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Radial velocity methods for detecting exoplanets keep improving. We’ve gone from the first main sequence star with a planet (51 Pegasi b) in 1995 to over 450 planets detected with RV, a technique that traces minute variations in starlight as a star nudges closer, then further from us as it is tugged by a planet. Radial velocity, then, sees gravitational effects while not directly observing the planet, which may in some cases be studied by its transits or direct imaging.

Image: 51 Pegasi b, also called “Dimidium,” was the first exoplanet discovered orbiting a star like our sun. This groundbreaking find in 1995 confirmed that planets around main sequence stars could exist elsewhere in the universe. Credit: NASA.

Transit methods have accounted for more planets, but radial velocity techniques are increasingly robust and continue to provide breakthroughs. Consider this morning’s news about HR 5183, which is now known to be orbited by a gas giant designated HR 5183 b. Astronomers at the California Planet Search have employed data from Lick Observatory in northern California, the W. M. Keck Observatory in Hawaii and McDonald Observatory in Texas to identify the planet around a star they have been studying since the 1990s. Even now, they do not have data corresponding to a single full orbit of the planet.

Thus the inherent dilemma of radial velocity work: To analyze Doppler data, observations taken over a planet’s entire orbital period are optimal, but as the distance between planet and host star widens, a full orbit can take decades or even centuries. But with data on a timescale of decades, the California Planet Search team identified this one because of its odd orbit. Andrew Howard (Caltech), who leads the effort, points to the unusual nature of its motion:

“This planet spends most of its time loitering in the outer part of its star’s planetary system in this highly eccentric orbit, then it starts to accelerate in and does a slingshot around its star. We detected this slingshot motion. We saw the planet come in and now it’s on its way out. That creates such a distinctive signature that we can be sure that this is a real planet, even though we haven’t seen a complete orbit.”

Just how HR 5183 b got into such an interesting orbit is an intriguing question, one that is most likely answered by gravitational interactions with a planet of roughly the same size. The scenario: One planet is pushed out of the system to become a ‘rogue’ planet without a star, while HR 5183 b is forced into the orbit we observe, one that takes somewhere between 45 to 100 years to complete. Needless to say, we have nothing like this in our Solar System, but the new world reminds us how a system can be shaped by encounters between giant planets.

Moreover, there is an interesting twist here, a possible stellar companion that the authors have identified in the form of HIP 67291, a K-class star on the order of 15,000 AU from HR 5183. At this distance, the companion star is too far away to affect HR 5183 b, but it has to be included in any discussion of this system’s evolution. From the paper:

The extreme eccentricity and decades-long orbital period of HR 5183 b, coupled with the existence of a widely separated, eccentric stellar companion…raise interesting questions about the system’s formation. High eccentricity is a signature of past dynamical interactions (Dawson et al. 2014). Moreover, recent dynamical simulations by Wang et al. (2018) revealed that systems hosting multiple young massive planets, presumably near their formation locations, are likely unstable on Gyr or shorter timescales. Therefore, the HR 5183 system might have initially contained multiple massive planets with moderate eccentricities.

All of which backs the theory of a slingshot effect, with one planet coming close to the other and, in Howard’s words, coming “in like a wrecking ball, knocking anything in its way out of the system.”

We’re also reminded that radial velocity methods are now moving deeper into stellar systems, helping us to learn about planets that do not necessarily transit but leave their signature on the host star’s motion. The paper goes on to point to another system, HR 8799, which contains at least four massive planets whose orbital motion was confirmed by direct imaging.

Planet-planet interactions in such a system could have ejected some planets and transferred angular momentum to the remaining planet(s), pumping their eccentricities. If this is true, the HR 5183 system could be viewed as the “fate” of systems like HR 8799. Dynamical work aiming to distinguish between this and other possible formation scenarios (for example, potential past interactions with HIP 67291) would be an excellent avenue for future studies. It will be interesting to learn whether HR 5183 b represents the eventual evolution of multiple giant planet systems like HR 8799, or if it is in a class all its own.

So radial velocity finds its way into a previously unexplored parameter space, the realm of long-period gas giants on eccentric orbits. Usefully, HR 5183 b is a candidate for future detection through high-contrast imaging and stellar astrometry, which would make it possible to measure its mass directly. The authors believe the planet will be detectable in data from Gaia, ESA’s space observatory designed to use astrometry to measure the positions and motions of stars with the highest precision yet. We’ll be hearing a good deal more about HR 5183 b.

The paper is Blunt et al., “Radial Velocity of an Eccentric Jovian World Orbiting at 18AU,” accepted at The Astronomical Journal (preprint).


Upwelling Oceans: Modeling Exoplanet Habitability

We usually talk about habitability in binary form — either a planet is habitable or it is not, defining the matter with a ‘habitable zone’ in which liquid water could exist on the surface. Earth is, of course, the gold standard, for we haven’t detected life on any other world.

But it is conceivable that there are planets where conditions are more clement than our own, as Stephanie Olson (University of Chicago) has recently pointed out. The work, presented at the just concluded Goldschmidt Geochemistry Congress in Barcelona, models circulatory patterns in oceans, some of which may support abundant life if they exist elsewhere. The emphasis here is not so much on surface ocean currents but upwelling water from deep below. Says Olson:

“We have used an ocean circulation model to identify which planets will have the most efficient upwelling and thus offer particularly hospitable oceans. We found that higher atmospheric density, slower rotation rates, and the presence of continents all yield higher upwelling rates. A further implication is that Earth might not be optimally habitable–and life elsewhere may enjoy a planet that is even more hospitable than our own.”

All this has implications for how we use the term ‘Earth-like,’ and reminds us to be careful, as Olson told a Los Angeles Times interviewer in 2018:

“The phrase Earth-like does not refer to a planet that necessarily resembles modern-day Earth at all… It’s actually a very broad term that encompasses a broad variety of worlds. It includes hazy worlds like the Archean; it includes icy worlds like the ‘snowball Earth’ intervals; it includes anoxic worlds with exclusively microbial ecosystems; it includes worlds with complex and intelligent life; and it includes worlds that we haven’t even seen yet.”

Image: Geophysicist Stephanie Olson. Credit: University of Chicago.

Stephanie Olson makes the case that life has to be far more common than what we can detect at our current stage of technology. An ecosystem beneath the surface of an icy moon may defeat our methods, as could microorganisms deep within a planet’s mantle. So what we need to do, in this scientist’s view, is build our target lists for future study around a subset of planets, those that meet the habitability demands of forms of life that are global, active and detectable. This also builds the list of those worlds for which a non-detection would be the most telling.

