospheres being stripped away. “We’re not so optimistic anymore about the chances of finding advanced life around many M stars,” Guinan said.

Guinan and Engle have been engaged in a project called GoldiloKs at Villanova, in which they work with undergraduate students to measure factors like age, rotation rate, and radiation exposure in a sampling of stars ranging across primarily G- and K-class stars. The Hubble instrument, Chandra X-ray Observatory, and ESA’s XMM-Newton satellite are involved in the observations, with Hubble particularly useful for assessing radiation from the K-dwarfs. Lacking the intense magnetic fields powering up X-ray and UV emissions, these stars produce a scant 1/100th as much radiation as would be received by a habitable zone world around an M-dwarf.

To Guinan, these orange dwarfs are indeed the Goldilocks stars:

“K-dwarf stars are in the ‘sweet spot,’ with properties intermediate between the rarer, more luminous, but shorter-lived solar-type stars (G stars) and the more numerous red dwarf stars (M stars). The K stars, especially the warmer ones, have the best of all worlds. If you are looking for planets with habitability, the abundance of K stars pump up your chances of finding life.”

Image: This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G stars; stars less massive and cooler than our Sun are K dwarfs; and even fainter and cooler stars are the reddish M dwarfs. The graphic compares the stars in terms of several important variables. The habitable zones, potentially capable of hosting life-bearing planets, are wider for hotter stars. The longevity for red dwarf M stars can exceed 100 billion years. K dwarf ages can range from 15 to 45 billion years. And, our Sun only lasts for 10 billion years. The relative amount of harmful radiation (to life as we know it) that stars emit can be 80 to 500 times more intense for M dwarfs relative to our Sun, but only 5 to 25 times more intense for the orange K dwarfs. Red dwarfs make up the bulk of the Milky Way’s population, about 73%. Sunlike stars are merely 6% of the population, and K dwarfs are at 13%. When these four variables are balanced, the most suitable stars for potentially hosting advanced life forms are K dwarfs. Credit: NASA, ESA, and Z. Levy (STScI).

We have about 1,000 orange dwarfs within 100 light years of the Sun, making these interesting targets for future study. Whereas our own planet will face a habitable zone that gradually moves outward as the Sun begins to swell — we’re in deep trouble in a billion years or so — K-dwarfs see much slower migration of the habitable zone, with an increase in brightness by about 10-15 percent over the Sun’s entire lifetime. No wonder Guinan and Engle single out K-star hosts like Kepler-442, Tau Ceti, and Epsilon Eridani for extra attention. Indeed, Kepler-442 b is a rocky world circling a K5 star that Guinan calls ‘a Goldilocks planet hosted by a Goldilocks star.’

All this reminds us of how our views of our own circumstances have changed over time. It was natural enough to believe that in seeking out life elsewhere in the universe, we would look for places like the one we knew supported it. But we’re beginning to ask whether, habitable though it obviously is, the Earth is as ideally habitable as it might be. Let me point you to René Heller (McMaster University) and John Armstrong (Weber State University), who raised similar issues in a 2014 paper in Astrobiology. The duo use the term ‘superhabitability,’ and, although looking primarily at planetary types, also ask about the host stars:

Higher biodiversity made Earth more habitable in the long term. If this is a general feature of inhabited planets, that is to say, that planets tend to become more habitable once they are inhabited, a host star slightly less massive than the Sun should be favorable for superhabitability. These so-called K-dwarf stars have lifetimes that are longer than the age of the Universe. Consequently, if they are much older than the Sun, then life has had more time to emerge on their potentially habitable planets and moons, and — once occurred — it would have had more time to ‘tune’ its ecosystem to make it even more habitable.

Back to Guinan and Engle, whose work over the past 30 years has included X-ray, UV and photometric studies of F- and G-class stars, a corresponding study of M-dwarfs that lasted a decade, and now the collection of similar data for K-dwarfs. My point here is that the K-dwarf work takes place within the context of a robust dataset painstakingly gathered across a wide range of spectral types, giving these two researchers’ conclusions substantial heft.

Is Earth, then, only ‘marginally habitable’ when compared to planets that could exist around stars more benign than our Sun? It’s a fascinating thought that demands we examine our own anthropocentrism while at the same time bolstering our target list for future observatories.

