Paul Gilster's Blog, page 42

Atmospheric Evolution on Hot Super-Earths

Hot Jupiters (notice I’ve finally stopped putting the term into quotation marks) were the obvious early planets to detect, even if no one had any idea whether such things existed. I suppose you could say Greg Matloff knew, at least to the point that he helped Buzz Aldrin and John Barnes come up with a plot scenario involving a planet that fit the description in their novel Encounter with Tiber (Grand Central, 1996), which was getting published just as the hot Jupiter 51 Pegasi b was being discovered. Otto Struve evidently predicted the existence of gas giants close to their star as far back as 1952, but it’s certainly true that planets like this weren’t in the mainstream of astronomical thinking when 51 Pegasi b popped up.

Selection effect works wonders, and it makes sense that radial velocity methods would bear first fruit with a large planet working its gravitational effects on the star it orbits closely. Today, using transits, gravitational microlensing, astrometry and even direct imaging, we’re uncovering a much more representative sample of what’s out there, and hot Jupiters are somewhere around 1 percent of the catalog. Similarly rare, I’m sure, are the planets classed as ultra-short period worlds (USP), though they’re much smaller, with radii in the range of 2 Earth radii and periods of less than a day. We now have evidence for somewhere around 100 of these planets.

Two recently discovered examples are the planet candidates TOI-1634b and TOI-1685b, both of them TESS catches and subsequently examined by the Subaru Telescope (Mauna Kea, Hawaiʻi), supplemented with data from other observatories. An international team of astronomers have been at work on this duo, led by Teruyuki Hirano (University for Advanced Studies, Tokyo). The planet candidates are in Perseus, the first about 114 light years away; the second is 122 light years out. Both have now been confirmed to be rocky super-Earths (and I’m no longer going to put that term in quotation marks either). Both are likewise USPs, with ultra-tight orbits of less than 24 hours.

Image: Artist’s conceptual image showing the sizes of the planets observed in this study. The radius of TOI-1634 is 1.5 times larger than Earth’s radius and TOI-1685 is 1.8 times larger. The planets would appear red, due to the light from the red dwarf stars they orbit. Credit: Astrobiology Center.

As you’ll note from the caption above, the Subaru work (using the InfraRed Doppler — IRD — spectrograph mounted on the telescope) has been able to measure the masses of these transiting worlds. Here the chief interest is the fact that both lack a primordial hydrogen-helium atmosphere, which may or may not be the result of their proximity to their host stars.

Are there secondary atmospheres here, made up of gases released from within the planets? It’s an interesting speculation, as examining their constituents would tell us a lot about how these planets formed. The origins of USPs have been discussed in the literature in terms of inward migration, their highly circularized orbits the result of tidal interactions. There has been some suggestion that they represent remnant cores from hot Jupiters whose atmospheres have dissipated, but their hosts stars have different metallicity values than hot Jupiter host stars.

TOI-1634b and TOI-1685b, both of them orbiting M-dwarfs and relatively close to the Earth, may help us move forward in probing these issues, because to date there are few well characterized planets in the USP category; in fact, only the red dwarfs LTT 3780 and GJ 1252 have USP planets whose masses have been precisely measured. The authors note that TESS has been deeply involved in the hunt for more USPs, with 151 candidates announced as of February of 2021. 31 of these are known to orbit M-dwarfs.

The authors drew data from instrumentation ranging from Subaru itself to the MuSCAT imager at the Okayama Astro-Complex in Japan, the TCS telescope at Teide Observatory in Spain, the Gemini North telescope and Keck Observatory on Mauna Kea. Thus we’re examining data from ground-based transit photometry to high-resolution imaging, reconnaissance spectroscopy and radial velocity measurements, refining the orbital periods of these planets by more than an order of magnitude. Their future utility in atmospheric studies is clear. From the paper:

TOI-1634b is one of the largest and most massive USP planets having an Earth-like composition, and therefore, would become a benchmark target to study the formation and evolution history of massive USP planets. Both planets are listed among the best suitable targets for future atmospheric studies of small rocky planets by emission spectroscopy thanks to the brightness of the host stars, which encourages future characterizations using large aperture telescopes including JWST. Although small USP planets (< 2 R⊕) are likely to have lost the primordial atmospheres dominated by H2 and He, one may be able to probe and constrain the secondary atmosphere formed via the outgassing from the planet interior.

The paper is Hirano et al, “Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions,” The Astronomical Journal Vol. 162, No. 4 (23 September, 2021). Abstract / Preprint.

Hit-and-Run: Earth, Venus and Planet-Shaping Impacts

The gradual accretion of material within a protoplanetary disk should, in conventional models, allow us to go all the way from dust grains to planetesimals to planets. But a new way of examining the latter parts of this process has emerged at the University of Arizona Lunar and Planetary Laboratory in Tucson. There, in a research effort led by Erik Asphaug, a revised model of planetary accretion has been developed that looks at collisions between large objects and distinguishes between ‘hit-and-run’ events and accretionary mergers.

The issue is germane not just for planet formation, but also for the appearance of our Moon, which the researchers treat in a separate paper to extend the model for early Earth and Venus interactions that appears in the first. In the Earth/Venus analysis, an impact might be a glancing blow that, given the gravitational well produced by the Sun, could cause a surviving large part of an Earth-impactor (the authors call this a ‘runner’) to move inward and subsequently collide with Venus. So we’re not talking about impacts alone, but about impact ‘chains.’ The implications of this multi-impact theory on planet composition may be profound.

Alexandre Emsenhuber (now at Ludwig Maximilian University, Munich) is lead author of the paper on Earth/Venus interactions, pointing to the different impact scenarios for Earth and Venus:

“The prevailing idea has been that it doesn’t really matter if planets collide and don’t merge right away, because they are going to run into each other again at some point and merge then. But that is not what we find. We find they end up more frequently becoming part of Venus, instead of returning back to Earth. It’s easier to go from Earth to Venus than the other way around.”

Image: The terrestrial planets of the inner solar system, shown to scale. According to ‘late stage accretion’ theory, Mars and Mercury (front left and right) are what’s left of an original population of colliding embryos, and Venus and Earth grew in a series of giant impacts. New research focuses on the preponderance of hit-and-run collisions in giant impacts, and shows that proto-Earth would have served as a ‘vanguard’, slowing down planet-sized bodies in hit-and-runs. But it is proto-Venus, more often than not, that ultimately accretes them, meaning it was easier for Venus to acquire bodies from the outer solar system. Credit: Lsmpascal – Wikimedia Commons.

