Paul Gilster's Blog, page 7

Pondering Life in an Alien Ocean

No one ever said Europa Clipper would be able to detect life beneath the ice, but as we look at the first imagery from the spacecraft’s star-tracking cameras, it’s helpful to keep the scope of the mission in mind. We’re after some critical information here, such as the thickness of the ice shell, the interactions between shell and underlying ocean, the composition of that ocean. All of these should give us a better idea of whether this tiny world really can be a home for life.

Image: This mosaic of a star field was made from three images captured Dec. 4, 2024, by star tracker cameras aboard NASA’s Europa Clipper spacecraft. The pair of star trackers (formally known as the stellar reference units) captured and transmitted Europa Clipper’s first imagery of space. The picture, composed of three shots, shows tiny pinpricks of light from stars 150 to 300 light-years away. The starfield represents only about 0.1% of the full sky around the spacecraft, but by mapping the stars in just that small slice of sky, the orbiter is able to determine where it is pointed and orient itself correctly. The starfield includes the four brightest stars – Gienah, Algorab, Kraz, and Alchiba – of the constellation Corvus, which is Latin for “crow,” a bird in Greek mythology that was associated with Apollo. Besides being interesting to stargazers, the photos signal the successful checkout of the star trackers. The spacecraft checkout phase has been going on since Europa Clipper launched on a SpaceX Falcon Heavy rocket on Oct. 14, 2024. Credit: NASA/JPL-Caltech.

Seen in one light, this field of stars is utterly unexceptional. Fold in the understanding that the data are being sent from a spacecraft enroute to Jupiter, and it takes on its own aura. Naturally the images that we’ll be getting at the turn of the decade will far outdo these, but as with New Horizons, early glimpses along the route are a way of taking the mission’s pulse. It’s a long hike out to our biggest gas giant.

I bring this up, though, in relation to new work on Enceladus, that other extremely interesting ice world. You would think Enceladus would pose a much easier problem when it comes to examining an internal ocean. After all, the tiny moon regularly spews material from its ocean out through those helpful cracks around its south pole, the kind of activity that an orbiter or a flyby spacecraft can readily sample, as did Cassini.

Contrast that with Europa, which appears to throw the occasional plume as well, though to my knowledge, these plumes are rare, with evidence for them emerging in Hubble data no later than 2016. It’s possible that Europa Clipper will find more, or that reanalysis of Galileo data may point to older activity. But there’s no question that in terms of easy access to ocean material, Enceladus offers the fastest track.

Enceladus flybys by the Cassini orbiter revealed ice particles, salts, molecular hydrogen and organic compounds. But according to a new paper from Flynn Ames (University of Reading) and colleagues, such snared material isn’t likely to reveal life no matter how many times we sample it. Writing in Communications Earth and Environment, the authors make the case that the ocean inside Enceladus is layered in such a way that microbes or other organic materials would likely break down as they rose to the surface.

In other words, Enceladus might have a robust ecosystem on the seafloor and yet produce jets of material which cannot possibly yield an answer. Says Ames:

“Imagine trying to detect life at the depths of Earth’s oceans by only sampling water from the surface. That’s the challenge we face with Enceladus, except we’re also dealing with an ocean whose physics we do not fully understand. We’ve found that Enceladus’ ocean should behave like oil and water in a jar, with layers that resist vertical mixing. These natural barriers could trap particles and chemical traces of life in the depths below for hundreds to hundreds of thousands of years.”

The study relies on theoretical models that are run through global ocean numerical simulations, plugging in a timescale for transporting material to the surface across a range of salinity and mixing (mostly by tidal effects). Remarkably, there is no choice of variables that offers an ocean that is not stratified from top to bottom. In this environment, given the transport mechanisms at work, hydrothermal materials would take centuries to reach the plumes, with obvious consequences for their survival.

From the paper:

Stable stratification inhibits convection—an efficient mechanism for vertical transport of particulates and dissolved substances. In Earth’s predominantly stably stratified ocean this permits the marine snow phenomena, where organic matter, unable to maintain neutral buoyancy, undergoes ’detrainment’, settling down to the ocean bottom. Meanwhile, the slow ascent of hydrothermally derived, dissolved substances provides time for scavenging processes and usage by life, resulting in surface concentrations far lower than those present nearer source regions at depth.

Although its focus is on Enceladus, the paper offers clear implications for what may be going on at Europa. Have a look at the image below (drawn not from the body of the paper but from the supplementary materials linked after the footnotes) and you’ll see the problem. We’re looking at these findings as applied to what we know of Europa.

Image: From part of Figure S7 in the supplementary materials. Caption: “Tracer age (years) at Europa’s ocean-ice interface, computed using the theoretical model outlined in the main text. Note that age contours are logarithmic.” Credit: Ames et al.

The figure shows the depth of the inversion and age of the ice shell for the same ranges in ocean salinity as inserted for Enceladus. Here we have to be careful about how much we don’t know. The ice thickness, for instance, is assumed as 10 kilometers in these calculations. Given all the factors involved, the transport timescale through the stratified layers of the modeled Europa is, as the figure shows, over 10,000 years. The same stratification layers impede delivery of oxidants from the surface to the ocean.

So there we are. The Ames paper stands as a challenge to the idea that we will be able to find evidence of life in the waters just below the ice, and likewise indicates that even if we do begin to trace more plumes from Europa’s ocean, these would be unlikely to contain any conclusive evidence about biology. Just what we needed – the erasure of evidence due to the length of the journey from the ocean depths to the ice sheet. Icy moons, it seems, are going to remain mysterious even longer than we thought.