Heller and Armstrong’s paper is “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014). Abstract available. I’m sure a paper from Guinan and Engle is in the works. For now, however, have a look at Cuntz & Guinan, “About Exobiology: The Case for Dwarf K Stars,” Astrophysical Journal Vol. 827, No. 1 10 August 2016 (abstract).

New Entry in High Precision Spectroscopy

As if I don’t have enough trouble figuring out acronyms, I now have to figure out how to pronounce acronyms. The issue comes up because a new NASA instrument now in use at Kitt Peak National Observatory is a spectrograph built at Penn State called NEID. Now NEID stands for NN-EXPLORE Exoplanet Investigations with Doppler spectroscopy. Here we have an acronym within an acronym, for NN-EXPLORE itself stands for the NASA-NSF Exoplanet Observational Research partnership that funds NEID.

Here’s the trick: The acronym NEID is not pronounced ‘NEE-id’ or ‘NEED’ but ‘NOO-id.’ The reason: Kit Peak is on land owned by the Tohono O’odham nation, and the latter pronunciation honors a verb that means something close to ‘to see’ in the Tohono O’odham language.

As a person fascinated with linguistics, I’m delighted to see this nod to a language whose very survival is threatened by the small number of speakers (count me as one infinitely cheered by the resurrection of Cornish, for example). And as one absorbed with exoplanet science, I note that NEID first light results were discussed at the recent meeting of the American Astronomical Society in Honolulu. The instrument is mounted on the WIYN 3.5-meter telescope at Kit Peak. The first observations were of 51 Pegasi, the first main sequence star found to host an exoplanet.

Here we’re in the realm of using radial velocity measurements to ferret out the slight stellar motion that indicates planets. The better we get at doing radial velocity calibration, the better, not only because we can discover new planets, but because we can use the method to characterize already known worlds. Thus TOI 700 d, that interesting habitable zone world we looked at yesterday, is a case of discovery by transit methods, but having measured its size, we can now use followup radial velocity readings to get a read on its density.

Image: The NEID instrument, mounted on the 3.5-meter WIYN telescope at the Kitt Peak National Observatory. The NASA-NSF Exoplanet Observational Research (NN-EXPLORE) partnership funds NEID (short for NN-EXPLORE Exoplanet Investigations with Doppler spectroscopy).Credit: NSF’s National Optical-Infrared Astronomy Research Laboratory/KPNO/NSF/AURA.

With NEID, we continue the movement in radial velocity studies down to measurements well below 1 meter per second. Long-time Centauri Dreams readers will know that for a long time, the HARPS spectrograph (High Accuracy Radial velocity Planet Searcher) at the ESO La Silla 3.6m telescope, has been considered the gold standard, taking us down to 1 meter per second, meaning that scientists could discern via Doppler methods the tiny pull of a planet on the star first towards us, and then away from Earth. The ESPRESSO instrument (Echelle Spectrograph for Rocky OxoPlanet and Stable Spectroscopic Observations) installed at the European Southern Observatory’s Very Large Telescope in Chile, takes us into centimeters per second range, which means detecting Earth-size habitable zone planets around Sun-like stars.

Image: The left side of this image shows light from the star 51 Pegasi spread out into a spectrum that reveals distinct wavelengths. The right-hand section shows a zoomed-in view of three wavelength lines from the star. Gaps in the lines indicate the presence of specific chemical elements in the star. Credit: Guðmundur Kári Stefánsson/Princeton University/NSF’s National Optical-Infrared Astronomy Research Laboratory/KPNO/NSF/AURA.

Can NEID likewise reach the realm of centimeters per second? At the AAS meeting, researchers described the instrument they are calling an ‘extreme precision Doppler spectrograph.’ Exploring radial velocity detection in this realm will demand the upgrades the NEID team has made to the observatory, allowing the spectrograph to achieve room temperature fluctuations below +/-0.2 degrees Celsius in the short term.

Vibration from the WIYN telescope also must be taken into account using high-precision accelerometers and speckle imaging data taken on-sky to achieve the needed precision. The team believes the instrument is capable of reaching 27 centimeters per second and perhaps lower. The goal of the ESPRESSO group is 10 centimeters per second. As we explore the capabilities of both instruments, we are revitalizing radial velocity and making it ever more relevant to the quest to discover and characterize small worlds that may support liquid water.