This work draws on a 2019 analysis by the same authors that first examined hit-and-run collisions and subsequent mergers of the two bodies. The authors point out that most simulations of this stage of planetary evolution assume perfect mergers for all impacts that are not completely catastrophic. Reflecting on this, they write:

Emsenhuber & Asphaug (2019a, hereafter Paper I) showed that this is not generally the case. They studied the fate of the runner following hit-and-runs into proto-Earths at 1 au, for thousands of geometries, and found that, contrary to expectation, only about half the time (depending on the runner’s egress velocity, which depends on the impact velocity and angle) do they return to collide again with proto-Earth. When they do, the return collision happens on a timescale of thousands to millions of years.

That work — fully treated in the first of the papers cited below — also revealed that the majority of the runners that did not return to the forming Earth would be likely to collide with Venus, given the assumption of their current masses and orbits. Those runners that did return would show an impact velocity in the second collision similar to the egress velocity after the first hit and run, thus slower than the original impact because of momentum loss. Follow-on collisions, then, are likely to be slow.

So we have a scenario in which the Earth takes repeated hits and spins off many impactors toward the inner system as they fall deeper into the Sun’s gravity well rather than eventually assimilating them itself. It’s an interesting notion given that, while Earth and Venus (so-called ‘sister planets’) have similar mass and density, Venus is nonetheless in a distinctly different state, its rotation retrograde compared to other planets, with a single rotation taking 243 days. There are also no moons at Venus. Do impacts during formation account for the differences?

To put the thesis to the test, the scientists built predictive models from 3D simulations of such impacts, drawing on machine learning techniques. They simulated terrestrial planet evolution over the course of 100 million years, calculating both hit-and-run collisions and those in which the impactor merged with the object struck.

The simulations explore the dynamical evolutions of remnants of hit-and-run collisions until the impactor is finally accreted or ejected.The different scenarios, says Asphaug, portray a sharply different formation history for the two worlds:

“In our view, Earth would have accreted most of its material from collisions that were head-on hits, or else slower than those experienced by Venus. Collisions into the Earth that were more oblique and higher velocity would have preferentially ended up on Venus…. We find that most giant impacts, even relatively ‘slow’ ones, are hit-and-runs. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision. To think of giant impacts, for instance the formation of the moon, as a singular event is probably wrong. More likely it took two collisions in a row.”

Image: The Moon is thought to be the aftermath of a giant impact. According to a new theory, there were two giant impacts in a row, separated by about 1 million years, involving a Mars-sized ‘Theia’ and proto-Earth. In this image, the proposed hit-and-run collision is simulated in 3D, shown about an hour after impact. A cut-away view shows the iron cores. Theia (or most of it) barely escapes, so a follow-on collision is likely. Credit: A. Emsenhuber/University of Bern/University of Munich.

Earth’s impact history thus has a telling influence on planetary composition. From the paper:

…if the terrestrial planets formed in multiple giant impacts, then Venus is significantly more likely than Earth to have accreted a massive outer solar system body during the late stage of planet formation. Earth, by contrast, has no terrestrial planet beyond its orbit to act as a vanguard. Mars is about the same mass as the late-stage projectiles…, 0.1 M⊕, and thus relatively inconsequential in terms of slowing them down through hit-and-run, so Earth has to do it on its own.

The late stage of terrestrial planet evolution in our own Solar System thus may hinge on how each world dealt with these impact runners. One thing that emphatically emerges from the work is that, according to these simulations, the terrestrial planets were hardly isolated during this period. Hit-and-run objects strike one planet, then the other, the probability of the impacts factored into the simulation via relative velocity and orbital configuration choices in the analysis.

In this study, Earth slows down projectiles, but accretes no more than half of them itself. Venus becomes a sink for these objects, retaining the majority of them in all simulations after their encounter with Earth as the slowed velocity of the runner allows for subsequent accretion. This would naturally lead to differences in composition between Venus and Earth and would account for differences in everything from Venus’ spin state, its formation (or lack of it) of moons, to its core-mantle dynamics. The authors promise a follow-up paper exploring these issues.

The papers are Emsenhuber et al., “Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus,” Planetary Science Journal Vol. 2, No. 5 (23 September, 2021), 199 (full text). The second paper is Asphaug et al., “Collision Chains among the Terrestrial Planets. III. Formation of the Moon,” Planetary Science Journal Vol. 2, No. 5 (23 September, 2021), 200 (full text)

Cloud Layers at WASP-127b

A ‘hot Saturn’ with a difference, that’s WASP-127b. Although it’s 525 light years away, we’ve learned a surprising amount about the planet’s atmosphere. Details come via the ongoing Europlanet Science Congress 2021, now being held virtually for pandemic reasons, at which Romain Allart (iREx/Université de Montréal and Université de Genève) spoke this week.

WASP-127b is quite an unusual planet with or without cloud cover. It’s orbiting its star in a scant four days, amped up by stellar irradiation levels 600 times what the Earth receives from the Sun. That would, the researcher points out, produce temperatures in the range of 1100 degrees Celsius (over 1370 Kelvin). The result of all these factors is a world with a fifth the mass of Jupiter actually inflating into a radius 1.3 larger than Jupiter. The word in vogue among astrophysicists for a planet like this seems to be ‘fluffy,’ which pretty much describes it.

Image: WASP-127b compared with planets of our Solar System. Credits: David Ehrenreich/Université de Genève, Romain Allart/Université de Montréal.

The work on WASP-127b’s atmosphere involves infrared observations from Hubble combined with data from the extraordinary ESPRESSO spectrograph at the European Southern Observatory’s Very Large Telescope in Chile. The method at the VLT is transmission spectroscopy, analyzing the light of the star during a transit as it passes through the planet’s atmosphere. Allart pointed out to the virtual Europlanet Science Congress attendees that surprises were in store:

“First, as found before in this type of planet, we detected the presence of sodium, but at a much lower altitude than we were expecting. Second, there were strong water vapour signals in the infrared but none at all at visible wavelengths. This implies that water-vapour at lower levels is being screened by clouds that are opaque at visible wavelengths but transparent in the infrared. We don’t yet know the composition of the clouds, except that they are not composed of water droplets like on Earth. We are also puzzled about why the sodium is found in an unexpected place on this planet. Future studies will help us understand not only more about the atmospheric structure, but about WASP-127b, which is proving to be a fascinating place.”

