Super-Earths, Magnetic Fields and Astrobiology

We’d like to know a lot more than we do about how planets create magnetic fields. After all, a major motivation for exoplanet research (though hardly the only one) is to find out whether there is other life in the universe. A magnetic field can protect planetary atmospheres from the effects of the host star’s stellar wind, a stream of charged particles that could disrupt life’s formation. Planets in close orbits of a central star are going to be particularly vulnerable.

But if protecting a planetary surface as well as keeping its atmosphere intact are powerful factors in understanding its evolution, learning more about planetary magnetic fields isn’t going to be easy. Consider a new paper from François Soubiran (École Normale Supérieure, Lyon) and Burkhard Militzer (UC-Berkeley). They’re digging into the question of magnetic fields on super-Earths, in this case planets up to three times the mass of our own world. The scientists believe that magnetic fields could emerge here, but in different ways than on Earth.

The density of some super-Earths, calculated by reference to known radius (from transit studies) and mass (from radial velocity investigations) implies that they are largely made of silicates. While we cannot know with certainty, modeling indicates that their interiors are probably much hotter than Earth’s and reach higher temperatures. The presence of large and persistent magma oceans in their mantles is likely. The question is, could such magma oceans in a state of constant churn generate the dynamo that would produce a magnetic field?

Soubiran and Militzer believe the answer is yes:

“This is a new regime for the generation of planetary magnetic fields,” said Militzer. “Our magnetic field on Earth is generated in the liquid outer iron core. On Jupiter, it arises from the convection of liquid metallic hydrogen. On Uranus and Neptune, it is assumed to be generated in the ice layers. Now we have added molten rocks to this diverse list of field-generating materials.”

Image: An artist’s concept of a super-Earth in the habitable zone of a star smaller and cooler than the sun. Such large planets could have long-lasting magma oceans that generate magnetic fields capable of protecting incipient life. The graphic was created to model Kepler-62f, one of many exoplanets discovered by NASA’s now inoperable Kepler space telescope. Credit: NASA Ames/JPL-Caltech/Tim Pyle).

On Earth, we locate the processes that generate the magnetic field in the molten-iron outer core, where in combination with the planet’s rotation, churning, electrically conducting liquid iron creates the needed dynamo. But between the core and the Earth’s crust is a mantle of silicates, the silicon-based materials that make up our planet’s interior. The researchers believe layers of material in the mantle of the early Earth may once have produced a convecting magma ocean and a magnetic field, one in place as long as enough material in the mantle remained liquid and convecting.

In the case of super-Earths, the same process could be in play, with the massive scale of these planets keeping their own mantles liquid and convecting for billions of years. The challenge is to test the theory, for working with silicates under conditions like these is, shall we say, problematic. We’re talking about internal planetary temperatures of 10,000 degrees Celsius and pressures up to millions of times greater than atmospheric pressure at the surface.

“At standard temperatures and pressures, silicates are completely insulating; the electrons are either tightly bound to the nuclei or they are in molecular bonds and not able to freely move and create macroscopic electric currents,” Soubiran said. “Even if the high internal pressure helps reduce the barriers for the electrons to move, it was not necessarily obvious silicates would be conducting in super-Earths.”

Thus the new paper, which recounts the results from the researchers’ modeling of minerals — quartz (silicon dioxide), magnesia (magnesium oxide) and a silicon-magnesium-oxide (post-perovskite) — at atomic scales, by way of making calculations of their conductivity. We learn that the silicates in question do become conductors as they make the transition to liquids at extremely high pressures and temperatures, enough so as to create a magnetic field.

Image: Layers of a possible super-Earth. The heat of formation of such a large planet could keep its magma oceans active for a billion years, generating its own magnetic field in addition to the magnetic field produced by an iron core. Credit: NASA image.

The rotation of a super-Earth is a factor here. From the paper:

Based on this analysis and with the estimated properties of the silicates mentioned above, to have a dominant dipolar component, one would need to have a rotation period of the planet shorter than 2 days. Such a short period is incompatible with a tidally locked planet in the habitable zone of any star but could be achieved for non-tidally locked planets. Magma oceans on tidally locked Super Earths are thus likely to generate multipolar and not dipolar magnetic fields.