TOI 700 d: A Possible Habitable Zone Planet

Among the discoveries announced at the recent meeting of the American Astronomical Society in Hawaii was TOI 700 d, a planet potentially in the habitable zone of its star. TOI stands for TESS Object of Interest, reminding us that this is the first Earth-size planet the Transiting Exoplanet Survey Satellite has uncovered in its data whose orbit would allow the presence of liquid water on the surface. The Spitzer instrument has confirmed the find, highlighting the fact that Spitzer itself, a doughty space observatory working at infrared wavelengths, is nearing the end of its operations. Thus Joseph Rodriguez (Center for Astrophysics | Harvard & Smithsonian):

“Given the impact of this discovery – that it is TESS’s first habitable-zone Earth-size planet – we really wanted our understanding of this system to be as concrete as possible. Spitzer saw TOI 700 d transit exactly when we expected it to. It’s a great addition to the legacy of a mission that helped confirm two of the TRAPPIST-1 planets and identify five more.”

Image: The three planets of the TOI 700 system, illustrated here, orbit a small, cool M dwarf star. TOI 700 d is the first Earth-size habitable-zone world discovered by TESS. Credit: NASA’s Goddard Space Flight Center.

TOI-700 is an M-dwarf star in the constellation Dorado, a southern sky object whose mass and size are roughly 40 percent that of the Sun, with half the Sun’s surface temperature. Remember that TESS monitors sky sectors in 27-day blocks, a period lengthy enough to spot the changes in stellar brightness that mark the transit of a planet across the star’s face as seen from Earth.

It’s interesting to note how any misclassification of stellar type can confound our conclusions about a transiting planet. In this case, the star was originally thought to be closer in size and type to the Sun, which would have meant planets that were larger and hotter than we now know are there. Correcting the problem revealed what looks to be a very interesting world.

“When we corrected the star’s parameters, the sizes of its planets dropped, and we realized the outermost one was about the size of Earth and in the habitable zone,” said Emily Gilbert, a graduate student at the University of Chicago. “Additionally, in 11 months of data we saw no flares from the star, which improves the chances TOI 700 d is habitable and makes it easier to model its atmospheric and surface conditions.”

Video: NASA’s Transiting Exoplanet Survey Satellite (TESS) has discovered its first Earth-size planet in its star’s habitable zone, the range of distances where conditions may be just right to allow the presence of liquid water on the surface. Scientists confirmed the find, called TOI 700 d, using NASA’s Spitzer Space Telescope and have modeled the planet’s potential environments to help inform future observations. Credit: NASA’s Goddard Space Flight Center.

What we now know about TOI 700 is that there are at least three planets here, with TOI 700 d being the outermost and the only one likely to be in the habitable zone. The planet is in a 37 day orbit and receives 86 percent of the insolation that the Sun gives the Earth. Here again we look to Spitzer, for its data allowed researchers not only to confirm the existence of TOI 700 d but also to tighten the constraints on its orbital period by 56% and its size by 38%. Further observations from the Las Cumbres Observatory network also tightened the orbital period.

The other worlds in the TOI 700 system are TOI 700 b, about Earth size and probably rocky, orbiting the star every 10 days, and TOI 700 c, 2.6 times larger than Earth and in a 16 day orbit. As to the intriguing TOI 700 d, let’s keep in mind that it’s relatively close at just over 100 light years, making it a potential target for follow-up observations by future space observatories, although not the James Webb Space Telescope, as I’ll explain in a moment. TOI 700 d is also likely to be, along with its planetary companions, in tidal lock with the star, keeping one side constantly in daylight, the other in perpetual night.

This nearby star appears to have low flare activity, adding to the potential that if TOI 700 d is truly in its habitable zone, any life developing there would not have to cope with severe doses of UV and X-rays. We should be able to get radial velocity information on this system that could firm up our assumptions about the composition of the three planets by determining their density when contrasted with the transit data that gives us their size.

For the time being, researchers at NASA GSFC have modeled 20 potential environments for TOI 700 d, using 3D climate models that consider various surface types and atmospheric compositions. Led by Gabrielle Engelmann-Suissa (a USRA visiting research assistant at GSFC), the team simulated 20 spectra for the 20 modeled environments. Dry, cloudless worlds and ocean-covered surfaces showed the range of possibilities. Such simulations can be of high value, as the paper on this modeling points out:

While the detection threshold of the spectral signals for this particular planet are most likely unfeasible for near-term observing opportunities, the end-to-end atmospheric modeling and spectral simulation study that we have performed in this work is an illustrative example of how global climate models can be coupled with a spectral generation model to assess the potential habitability of any HZ terrestrial planets discovered in the future, as we have done here with the exciting new discovery, TOI-700 d. With more discoveries on the horizon with TESS and ground-based surveys, we hope that this methodology will prove useful for not only predicting the observability of HZ planets but also for interpreting actual observations in the years to come.