The zone within the atmosphere that the scientists were able to probe with their combined datasets proved to be clouds in an atmospheric layer with a pressure ranging between 0.3 and 0.5 millibars. We also know that this is a planet in a retrograde orbit, something Allart says is unexpected for a hot Saturn in a stellar system that is thought to be a whopping 10 billion years old. If things have not settled down by this time (and the planet orbits as well in a different plane than equatorial), it may point to the presence of an unseen planetary companion.

Is planetary migration a possibility here? From the paper:

Our best-fit model shows that the old star WASP-127 is a slow rotator while its planet has a misaligned retrograde orbit (a view of the system is represented in Fig. 4). We also note that the WASP-127 system does not fit in the known dichotomy of hot exoplanets. Winn et al. (2010); Albrecht et al. (2012) have reported that stars with Teff below 6250 K have aligned systems which is not the case of WASP-127b (Teff = 5842 ± 14 K). One possible scenario is that WASP-127b remained trapped in a Kozai resonance with an outer companion for billions of years, and only recently migrated close to its star (see e.g. the case of GJ436b, Bourrier et al. (2018)). An in depth analysis of the system dynamic is beyond the scope of the present study.

Image: This is Figure 4 from the paper. Caption: Fig. 4: View of the star planet system at the ingress transit. The stellar disk velocity has a color gradient from blue to red for negative to positive stellar surface velocity. The stellar rotation axis is shown as the black top arrow. The planet is represented by the black disk and occults first the red part of the star and then the blue part, following its misaligned orbit shown in green. The green arrow represents the orbital axis. Credit: Allart et al.

So we’ve reached the point where we can tell the difference between cloudy worlds and their cloud-free counterparts by measuring water content, acquiring data that also reveals the pressure of the cloud deck from which the measurements are taken. It’s early days in this kind of analysis, but we can assume that these methods will work their way into future surveys on planetary atmospheres. Understanding atmospheric structure and composition is a path into the formation history and evolution of any exoplanets. Beginning with planets in extreme conditions, we can look forward to future surveys as these techniques are applied to smaller worlds.

The paper is Allart et al., “WASP-127b: a misaligned planet with a partly cloudy atmosphere and tenuous sodium signature seen by ESPRESSO,” Astronomy & Astrophysics Vol. 644 (December, 2020), A155 (abstract / preprint).

Keeping an Eye on Psyche

What makes the asteroid 16 Psyche interesting is that it may well be the exposed core of a planet from the early days of Solar System formation, a nickel-iron conglomeration that normally would lie well below a surface mantle and crust. It’s also an M-class asteroid, a category of which it is the largest known sample. These are mostly made of nickel-iron and thought to be fragmented cores, though many have a composition that has not yet been determined.

Image: Deep within the terrestrial planets, including Earth, scientists infer the presence of metallic cores, but these lie unreachably far below the planets’ rocky mantles and crusts. Because we cannot see or measure Earth’s core directly, asteroid Psyche offers a unique window into the violent history of collisions and accretion that created the terrestrial planets. Credit: University of Arizona.

M-class asteroids have been imaged before — the Rosetta spacecraft imaged the non-metallic 21 Lutetia in 2010, and 216 Kleopatra has been imaged by ES0’s 3.6 meter telescope at La Silla as well as Arecibo — but now we’ll see one from close orbit.

For Psyche will be visited by a spacecraft of the same name, slated for launch in 2022 via Falcon Heavy, with arrival in 2026 after a 3.5-year cruise under solar electric propulsion. A single Mars flyby will occur along the way. The plan is to spend 21 months in orbit around the asteroid. You’ll recall that this mission will be testing NASA’s DSOC package (Deep Space Optical Communication), using laser techniques to communicate with Earth, See Deep Space Network: A Laser Communications Future for more on the DSOC and its capabilities.

The Psyche spacecraft’s Hall thrusters, using solar arrays as the power source, will be used beyond lunar orbit for the first time in this mission. The propellant is xenon, a neutral gas that will be accelerated and expelled from the spacecraft by electromagnetic fields after being ionized, producing a now familiar blue beam. The spacecraft will carry 922 kilograms of xenon, enough to run the Hall thrusters for years without exhausting available fuel supplies. NASA engineers estimate that chemical methods would require about five times that amount of propellant to achieve the same mission.

Image: The photo on the left captures an operating electric Hall thruster identical to those that will propel NASA’s Psyche spacecraft, which is set to launch in August 2022 and travel to the main asteroid belt between Mars and Jupiter. The xenon plasma emits a blue glow as the thruster operates. The photo on the right shows a similar non-operating Hall thruster. The photo on the left was taken at NASA’s Jet Propulsion Laboratory in Southern California; the photo on the right was taken at NASA’s Glenn Research Center.

We have fairly scant information about Psyche, as witnessed by the image below, which comes out of an asteroid-imaging project nicknamed HARISSA that is being run by the European Southern Observatory, using adaptive optics on the Very Large Telescope. The survey data along with ground-based radar imaging has determined that Pyche is roughly 226 kilometers wide and contains two interesting surface features, the first of which is a bright area christened Panthia. The second is a huge crater about half the size of the asteroid itself, which the ESO team calls Meroe. No moons have turned up in this work, at least none larger than one kilometer, eliminating one marker for determining the asteroid’s mass.

Image: Views of Psyche from the HARISSA survey, with Meroe and Panthia highlighted. Credit: ESO/LAM.

Orbiting between 378 and 497 million kilometers from the Sun, between the orbits of Mars and Jupiter, Psyche takes five years to complete an orbit, with a rotation period of a little over four hours. If the asteroid is the core of what would have been a planet-sized object, the mission’s science instruments should be able to make the call. They include a multi-spectral imager, a magnetometer, a radio instrument for gravity measurement (tricky at this potato-shaped object) and a gamma-ray and neutron spectrometer. We can hope this will be sufficient to untangle a past likely marked by violent collisions in the era in which the planets were forming.