In general, our developing models for habitability have tracked our interest in finding atmospheric biosignatures, for we are closing in on the capability of doing this for small, rocky worlds circling nearby M-dwarf stars. The complexities of ocean dynamics have been left out of the picture other than when used as a mechanism for climate regulation or heat transport.

In her conference abstract at the Goldschmidt conference, Olson argues that the implications of circulatory patterns in oceans should be folded into the habitability question. Cycles of ocean upwelling driven by winds can recycle nutrients from the deep ocean back to shallower waters where they can play a role in stimulating photosynthesis. From the abstract:

Photosynthesis,,,provides energy in the form of chemical disequilibrium that sustains life more broadly on our planet. Ocean circulation is thus a first-order control on the productivity and distribution of life on Earth today and throughout our planet’s history. Moreover, ocean circulation patterns, sea ice coverage, and sea-to-air gas exchange kinetics modulate the extent to which biological activity within the ocean is communicated to the atmosphere. The chemical evolution of Earth’s atmosphere has ultimately been an imperfect reflection of the evolution of Earth’s marine biosphere owing to these oceanographic phenomena.

Models of Habitability

Olson’s tool for exploring ocean dynamics on a range of modeled, habitable exoplanets is a global circulation model (GCM) called ROCKE-3D. The software is designed to examine different periods in the evolution of terrestrial-class planets, with the goal of finding what kind of techniques might flag the presence of life in these environments. You can have a look at ROCKE-3D in action in this NASA page on the simulation of planetary climates. Different parameters can be selected on a form to create maps of a number of climate variables.

Below is an example of one of these maps, as created by the ROCKE-3D software.

Image: The discovery of the planet Proxima Centauri b orbiting the star closest to Earth has generated much research about whether it has a chance to be habitable. With ROCKE-3D we have imagined Proxima Centauri b as an “aquaplanet” covered by water. Because the planet is close to its star, it may show the same face to the star all the time, as the Moon does to the Earth. If so, the dayside remains a few degrees above freezing (yellow colors). Elsewhere, the ocean is perpetually covered by ice (dark blue colors), except near the equator where winds and ocean currents push sea ice eastward onto the dayside where it breaks up and melts (pale blue to light yellow colors). Credit: NASA Nexus for Exoplanet System Science (NExSS) / NASA Goddard Institute for Space Studies (GISS).

Three-dimensional planetary general circulation models have been used to project climate change into future decades, but have matured to the point that they can probe habitability questions such as how a planet can become habitable under variations in stellar radiation and atmospheric chemistry. The NASA Nexus for Exoplanet System Science (NExSS) effort works on these matters in a cross-disciplinary effort to parse habitability in terms of the factors that make it happen, from host stars to protoplanetary disks and rocky planet atmospheres.

ROCKE-3D stands for Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics, now developing as a collaborative investigation within NExSS. At NASA GSFC, the Goddard Institute for Space Studies (GISS) developed ROCKE-3D to run global circulation model simulations deploying and manipulating past climates of Earth and other planets by way of analyzing climates and ocean habitats. The idea is to produce model spectra and phase curves for future observations. Let me quote from the GISS website:

Our project uses solar radiation patterns and planetary rotation rates from simulations of spin-orbit dynamical evolution of planets over Solar System history provided by our colleagues at the Columbia Astrobiology Center and at other institutions that are part of our NExSS team. In turn, the synthetic disk-integrated spectra we produce from the GCM will be used as input to a whole planetary system spectral model that emulates observations that candidate future direct imaging exoplanet missions might obtain…

Here you can see the direction of this work. What these teams are trying to do is model what future observatories may see when we become capable of directly imaging rocky exoplanets. We need to learn what kind of signals may be detectable as we allocate precious observing time to those targets most likely to repay the effort. Here theory about the kind of spectral details that life may produce is the foundation for later direct observational data.

Olson’s Oceans

Back to Olson, who wants to fold ocean dynamics into this effort and consider how they may be manifested on habitable exoplanets. Can features of ocean circulation that we cannot observe be inferred from atmospheric properties we can see? Olson’s work is an attempt to link ocean circulation with key planetary parameters, invoking the biological constraints differing ocean habitats may place on worlds around a variety of stars.

We can’t say how this work will develop, but there is the real prospect for the telescope design of future missions — think LUVOIR (Large UV/Optical/IR Surveyor) or HabEx (Habitable Exoplanet Observatory) — to be affected as we learn more about what we need to look for. Adds Olson:

“Our work has been aimed at identifying the exoplanet oceans which have the greatest capacity to host globally abundant and active life. Life in Earth’s oceans depends on upwelling (upward flow) which returns nutrients from the dark depths of the ocean to the sunlit portions of the ocean where photosynthetic life lives. More upwelling means more nutrient resupply, which means more biological activity. These are the conditions we need to look for on exoplanets.”

Immense effort is going into modeling planetary climate and evolution to guide our investigation of habitability. It will be fascinating to watch the trajectory of these studies as we begin to deploy advanced space-based resources to probe for biosignatures. My guess is that we will see early detections of potential biosignatures — these will receive huge press coverage — but we will not find anything that is unambiguous.

That may seem like a letdown when it happens, but ruling out abiotic mechanisms for possible biosignatures is equally a part of global circulation modeling, and this work will take time.


JUICE: Targeting Three Icy Moons

Because Europa Clipper has been on my mind, what with the confirmation of its next mission phase (see Europa Clipper Moves to Next Stage), we need to continue to keep the mission in context. What is playing out is a deepening of our initial reconnaissance of the Jovian system, and the JUICE mission (Jupiter Icy Moons Explorer) is a significant part of that overall effort. The European Space Agency has the spacecraft under development, with Airbus Defence and Space as the primary contractor.

We saw last week that while Europa Clipper will use flybys of Ganymede and Callisto for gravity maneuvers intended to refine its orbit, the latter two moons are not science priorities. JUICE, on the other hand, focuses on all three, each thought to house liquid water beneath the surface. JUICE is slated for a June, 2022 launch, reaching Jupiter in 2029 with the help of five gravity assists along the way, so its operations will overlap with Europa Clipper, the NASA craft launching in 2023. The coming decade will be busy indeed as we journey to and explore these compelling icy targets.

The orbital maneuvers chosen for JUICE are intriguing, for after its first flyby of Europa in 2030, the spacecraft is to enter a high-inclination orbit to study Jupiter’s polar regions and magnetosphere. Repeated flybys of Europa, Ganymede and Callisto are planned, and following a Callisto flyby in 2031, the spacecraft will actually enter orbit around Ganymede, making it the first spacecraft to orbit a moon other than our own. I’m simplifying these complicated orbital maneuvers for the sake of brevity, but the point is that JUICE will greatly expand our datasets on all three moons.