The authors believe that a multipolar magnetic field would be that much more difficult to detect, though the paper does not go into detection methods. And things get more complicated still: While a magnetic field could emerge from a magma ocean alone, interactions difficult to predict are also possible between that ocean and a liquid iron core. What the paper gives us, however, is a calculation of high temperature and pressure conductivity of liquid silicates, showing that with enough convection, they can be conductive enough to support a magnetic field.

The paper is Soubiran & Militzer, “Electrical conductivity and magnetic dynamos in magma oceans of Super-Earths,” Nature Communications 9, Article 3883 (2018). Full text.


Beyond TESS: Looking Toward CHEOPS

As the exoplanet hunt deepens, we’re seeing how research efforts build upon each other, and how the findings of one investigation play into the planning for another. Kepler candidate planets, for example, have been confirmed using ground-based telescopes in radial velocity investigations, giving an independent check that the putative world is really there. TESS (Transiting Exoplanet Survey Satellite) will find planets that refine the target list for the James Webb Space Telescope, with extremely large telescope technology already in the wings.

What we sometimes forget is that this collaborative effort has already built up a healthy momentum. Having maxed out Kepler (and K2 was an outstanding rehabilitation of a damaged spacecraft), the operations of TESS will focus on bright, nearby stars. The momentum of TESS and its contributions to the upcoming JWST should remind us that we then have the European Space Agency’s CHEOPS (CHaracterising ExOPlanet Satellite) mission queuing up for launch.

ESA has just announced 15 October to 14 November of 2019 as the launch window for CHEOPS, whose ancient name bears parallels to NASA’s Lucy mission. Whereas the inspiration for Lucy was a four-million year old ancestor of modern humans who lived in what is now Ethiopia, CHEOPS bears the name of an ancient Egyptian monarch named Khufu — he was known to the Greeks as Cheops — who lived in the Old Kingdom period in the 26th Century BC, and who may well have commissioned the Great Pyramid of Giza.

The link to ancient humanity, whether intentional in the case of the CHEOPS acronym or not, is a fitting perspective enhancer for a mission that involves expanding our understanding of our place in the universe. The purpose of the mission is to provide precise radius information for exoplanets that have been identified by earlier missions like TESS, using a 30 cm optical telescope working in a Sun-synchronous orbit about 800 kilometers above the Earth. CHEOPS will also target exoplanets found by radial velocity methods and larger worlds found in ground-based transit work.

Image: Artist’s impression of CHEOPS. Credit: ESA – C. Carreau

Tightening up our radius estimates of known exoplanets by observing multiple transits of each planet will help to establish the density of these worlds by comparison with mass estimates provided by radial velocity studies. We wind up with constraints on their composition, with the focus on planets ranging from super-Earths to Neptune-class. Out of all this will surely come candidates for follow-up spectroscopic analysis by the instrumentation that will follow.

Having just completed its environmental test campaign at ESA’s technical centre in the Netherlands, CHEOPS is currently at Airbus Defence and Space in Spain for final testing before being declared fit for its 2019 launch. With this in mind, we also look forward to ESA’s PLAnetary Transits and Oscillations of stars (PLATO) mission, whose planned transit work will target one million stars, emphasizing rocky planets in the habitable zone. With launch scheduled for 2026, PLATO is to be followed by ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey), which will survey the chemistry of roughly 1,000 exoplanet atmospheres.

Those of us whose memories of Apollo are vivid sometimes despair when we see the slow pace of human space exploration in the decades since, but we do see momentum building in the exoplanet realm as developing technologies portend serious breakthroughs ahead. A great age of discovery is upon us even if finding approval for the needed resources isn’t always easy. How the exoplanet revolution unfolds will shape our understanding of our place in the cosmos.


CubeSats Deliver at Mars

I never saw the 2008 film WALL·E, which was all the rage not long after its release. A computer animated science fiction movie, WALL·E won a slew of awards including a Golden Globe for best animated feature, a Nebula for best script, and an Academy Award, as well as making Time‘s list of best movies of the decade. Bringing it to mind this morning, though, is the recent success of the InSight mission at landing on Mars, and the support technologies that flew with it.

Thus the image below, which in its own way is iconic. It’s from a craft nicknamed WALL·E after the star of the film, a CubeSat no larger than a briefcase that flew all the way to Mars in a seven month journey that demonstrated what miniaturized technologies can do. WALL·E is formally known as MARCO-B, the partner to MARCO-A (nicknamed EVE, another star of the film).