To keep up with Psyche mission developments from the inside, check the Psyche blog at Arizona State. Here’s a snip from the most recent entry (June 14), from Paige Arthur, who captures some of the excitement of being involved in a deep space mission at building 179, the famed home of JPL’s Spacecraft Assembly Facility:

Engineers and technicians adorned in lab coats, gloves, masks, and hair nets meander around on the floor below, taking measurements with multimeters and mating connectors not too different from the ones Will and I had been handling in the testbed moments before. Along the opposite wall are plaques commemorating all of the spacecraft that have been assembled there — Mariner, Ranger, Voyager, Galileo, Cassini, Curiosity, Opportunity, and, most recently, Perseverance. But in this moment I don’t notice any of it, because my attention is totally captured by the massive spacecraft suspended in the middle.

The Psyche Chassis is a huge black, silver, and gold box the size of a car with an antenna dish fixed to one end. Long struts surround the dish like the spindly legs of a massive aluminum spider, and red electronic boards connected by thick, snaking cords populate the sides. I’ve spent the last two and a half years at JPL working with software simulations and engineering models, with bits of pieces of Psyche’s brain, eyes, and heart, but seeing the physical manifestation of all that work suspended before me in the form of such a tremendous machine still brings me chills.

I’ve never worked on a spacecraft, but I’ve felt the chills, particularly at JPL one day many years ago when I watched Spirit and Opportunity being readied for shipment to Florida. Go Psyche.

Pondering SETI Strategy

I try to keep my ear to the ground (rather than my eye to the sky) when it comes to SETI. What I mean is that there are enough scientists working SETI issues that it’s a challenge to know who is doing what. I try to track ongoing discussions even when, as at a conference, people keep ducking into and out of audibility. Hence the possibility of overlap in SETI efforts and, as Jason Wright points out in a discussion on his AstroWright site, the circulation of the same ideas without moving the ball forward.

This is hardly a new phenomenon, as a look back at my own grad school experience in a much different area reveals. I was a medievalist with an ear for language, and I was always struck by how compartmentalized we tended to be when discussing medieval linguistics. At that time, northern European tongues like Gothic, Old Icelandic, Anglo-Saxon and Old Saxon formed a scholarly thicket I happily wandered through, but in the absence of computerized resources back in the day, the Gothic scholars had a hard time keeping current with the Old Icelandic papers, and new work on the Anglo-Saxon alliterative line arrived mostly by rumor picked up at coffee time. These Germanic languages were definitely not talking to each other.

Bear in mind, that was a small, tightly focused community of scholars working on very esoteric stuff, and even then it was hard to keep up with the various strands of the tapestry. SETI’s problems are of a different sort. Here, the work is scattered across numerous journals and in particular, a wide range of disciplines — you might, for example, find a SETI paper in a journal of anthropology, well outside an astrophysicist’s normal range of sampling. Wright also makes the good point that SETI suffers from a lack of a curriculum, though he himself is working to change that at Penn State. What SETI does have going for it that my grad experience didn’t have is the proliferation of online resources, even if many are walled away like medieval monks behind monastic firewalls. Online access remains an emphatically moving target.

A useful paper that is helping to stabilize things is available on the arXiv site under the title “Furthering a Comprehensive SETI Bibliography.” It’s the background on the discovery process and categorization issues of an ongoing bibliographic update called SETI.news, which Centauri Dreams readers will want to know about. The newsletter began in 2016 as a way of gathering academic articles and occasional blog posts with high relevance to the SETI effort and I’m finding it indispensable.

The combined July/August SETI.news mailing is here, thanks to the efforts of Wright and Macy Huston. I should also mention while we’re talking about resource gathering that a bibliographic effort useful to those of us following interstellar exploration is available through the Interstellar Research Group. The database is searchable; updates appear every weekday. The IRG’s remit is obviously broad in scope, ranging through everything from propulsion issues to astrobiology, unlike the highly focused SETI.news. But I think both will be of interest to the Centauri Dreams audience.

Image: The Very Large Array (VLA) is a collection of 27 radio antennas located at the NRAO site in Socorro, New Mexico, since early 2020 a participant in the hunt for technosignatures. Credit: Alex Savello/NRAO.

I don’t want to leave Wright’s interesting Strategies for SETI III: Advice post without mentioning his comments on the Fermi Paradox and the Drake Equation. He’s wondering whether we haven’t in some ways exhausted the discussion, quoting Kathryn Denning on the matter:

Thinking about that future [of contact with ETI] was itself an act of hope. Perhaps it still is. But I want to suggest something else here: that the best way to take that legacy forward is not to keep asking the same questions and elaborating on answers, the contours of which have long been established, and the details of which cannot be filled in until and unless a detection is confirmed. Perhaps this work is nearly done.

Have we driven the Fermi question into the ground? As per the earlier part of this post, it does seem that the discussion ranges around to the same issues without moving the ball forward, but then, a ball this theoretical is a hard thing to push down-field! The Drake Equation reliably gets an indignant rebuttal every now and then in my email from people who don’t realize that it is, as Wright points out, a heuristic tool that is designed to make us ponder the odds. So when someone writes me to point out that it is not possible to solve the equation, I think I’ll start quoting Wright’s comment:

[The Drake Equation is]…not a foundational equation like the Schrodinger equation from which one derives results, it’s more like a schematic map of the landscape to help orient yourself.

Schematic maps can be quite useful, and the Drake Equation has served its role. Everything in its place as we move forward. My own interest is more tightly focused on papers that can help with actual search efforts rather than theoretical ones, papers that, as Wright puts it, ‘stay close to the data.’ Which is not to underplay the sheer fascination of the topic, but only to say that the nuts and bolts have to be tightened and maintained, and a contribution like SETI.news is tangible and productive.

Tracking Missing Ammonia on the Ice Giants

Something interesting always comes out of the Europlanet Science Congress (probably better known to Centauri Dreams readers under its former name, the European Planetary Science Congress), and this year is no exception. This is the largest planetary science meeting in Europe, normally drawing about 1000 participants, though last year and this year as well have been virtual meetings, the latter ongoing as I write this and running until September 24.