The paper is Ames et al., “Ocean stratification impedes particulate transport to the plumes of Enceladus,” Communications Earth & Environment 6 (6 February 2025), 63 (full text).

Putting AI to Work on Technosignatures

As a quick follow-up to yesterday’s article on quantifying technosignature data, I want to mention the SETI Institute’s invitation for applicants to the Davie Postdoctoral Fellowship in Artificial Intelligence for Astronomy. The Institute’s Vishal Gajjar and his collaborators both in the US and at IIT Tirupati in India will be working with the chosen candidate to focus on neural networks optimized for processing image data, so-called ‘CNN architectures’ that can uncover unusual signals in massive datasets.

“Machine learning is transforming the way we search for exoplanets, allowing us to uncover hidden patterns in vast datasets,” says Gajjar. “This fellowship will accelerate the development of advanced AI tools to detect not just conventional planets, but also exotic and unconventional transit signatures including potential technosignatures.”

As AI matures, the exploration of datasets is a critical matter as these results from missions like TESS and Kepler are packed with both exoplanet data as well as stellar activity and systematics that can mislead investigators. Frameworks for sifting out anomalies should help us distinguish unusual candidates including disintegrating objects, planets with rings, exocomets and perhaps even megastructures and other technosignatures, all flagged by their deviation from our widely used transit models.

The data continue to accumulate even as our AI tools sharpen to look for anomalies. I can think of several Centauri Dreams readers who should find this work right up their alley. If you’re interested, you can find everything you need to apply for the fellowship here. The deadline for applications is March 15, 2025.

Quantifying the Chances of a Technosignature

It’s one thing to talk about technology as we humans know it, but applying it to hypothetical extraterrestrials is another matter. We have to paint with a broad brush here. Thus Jason Wright’s explanation of technosignatures as conceived by SETI scientists. The Penn State astronomer and astrophysicist defines technology in that context as “the physical manifestations of deliberate engineering.” That’s saying that a technology produces something that is in principle observable. Whether or not our current detection methods are adequate to the task is another matter.

Image: Artist’s concept of an interesting radio signal from galactic center. But the spectrum of possible technosignature detections is broad indeed, extending far beyond radio. Credit: UCLA SETI Group/Yuri Beletsky, Carnegie Las Campanas Observatory.

A technosignature need not be the sign of industrial or scientific activity. Consider: In a new paper in The Astronomical Journal, Sofia Sheikh (SETI Institute) and colleagues including such SETI notables as Wright himself, Ravi Kopparapu and Adam Frank point out that the extinction of ancient megafauna some 12,800 years ago may have contributed to changes in atmospheric methane that fed into a period of cooling known as the Younger Dryas, to be followed by growing human agricultural activity whose effects on carbon dioxide and methane in the atmosphere would be detectable.

As a technosignature, that one has a certain fascination, but it’s not likely to be definitive in ferreting out extraterrestrials, at least not at our stage of detection technology. But Sheikh and team are really after a much less ambiguous question. We know what our own transmission and detection methods are. How far away can our own technosignature be detected? By studying the range of technosignatures we are producing on Earth, the authors produce a scale covering thirteen orders of magnitude in detectability, with radio still at the top of the heap. The work establishes quantitative standards for detectability based on Earth’s current capabilities.

We might, for example, use the James Webb Space Telescope and the upcoming Habitable Worlds Observatory to provide data on atmospheric technosignatures as far out as 5.7 light years away. That takes us interstellar, with that interesting system at Proxima Centauri in range. Let’s tarry a bit longer on this one. While carbon dioxide is implicated in manmade changes to Earth’s atmosphere, the paper points to other sources, zeroing in on one in particular:

…there are other atmospheric technosignatures in Earth’s atmosphere that have very few or even no known nontechnological sources. For example, chlorofluorocarbons (CFCs), a subcategory of halocarbons, are directly produced by human technology (with only very small natural sources), e.g., refrigerants and cleaning agents, and their presence in Earth’s atmosphere constitutes a nearly unambiguous technosignature (J. Haqq-Misra et al. 2022). Nitrogen dioxide (NO2), like CO2, has abiotic, biogenic, and technological sources, but combustion in vehicles and fossil-fueled power plants is a significant contributor to the NO2 in Earth’s atmosphere (R. Kopparapu et al. 2021).

And indeed nitrogen dioxide (NO2) is what the authors plug into this study, drawing on earlier work by some of the paper’s authors. Note the fact that biosignatures and technosignatures overlap here given how much work has proceeded on characterizing exoplanet atmospheres. It turns out that the wavelength bands that the Habitable Worlds Observatory will see best in its search for biosignatures are also those that include the NO2 technosignature, a useful example of piggybacking our searches.

But of course the realm of technosignatures is wide, including everything from the lights of cities to ‘heat islands’ (inferring cities), orbiting satellites, radio transmissions and lasers. I’m aware of no other study that combines the various forms of technosignature in a single analysis. If you start looking at the full range of technosignatures according to distance, you find objects on a planetary surface to be the toughest catch, with heat islands swimming into focus only from a distance as far as outer planetary orbits in the Solar System. The current technology with the most punch is planetary radar, whose pulses should be detectable as much as 12,000 light years away, although such a signal would be a fleeting and non-repeating curiosity.

SETI does find signals like that now and then. But precisely because they are non-repeating, we simply don’t know what to make of them.