Both these craft proved successful at their mission, which was to offer Earthside engineers the opportunity to monitor the InSight landing in ways that hadn’t been attempted before. The CubeSats relayed information during the harried minutes of InSight’s descent and touchdown, returning data to the landing team in the time it took radio signals to travel between Earth and Mars. That was an improvement over using Mars orbiters to do the job, as these weren’t positioned to observe the entire landing sequence and get information quickly back to Earth.

“WALL-E and EVE performed just as we expected them to,” said MarCO chief engineer Andy Klesh of NASA’s Jet Propulsion Laboratory in Pasadena, California, which built the CubeSats. “They were an excellent test of how CubeSats can serve as ‘tag-alongs’ on future missions, giving engineers up-to-the-minute feedback during a landing.”

Image: MarCO-B, one of the experimental Mars Cube One (MarCO) CubeSats, took this image of Mars from about 7,600 kilometers (4,700 miles) away during its flyby of the Red Planet on Nov. 26, 2018. MarCO-B was flying by Mars with its twin, MarCO-A, to attempt to serve as communications relays for NASA’s InSight spacecraft as it landed on Mars. This image was taken at about 12:10 p.m. PST (3:10 p.m. EST) while MarCO-B was flying away from the planet after InSight landed. Credit: NASA/JPL-Caltech.

Not everything went smoothly aboard the CubeSats. While MARCO-B was able to image Mars in a sequence of shots, similar imagery from MARCO-A fell victim to camera problems. In keeping with a key advantage of CubeSats, their low cost of assembly, the imagery we do see came from consumer-grade cameras, and the cause of failure on MARCO-A isn’t yet known. But MARCO-B obviously succeeded and we may have further photos in the pipeline.

Meanwhile, an unusually young team of spacecraft designers can take pride in their accomplishment:

“MarCO is mostly made up of early-career engineers and, for many, MarCO is their first experience out of college on a NASA mission,” said Joel Krajewski of JPL, MarCO’s project manager. “We are proud of their accomplishment. It’s given them valuable experience on every facet of building, testing and operating a spacecraft in deep space.”

The point of the photo above isn’t that CubeSats are ready to replace larger, more capable spacecraft. Rather, it’s the fact that in this early assignment in a mission-supporting role, the diminutive craft have proven their worth. As trends in miniaturization continue, we can look forward to putting more functionality into smaller packages, which means increasingly sophisticated payloads with lowering costs for instrumentation and launch.

Coupling CubeSat technologies with solar sails is something we’re learning how to do, and we can now anticipate The Planetary Society’s LightSail 2. We’re shaking out new technologies all around, and as we apply the lessons learned in these missions, we can look forward to a future in which small craft operating in swarm fashion, perhaps driven by solar sails, investigate the outer planets in support of larger spacecraft. Previous paradigms of size and cost will adjust accordingly.


Modeling Climates at TRAPPIST-1

It’s a long name, but with the successful arrival of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander on Mars, we now go to work on the planet’s deep interior. With Centauri Dreams‘ deep space perspective, my thoughts quickly turn to other stellar systems. We’ve all seen how hard it is to land on Mars, and have looked up into the night sky to find the ruddy pinprick that marks its naked eye presence. Given our Solar System’s scale, the task of getting humans to Mars looms as a major challenge.

Image: Who can resist the first clear photo from a Mars mission? Not me. Credit: NASA.

But suppose we were on a planet in the TRAPPIST-1 system. Here we have roughly Earth-sized planets packed into tight proximity around the parent red dwarf. TRAPPIST-1b is at 0.011 AU, while TRAPPIST-1c is at 0.015 AU. Even the most distant from the star, TRAPPIST-1h, orbits at 0.062 AU, so that these seven worlds are all closer to the host than Mercury in our system. TRAPPIST-1b and TRAPPIST-1c are no more than 1.6 times the distance between the Earth and the Moon apart.

With significant celestial targets this numerous and this obvious in the sky, would any civilization emerging in such a system not have a greater incentive to become spacefaring at an early stage in its development? Imagine another planet, perhaps with atmosphere and ecosystem of its own, looming larger than the Moon in our skies. And others not so much farther away.

Climate Among the Seven Worlds

Space telescopes have limited resources and they’re expensive to operate. Better, then, that we figure out as much as we can about potential objects of study before we even launch such tools as the James Webb Space Telescope, now expected to be sent aloft in 2021. Thus climate models of the TRAPPIST-1 planets are becoming something of a cottage industry.