The paper on the modeling is one of three describing the work on TOI 700 d. It makes clear that the noise floor of JWST, which takes into account instrument noise aboard the telescope, makes it unlikely the observatory will be able to characterize TOI 700 d. Similarly, direct imaging even by next-generation extremely large telescopes (ELTs) is challenging. Thus the paper’s conclusion: “Significant characterization efforts will therefore require future space-based IR interferometer missions such as the proposed LIFE (Large Interferometer For Exoplanets) mission.”

It’s good, then, to have TOI 700 d in our catalogs, but it’s not going to be the first exoplanet whose atmosphere we can probe for potential biosignatures.

Three papers describe this work. They are Gilbert et al., “The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System,” submitted to AAS Journals (preprint); Rodriguez et al., “The First Habitable Zone Earth-Sized Planet From TESS II: Spitzer Confirms TOI-700 d,” submitted to AAS Journals (preprint); Engelmann-Suissa et al., “The First Habitable Zone Earth-sized Planet from TESS. III: Climate States and Characterization Prospects for TOI-700 d,” submitted to the Astrophysical Journal (preprint).

Discovery of TESS Mission’s First Circumbinary Planet

TOI 1338b is a great catch, a circumbinary world that turned up in TESS data and was announced at the ongoing meeting of the American Astronomical Society in Hawai’i. Ravi Kopparapu (NASA GSFC) describes the discovery process in the essay below. The system lies 1,300 light years out in the constellation Pictor, with the planet transiting the larger star. Dr. Kopparapu’s work on exoplanet habitability is well known to Centauri Dreams readers. See, for example, his How Common Are Potential Habitable Worlds in Our Galaxy?, which ran in 2014. He followed this up with a look at an unusual multi-planet system (Ravi Kopparapu: Looking at K2-72). Analyzing habitable zone possibilities around different kinds of stars, as well as modeling and characterizing exoplanet atmospheres, plays a major role in his research interests. Here Dr. Kopparapu tells us about the new world and the significant role of an intern in its discovery, reminding us that the opportunities for young scientists to make a difference are abundant in this burgeoning field.

by Ravi Kumar Kopparapu

Back in December 2015 a couple of my colleagues, including Dr. Veselin Kostov from SETI Institute (who is now based at NASA Goddard) and Prof. Bill Welsh from San Diego State University (SDSU), met at a conference in Hawai’i. Veselin and Bill are astronomers well-known for discoveries of planets around binary star systems in the Kepler mission data. The topic of the discussion was submitting a NASA proposal to study the habitability of planets around binary stars. Planets in multiple star systems experience different kinds of illumination from their Suns (plural), varying periodically as the stars orbit each other, so the seasons or climate may be completely different than ours. It was decided that my theory colleague, Dr. Jacob Haqq-Misra from Blue Marble Space Institute of Science (BMSIS), will lead the proposal. A year or so after we submitted the proposal, we received a notification from NASA that our proposal was accepted. We were thrilled about this outcome and looked forward to start working soon.

About 2 years later, in January 2018, I received an email from a high school student named Wolf Cukier, asking if I could be his summer (2018) mentor for a high school project. His resume looked great, and he did his homework related to our group’s research. All was set and Wolf arrived in the summer of 2018.

Image: TOI 1338 b is silhouetted by its host stars. TESS only detects transits from the larger star. Credit: NASA’s Goddard Space Flight Center/Chris Smith.

Wolf worked with me on the project that we got funded through NASA. The project was to identify the habitable zones (HZs) of an Earth-like planet orbiting around two stars, which themselves are orbiting each other. These are called circumbinary planets (CBPs). We used a climate model to estimate the HZs around a variety of CBPs. I found that Wolf was a quick study and is capable of handling far more complex assignments. However, it was the end of his summer internship, so he had to leave. To tie up Wolf’s project with me, he and I started drafting a paper for publication, while Wolf was attending his school.

Image: NASA intern Wolf Cukier. Credit: Ravi Kopparapu.