Image: The maximum distances that each of Earth’s modern-day technosignatures could be detected at using modern-day receiving technology, in visual form. Also marked are various astronomical objects of interest. Credit: Sheikh et al.

Think back to the early days of SETI and ponder how far we’ve come in trying to understand what other civilizations might do that could get us to notice them. SETI grew directly out of the famous work by Giuseppe Cocconi and Philip Morrison that laid out the case for artificial radio signals in 1959, followed shortly thereafter by Frank Drake’s pioneering work at Green Bank with Project Ozma. Less known is the 1961 paper by Charles Townes and R. N. Schwartz that got us into optical wavelengths.

And while ‘Dysonian SETI’, which explicitly searches for technosignatures, is usually associated with vast engineering projects like Dyson spheres, the point here is that a civilization will produce evidence for an outside observer that will continue to deepen as that observer’s tools increase in sophistication. The search for technosignatures, then, actually grows into a multi-wavelength effort, but one that spans a vast range. Making all this quantitative involves a ‘detectability distance scale.’ The authors choose one known as an ichnoscale. Here’s how the paper describes it:

Using Earth as a mirror in this way, we can employ the concept of the ichnoscale (ι) from H. Socas-Navarro et al. (2021): “the relative size scale of a given technosignature in units of the same technosignature produced by current Earth technology.” An ι value of 1 is defined by Earth’s current technology. This necessarily evolves over time—for this work, we set the ichnoscale to Earth-2024-level technology, including near-future technologies that are already in development.

Considering how fast our detection methods are improving as we build extremely large telescopes (ELTs) and push into ever more sophisticated space observatories, learning the nature of this scale will become increasingly relevant. While we realized in the mid-20th Century that radio was detectable at interstellar distances, we’re now able to detect not just an intentional signal but radio leakage, at least from nearby stellar systems. That’s an extension of the parameter space that involves levels of power we have already demonstrated on Earth. The ichnoscale framework quantifies these signatures that will gradually become possible to detect as our methods evolve.

We see more clearly which methods are most likely to succeed. This is an important scaling because the universe we actually live in may not resemble the one we construct in our imaginations. Let me quote the paper on this important point:

…the focus on planetary-scale technosignatures provides very specific suggestions for which searches to pursue in a Universe where large-scale energy expenditures and/or travel between systems is logistically infeasible. While science fiction is, for example, replete with mechanics for rapid interstellar travel, all current physics implies it would be slow and expensive. We should take that constraint seriously.

And with this in mind we can state key results:

1. Radio remains the way that Earth is most detectable at ι = 1.
2. Investment in atmospheric biosignature searches has opened up the door for atmospheric technosignature searches.
3. Humanity’s remotely detectable impacts on Earth and the solar system span 12 orders of magnitude.
4. Our modern-day planetary-scale impacts are modest compared to what is assumed in many technosignature papers.
5. We have a multiwavelength constellation of technosignatures, with more of the constellation becoming visible the closer the observer becomes.

Let’s pause on item 4. The point here is that most notions of technosignatures assume technologies visible on astronomical scales, and indeed it is usually assumed that our first SETI detections, when and if they come, will involve civilizations vastly older and superior in technology than ourselves. Planets bearing technologies like those we have today are a supremely difficult catch, because the technosignatures we are throwing are tiny and all but trivial compared to the Dyson spheres and starship beaming networks we typically consider. And this point seems overwhelmingly obvious:

We should be careful about extrapolating current technosignatures to scales of ιTS = 10 (or even ιTS = 2) without considering the changing context in which these technologies are being developed, used, and (sometimes) mitigated or phased out (e.g., the recovery of the ozone hole; J. Kuttippurath & P. J. Nair 2017). As another example, we are becoming aware of the negative health effects of the UHI [urban heat index] (as detailed in, e.g., A. Piracha & M. T. Chaudhary 2022); thus, work may be done to mitigate the concentrated regions of high infrared flux discussed in Section 4.3.

Indeed. How many of the technosignatures we are producing are stable? Chlorofluorocarbons in the atmosphere are subject to adjustment on astronomically trivial timeframes. The chances of running into a culture that is about to realize it is polluting itself just before it takes action to mitigate the problem seem remote. So all these factors have to be taken into account as we rank technosignature detection strategies, and it’s clear that in this “multiwavelength constellation of technosignatures” the closer we are, the better we see. All the more reason to continue to pursue not just better telescopes but better ways to get ever improving platforms into the outer Solar System and beyond. Interstellar probes, anyone?

The paper is Sheikh et al., “Earth Detecting Earth: At What Distance Could Earth’s Constellation of Technosignatures be Detected with Present-day Technology?” Astronomical Journal Vol. 169, No. 2 (3 February 2025), 118 (full text). The Cocconi and Morrison paper is “Searching for Interstellar Communications,” Nature 184 (19 September 1959), 844-846 (abstract). The 1961 paper on laser communications is Schwartz and Townes, “Interstellar and Interplanetary Communication by Optical Masers,” Nature 190 (15 April 1961), 205-208 (abstract).

A Fast Radio Burst in a Dead Elliptical Galaxy

Work is healing, so let’s get back to it. I’m enthralled with what we’re discovering as we steadily build our catalog of fast radio bursts (FRB), close to 100 of which have now been associated with a galaxy. These are transient radio pulses of short duration (down to a fraction of a millisecond, though some last several seconds), the first being found in 2007 by Duncan Lorimer, as astronomer at West Virginia University. Sometimes FRBs repeat, although many do not, and one is known to repeat on a regular basis.