In June of 2019, engineers at Airbus Defence and Space’s site in Toulouse tested the navigation camera that will be essential for radio tracking and position and velocity information of the spacecraft relative to the moon it is currently studying. Given the powerful radiation found near Jupiter, the spacecraft will, like Europa Clipper, be radiation-hardened, allowing it to operate between 200 and 400 kilometers from its targets at closest points of rendezvous. The pointing accuracy demanded of NavCam during fast and close approaches like these is critical to the mission’s success.

The June tests looked at the NavCam engineering model in real sky conditions, the point being to stress the hardware and software interfaces to validate their design, as well as to prepare the image processing and onboard navigation software that JUICE will use to acquire its images. The engineers observed Earth’s own moon and a variety of sky objects included Jupiter itself as part of these tests, running NavCam in ‘imaging mode’ and ‘sky centroiding mode’ as part of fine tuning attitude control software.

Image: The Navigation Camera (NavCam) of the Jupiter Icy Moons Explorer (JUICE) has been given its first glimpse of the mission’s destination while still on Earth. The camera was mounted to an equatorial mount and pointed towards different targets, including bright stars, Jupiter and its moons in order to exercise its ‘Imaging Mode’ and ‘Stars Centroiding Mode’. The integration time was optimized for capturing the stars and moons acquisition, so Jupiter appears saturated. In this annotated image the size of Jupiter is indicated. Credit & Copyright: Airbus Defence and Space.

“Unsurprisingly, some 640 million kilometres away, the moons of Jupiter are seen only as a mere pixel or two, and Jupiter itself appears saturated in the long exposure images needed to capture both the moons and background stars, but these images are useful to fine-tune our image processing software that will run autonomously onboard the spacecraft,” says Gregory Jonniaux, Vision-Based Navigation expert at Airbus Defence and Space. “It felt particularly meaningful to conduct our tests already on our destination!”

Image: Impressions of how the Jupiter Icy Moons Explorer will see moons Europa (left), Ganymede (middle) and Callisto (right) with its Navigation Camera (NavCam). To generate these images, the NavCam was fed simulated views – based on existing images of the moons – to process realistic views of what can be expected once in the Jupiter system. Credit & copyright: Airbus Defence and Space.

The actual flybys will provide close inspection of surface features on Europa, Ganymede and Callisto. In the suite of tests, NavCam also received simulated views of the three moons in order to process the kind of imagery it will acquire in Jupiter space. NavCam will capture imagery that will be greatly augmented by the high-resolution camera suite that will give us our best views of the icy surfaces below.

By the end of 2019, the test NavCam will be augmented with a full flight representative performance optics assembly that will support onboard tests of the complete spacecraft. Meanwhile, a test version of the spacecraft’s 10.5-m long magnetometer boom developed by SENER in Spain has undergone testing at ESA’s Test Centre in the Netherlands, as part of what ESA is describing as “… the most powerful remote sensing, geophysical, and in situ payload complement ever flown to the outer Solar System.”

Image: Magnetometer boom built for ESA’s mission to Jupiter by European Space Agency. Credit: ESA–G. Porter, CC BY-SA 3.0 IGO.


Going Deep into Jupiter’s Storms

Having just looked at events that may have shaped Jupiter’s core, it seems a good time to note the new Hubble image of the planet, taken on June 27, 2019. A couple of things to focus on in the image below: The vast anticyclonic storm we call the Great Red Spot, about the diameter of the Earth, is evident as it rolls counterclockwise between bands of clouds moving in opposite directions toward it.

We still don’t know why, but the storm itself continues to shrink. Smaller storms show up vividly as white or brown ovals, some of which dissipate within hours, while others may be as long lasting as the Great Red Spot, which has dominated Jupiter’s face for at least 150 years. Note the cyclone showing up south of the Spot, visible as a worm-shaped feature. You can also see other anticyclones, appearing as white ovals.

Image: The NASA/ESA Hubble Space Telescope reveals the intricate, detailed beauty of Jupiter’s clouds in this new image taken on 27 June 2019. It features the planet’s trademark Great Red Spot and a more intense colour palette in the clouds swirling in the planet’s turbulent atmosphere than seen in previous years. This image was captured by Hubble’s Wide Field Camera 3, when the planet was 644 million kilometres from Earth. Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley).

But let’s move beyond Hubble. New work on the storm clouds of Jupiter swirling through the planet’s atmospheric belts has just appeared, drawing not only on space-based resources but also a mix of optical and radio telescopes that have gone into recent tracking of their activity. In January of 2017, Australian amateur astronomer Phil Miles observed the bright plume of a storm that was subsequently picked up by observations with the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile. The latter work was led by UC-Berkeley astronomer Imke de Pater, producing a paper that has been accepted at the Astronomical Journal (citation below).

Things happen quickly enough on Jupiter that we can track them by daily observation, and Hubble images taken a week after the ALMA work showed that what had been a single plume had spawned a second plume and left visible downstream changes in Jupiter’s south equatorial belt. Moreover, four bright spots seen three months earlier in the north equatorial belt had disappeared, while the belt itself had widened as well as changed colors, from a striking white to orange-brown. This may be the result of gas from plumes now depleted of ammonia falling back into the lower atmosphere.

“If these plumes are vigorous and continue to have convective events, they may disturb one of these entire bands over time, though it may take a few months,” says de Pater. “With these observations, we see one plume in progress and the aftereffects of the others.”

The paper posits that plumes like these emerge about 80 kilometers below the cloud tops, in a region where clouds of liquid water droplets are common. Jupiter’s atmosphere is primarily hydrogen and helium, with trace amounts of methane, ammonia, hydrogen sulfide and water. What we’re seeing at the top-most cloud layer, with its brown belts and white zones, is largely made up of ammonia ice. A layer of solid ammonium hydrosulfide particles is found below this in the upper cloud deck.

Image: ALMA’s view of Jupiter at radio wavelengths (top) and the Hubble Space Telescope’s view in visible light (bottom). The eruption in the South Equatorial Belt is visible in both images: a dark spot in radio, a bright spot in visible. Credit: ALMA image by Imke de Pater and S. Dagnello; Hubble image courtesy of NASA.

The radio telescopes of ALMA are able to see beneath the upper ammonia clouds that are opaque in visible frequencies, but de Pater’s team also brought data from Hubble, the Very Large Array, the Gemini, Keck and Subaru observatories in Hawaii and the Very Large Telescope (VLT) in Chile into the mix, homing in on the storm seen above as it emerged from the lower cloud levels into the upper ammonia ice clouds.

Image: A closeup of the two bright white plumes (center) in the South Equatorial Belt of Jupiter and a large downstream disturbance to their right. Credit: Imke de Pater, UC Berkeley; Robert Sault, University of Melbourne; Chris Moeckel, UC Berkeley; Michael Wong, UC Berkeley; Leigh Fletcher, University of Leicester.