As the conference proceeds, my eye was drawn to a study by Tristan Guillot on the ice giants Uranus and Neptune, targets of (let’s hope) future space missions that can help us resolve the differences between this class of world and gas giants like Jupiter and Saturn. Guillot (CNRS, Laboratoire Lagrange, Nice) targets the odd fact that both of these planets have recently been found to be deficient in ammonia in their atmospheres as compared to the gas giants. Astronomers are puzzled because other compounds such as methane, found in the primordial cloud as the young system coalesced, are clearly there in abundance.

Let’s pause briefly on this point. We’ve talked often about the ‘snowline,’ beyond which materials like methane, ammonia and other trace compounds — ‘ices’ in astronomical parlance — transition from being gases to solids during system formation. The ice giants differ from the primarily hydrogen and helium-based gas giants in being mostly composed of ices beyond the snowline during formation. Recent work at infrared and radio wavelengths indicating the lack of ammonia at Uranus and Neptune thus needs an explanation. The current formation model suggests it should be there.

Image: Composite image of Neptune, Uranus, Saturn and Jupiter. Credits: Jupiter from Juno: NASA/SwRI/MSSS/Gerald Eichstädt/Seán Doran; Saturn from Cassini: NASA/JPL-Caltech/Space Science Institute; Uranus and Neptune from HST: NASA/ESA/A. Simon (NASA Goddard Space Flight Center), and M.H. Wong and A. Hsu (University of California, Berkeley).

Guillot proposes that the ammonia is actually present at both planets in the form of slushy hailstones of ammonia and water, which would carry it far below the limits of detection within the ice giant atmospheres. The scientist is drawing on data from the highly productive Juno mission to Jupiter, where ammonia turns up at deeper levels than expected because of similar ‘hailstone’ formation.

Is the process common in the outer planets? The Juno data have revealed ammonia-water hailstones forming rapidly in Jupiter’s atmosphere during storms, a fact Guillot attributes to ammonia’s ability to liquefy water ice crystals at low temperatures. Models drawing on the Juno findings make these hailstones out to be up to a kilogram or more in weight, larger than we find on Earth, and they act as a transport mechanism for moving ammonia well below the cloud base. However, at Jupiter the effect is more confined. Says Guillot:

“Thermodynamic chemistry implies that this process is even more efficient in Uranus and Neptune, and the mushball seed region is extended and occurs at greater depths. Thus, ammonia is probably simply hidden in the deep atmospheres of these planets, beyond the reach of present-day instruments.”

Image: Artist’s impression showing how mushballs form in giant planets’ atmospheres. Credit: NASA/JPL-Caltech/SwRI/CNRS.

The Juno findings become a plausible solution to the ammonia deficit at Uranus and Neptune, but here we run into the limits of our instrumentation. Just how deep is the ammonia taken by this process? Guillot is hardly the first to point to the need for a dedicated mission, preferably two, in the form of orbiters for each of the ice giants. Only then will we begin to get a handle on mixing in hydrogen atmospheres in the interesting realm of ice giants, which are turning up in many other stellar systems as well.

Guillot adds: “Neptune and Uranus are a critical link between giant planets, like Jupiter and Saturn, and ice giant exoplanets that we are discovering in the galaxy. We really need to go there!”

Dr. Guillot’s paper is still in preparation. The reference for today is Guillot, “Mushballs and the lack of Ammonia in Uranus and Neptune,” Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-422. https://meetingorganizer.copernicus.org/EPSC2021/session/41678#Oral_and_Poster_presentations_and_abstracts

Adjusting the Clock: Hydrogen Burning in White Dwarfs

White dwarfs have turned out to be more interesting than I had imagined. We know how they form: A star like the Sun exhausts the hydrogen in its core and swells into a red giant, a scenario that is a trope in science fiction, as it posits an Earth of the far-future incinerated by its star. Losing its outer layers near the end of nuclear burning, a red giant ultimately leaves behind an object with much of the mass of the Sun now crammed into a white dwarf that is about the size of the Earth. For years I assumed white dwarfs were dead ends, a terminus for life whose only function seemed to be in binary systems, where they could be the locus, through accretion from the other star, of a stellar explosion in the form of a nova.

Lately we’ve been learning, though, that through analysis of their atmospheres, white dwarfs can yield information about objects that have fallen into them, such as remnants of the original stellar system. Some white dwarfs may have habitable zones lasting several billion years, an interesting thought if surviving planets in the system migrate inward. And now, thanks to Hubble data, we are learning that white dwarfs may have a longer lifetime than previously thought. Continuing to burn hydrogen, some of these stars may be older than they look.

The work was coordinated by Francesco Ferraro (Alma Mater Studiorum Università di Bologna / Italian National Institute for Astrophysics), who points to the useful role of white dwarfs in determining the ages of stars. The white dwarf cooling rate has been used as a natural clock as a way of calibrating the age of star clusters, as their cooling seemed easy to model by following the relationship between age and temperature. But the new study finds that white dwarf aging is nowhere near as simple as this, so an accepted method of measuring cluster ages may have to be re-examined.

To investigate white dwarf aging, the scientists looked at the globular clusters M3 in the constellation Canes Venatici and M13 in Hercules, in which stars in aggregate show common values of metallicity and age. As always in astronomy, the word ‘metals’ refers to elements higher than hydrogen and helium, with the bulk of a star like the Sun being made up of the latter two, while a scant 1.3 percent is given over to metals.

Ferraro’s team compared cooling white dwarfs in the two clusters and found that among the stars that will eventually become white dwarfs, the color of stars in the Horizontal Branch is bluer in M13 than in M3. The Horizontal Branch is a stage in the evolution of a star in which it has begun helium burning in the core, a stage that is flagged by a reduction in luminosity and increases in surface temperature. So the enhanced blue color in M13 is indicative of hotter stars on their way toward white dwarf status.

Image: To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity, but the populations of stars which will eventually give rise to white dwarfs are different. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool. Credit: ESA/Hubble & NASA, G. Piotto et al.

The researchers consider the M3 and M13 clusters to be “a classical horizontal branch (HB) morphology pair” because they share many physical properties including metallicity, making the color difference the salient feature. Homing in on the white dwarfs among this population, the team finds the outer envelope of hydrogen in these bluer white dwarfs allows them to burn for longer and cool more slowly than the standard white dwarf model. Using near-ultraviolet data from Hubble’s Wide Field Camera 3, the team compared more than 700 white dwarfs in the two clusters.