What kind of astrophysical processes might be driving such a phenomenon? The leading candidate appears to be supernovae in a state of core collapse, producing vast amounts of energy as stars more massive than the Sun end their lives. Out of such catastrophic events a type of neutron star called a magnetar may be produced, its powerful magnetic field pumping out X-ray and gamma ray radiation. Young, massive stars and regions of active star formation are implicated under this theory. But as we’re learning, magnetars are only one of a possible range of candidates.

For the event known as FRB 20240209A, detected in 2024 by the Canadian Hydrogen Intensity Mapping Experiment (CHIME), has dealt us a wild card. Remember, a single FRB can produce more energy in a quick burst than our Sun emits in an entire year. This one has repeatedly fired up, producing 21 pulses between February and July of last year. And the problem with it is that it has been traced to a galaxy in which star formation has ceased. That finding is verified by data from the Gemini North telescope and the Keck Observatory using its Low Resolution Imaging Spectrometer (LRIS).

Yuxin (Vic) Dong is an NSF Graduate Research Fellow and second author on one of two papers recently published on the event:

“For nearby galaxies, there is often archival data from surveys available that tells you the redshift — or distance —to the galaxy. However, in some cases, these redshift measurements may lack precision, and that’s where Keck Observatory and the LRIS instrument becomes crucial. Using a Keck/LRIS spectrum, we can extract the redshift to a very high accuracy. Spectra are like fingerprints of galaxies, and they contain special features, called spectral lines, that encode tons of information about what’s going on in the galaxy like the stellar population age and star formation activity. What’s really fascinating in this case is that the features we saw from the Keck/LRIS spectrum revealed that this galaxy is quiescent, meaning star formation has shut down in the galaxy. This is strikingly different from most FRB galaxies we know which are still actively making new stars.”

Image: The ellipse shows the location of the FRB and the crosshairs point to its host galaxy, taken with the Gemini North telescope from Maunakea. Credit: Shah et al.

It turns out that FRB 20240209A is coming from a galaxy fully 11.3 billion years old some 2 billion light years from Earth. This is painstaking work and quite productive, for the papers’ authors report that the galaxy is both extremely luminous and the most massive FRB host galaxy yet found. Moreover, while FRBs that have been associated with their host galaxies are usually located deep within the galaxy, this one occurs 130,000 light years from galactic center, in a region with few stars nearby.

When you’re dealing with a new phenomenon, finding similar events can be productive. In this case, there is one other FRB that can be placed in the outskirts of a galaxy, the spiral M81. While FRB 20240209A occurred in an ancient elliptical galaxy, it like the M81 event is far from areas of active star formation, again raising the possibility that FRBs have causes we have yet to pin down. From the Eftekhari et al. paper:

Since the first host associations, investigations into FRB host demographics have offered valuable insights into the origins of FRBs and their possible progenitor systems. Such studies remain in their infancy, however. With the development of interferometric capabilities for various FRB experiments and the promise of hundreds of precisely localized events, the discovery landscape for new and unforeseen hosts and environments presents considerable potential.

And as the paper notes, FRB 20240209A isn’t the first FRB that challenges our assumptions:

Indeed, the connection of a few FRBs with remarkable environments, including dwarf galaxies (S. Chatterjee et al. 2017; C. H. Niu et al. 2022), a globular cluster (F. Kirsten et al. 2022), and the elliptical host of FRB 20240209A, implicate exotic formation channels as well as older stellar populations for some FRBs and demonstrate that novel environments offer significant constraining power for FRB progenitors. A larger sample of host associations will further uncover intriguing diversity in host environments and may identify interesting subpopulations or correlations with FRB repetition, energetics, or other burst characteristics, contributing to a clearer understanding of FRB origins.

Image: CHIME detectors. Credit: CHIME, Andre Renard, Dunlap Institute for Astronomy & Astrophysics, University of Toronto.

Clearly we have a long way to go as the FRB catalog grows. Senior author Wen-fai Fong, who was involved in both papers, likes to talk about the surprises the universe has in store for us, disrupting any possibility of scientific complacency. Instead, we are often confronted with yet another reason to revise our thinking, in what Fong refers to as “a ‘dialogue’ with the universe” as we pursue time-domain astronomy, the analysis of changes in brightness and spectra over time so suited for mysterious FRBs.

The papers are Shah et al., “A repeating fast radio burst source in the outskirts of a quiescent galaxy,” Astrophysical Journal Letters Vol. 979, No. 2 (21 January 2025) (full text) and Eftekhari et al., “The massive and quiescent elliptical host galaxy of the repeating fast radio burst FRB 20240209A,” Astrophysical Journal Letters Vol. 979 No. 2 (21 January 2025). Full text.

Centauri Dreams to Resume Soon

I’d like to thank all of you who wrote comments and emails about the recent pause in Centauri Dreams. My beautiful wife Eloise passed away on January 17. It was as peaceful a death as can be imagined, and I am so pleased to say that she was able to stay at home until the end. As she had battled Alzheimer’s for eleven gallant years, death was simply a bridge that now had to be crossed. As she did with everything else in her life, she did it with class.

This is to let you know that I will be getting Centauri Dreams back into action again in about three weeks. When I began the site in 2004, my primary goal was to teach myself as much as I could about the topics we address by writing about them, which is how I’ve always tended to learn things. I’ve always welcomed comments that informed me, caught my errors and extended the discussion into new realms. No one could work with a better audience than the readers I’ve been privileged to address, and for this I will always be profoundly grateful.