Meanwhile at NASA, I mentioned to Veselin that Wolf would be very well suited to look for planets in the TESS mission data, particularly to search for circumbinary planets. At that time, only the Kepler mission had discovered transiting CBPs (about 10 of them). As exciting as these are, the small number leaves a vast gap in our understanding of this new class of worlds, not unlike the state of exoplanet science 20 years ago, when only a handful of hot-Jupiter exoplanets were known. Among the unknowns are the formation and migration efficiency of CBPs, their orbital architectures and occurrence rates. Therefore, discovering more CBPs in TESS mission data will open opportunities to answer these questions.

Veselin agreed that it would be a good project, particularly considering that no CBP was discovered by the TESS mission yet. Consequently, I offered Wolf another summer internship opportunity for 2019, with paid work through the NASA intern program. However, by summer, my calendar was booked with conference travel. It became so busy that I was not even going to be in town when Wolf would arrive to sign him in. I requested Veselin to take Wolf under his wing, which was going to happen anyway because of the work, but much earlier than Veselin expected. Being a great gentleman, and one of the nicest people I know, Veselin agreed.

I came back to my office from one of my conference travels, two days after Wolf joined, and was getting ready for my next conference travel. The next day, I got a cryptic message from Veselin asking if we all three could meet. I was concerned. What could it be? It couldn’t possibly be a discovery because Wolf just started 2 days ago. It would take weeks, sometimes months to even find a candidate planet, and that too for a high school student who is just learning data analysis. Was it not working out between Wolf and Veselin?

They came to my office the next day. I could see both were trying to hide something because both of them were containing, or trying to contain, their smiles. A small suspicion deep inside my mind started taking root. Veselin started, “We got something,” and quickly added “but we have to make sure”. Apparently within three days since he arrived at NASA, Wolf noticed that in one of the light curves there was both a prominent primary eclipse and an additional unknown feature.

This was initially flagged as an eclipsing binary on the Planet Hunters TESS platform. Planet Hunters is a citizen science project where in addition to primarily tagging transit-like features, volunteers may tag targets as various phenomena, including eclipsing binaries, variable stars, etc., thus effectively creating informal catalogs. These catalogs will be later followed-up by professional astronomers, like Veselin, to see if they are indeed candidate planets. Planet Hunters has already successfully contributed to the field of circumbinary planets through the independent discovery of Kepler-64 (also known as Planet Hunters-1).

Wolf immediately notified Veselin about the unknown feature flagged in the Planet Hunters catalog, and Veselin dutifully followed up to verify the authenticity with the help of his fellow astronomers Jeremy Orosz, Adina Feinstein and Bill Welsh. Veselin has a reputation of being extremely thorough and incredibly careful in analyzing candidate planets, so his standards for confirmation of a planet are pretty high, and if he says there is a planet in the data, you can take it to the bank. When Veselin came to my office to tell me that they may have discovered TESS’s first circumbinary planet, with the help of a high school student, I paid full attention.

They found a Saturn-sized planet in a 95-day orbit around both the stars, which means the planet is the longest period circumbinary planet found by TESS. The stars themselves orbit each other in 15 days, with one star being Sun-like and the other one a smaller, cooler star (effective temperatures of 5976 K and 3657 K, respectively. For comparison, the Sun is 5780 K). The planet is not in the habitable zone of its host stars (which Wolf verified based on his work from earlier summer). The system itself is estimated to be 4.4 Gyr old.

Video: Researchers working with data from NASA’s Transiting Exoplanet Survey Satellite (TESS) have discovered the mission’s first circumbinary planet, a world orbiting two stars. The planet, called TOI 1338 b, is around 6.9 times larger than Earth, or between the sizes of Neptune and Saturn. It lies in a system 1,300 light-years away in the constellation Pictor. The stars in the system make an eclipsing binary, which occurs when the stellar companions circle each other in our plane of view. One is about 10% more massive than our Sun, while the other is cooler, dimmer and only one-third the Sun’s mass. TOI 1338 b’s transits are irregular, between every 93 and 95 days, and vary in depth and duration thanks to the orbital motion of its stars. TESS only sees the transits crossing the larger star — the transits of the smaller star are too faint to detect. Its orbit is stable for at least the next 10 million years. The orbit’s angle to us, however, changes enough that the planet transit will cease after November 2023 and resume eight years later. Credit: NASA’s Goddard Space Flight Center.