The result: M3’s white dwarfs follow the expected model, consisting of predictably cooling stellar cores and no stable thermonuclear activity. But in M13, two populations of white dwarfs can be found, the second being those that have retained an outer hydrogen envelope, continue thermonuclear burning and therefore cool at a slower pace. In fact, 70 percent of the white dwarfs in M13 appear to be burning hydrogen on their surface. This appears to be the only viable explanation for the ‘blue tail’ found in the Hubble data that distinguishes the two clusters.

From the paper:

At the moment, this [hydrogen burning on the surface] appears to be the most viable and natural explanation, while alternative scenarios should invoke ad hoc and unknown mechanisms able to increase the production or slow down the cooling process of the WDs in M13, and not in M3. The discovery reported in this paper represents the first direct evidence for the occurrence of stable nuclear burning in the residual hydrogen envelope of cooling WDs and offers an empirical measure of the delay in the flow of time marked by the WD clock in the presence of slowly cooling WDs.

The authors believe the road ahead should involve studying other clusters that show differences when at the Horizontal Branch of stellar evolution, while also examining clusters with different degrees of metallicity to determine the role it plays. Clarifying how white dwarfs evolve will force us to adjust the use of these stellar remnants in calibrating age, where current uncertainties can be as large as a billion years.

Adds Ferraro:

“Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old. We are now investigating other clusters similar to M13 to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly.”

The paper is Chen et al., “Slowly cooling white dwarfs in M13 from stable hydrogen Burning,” Nature Astronomy 6 September 2021 (abstract).

Predicting a Supernova in 2037

Here’s a story that’s both mind-bending and light-bending. It involves a supernova that, on the one hand, happened 10 billion years ago, and on the other hand, has appeared in our skies not once but three times, with a fourth in the works. In play here is gravitational lensing, in which light from a background galaxy bends around a foreground galactic cluster known as MACS J0138.0-2155. Out of this we get multiple mirror images, and researchers predict another supernova appearance in the year 2037.

Three of the appearances of the supernova, labeled AT 2016jka and nicknamed ‘Requiem,’ are in the image below, a Hubble view from 2016, all three circled for ease of identification. The light of the supernova has been split into different images by the lensing effect. Using archival data, researchers led by Steve Rodney (University of South Carolina) have analyzed differences in brightness and color that reflect different phases of the event as the supernova faded.

“This new discovery is the third example of a multiply imaged supernova for which we can actually measure the delay in arrival times,” says Rodney. “It is the most distant of the three, and the predicted delay is extraordinarily long. We will be able to come back and see the final arrival, which we predict will be in 2037, plus or minus a couple of years.”

Image: Three views of the same supernova appear in the 2016 image on the left, taken by the Hubble Space Telescope. But they’re gone in the 2019 image. The distant supernova, named Requiem, is embedded in the giant galaxy cluster MACS J0138. The cluster is so massive that its powerful gravity bends and magnifies the light from the supernova, located in a galaxy far behind it. Called gravitational lensing, this phenomenon also splits the supernova’s light into multiple mirror images, highlighted by the white circles in the 2016 image. The multiply imaged supernova disappears in the 2019 image of the same cluster, at right. The snapshot, taken in 2019, helped astronomers confirm the object’s pedigree. Supernovae explode and fade away over time. Researchers predict that a rerun of the same supernova will make an appearance in 2037. The predicted location of that fourth image is highlighted by the yellow circle at top left. The images were taken in near-infrared light by Hubble’s Wide Field Camera 3. Image processing credit: Joseph DePasquale (STScI).

Cluster and supernova are at vastly different distances from us, with the light from the lensing cluster MACS J0138.0-2155 taking about four billion years to reach us, while the light from the supernova has traveled an estimated 10 billion years. Computer modeling makes the call on the supernova’s return appearance as researchers untangle the complex path followed by the light.

In fact, says Rodney, the longer delay in the predicted 2037 light is the result of its traveling through the middle of the cluster and thus encountering the densest amount of dark matter. While dark matter remains controversial in many ways, it’s telling that the assumption of dark matter in the cluster explains the current three images and makes the call on the upcoming fourth. There appears to be a likelihood for a fifth appearance some time after the 2037 event, although the prediction is that it will be extremely hard to detect.

The lensing supernova images were discovered by Gabe Brammer (Niels Bohr Institute, University of Copenhagen), who found the three mirrored images while analyzing lensing magnification effects for the REQUIEM (REsolved QUIEscent Magnified Galaxies ) program, which uses Hubble data. Comparing the 2019 data with data from three years earlier showed that what he thought was a single image of a lensed galaxy had disappeared. Says Brammer:

“But then, on further inspection of the 2016 data, I noticed there were actually three magnified objects, two red and a purple. Each of the three objects was paired with a lensed image of a distant massive galaxy. Immediately it suggested to me that it was not a distant galaxy but actually a transient source in this system that had faded from view in the 2019 images like a light bulb that had been flicked off.”

Co-author Johan Richard (University of Lyon) developed a map of the amount of dark matter in the foreground cluster that drew on inferences from the lensing effects found in Brammer’s data. The map fits with the locations of the lensed objects based on Richard’s assumptions. Analyzing the fourth image in 2037, assuming all happens as expected, will allow astronomers to more accurately measure the time delays between the four images, which in turn will yield further data on the distortions to spacetime through which the light transited. Adds Rodney:

“These long time delays are particularly valuable because you can get a good, precise measurement of that time delay if you are just patient and wait years, in this case more than a decade, for the final image to return. It is a completely independent path to calculate the universe’s expansion rate. The real value in the future will be using a larger sample of these to improve the precision.”

In the excerpt from the paper below, MRG-M0138 refers to the background galaxy containing the supernova, which is being lensed by the foreground galaxy cluster MACS J0138.0-2155:

We model the mass distribution in the cluster core as the combination of a cluster-scale and galaxy-scale potentials… From this model we derive estimates for the lensing magnification and time delay of each of the SN images, including two predicted future images… The lens model predicts that the SN should appear in the fourth MRG-M0138 image in the year 2037±2, demagnified with µ = 0.4 ± 0.2. A fifth image will also appear at a still later date, located near the center of the cluster and much more significantly demagnified, so it will not be easily observable. We anticipate that future lens modelling of the cluster will improve on these predictions primarily by exploring a wider range of mass models and incorporating more observational constraints.