A Necessary Break

It’s time to write a post I’ve been dreading to write for several years now. Some of my readers already know that my wife has been ill with Alzheimer’s for eleven years, and I’ve kept her at home and have been her caregiver all the way. We are now in the final stages, it appears, and her story is about to end. I will need to give her all my caring and attention through this process, as I’m sure you’ll understand. And while I have no intention of shutting down Centauri Dreams, I do have to pause now to devote everything I have to her. Please bear with me and with a bit of time and healing, I will be active once again.

Recalibrating ‘Hot Jupiter’ Migration

What catches your eye in this description of an exoplanetary system? Start with a ‘hot Jupiter,’ with a radius 0.87 times that of our Jupiter and an orbit of 7.1 days. This is WASP-132b, confirmed in 2016, and first discovered through the labors of the Wide-Angle Search for Planets program. Subsequent confirmation came through the CORALIE spectrograph installed on the Euler telescope at the European Southern Observatory’s La Silla site. This world orbits a K-class star 403 light years out in Lupus.

The CORALIE measurements gave hints of another giant planet in a long period orbit. The system came still further into focus in 2021, when observations from TESS (Transiting Exoplanet Survey Satellite) showed a transiting super-Earth with a diameter of 1.8 Earth radii in a tight orbit of 1.01 days. The mass of the planet, as measured by the HARPS spectrograph at La Silla, is six times that of Earth. So we have both a hot-Jupiter and a super-Earth hugging the star, along with an outer gas giant.

Image: The WASP-132 system was known to harbour WASP-132b, here in the foreground, a hot Jupiter planet orbiting around a K-type star in 7.1 days. New data confirms the system has more planets, including an inner super-Earth, here seen transiting in front of the orange host-star. Visible as a pale blue dot near the top right corner is also the giant planet WASP-132d discovered in the outskirts of the system. © Thibaut Roger – Université de Genève.

While the European Space Agency’s Gaia satellite continues to take astrometrical data on WASP-132, follow-up work has shown the super-Earth to have a density similar to Earth’s and a composition of metals and silicates fairly similar to our planet (remember, we have both radius and mass measurements to work with because of the multiple datasets from different detection methods). Meanwhile, the problem here should be apparent.

Ravit Helled (University of Zurich) and a co-author of the study offers this:

‘‘The combination of a Hot Jupiter, an inner Super-Earth and an outer giant planet in the same system provides important constraints on theories of planet formation and in particular their migration processes. WASP-132 demonstrates the diversity and complexity of multi-planetary systems, underlining the need for very long-term, high-precision observations.’’

All true, of course, but it doesn’t get across how unusual this finding is. For hot Jupiters as thus far observed have been relatively isolated from planets further out in their systems. That makes sense because the model for their formation involves migration, with the giant worlds forming far enough out from the star to feed off plentiful gas and dust in the protoplanetary disk, and then moving inward as the system takes form. Woe to inner planets, whose fate might include ejection from the system entirely.

WASP-132 shouldn’t have the system architecture it does given this theory of migration, meaning we have to re-examine the nature of migration, or ponder ways to achieve a planet of this size in a tight stellar orbit that leave migration behind altogether. The hot Jupiter here leads François Bouchy (University of Geneva), a co-author of the study, to say this:

“The WASP-132 system is a remarkable laboratory for studying the formation and evolution of multi-planetary systems. The discovery of a Hot Jupiter alongside an inner Super-Earth and a distant giant planet calls into question our understanding of the formation and evolution of these systems.”

To my knowledge, WASP-132 is the second example of a planetary system in this configuration. WASP-47 takes precedence in terms of discovery, having been first analyzed by the WASP team in 2012 (discovery of the hot Jupiter) and then expanded through work with K2 data in 2015. WASP-47, a G-class star in Aquarius some 880 light years away, hosts a super-Earth inside the hot Jupiter’s orbit, a hot Neptune outside its orbit, and an outer gas giant (‘warm Saturn’) within the habitable zone. The discovery paper of the smaller worlds at WASP-47 is worth quoting:

The continued existence of the companions in this system indicates that HEM [high eccentricity migration] ] cannot serve as the sole formation mechanism for hot Jupiters. HEM would likely have disrupted the orbits of the smaller planets. It is quite possible that there is more than one potential formation mechanism for hot Jupiters. Additionally, recent observations have identified an additional Jupiter-mass planet in a 571-day orbit (called WASP-47c; Neveu-VanMalle et al. 2015) in this system, making this the first hot Jupiter with both close-in companions and an external perturber. Future dynamical work will place limits on the architecture of this system.

The paper is Thibaut et al., ”Discovery of a cold giant planet and mass measurement of a hot super-Earth in the multi-planetary system WASP-132′,” Astronomy and Astrophysics Vol. 693 (15 January 2025), A144 (full text). On WASP-132, see Becker at al., “WASP-47: A Hot Jupiter System With Two Additional Planets Discovered by K2,” Astrophysical Journal Letters Vol. 812, No. 2 (12 October 2015), L18 (full text).