The storm clouds, reaching Jupiter’s tropopause, where the atmosphere is at its coldest, spread out in much the same way as the anvil-shaped formations of thunderstorms we see in Earth’s atmosphere. The ALMA data were sufficient to show that high concentrations of ammonia gas are forced upward during an eruption like this. Convection, the scientists believe, brings both ammonia and water vapor high enough for the water to condense into liquid droplets, releasing heat along the way.

Now we have a plume with enough momentum that, as heat is released from condensing water, can break out above the clouds of the upper deck, where the ammonia will freeze to create the white plume of these images.

“We were really lucky with these data, because they were taken just a few days after amateur astronomers found a bright plume in the South Equatorial Belt,” adds de Pater. “With ALMA, we observed the whole planet and saw that plume, and since ALMA probes below the cloud layers, we could actually see what was going on below the ammonia clouds.”

Image (click to enlarge): An illustration of “moist convection” in Jupiter’s atmosphere shows a rising plume originating about 80 kilometers below the cloud tops, where the pressure is five times that on Earth (5 bar), and rising through regions where water condenses, ammonium hydrosulfide forms and ammonia freezes out as ice, just below the coldest spot in the atmosphere, the tropopause. Credit: Adapted from illustration by Leigh Fletcher, University of Leicester.

This useful analysis was made possible because of simultaneous observations at different wavelengths, in this case homing in on transient events and showing us how the atmosphere at different levels, from cloud tops to deep below, responds to them. This is new ground in the study of Jupiter’s weather, as the paper notes:

These data are the first to characterize the atmosphere below the cloud layers during/following such outbreaks. Aided also by observations ranging from uv to mid-infrared wavelengths, we have shown that the eruptions are consistent with models where energetic plumes are triggered via moist convection at the base of the water cloud. The plumes bring up ammonia gas from the deep atmosphere to high altitudes, where NH3 gas is condensing out and the subsequent dry air is descending in neighboring regions. The cloud tops are cold, as shown by mid-infrared data, indicative of an anticyclonic motion, which causes the storm to break up, as expected from similarities to mesoscale convective storms on Earth. The plume particles reach altitudes as high as the tropopause.

The paper is de Pater et al., “First ALMA Millimeter Wavelength Maps of Jupiter, with a Multi-Wavelength Study of Convection” (preprint).


Giant Jovian Impact Could Explain Juno Data

Impacts seem to have run rampant in the early Solar System, to judge from what we keep uncovering as we survey today’s evidence. The Moon is widely considered to be the result of Earth’s impact with a Mars-class object, while Mercury’s big iron core may show what happens when a larger world is stripped of much of its mantle in another ‘big whack.’ Then there’s Uranus, spinning lopsidedly in the outer system.

We also know that impacts continue to make their mark. They’re shown up on Jupiter at a fairly brisk pace, with Shoemaker-Levy striking the gas giant in 1994, and another evident impact from an asteroid earlier this month, creating a definitive flash.

For that matter, we have a Hubble image from 2009 showing an impact, an expanding spot twice the length of the United States. That one was discovered by Australian amateur astronomer Anthony Wesley. Later observations allowed scientists to estimate the impactor’s diameter at 200 to 500 meters, with an explosion thousands of times more powerful than the Tunguska event in 1908. Juno mission scientist Ravit Helled (University of Zurich) jokes that when planetary scientists lack a solution, they tend to invoke a giant impact. If so, it seems to be an understandable assumption.

Image: Hubble’s view of the 2009 impact event on Jupiter. Credit: NASA, ESA, and H. Hammel (Space Science Institute, Boulder, Colo.), and the Jupiter Impact Team.

But Helled and colleagues in the National Centre of Competence in Research PlanetS (Bern, Switzerland) aren’t joking when it comes to results from Juno that are forcing them to change their views of Jupiter’s core. The gravity data demand explanation, which may emerge in a massive impact early in the history of Jupiter’s formation:

“Instead of a small compact core as we previously assumed, Jupiter’s core is ‘fuzzy,’” Helled explains. “This means that the core is likely not made of only rocks and ices but is also mixed with hydrogen and helium and there is a gradual transition as opposed to a sharp boundary between the core and the envelope.”

In a paper just published in Nature, Helled and an international team led by Shang-Fei Liu (Sun Yat-sen University in Zhuhai, China) present the results of simulated collisions between an early Jupiter and planetary embryos. They worked with software code developed by PhD student Simon Müller that probed planetary evolution. Particularly puzzling is the thermal evolution of the planet after the impact. Could the diluted core Juno found really persist for billions of years until today?

Giant impacts, the authors argue, are most likely to occur not long after runaway gas accretion. This is when the gravitational effects of the growing planet increase 30-fold in the space of a few million years, destabilizing the orbits of nearby embryos. The team’s simulations show that Jupiter’s gravitational effect on nearby planetary embryos would have been profound, with at least a 40 percent chance that a large embryo would hit Jupiter within the first few million years.

The scientists worked through tens of thousands of simulations to model this effect, and went on to use separate computer code to investigate what these impacts would do to Jupiter’s internal structure. To produce the diluted core we see today, heavy elements in the core and the embryo need to mix with the surrounding gas envelope.

Analyzing heat transport and heavy element mixing, the team finds that it would take an impactor with about 10 times Earth’s mass to stir Jupiter’s core, mixing denser layers with less dense layers above. The team’s 3D models show the effects of a major hit below.

Image: This is Figure 3 from the letter in Nature. Caption: Three-dimensional cutaway snapshots of density distributions during a merger event between a proto-Jupiter with a 10M rock/ice core and a 10M impactor. a, Just before the contact. b, The moment of core–impactor contact. c, 10 h after the merger. Owing to impact-induced turbulent mixing, the density of Jupiter’s core decreases by a factor of three after the merger, resulting in an extended diluted core… Credit: Shang-Fei Liu/Sun Yat-sen University.

The authors go on to compute the thermal evolution following the impact of a 10 Earth mass impactor until the present day, a span of 4.56 billion years. There is only one solution that produces a diluted core like that found by Juno. From the paper:

We conclude that Jupiter’s diluted-core structure could be explained by a giant impact event, but only under specific conditions including a head-on collision with a massive planetary embryo, a post-impact central temperature of about 30,000 K or an initial thermal structure created by the accretion shock during the runaway phase. Indeed, the hydrodynamic simulation suggests that most of the impact energy is not deposited in the deep interior, and therefore the central temperature is unlikely to increase substantially, supporting the diluted core solution.