The chain of events started in Hawai’i with a dinner chat, which led to a NASA grant. It in turn helped to recruit a diligent high school student, who helped meticulous astronomers confirm the first circumbinary planet discovered by the TESS mission. This is an exemplary example of how teamwork and valuable contributions from the most junior scientist to the senior, can produce high impact science.

Some Thoughts on Science Fiction Visuals

With the conclusion of The Man in the High Castle’s TV version, I’ve been having a few conversations about the ins and outs of turning the novel into a considerably bloated series. Or maybe I should say simply that when I realized at the end of the first season that, having made their choices and essentially filmed their version of the book, the producers were now going to go for further seasons, I was dismayed. Who would be making the choices now that the original author was not available, and how would the plot unfold? An ongoing series can do this well, of course — consider the absorbing tale unfolding in The Expanse — but going well outside the boundaries of a foundational novel can often be asking for trouble.

While I wasn’t much taken with the way The Man in the High Castle’s plot played out on TV, I did go ahead and watch every episode because I found the visuals so entrancing. The idea of a Japanese occupied California was fully realized, with touches like the Japanese fascination with American pop culture and antiques, the use of the I Ching by the trade minister (Dick’s fascination with and use of the book in his plots is well documented), and the interactions between the Nazi government in America and the Japanese one, with glimpses of Concord-like airliners bearing swastikas, were compelling and compulsively watchable.

So this is a post about film visuals, and I’m thinking about the topic because another Philip K. Dick novel, Do Androids Dream of Electric Sheep? is suddenly back in the news thanks to the death of a key figure in the movie made from it, Bladerunner. This was Syd Mead, who worked on the 1982 classic directed by Ridley Scott (who also had a hand in Amazon’s take on The Man in the High Castle). Mead served as a concept designer on the movie, a role he had also played in Hollywood creations like Aliens, Star Trek: The Movie and Tron. Here, as with The Man in the High Castle, I am drawn into concept as expressed in breathtaking design, for both film and TV series presented visual delights (although Mead was not involved with High Castle).

So let’s talk about Bladerunner. I think back to watching the film when it was released, having gone in without any knowledge of it other than that it was set in the future (I had seen some clips on one of the morning TV shows, but had not read any reviews, and didn’t even know it was drawn on a Philip K. Dick novel). I found myself pulled deeply, immersively, into this dystopian 2019 (the year the film was set). This was a Los Angeles like no other, with colossal buildings and streets so far below they were almost subterranean. All wrapped around a plot with elements of Raymond Chandler, and the Harrison Ford voiceover of the original (not the director’s cut). Speaking of that, I am one of the apparent minority that preferred the voiceover version, probably because I’m such a fan of 1940s era film noir.

And then there were those vehicles, all drawing on Mead’s vast experience as a designer for companies like Ford and Philips. Mead spent two years developing concept cars — i.e., vehicles of the future — for Ford Motor Company’s Advanced Styling Studio, and launched his own design firm in 1970. He had clients worldwide, particularly in Japan (Sony, NHK and Honda, among others). His film and animation projects made him wildly popular in Japan, a place that obviously fascinated the man, as witness the Japanese aura of Bladerunner.

Image: A corporate ‘spinner’ in an early concept drawing from the Bladerunner Sketchbook.

‘Spinners’ wafted the wealthy in the stratified upper reaches of the city wherever they wanted to go, and served the police for their patrols, but these were not the flying cars that have often been conceptualized in science fiction art. Mead’s vehicles were realistic and looked like they had seen plenty of use, utilitarian workhorses with gull-wing, vertically opening doors, with a look that made them equally at home on the street or in the air, where the wheel covers had rotated and the device lifting the vehicle was enclosed within it — no wings or propellers here.

Mead said that Bladerunner was “not a ‘hardware’ movie.’ It’s not one of those gadget-filled pictures where the actors seem to be there only to give scale to the sets, props and effects. We’ve created an environment to make the story believable. The tools and machinery appear only when needed and fit tightly into the plot.”

That quote can be found in the Bladerunner Sketchbook, where you can see how everything from skyscrapers to parking meters emerged, based on principles woven around a fully realized model of the future, one drawing on a career in industrial design. Many of the original production designs here were subsequently modified, offering a rare chance to see how a film’s visuals evolve even as it is in the latter stages of production.