The building of the model that makes the 2037 prediction is fascinating, fully explicated in the ‘Methods’ section of the paper, as the researchers use a software program called LENSTOOL to develop five lens models that would yield the lensing effects seen in the data. The best fit model was then used to predict the magnification and time delays for the three images observed by Hubble, as well as the location of the fourth and fifth images, which have yet to appear. An education on lens modeling is available here for those interested in digging into the details.

The name Requiem comes into use for a reason beyond the reference to the REQUIEM program. I like the note at the end of the paper that explains it:

HST observations enabled us to find this SN. We anticipate that HST may be deorbited and make its final plummet to Earth around the time of the reappearance of AT 2016jka, so we coin the name SN Requiem as an ode to the vast new discovery space that HST continues to unveil.

Nicely put.

The paper is Rodney et al., “A gravitationally lensed supernova with an observable two-decade time delay,” Nature Astronomy 13 September 2021 (abstract / preprint).

Exomoons: The Binary Star Factor

Centauri Dreams readers will remember Billy Quarles’ name in connection with a 2019 paper on Alpha Centauri A and B, which examined not just those stars but binary systems in general in terms of obliquity — axial tilt — on potential planets as affected by the gravitational effects of their systems. The news for habitability around Centauri B wasn’t good. Whereas the Moon helps to stabilize Earth’s axial tilt, the opposite occurs on a simulated Centauri B planet. And without a large moon, gravitational forcing from the secondary star still causes extreme obliquity variations.

Orbital precession induced by the companion star is the problem, and it may be that Centauri A and B are simply too close together, whereas more widely separated binaries are less disruptive. I’ll send you to the paper for more (citation below), but you can get an overview with Axial Tilt, Habitability, and Centauri B. It’s exciting to think that our ongoing investigations of Centauri A and B will, one of these days, be able to confirm these results or cause them to be reassessed, assuming we find planets there.

Exomoons in Binary Systems

Quarles (Georgia Institute of Technology) continues to use numerical methods to look at the dynamics in both single and multiple star systems, with his interest in Centauri A and B undiminished. His most recent paper looks at exomoon possibilities at binary star systems, homing in on orbital stability in systems where a companion star forces greater eccentricity. We can look for such moons using transit-timing variations as well as variations in the durations of a transit. All of this gives us no more than hints of a moon, which is too small to be seen, but it opens up new space for such detections.

Co-author Siegfried Eggl (University of Illinois Urbana-Champaign) explains:

“We first had to determine the orbital resonances in the systems we looked at. When moons and planets have slightly elliptical orbits, they don’t always move at the same speed. The more eccentric an orbit, the more frequencies can be excited, and we see these resonances become more and more important. At some point there will be overlapping resonances that can lead to chaos in the system. In our study we have shown, however, that there is enough stable ‘real estate’ to merit a thorough search for moons around planets in double star systems.”

Image: In this map of overlapping orbital resonances, the regions between resonances are colored black and could allow for stable satellite orbits under optimal conditions. The light green curve connects the first point of intersection between adjacent resonances and marks a stability boundary within the three body problem. Credit: Quarles et al.

Transit timing variations (TTV) and variations in the actual duration of the transit (TDV) are the most readily observable effects on the table. TTVs are variations in timing as the planet transits its star. Does the transit show strict periodicity, or is there some variation from one transit to the next? TTVs can be used to demonstrate the presence of other gravitational influences, an unseen planet, or a moon. Transit duration variation measures the time during which any part of the planet obscures the stellar disk. Variations in duration occur as planet and moon orbit a common center of mass.

David Kipping (Columbia University) has been looking at transit timing variations and other factors for a long time in connection with the quest for an exomoon detection, a quest he began as a grad student and continued with his project The Hunt for Exomoons with Kepler. HEK uses dynamics and Kepler photometry in combination, modeling observable effects of a moon on a transit as well as the dynamical perturbations that can be revealed by transit timing and transit duration variations.

Quarles and team have taken the exomoon hunt explicitly into the realm of binary stars, where a stellar companion forces its own perturbations on moons orbiting planets there, affecting their occurrence and orbital evolution. The researchers have applied their findings to hypothetical Earth-Moon analogues at Centauri A and B, and have set up orbital stability limits for exomoons in binary star systems in general.

Gravitational interactions with a companion star can foster greater eccentricity in planetary orbits, with resulting stability issues for moons and implications for detecting them through TTVs. In typical binary systems, “TTV (rms) amplitudes induced by exomoons in binary systems are ≲10 minutes and appear more likely for planets orbiting the less massive stellar component.”

In some systems, say the researchers, we would expect the Hill radius — the region around a planetary body where its own gravity, as compared to that of other nearby bodies, is the dominant force in attracting satellites — to shrink, which could cause moons to become unstable. If too close to the host star, the moon could be ejected from its planetary orbit and flung outwards. Zeroing in on Alpha Centauri:

The truncation of the Hill radius through secular eccentricity oscillations and outward tidal migration can influence potential observations of exomoons through TTVs… The TTV (RMS) amplitude is largest when satellites are close to their outer stability boundaries. These mechanisms limit the outer stability limit and can constrain the range of tidal dissipation allowed. The maximum TTV amplitude in a system like α Cen AB is ∼40 min, where we find that an Earth-Moon analog would exhibit ∼2 min TTV signature.

The acronym RMS above stands for ‘root mean square,’ a reference to the value of the total waveform of the transit data, but let’s not get too deep into the weeds. The point is that a delicate balance needs to be struck so that the moon can survive. This is what Eggl refers to above as ‘stable real estate.’ But to detect an exomoon, we first have to find planets in the Alpha Centauri system, about which the authors have this to say:

The primary star of α Cen AB would be a good candidate for searching for TTV inducing exomoons if transiting Earth-analogs were present. However, surveys of α Cen AB for planets are difficult because of pixel saturation in photometric observations (Demory et al. 2015) and astrophysical noise in radial velocity observations…

And further on:

Observations of α Cen AB with the Very Large Telescope (VLT) have suggested that any exoplanets there need to be ≲20 M⊕ (Kasper et al. 2019), which bodes well for the potential for terrestrial planets. The first results of the New Earths in the α Centauri Region (NEAR) experiment on VLT uncovered a direct imaging signature of a roughly Neptune-sized planet orbiting α Centauri A (Wagner et al. 2021), but these early results still await confirmation. Detecting exoplanets in binary star systems is a crucial step in the search for exomoons, where a wide array of methods (including TTVs) can be employed.