Interestingly, such an impact would demand the collision be head-on, for grazing impacts would not produce the core-density profile that Juno has now measured. Even a grazing embryo of 10 Earth masses, in this scenario, would be disrupted while sinking to the center of the planet. Meanwhile, smaller impactors (1 Earth mass or less) disintegrate in the envelope of the gas giant before they ever reach its center.

How definitive is this impact solution? The paper points out in its conclusion that a gradual accretion of planetesimals along with runaway gas accretion could produce a disrupted core, but the authors question whether this would allow a diluted core to be preserved to the present day. They also note that giant impacts like the one they model here may be producing an observational signature in extrasolar gas giants, in the form of the high metallicity found in some of these worlds. And this is interesting:

Since impacts of planetary embryos are expected to be frequent after a gas giant’s runaway gas accretion phase, such an event with different impact conditions (such as a small impactor or an oblique collision) may have also happened to Saturn, and could in principle explain the differences between the internal structures of Jupiter and Saturn.

Shang-Fei Liu et al., “The formation of Jupiter’s diluted core by a giant impact,” Nature 572 (15 August 2019), pp. 355–357 (abstract).


Europa Clipper Moves to Next Stage

Europa Clipper stays on my mind, with the intent of digging deeper into the spacecraft as development moves forward. We are talking about a craft that is by necessity radiation-tolerant as it will make a series of close flybys of Europa during its long orbit of Jupiter. 45 such flybys are in the cards, at altitudes varying from 2700 to 25 (!) kilometers, with flybys of Ganymede and Callisto in the mix as well. The latter are considered gravitational maneuvers intended to refine Europa Clipper’s orbit, and while they should be productive, they are not science priorities.

Image: Because Europa lies well within the harsh radiation fields surrounding Jupiter, even a radiation-hardened spacecraft in near orbit would be functional for just a few months. Studies by scientists from the Jet Propulsion Laboratory show that by performing several flybys with many months to return data, the Europa Clipper concept would enable a $2B mission to conduct the most crucial measurements of the cancelled $4.3B Jupiter Europa Orbiter concept. Here we see how the mission can achieve global coverage during successive flybys. Credit: NASA/JPL.

NASA has now announced confirmation of Europa Clipper’s next mission phase, which means we proceed to completion of the final design, which will in turn be followed by construction and testing of the spacecraft and its science payload. NASA associate administrator for the Science Mission Directorate Thomas Zurbuchen frames Europa Clipper within the sequence of outer system missions that most recently has included Cassini’s operations at Saturn:

“We are all excited about the decision that moves the Europa Clipper mission one key step closer to unlocking the mysteries of this ocean world. We are building upon the scientific insights received from the flagship Galileo and Cassini spacecraft and working to advance our understanding of our cosmic origin, and even life elsewhere.”

Image: This artist’s rendering shows NASA’s Europa Clipper spacecraft, which is being developed for a launch sometime in the 2020s. Credit: NASA/JPL.

As the concept evolves, we’ll see how closely it tracks the image above, in which the ice-penetrating radar antennae are attached to the solar arrays extending from the spacecraft. The magnetometer boom and round high-gain antenna are visible on the side of the spacecraft, with a remote-sensing palette housing the rest of the instrument payload on the left.

The instruments NASA has selected to study Europa include nine of the thirty-three originally proposed. As you would imagine, they include a thermal instrument that will search the surface for recent eruptions of warmer water even as other instruments look for tiny particles in the thin atmosphere around the moon. It was back in 2012 that Hubble data indicated water vapor above the south polar region, giving us the possibility of water plumes linked to the subsurface ocean. As at Enceladus, that would open sampling options without drilling through the ice.

Ice-penetrating radar will be used to determine the thickness of the ice shell while also looking for the kind of subsurface lakes found beneath Antarctica. The matter has been the subject of controversy for years and clearly determines what is and is not possible in terms of ocean sampling from the surface, although collection of materials near Europa’s chaos regions, where the surface has been deformed, may one day allow a lander to study frozen ocean brines.

Long linear cracks on the surface seem to be the result of tidal forces causing the ice shell to flex. The constant gravitational interaction with Jupiter could provide enough heat energy to enable chemical reactions in the interior that, through volcanoes or hydrothermal vents, recycle nutrient-rich water between the ocean and the rocky interior. How well Europa’s different layers move material between them may determine whether living organisms can flourish here.

Image: This view of the Conamara Chaos region on Jupiter’s moon Europa taken by NASA’s Galileo spacecraft shows an area where the icy surface has been broken into many separate plates that have moved laterally and rotated. These plates are surrounded by a topographically lower matrix. This matrix material may have been emplaced as water, slush, or warm flowing ice, which rose up from below the surface. One of the plates is seen as a flat, lineated area in the upper portion of the image. Below this plate, a tall twin-peaked mountain of ice rises from the matrix to a height of more than 250 meters. The matrix in this area appears to consist of a jumble of many different sized chunks of ice. Though the matrix may have consisted of a loose jumble of ice blocks while it was forming, the large fracture running vertically along the left side of the image shows that the matrix later became a hardened crust, and is frozen today. Credit: NASA/JPL.

Europa Clipper will also carry cameras and spectrometers to produce high-resolution images and map surface composition, along with a magnetometer to measure the moon’s magnetic field, offering insights into the depth and salinity of the ocean. NASA announced in March that it was going to replace the earlier magnetometer designed for the mission — Interior Characterization of Europa Using Magnetometry, or ICEMAG — with a less complex (read ‘expensive’) instrument. The current list of instruments can be accessed here.


LHS 3844b: Rocky World’s Atmosphere Probed

These days we have a keen interest in small red dwarf stars (M-dwarfs) not only because they’re ideal for study, with deep transits of worlds in their habitable zones and the prospect of future analysis of their atmospheres, but also because they are so plentiful. Comprising perhaps 80 percent of all stars, they may well be home to the great majority of planets in the galaxy. And while they are common, they’re also long-lived, so that life would have plenty of opportunity to develop.

Now we have word of new work using both the Transiting Exoplanet Survey Satellite (TESS) and the Spitzer Space Telescope. TESS is, of course, a transit hunter, looking for the telltale dips in light from a parent star when a planet passes in front of it. The planet in question is LHS 3844b, about 48.6 light years out, and discovered by TESS in 2018. Follow-up observations in the infrared with Spitzer have detected light from the surface of this newly discovered world, allowing study of its atmosphere and composition. Note: This is not direct imaging; see below for more on the techniques used.

LHS 3844b orbits its star in 11 hours, making it almost certainly tidally locked; i.e., with one side always facing the star. The Spitzer data show that the dayside here reaches 770 degrees Celsius, while the nightside temperature is consistent with 0 Kelvin. In other words, the researchers could detect no heat being transferred from one side to the other, a process we would expect in the presence of an atmosphere.