Syd Mead started doing movie concept work in 1978, four years before Bladerunner. As an aside, my friend Al Jackson, who is as good at ferreting out information about writers as associate editor Larry Klaes is at reverse-engineering films, told me that Mead did some concept art for a planned remake of Forbidden Planet that was never made. I wonder where that artwork is today? Al describes Mead’s earlier work as looking a bit like Saturday Evening Post illustrations, but adds that he then began to produce an other-worldly science fictional art unlike anything else in the market at that time. Al sends along a piece called “Race of DGXXX,” though neither of us knows where it originally ran.

The image below is the one that caught my eye on the morning TV show that was talking about Bladerunner. I was in a motel room somewhere in the Cumberlands in 1982, getting ready to check out and head west, and I remember standing there with my suitcase open on the bed behind me mesmerized by the visuals. This was a movie I had to see, and I did so as soon as possible.

I had never heard of Syd Mead back then and only found out that Philip K. Dick had died by watching the final credits on the film. But I had never seen as fully visualized and boldly executed a vision of the future as this one. Those of us wrapped up in the literature of the imagination and the filming of same have suffered a grievous loss with the death of Syd Mead.

A final thought: Apple TV+ is in the pre-production phase for its version of Asimov’s Foundation, with Jared Harris playing Hari Seldon. It will be interesting to see how the visuals on Trantor turn out!

Mapping Asteroid Bennu

The holiday season seems an appropriate time to thank not only my Centauri Dreams readers for their continued high level of discussion in these pages, but also the army of citizen scientists who are out there working on everything from exoplanet detection to asteroid mapping. We saw recently how valuable the work of amateurs like Thiam-Guan Tan can be in confirming a possible exoplanet, while projects like the Habitable Exoplanet Hunting Project continue coming online to push the boundaries of what amateur equipment can do.

Now comes word of the signal contribution made to OSIRIS-REx and its mission to asteroid Bennu. You’ll recall that when the spacecraft arrived at the asteroid, the surface was found to be far more littered with rocks and boulders than anyone had foreseen. Finding a spot for landing and retrieving samples would be no easy task, but it was made substantially more manageable by a team of 3,500 people using their PCs to join in analysis and characterization of the asteroid surface.

These volunteers measured boulders and marked craters, eventually tallying over 14 million annotations of features on Bennu’s emerging global map. Behind all this work was CosmoQuest, a project based at the Planetary Science Institute in Tucson, Arizona.

“It is amazing that more than 3,500 citizen scientists participated in CosmoQuest’s project to map Bennu and help mission scientists find the best place for OSIRIS-REx to collect a sample,” said Pamela L. Gay, Senior Scientist and Senior Education and Communication Specialist at PSI. “This kind of a volunteer effort makes it easier to find safe places to sample and scientifically interesting places to explore.”

Image: This image shows sample site Nightingale, OSIRIS-REx’s primary sample collection site on asteroid Bennu. The image is overlaid with a graphic of the OSIRIS-REx spacecraft to illustrate the scale of the site. Credit: NASA/Goddard/University of Arizona.

During the four month period needed to complete the mapping, some volunteers marked more than 500 images (the average was closer to 10), with each image taking up to 45 minutes to complete. It seems worthwhile to list the usernames of those with the greatest number of contributions: MikeCassidy, Nilium, bc2callhome, zathras, joed, dpi209, and pattyg. PSI’s CosmoQuest team will continue working with the Bennu science team to generate science drawn from the mapping data now that the initial site selection has been performed.

If you missed out on the Bennu mapping but would like to get involved, CosmoQuest intends to be launching new citizen science projects some time in 2020, so keep an eye on the site. A good New Year’s resolution might be to get involved in one or more of the many sites catering not just to amateur astronomers but interested laypeople willing to devote time to image analysis. Have a look, for example, at Zooniverse’s list of projects on physics to get an idea of the range. It’s clear that space missions draw real value out of the kind of citizen participation that, not so many years ago, was limited to watching images on a television. Actually joining in efforts that can assist a mission or discover new worlds through its data is no longer a novelty.

Image: All 3,640 names of the Bennu Mappers are superimposed on this Global Mosaic of the Bennu Asteroid that was acquired by the OSIRIS-REx Mission (the image has to be blown up several times to actually see the names). Credit: Created using sources images from NASA/Goddard/University of Arizona.

Let me wish all of you a wonderful holiday and an energized return to work when the season ends. Working on behalf of ideas one believes in is a high vocation. Let’s continue to focus in 2020 on pushing the seemingly intractable problem of interstellar flight forward with new ideas and clarifications of the old.