A crucial step indeed, but detecting an exomoon is an even tougher task, whether in a binary system or not. In 2020, for example, Chris Fox and Paul Wiegert (University of Western Ontario) theorized that six exoplanets found by Kepler could be hosting exomoons. Studying TTVs in the data, the astronomers noted that these were indirect detections, and that nearby planets could also be responsible for the TTVs.

We’re reminded that this is truly a frontier. Having examined the data, Quarles found that four of these six systems would tidally disrupt their exomoons or lose them to outward migration. He is quoted elsewhere as saying of the six possible moons:

“Could they (exomoons) exist physically? Four (candidate systems) of the six could not, two of the six are possible but the signature they produced aren’t produced by the data. Those two probably aren’t moons.”

Exomoon hunter Kipping found no compelling evidence for any of the six exomoons based on his own work. Moons around gas giants could be interesting venues for habitability, and we know the investigation of such will continue. You’ll recall evidence of a moon forming around the planet PDS 70c, an encouraging sign that a confirmed exomoon is getting closer. So it’s a fascinating part of the process that we now examine forced resonances in binary systems as another way into this daunting problem.

The paper is Quarles et al., “Exomoons in Systems with a Strong Perturber: Applications to α Cen AB,” Astronomical Journal Vol. 162, No. 2 (14 July 2021) 58 (abstract / preprint). The Quarles paper on orbital obliquity is “Obliquity Evolution of Circumstellar Planets in Sun-like Stellar Binaries,” Astrophysical Journal Vol. 886, No. 1 (19 November 2019). Abstract / Preprint. The Fox & Wiegert paper on exomoon detection cited above is “Exomoon candidates from transit timing variations: eight Kepler systems with TTVs explainable by photometrically unseen exomoons.” Monthly Notices of the Royal Astronomical Society Vol. 501, Issue 2 (February 2021) 2378-2393 (abstract).

1001 Near Earth Objects for Planetary Radar

A century ago, when American magazine science fiction was developing, the Solar System seemed a relatively tidy place. At least, it did in comparison to today. The first issue of Hugo Gernsback’s Amazing Stories serialized a reprint of Jules Verne’s 1877 novel Off on a Comet and, indeed, in those days comets were the objects most likely to move around the system. The asteroids seemed distant in their belt and in stable orbits and there was little else between the planets. There was no Pluto.

Today, of course, we seem to have debris everywhere. The main belt asteroids are joined by trojan objects like the large population around Jupiter, and there is another belt of ancient material out beyond Neptune, the Kuiper Belt. In Earth’s neighborhood, interesting objects like 2021 PJ1, whose approach to our planet occurred on August 14 at 1.7 million kilometers, remind us that there is a large population of asteroids that move in orbits well inside the main belt, and could conceivably present a danger to us, at least enough of one to demand that we keep a close eye on their trajectories.

2021 PJ1 has a certain claim to fame, being the 1,000th near-Earth asteroid to be observed by planetary radar in the past 50 years. In this technique, we bounce a radar signal off an object and examine the photonic echo. The first asteroid to be viewed in this way was 1566 Icarus, all the way back in 1968, ‘painted’ by Goldstone radar, the same facility near Barstow, California that produced the PJ1 data this summer.

Image: The 70-meter Deep Space Station 14 (DSS-14) antenna at the Deep Space Network’s Goldstone Deep Space Complex near Barstow, California, was able to measure the Doppler frequency of the radio waves that reflected off asteroid 2021 PJ1’s surface. The figure shows radar echo signal strength on the vertical axis versus Doppler frequency (in units of hertz, or Hz) on the horizontal axis. The strong spike at a value of minus 70 Hz is the reflected signal (or “echo”) from 2021 PJ1; the other, smaller spikes are receiver noise. Credit: NASA/JPL-Caltech.

I always think primarily of Arecibo when planetary radar comes to mind, and in fact its radar capabilities were a prime reason for fighting to sustain its funding before its collapse in 2020. Well over half of existing NEO radar observations were made by its 305-meter dish. But the tally of the Goldstone Deep Space Complex is impressive via its DSS-14 70-meter and 34-meter DSS-13 antennae, with 374 near-Earth asteroids to date. Moreover, the Deep Space Network’s Canberra site, working with Australian observatories including Parkes, has notched up another fourteen.

NEA 1,001 came only a week after 2021 PJ1, an object labeled 2016 AJ193 that moved by at about 3.4 million kilometers. While 2021 PJ1 was small — between 20 and 30 meters wide — 2016 AJ193, although more distant, was a much easier catch because it’s some 40 times larger, with a diameter in the range of 1.3 kilometers. Originally observed by the NEOWISE mission, this asteroid gave us a lot more than plots on a graph: Ridges, hills, concavities and possible boulders appeared in the Goldstone observations, which also determined that it rotates with a period of 3.5 hours.

Image: This animation shows asteroid 2016 AJ193 rotating as it was observed by Goldstone’s 70-meter antenna on Aug. 22, 2021. 1.3-kilometers wide, the object was the 1,001st near-Earth asteroid to be measured by planetary radar since 1968. Credit: NASA/JPL-Caltech.

The 2016 AJ193 observations were led by Shantanu Naidu (JPL), who says:

“The 2016 AJ193 approach provided an important opportunity to study the object’s properties and improve our understanding of its future motion around the Sun. It has a cometary orbit, which suggests that it may be an inactive comet. But we knew little about it before this pass, other than its size and how much sunlight its surface reflects, so we planned this observing campaign years ago.”

The significance of planetary radar for simple security is obvious. Including telescopes on the ground and in space, we’re tracking close to 27,000 near-Earth objects, characterizing them through observations like the recent Goldstone work. The more we learn about the size, shape and composition of NEOs, the better we’ll be able to resolve questions about their trajectories and any future danger they might pose. This, in company with data from asteroid missions like Hayabusa2 and OSIRIS-REx, will help us tune our threat mitigation strategies if we ever do have to nudge an NEO.

Image: This series of images captured on Aug. 22, 2021, shows asteroid 2016 AJ193 rotate as it was observed by Goldstone’s 70-meter antenna. Credit: NASA/JPL-Caltech.