Heat transfer is a mechanism that could ameliorate the effects of tidal lock, spreading warmth to the dark side and moderating global temperatures, but it takes an atmosphere to do that. We learn, then, that LHS 3844b is an object something like the Moon, or at any rate, a large version of it. Laura Kreidberg (Harvard-Smithsonian Center for Astrophysics), lead author of the paper that appears in Nature, says that this planet “…matches beautifully with our model of a bare rock with no atmosphere.” The scientist continues:

“We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically. Now, with LHS 3844b, we have a terrestrial planet outside our solar system where for the first time we can determine observationally that an atmosphere is not present.”

Image: This artist’s illustration depicts the exoplanet LHS 3844b, which is 1.3 times the mass of Earth and orbits an M dwarf star. The planet’s surface may be covered mostly in dark lava rock, with no apparent atmosphere, according to observations by NASA’s Spitzer Space Telescope. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).

This is painstaking analysis indeed, drawing on phase curve data from the planet’s transits. Phase curves are a combination of reflected light and thermal emission from the planet. Unable to resolve the planet from the host star, astronomers must work with their combined light, and observe light variations of exquisite subtlety as planets go through phase changes as they orbit. A phase curve, then, is the time-dependent change in the brightness of a planet as seen from Earth during one orbital period.

Image: Detecting Light from Exoplanet LHS 3844b. Credit: NASA/JPL-Caltech/L. Kreidberg (CfA | Harvard & Smithsonian).

Learning about the atmosphere (or lack thereof) of a small rocky world — LHS 3844b has a radius 1.3 times that of Earth — is therefore something of a coup, and bodes well for future discovery. The authors infer from the planet’s reflectivity (albedo) that it is covered with basalt, much like the mare of the Moon, which is probably an indication of volcanic activity in the distant past. From the paper:

We modeled the emission spectra of several rocky surfaces and compared with the measured planet-to-star flux… We considered multiple geologically plausible planetary surface types, including primary crusts that form from solidification of a magma ocean (ultramafic and feldspathic), secondary crust that forms from volcanic eruptions (basaltic), and a tertiary crust that forms from tectonic re-processing (granitoid). Governed by the reflectivity in the visible and the near-infrared and the emissivity in the mid-infrared, the surface types have distinct emission spectra. The measured planet-to-star flux for LHS 3844b is most consistent with a basaltic composition. Such a surface is comparable to the lunar mare and Mercury, and could result from widespread extrusive volcanism.

But this is a world much larger than the Moon, so what happened to its atmosphere? M-dwarf flare activity is thought to erode early planetary atmospheres, especially given how closely worlds like this orbit their star. The researchers rule out an atmosphere of over 10 bars (Earth’s atmospheric pressure at sea level is about 1 bar), and largely rule out one between 1 and 10 bars. They believe stellar winds and flares are the culprit. Modeling atmospheric escape over time, they assess how an early atmosphere dissolves within a magma ocean during planet formation or photolyzes into hydrogen and oxygen because of the intense bombardment of X-rays and UV from flares.

Thus a thick atmosphere is ruled out by the data, while stellar winds could account for further erosion of a thin atmosphere, all leading to the conclusion that LHS 3844b is a bare rock unless a thin atmosphere is replenished over time. Should we assume that hot terrestrial planets orbiting well inside the habitable zone of M-dwarfs are all devoid of atmospheres? Perhaps, but these stars may still be of astrobiological interest:

The results presented here motivate similar studies for less-irradiated planets orbiting small stars. Cooler planets are less susceptible to atmospheric escape and erosion, and may provide a friendlier environment for the evolution of life. In coming years this hypothesis can be tested, thanks to the infrared wavelength coverage of the James Webb Space Telescope and the influx of planet detections expected from current and future surveys.

The paper is Kreidberg et al., “Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b,” Nature 19 August 2019 (abstract / preprint)


Heliophysics with Interstellar Implications

You would think that heading toward the Sun, rather than away from it, would not necessarily fall under Centauri Dreams’ purview, but missions like the Parker Solar Probe have reminded us that extreme environments are ideal testing grounds for future missions. Build a heat shield that can take you to within 10 solar radii of our star and you’re also exploring possibilities in ‘sundiver’ missions that all but brush the Sun in a tight gravity assist.

Or consider the two proposals NASA has just selected in the area of small satellite technologies, which grow directly out of its heliophysics program. Here, the study of the Sun’s interactions with the Solar System, and the consideration of Sun, planets and heliosphere as a deeply interconnected system, takes pride of place. Let’s start with a mission called SETH — Science-Enabling Technologies for Heliophysics. One of its two technology demonstrators, called the HELio Energetic Neutral Atom (HELENA) detector, involves solar energetic neutral atoms, which can provide advanced warnings of potential radiation threats to astronauts.

The other demonstrator aboard SETH is an optical communications technology expressly designed for CubeSats and other small satellites, one that could allow a hundred-fold increase in the return of deep space data. Building out a robotic infrastructure in the Solar System will involve increasingly miniaturized technologies. We can envision small satellite constellations that can network and operate one day in ‘swarm’ fashion to create a continuous presence around targets ranging from asteroids to the gas and ice giants that can shape their orbits.

Image: NASA has selected two proposals to demonstrate technologies to improve science observations in deep space. The proposals could help NASA develop better models to predict space weather events that can affect astronauts and spacecraft, such as coronal mass ejections (CMEs). In this image, taken by the Solar and Heliospheric Observatory on Feb. 27, 2000, a CME is seen erupting from the Sun, which is hidden by the disk in the middle, so the fainter material around it can be seen. Credit: ESA/NASA/SOHO.

Toward a Large Solar Sail

But if you’re looking for a mission with real interstellar punch, consider Solar Cruiser, whose two technology demonstrations involve measurements of the Sun’s magnetic field structure and the velocity of coronal mass ejections (CMEs), those vast explosions of plasma that can create space weather nightmares for utility grids on Earth. Making this mission possible will be a solar sail of almost 1,700 square meters. The timing on this proposal seems propitious given The Planetary Society’s recent success at raising the orbit of LightSail-2 using sunlight. Pushing toward much larger designs is the next step.

“This is the first time that our heliophysics program has funded this kind of technology demonstration,” said Peg Luce, deputy director of the Heliophysics Division at NASA Headquarters. “Providing the opportunity to mature and test technologies in deep space is a crucial step towards incorporating new techniques into future missions.”