Planet Population around Orange Dwarfs

Last Friday’s post on K-dwarfs as home to what researchers have taken to calling ‘superhabitable’ worlds has caught the eye of Dave Moore, a long-time Centauri Dreams correspondent and author. Readers will recall his deep dives into habitability concepts in such essays as The “Habitability” of Worlds and Super Earths/Hycean Worlds, not to mention his work on SETI (see If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare). Dave sent this in as a comment but I asked him to post it at the top because it so directly addresses the topic of habitability prospects around K-dwarfs, based on a quick survey of known planetary systems. It’s a back of the envelope overview, but one that implies habitable planets around stars like these may be more difficult to find than we think.

by Dave Moore

To see whether K dwarfs made a good target for habitable planets, I decided to look into the prevalence and type of planets around K dwarfs and got carried away looking at the specs for 500 systems of dwarfs between 0.6 mass of the sun and 0.88.

Some points:

i) This was a quick and dirty survey.

ii) Our sampling of planets is horribly skewed towards the massive and close, but that being said, we can tell if certain types of planets are not in a system. For instance Jupiter and Neptune sized planets at approximately 1 au show up, so if a system doesn’t show them after a thorough examination, it won’t have them.

iii) I had trouble finding a planet list that was configurable to my needs. I finally settled on the Exoplanets Data Explorer configured in reverse order of stellar mass. This list is not as comprehensive as the Exosolar Planetary Encyclopedia.

iv) I concocted a rough table of the inner and outer HZ for the various classes of K dwarfs. Their HZs vary considerably. A K8 star’s HZ is between 0.26 au and 0.38 au while a K0’s HZ is between 0.72 au and 1.04 au. This means that you can have two planets orbiting at the same distance around a star and one I will classify as outside the HZ and the other inside the HZ.

v) Planets below 9 Earth mass I classified as Super-Earth/Sub-Neptune. Planets between 9 Earth masses and 30 are classified as Neptunes. Planets over that size are classified as Jupiters.

Image: An array of planets that could support life are shown in this artist’s impression. How many such worlds orbit K-dwarf stars, and are any of them likely to be ‘superhabitable’? Credit: NASA, ESA and G. Bacon (STScI).

What did I find:

By far the most common type are hot Super-Earths/Sub-Neptunes (SE/SNs). These are planets between 3 EM (Earth mass) and 6 EM. It is amazing the consistency of size these planets have. They are mostly in close (sub 10 day) orbits. There also appears to be a subtype of sub 2EM planets in very tight orbits (some quoted in hours) and given some of these were in multi-planet systems of SE/SNs, I would say these were SE/SNs, which have been evaporated down to their cores.

I also found 7 in the HZ and 2 outside the HZ.

I found 52 hot Jupiters and what I classified as 43 elliptical orbit Jupiters. These were Jupiter-sized planets in elliptical orbits under 3 au.

There were also 10 Jupiter classification planets in circular orbits under 3 au. and 3 outside that limit in what could be thought of as a rough analog of our system.

There were also 46 hot Neptunes and 14 in circular orbits further out, only one outside the habitable zone.

Trends:

At the lower mass end of the scale, K dwarf systems start off looking very much like M dwarfs except that everything, even those in multi-planet systems, is inside the habitable zone.

As you work your way up the mass scale, there is a slight increase in the average mass of the SE/SNs with 7-8 EM planets becoming more prevalent. More and more Jupiters appear, and Neptune-sized planets appear and become much more frequent. Also, you get the occasional monster system of tightly packed Jupiters and Neptunes like 55 Cancri.

An interesting development begins at about the mid mass range. You start getting SE/SNs in nice circular longer period orbits but still inside the HZ (28 in 20-100d orbits.)

Conclusions:

If we look at the TRAPPIST-1 system around an M-dwarf, its high percentage of volatiles (20% water/ice) implies that there is a lot of migration in from the outer system. If a planet has migrated in from outside the snow line, then there’s a good chance that even if it’s in the habitable zone, it will be a deep ocean planet.

Signs of migration are not hard to find. Turning back to the K-dwarfs, if we look at the Jupiters, only three show signs of little migration (analogs of our system). Ten migrated in smoothly but sit at a distance likely to have disrupted a habitable planet. Forty-three are in elliptical orbits, which are considered signs of planet-planet scattering.

Hot Jupiters can be accounted for by either extreme scattering or migration. As to inward migration, Martin Fogg did a series of papers showing that as Jupiter mass planets march inwards they scatter protoplanets, but these can reform behind the giant, and so Earth-like planets may occur outside of the hot Jupiter.

Neptunes in longer period circular orbits and the longer period SE/SNs all point to migration. These last groups are intriguing as they point to a stable system with the possibility of smaller planets further out. I would include the 7 planets in the habitable zone in this group. But if these planets all migrated inwards they may well be ocean planets.

K dwarfs have an interesting variety of systems, so they’d be useful to study, but I don’t see them as the major source of Earth analogs—at least not until we learn more.

Superhabitability around K-class Stars

We think of Earth as our standard for habitability, and thus the goal of finding an ‘Earth 2.0’ is to identify living worlds like ours orbiting similar Sun-like stars. But maybe Earth isn’t the best standard. Are there ways planets can be more habitable than our own, and if so where would we find them? That’s the tantalizing question posed in a paper by Iva Vilović (Technische Universität Berlin), René Heller (Max-Planck-Institut für Sonnensystemforschung) and colleagues in Germany and India. Heller has previously worked this issue in a significant paper with John Armstrong (citation below); see as well The Best of All Possible Worlds, which ran here in 2020.