Lots to work with here, and I’m drawing together more information about Solar Cruiser, which would not only be by far the largest solar sail yet deployed, but would also experiment with using the momentum of sunlight to continuously modify its orbit. This would allow us to obtain views of the Sun that orbits involving gravity alone would not make possible. Robert Forward explored the original concept and introduced it to the public first in the pages of Analog and then in his book Indistinguishable from Magic (1995), where he considered how we might use such spacecraft near the Earth. He called a spacecraft that uses a solar sail to hover over a region rather than orbiting the Earth a ‘statite,’ and explained it this way:

…I have the patent on it — U.S. Patent 5,183,225 “Statite: Spacecraft That Utilizes Light Pressure and Method of Use”… The unique concept described in the patent is to attach a television broadcast or weather surveillance spacecraft to a large highly reflective lightsail, and place the spacecraft over the polar regions of the Earth with the sail tilted so the light pressure from the sunlight reflecting off the lightsail is exactly equal and opposite to the gravity pull of the Earth.

Here we are using a solar sail for station-keeping rather than transport, and Solar Cruiser may turn out to be the first time we experiment with the technique, which offers options that other kinds of satellite do not:

With the gravity pull nullified, the spacecraft will just hover over the polar region, while the Earth spins around underneath it. Since the spacecraft is not in orbit around the Earth, it is technically not a satellite, so I coined the generic term ‘statite’ or ‘-stat’ to describe any sort of non-orbiting spacecraft (such as a ‘weatherstat’ or ‘videostat’ or ‘datastat’).

Image: Analog‘s December, 1990 issue contained an article by Robert Forward describing the ‘polesitter’ concept, one of many innovative ideas the scientist introduced to a broad audience. Credit: Condé Nast.

Can Solar Cruiser push these ideas forward in orbits near the Sun? Forward called orbits that are non-Keplerian ‘displaced orbits’ and also referred to such satellites as ‘polesitters.’ It will be fascinating to see how far Solar Cruiser will explore such capabilities as part of its larger mission, which should also teach us much about large sail materials and deployment.

What will follow is a nine-month study period, with both proposals funded at $400,000 for concept studies, after which one of the two proposals will be selected to go into space. Launch will take place in October of 2024 as a secondary payload along with the Interstellar Mapping and Acceleration Probe (IMAP) probe, another mission we’ll be following closely as it investigates the interactions of the solar wind with the local interstellar medium (the spacecraft will orbit the Sun-Earth L1 Lagrangian point and will also be used to monitor space weather).

Also of interest: Baig and McInnes, “Light-Levitated Geostationary Cylindrical Orbits are Feasible,” Journal of Guidance, Control and Dynamics, Vol. 33, No. 3 (2010), pp. 782-793 (abstract).


Looking for Life Under Flaring Skies

The faint glow of a directly imaged planet will one day have much to tell us, once we’ve acquired equipment like the next generation of extremely large telescopes (ELTs), with their apertures measuring in the tens of meters. Discovering the makeup of planetary atmospheres is an obvious deep dive for biosignatures, but there is another. Biofluorescence, a kind of reflective glow from life under stress, could be detectable in some conditions at astronomical distances.

New work on the matter is now available from Jack O’Malley-James and Lisa Kaltenegger, at Cornell University’s Carl Sagan Institute. The duo have been on the trail of biofluorescence for some time now, and in fact their paper in Monthly Notices of the Royal Astronomical Society picks up on a 2018 foray into biosignatures involving the phenomenon (citation below). Here the question is detectability in the context of biofluorescence as a protective mechanism, an ‘upshift’ of damaging ultraviolet into longer, safer wavelengths.

“On Earth, there are some undersea corals that use biofluorescence to render the sun’s harmful ultraviolet radiation into harmless visible wavelengths, creating a beautiful radiance,” says Kaltenegger.” Maybe such life forms can exist on other worlds too, leaving us a telltale sign to spot them.”

Image: An example of coral fluorescence. Coral fluorescent proteins absorb near-UV and blue light and re-emit it at longer wavelengths. Credit: Available under Creative Commons CC0 1.0 Universal Public Domain Dedication.

Biofluorescence in vegetation is an effect that is detectable from Earth orbit. Here the effect is comparatively small, accounting for 1-2 percent of the vegetation reflection signal, but of course it is also widespread given the coverage of vegetation over much of Earth’s surface. The phenomenon is also seen in corals, which produce a higher degree of fluorescence. Earth levels of biofluorescence are clearly too small to act as useful biosignatures for exoplanets, but higher levels may well occur elsewhere.

That’s because our early work on the atmospheres of Earth-sized planets will delve into systems around small M-dwarf stars. Such worlds are plentiful and the Transiting Exoplanet Survey Satellite (TESS) is expected to add to our inventory of habitable zone examples nearby. Even now, we have interesting targets: Proxima Centauri b, Ross 128b, TRAPPIST-1e, -f, -g, LHS 1140-b, for example, all of which orbit M-dwarf stars. And M-dwarf stars are known to flare.

This, in fact, is one of the cases originally made against life in such systems, for extreme X-Ray and UV radiation would create a challenging environment for even the simplest lifeforms. M-dwarfs vary in terms of the amount of radiation they produce; indeed, a planet around an inactive M-dwarf receives a lower dose of ultraviolet than Earth. But planets around active stars, particularly given the fact that the habitable zone is so close to the small star, receive much higher flux, and such flaring remains active for longer on M-dwarfs than on stars like the Sun.

Life could conceivably flourish underground on such worlds, or within oceans, but even on Earth, the authors note, we see biological responses like protective pigments and DNA repair pathways that are ways of mitigating radiation damage. The corals mentioned above use biofluorescence to reduce the risk of damage to symbiotic algae, absorbing blue and ultraviolet photons and re-emitting them at longer wavelengths.

Corals that display this phenomenon cover a mere 0.2 percent of the ocean floor, making for only a tiny change in our planet’s visible flux. But the situation could be different in actively flaring M-dwarf systems. To study the matter, O’Malley-James and Kaltenegger begin with a biofluorescent surface biosphere in a shallow, transparent ocean, adjusting the variables to simulate different ocean conditions. They vary fluorescent protein absorption and emission to produce values for reflected and emitted light. The assumption is that life evolving in conditions of extreme UV flux will produce ever more efficient fluorescence, in terms of absorbed vs, emitted photons (at maximum efficiency, all photons are absorbed and re-emitted).

The authors calculate the UV flux for different classes of M-dwarfs and quantify the outgoing emissions of common pigments during fluorescence. The model also includes atmospheric effects with varying cloud coverage, generating spectra and colors for hypothetical planetary conditions. False positives from mineral fluorescence are considered, as are signals produced by surface vegetation, with different fractions of surface coverings and biofluorescent life.

But let’s cut to the chase. A biosphere otherwise hidden from us could be revealed through the temporary glow resulting from the flare of an M-dwarf. From the paper:

Depending on the efficiency of the fluorescence, biofluorescence can increase the visible flux of a planet at the peak emission wavelength by over an order of magnitude during a flare event. For comparison, the change in brightness at peak emission wavelengths caused by biofluorescence could increase the visible flux of an Earth-like planet by two orders of magnitude for a widespread biofluorescent biosphere and clear skies, with low-cloud scenarios being more likely for eroded atmospheres. In an M star system, the reflected visible flux from a planet will be low due to the host star’s low flux at these wavelengths; however, the proposed biofluorescent flux is dependent on the host star’s UV flux, resulting in additional visible flux that is independent of the low stellar flux at visible wavelengths. This suggests that exoplanets in the HZ of active M stars are interesting targets in the search for signs of life beyond Earth.

Video: Lisa Kaltenegger, director of Cornell University’s Carl Sagan Institute, explains why studying bioluminescence on Earth can guide the way humans search for life on other planets.

If biofluorescence can evolve on the planets of active M-dwarf stars, it may turn out that high ultraviolet flux could be the key to reveal its existence. We need to quantify such effects because our ground- and space-based assets in the hunt for biosignatures are going to be homing in on the most readily studied planets first, and that means nearby worlds around red dwarfs. What O’Malley-James and Kaltenegger are doing is charting one possible signature that we may or may not find, but one which we now know to include in our investigative toolkit.

The paper is O’Malley-James and Kaltenegger, “Biofluorescent Worlds – II. Biological fluorescence induced by stellar UV flares, a new temporal biosignature,” Monthly Notices of the Royal Astronomical Society 13 August 2019 (abstract/full text). The earlier paper on this issue is O’Malley-James and Kaltenegger, “Biofluorescent Worlds: Biological fluorescence as a temporal biosignature for flare stars worlds,” accepted at MNRAS (preprint).


Modeling Early JWST Work on TRAPPIST-1

So much rides on the successful launch and deployment of the James Webb Space Telescope that I never want to take its capabilities for granted. But assuming that we do see JWST safely orbiting the L2 Lagrange point, the massive instrument will stay in alignment with Earth as it moves around the Sun. allowing its sunshield to protect it from sunlight and solar heating.

Thus deployed, JWST may be able to give us information more quickly than we had thought possible about the intriguing system at TRAPPIST-1. In fact, according to new work out of the University of Washington’s Virtual Planetary Laboratory, we might within a single year be able to detect the presence of atmospheres for all seven of the TRAPPIST-1 planets in 10 or fewer transits, if their atmospheres turn out to be cloud-free. Right now, we have no way of knowing whether any of these worlds have atmospheres at all. A thick, global cloud pattern like that of Venus would take longer, perhaps 30 transits, to detect, but is definitely in range.

“There is a big question in the field right now whether these planets even have atmospheres, especially the innermost planets,” says Jacob Lustig-Yaeger, a UW doctoral student who is lead author of the paper on this work. “Once we have confirmed that there are atmospheres, then what can we learn about each planet’s atmosphere — the molecules that make it up?”

Image: New research from UW astronomers models how telescopes such as the James Webb Space Telescope will be able to study the planets of the intriguing TRAPPIST-1 system. Credit: NASA.

Working with Lustig-Yaeger are UW’s Victoria Meadows, principal investigator for the Virtual Planetary Laboratory, and doctoral student Andrew Lincowski. The latter should be a familiar name if you’ve been following TRAPPIST-1 studies, because back in November of 2018 he was lead author on a paper on climate models for this fascinating system (see Modeling Climates at TRAPPIST-1).

We’ll now be hoping to follow up that work with early JWST data. Briefly, Lincowski and team pointed to the extremely hot and bright early history of TRAPPIST-1, a tiny M-dwarf 39 light years out with a radius not much bigger than Jupiter (although with considerably more mass — the star is about 9 percent the mass of the Sun). These early conditions could produce planetary evolution much like Venus, with evaporating oceans and dense, uninhabitable atmospheres. The Lincowski paper, though, did point to TRAPPIST-1 e as a potential ocean world.

These findings were in the context of a system among whose seven transiting worlds are three — e, f and g — that are positioned near or in the habitable zone, where liquid water might exist on the surface. Now we have Lustig-Yaeger and company modeling our early JWST capabilities. The paper finds that beyond the presence of an atmosphere, we may be able to draw further conclusions, particularly with regard to the evolution of what gas envelopes we find.

Although oxygen as a biosignature may not be detectable for the potentially habitable TRAPPIST-1 planets, oxygen as a remnant of pre-main-sequence water loss may be easily detected or ruled out… the 1.06 and 1.27 µm O2-O2 CIA [collisionally-induced absorption] features are key discriminants of a planet that has an oxygen abundance greatly exceeding biogenic oxygen production on Earth and may therefore indicate a planet that has undergone vigorous water photolysis and subsequent loss during the protracted super-luminous pre-main-sequence phase faced by late M dwarfs,,,

Such features could be detected fairly quickly:

… in as few as 7-9, 15, 8, 49-67, 55-82, 79-100, and 62-89 transits of TRAPPIST-1b, c, d, e, f, g, and h, respectively, should they possess such an atmosphere. These quoted number of transits may be sufficient to rule out the existence of oxygen-dominated atmospheres in the TRAPPIST-1 system. Additional evidence of ocean loss could be provided by detection of isotope fractionation, which may also be possible in as few as 11 transits with JWST.

Moreover, the authors find that water detection could help to pare down various evolutionary scenarios on these worlds, particularly for TRAPPIST-1 b, c and d, assuming atmospheres high in oxygen content that have not been completely desiccated by the star’s early history. Thus we are probing planetary evolution, but assessments of habitability are going to be tricky, and it seems clear that we will need to turn such analysis over to future direct-imaging missions.

On balance, we are talking about getting useful results with a fairly low number of transits. JWST’s onboard Near-Infrared Spectrograph will use transmission spectroscopy — where the star’s light passes through a planet’s atmosphere to reveal its spectral ‘fingerprint’ — to detect the presence of an atmosphere via the absorption of CO2. Such analysis can likewise either detect or rule out oxygen-dominated atmospheres, while constraining the extent of water loss through measurements of H2O abundance. All of this provides fodder for other, still evolving observing strategies using the JWST instrument package that can begin the characterization of these compelling worlds.

The paper is Lustig-Yaeger et al., “The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST,” Astronomical Journal Vol. 158, No. 1 (21 June 2019). Abstract / preprint. The Lincowski paper referenced above is “Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System,” Astrophysical Journal Vol. 867, No. 1 (1 November 2018). Abstract / Preprint.