The term for the kind of world we are looking for is ‘superhabitable,’ and the aim of this study is to extend the discussion of K-class stars as hosts by modeling the atmospheres we may find on planets there. While much attention has focused on M-class red dwarfs, the high degree of flare activity coupled with long pre-main sequence lifetimes makes K-class stars the more attractive choice, although less susceptible to near-term evaluation, as the paper shows in its sections on observability. It’s intriguing, for example, to realize that K-class stars are expected to live significantly longer than the Sun, as much as 100 billion years, and because they are cooler and less luminous than G-class stars, their habitable zone planets produce more frequent transits.

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 50 to 100 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 8% 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. Image credit: NASA, ESA, and Z. Levy (STScI).

Let’s dig into this a little further. The contrast in brightness between star and planet is enhanced around K-dwarfs, and spectroscopic studies are aided by lower levels of stellar activity, which also enhances the habitability of planets. While an M-dwarf may be in a pre-Main Sequence phase for up to a billion years, K stars take about a tenth of this. They emit lower levels of X-rays than G-type stars and are also more abundant, making up about 13 percent of the galactic population as opposed to 8% for G-stars. With luminosity as low as one-tenth of a star like the Sun, they offer better conditions for direct imaging and their planets are far enough from the host to avoid tidal lock.

So we have an interesting area for investigation, as earlier studies have shown that photosynthesis works well under simulated K-dwarf radiation conditions. The authors go so far as to call these ‘Goldilocks stars’ for life-bearing planets, and there are about 1,000 such stars within 100 light years of the Sun, Thus modeling superhabitable atmospheres to support future observations stands as a valuable contribution.

The authors model these atmospheres by drawing on Earth’s own history as well as astrophysical parameters, finding that a superhabitable planet would be somewhat more massive than Earth so as to retain a thicker atmosphere to support a more extensive biosphere. Plate tectonics and a strong magnetic field are assumed, as are elevated oxygen levels that would “enable more extensive metabolic networks and support larger organisms.” Surface temperatures are some 5 degrees C warmer than present day Earth and increased atmospheric humidity supports the ecosystem.

The paper continues:

In terms of the atmospheric composition, key organisms and biological sources affecting Earth’s biosphere and their atmospheric signatures are considered. A superhabitable atmosphere would have increased levels of methane (CH4) and nitrous oxide (N2O) due to heightened production by methanogenic microbes, as well as denitrifying bacteria and fungi, respectively (Averill and Tiedje 1982, Wen et al. 2017). Furthermore, it would have decreased levels of molecular hydrogen (H2) due to higher enzyme consumption (Lane et al. 2010, Greening and Boyd 2020). Lastly…molecular oxygen (O2) levels could increase from present-day 21% by volume on Earth to 25% to reflect a thriving photosynthetic biosphere (Schirrmeister et al. 2015).

Given these factors, the authors deploy simulations using three different modeling tools (Atmos, POSEIDON and PandExo, the latter two to examine observability of transiting planets). Using Atmos, they simulate three pairs of superhabitable planets in differing locations in K-dwarf habitable zones, varying stellar radii and masses and star age. They focused on organisms and biological sources that had influenced Earth’s biosphere, including O2, H2, CH4, N2O and CO2 at a variety of surface temperatures.

The results offer what the authors consider the first simulated data on superhabitable atmospheres and assessments of the observability of such life. What stands out here is the optimum positioning of a superhabitable world around its star. Note this:

We find that planets positioned at the midpoint between the inner edge and center of the habitable zone, where they receive 80% of Earth’s solar flux, are more conducive to life. This contrasts with previous suggestions that planets at the center of the habitable zone—where our study shows they receive about 60% of Earth’s solar flux—are the most favorable for life (Heller and Armstrong 2014). Planets at the midpoint between the center and the inner edge need less CO2 for temperate climates and are more observable due to their warmer atmospheric temperatures and larger atmospheric scale heights. We conclude that a superhabitable planet orbiting a 4300K star with 80% of the solar flux offers the best balance of observability and habitability.

Image: An artist’s concept of a planet orbiting in the habitable zone of a K-type star. Image credit: NASA Ames/JPL-Caltech/Tim Pyle.

Observability presents a major challenge. Using the James Webb Space Telescope, a biosignature detection at 30 parsecs requires 150 transits (43 years of observation time) as compared to 1700 transaits (1699 years) for an Earth-like planet around a G-class star. That would be a mark in favor of K-stars but it also underlines the fact that studies of that length are impractical even with the anticipated Habitable Worlds Observatory. The JWST is working wonders, but clearly we are talking about next-generation telescopes – or the generation after that – when it comes to biosignature detection on potential superhabitable planets.

So what we have is encouraging in terms of the chances for life around K-class stars but a clear notice that observing the biosignatures of these planets is going to be a much harder task than doing the same for nearby M-class dwarfs, where extremely close habitable zones also give us a much larger number of transits over time.

The paper is Vilović et al., “Superhabitable Planets Around Mid-Type K Dwarf Stars Enhance Simulated JWST Observability and Surface Habitability,” accepted at Astronomical Notes and now available as a preprint. The earlier Heller and Armstrong paper is “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014). Abstract. Another key text is Schulze-Makuch, Heller & Guinan, “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World,” Astrobiology 18 September 2020 (full text), which looks at candidates. Cuntz & Guinan, “About Exobiology: The Case for Dwarf K Stars,” Astrophysical Journal Vol. 827, No. 1 10 August 2016 (full text) should also be in your quiver.

A Manhole Cover Beyond the Solar System?

Let’s start the year with a look back in time to 1957, a time when nuclear bombs were being tested underground for the first time at the Nevada test site some 105 kilometers northwest of Las Vegas. If this seems an unusual place to launch a discussion on interstellar matters, consider the story of an object that some argue became the fastest manmade artifact in history, an object moving so fast that it would have passed the orbit of Pluto four years after ‘launch,’ in the days of Yuri Gagarin and Project Mercury.

I’m bringing it up because the tale of the nuclear test known as Plumbob Pascal B is again active on the Internet, and it’s a rousing tale. Operation Plumbob involved a series of 29 nuclear tests that fed the development of missile warheads both intercontinental and intermediate. The history of such underground nuclear testing would make for an interesting book and indeed it has, in the form of Caging the Dragon (Defense Nuclear Agency, 1995), by one James Carothers.

But let’s narrow our focus to the nuclear devices known as Pascal A and B, the former used used in the first nuclear test below ground. This would have been the first such test in history, as the Soviet Union did not begin its underground program until 1961.

Image: The scene following the detonation of Ranier, an underground nuclear test similar to Pascal B. Credit: Plane Encyclopedia.

The key player here was Robert Brownlee (Los Alamos National Laboratory), who supervised the detonation of Pascal A and duly noted the fact that the yield was much greater than anticipated, so that a column of flame shot into the sky. The blast was not remotely contained. Pascal B was partially an attempt to fix that problem by lowering a 900-kilogram, 4-inch thick iron lid over the borehole. It seemed sensible to at least some at the time, but Brownlee himself evidently did not believe it would work to contain the blast, as indeed it did not.

The detonation of Pascal B caused the blast, like its predecessor, to climb straight up the borehole and escape. The interesting part is that the lid was never found. The only camera footage of the event caught the iron plate in only one frame, and that fact seems to be the source of the current interest. For Brownlee, extrapolating from the speed of the filming (one frame per millisecond) attempted a calculation on the speed of the object. He wound up with something on the order of six times Earth’s escape velocity, which would be 241,920 kilometers per hour, or 67.2 kilometers per second.

That’s an interesting figure! Voyager 1 is moving at about 17.1 kilometers per second and is more or less the yardstick for our thinking about where we are today in achieving deep space velocities. So what is commonly being described as a ‘manhole cover,’ which is pretty much what this object was, is conceivably the fastest moving object humans have ever produced.

Brownlee, recalling these events in 2002, described the iron cap as requiring a lot of ‘man-handlng’ to get it into place. And he goes on to say this:

For Pascal B, my calculations were designed to calculate the time and specifics of the shock wave as it reached the cap. I used yields both expected and exaggerated in my calculations, but significant ones. When I described my results to Bill Ogle [deputy division leader on the project], the conversation went something like this.

Ogle: “What time does the shock arrive at the top of the pipe?”
RRB: “Thirty one milliseconds.”
Ogle: “And what happens?”
RRB: “The shock reflects back down the hole, but the pressures and temperatures are such that the welded cap is bound to come off the hole.”
Ogle: “How fast does it go?”
RRB: “My calculations are irrelevant on this point. They are only valid in speaking of the shock reflection.”
Ogle: “How fast did it go?”
RRB: “Those numbers are meaningless. I have only a vacuum above the cap. No air, no gravity, no real material strengths in the iron cap. Effectively the cap is just loose, traveling through meaningless space.”
Ogle: And how fast is it going?”

This last question was more of a shout. Bill liked to have a direct answer to each one of his questions.

RRB: “Six times the escape velocity from the earth.”

Image: Los Alamos’ Robert Brownlee (1924-2018). Credit: American Astronomical Society.

According to Brownlee, the answer delighted Ogle, who had never heard of a velocity given in terms of escape velocity from the Earth. Brownlee himself notes that because the object was only caught in one camera frame, there was no direct velocity measurement. He could only summarize the situation by saying that the ‘manhole cover’ was “going like a bat!” But he also notes that neither he nor Ogle believed that the cap would actually have made it into space. And the story doesn’t end just yet.

As passed along by my ever-reliable buddy Al Jackson (Centauri Dreams readers will know of Al as astronaut trainer on the Lunar Module Simulator during the Apollo era, and as the author of numerous papers on interstellar propulsion), I point to a set of calculations by one R. Finden titled “The Fastest Object Ever: The Manhole Cover,” evidently sent in response to an article in a magazine called Business Insider in 2016. The note appeared originally on a Reddit thread. Finden notes that his or her work should be considered as a rough estimate because “flight at a mach number upwards of 200 has not been studied and may never be.” Good point.

Finden’s calculations show that the cover would have reached temperatures five times its melting point before it could ever escape the atmosphere. And then this:

If the steel plate were magical and did not burn in the atmosphere, it would have escaped the upper stratosphere (50km) at 53 km/s just 934ms from launch. This not only means it would have made it to space, but it would have eventually escaped our solar system (depending on the time of day at launch). What likely happened was the plate was initially launched parallel to the ground and rotated with oscillation into the upright position, and by that time the drag from the first second of flight decreased its speed enough to prevent it from entering the upper stratosphere.

Conclusion: No manhole cover in space. It’s worth recalling, the Finden note adds, that the Chelyabinsk meteor was moving at only one-third the speed and had 13,900 times the weight of the flying cover, and even this mass was unable to survive Earth’s atmosphere. I dislike this result, as the idea of an object ‘launched’ in 1961 escaping the Solar System while we were still trying to get to the Moon is utterly delightful. And because R Finden’s math skills are well beyond my pay grade, I can’t reach a definitive conclusion about the result. So maybe we can still dream of flying manhole covers even if the odds seem long indeed.