Now we have a new paper out of the University of Washington that offers rigorous physical modeling both of the radiation environment and chemistry in the system. The models create spectral signatures for each of the possible gases in TRAPPIST-1 atmospheres.

“We are modeling unfamiliar atmospheres, not just assuming that the things we see in the solar system will look the same way around another star,” said Andrew Lincowski, UW doctoral student and lead author of a paper published Nov. 1 in the Astrophysical Journal. “We conducted this research to show what these different types of atmospheres could look like.”

Image: The small, cool M dwarf star TRAPPIST-1 and its seven worlds. New research from the University of Washington speculates on possible climates of these worlds and how they may have evolved. Credit: NASA.

It’s important to bear in mind that we can’t make G-class star assumptions about the planets orbiting this M-dwarf, an ultra-cool object not much larger than Jupiter in size and less than a tenth of the mass of the Sun. So when we talk about three of the TRAPPIST-1 planets being near or in the habitable zone where liquid water could exist at the surface, the statement acknowledges how little we know of conditions on any of these worlds.

We have to include the high degree of stellar activity we find on M-dwarf stars, which could disrupt the early atmosphere as well as destroying ozone that could protect life from UV radiation. In its early stages of development, such a star could put a rocky world with an ocean into a runaway greenhouse condition that might persist for hundreds of millions of years before, as the star gradually dims and enters the main sequence, the planet emerges into the habitable zone.

So we can’t expect terrestrial-class planets around these stars to go through the same development as planets around our G-class Sun. When next-generation searches of small rocky worlds finally occur, they will be our first spectrographic analyses of distant atmospheres looking not only for water vapor but a wide range of biosignature gases. Planetary evolution at M-dwarfs clearly needs to be understood to ensure accuracy.

What happens to planetary atmospheres around dim red stars like this one? Using the Hyak supercomputer system at the University of Washington, Lincowski and team modeled the TRAPPIST-1 planets drawing on the methods of terrestrial climate modeling and infusing into them photochemistry models that they believe provide as good a simulation of planetary conditions here as we have yet seen. The number of possibly ‘habitable’ worlds decreases to one. For it turns out that any or all of the TRAPPIST-1 planets could have stronger resemblance to Venus than to Earth, with whatever water once existed at the surface long departed.

“This may be possible if these planets had more water initially than Earth, Venus or Mars,” Lincowski adds. “If planet TRAPPIST-1 e did not lose all of its water during this phase, today it could be a water world, completely covered by a global ocean. In this case, it could have a climate similar to Earth.”

The team’s work models atmospheric conditions after extreme loss of volatiles early in planetary evolution, discriminating between oxygen- and carbon dioxide-dominated atmospheres and including interior outgassing as a contributor to the final composition. For depending on early conditions, water can be broken by ultraviolet light into its constituents. Hydrogen is thus released, which is light enough to escape the planet’s gravity. Oxygen dominates the thick atmosphere left behind, a remnant that has little to do with life. We have no analog to this kind of atmosphere in our own Solar System.

Differentiating among these worlds, the researchers’ modeling offers the insight that if one of these planets is likely to host life, it is TRAPPIST-1e, the world we’ll want to focus on in future astrobiological studies. TRAPPIST-1b appears hotter than Venus. Planets c and d, further out, still receive enough energy from the host to be Venus-like, with any atmosphere likely dense over an uninhabitable surface. As for planets f, g and h, the spread is wide. They could be frozen worlds or, depending on the amount of water at formation, Venus-like themselves.

The amount of water available to these worlds early in their formation is key here, and plays against the team’s calculations of ocean loss and oxygen accumulation for all seven of the TRAPPIST-1 planets. From the paper:

Our evolutionary modeling suggests that the current environmental states can include the hypothesized desiccated, post-ocean-runaway O2-dominated planets, with at least partial ocean loss persisting out to TRAPPIST-1 h. These O2-dominated atmospheres have unusual temperature structures, with low-altitude stratospheres and no tropospheres, which result in distinctive features in both transmission and emission, including strong collision-induced absorption from O2.

Thus we have a possible signature to look for in future observations. Or we could get atmospheres much more similar to Venus:

Alternatively, if early volatile outgassing (e.g. H2O, SO2, CO2) occurred, as was the case for Earth and Venus, Venus-like atmospheres are possible, and likely stable, throughout and beyond the habitable zone, so the maximum greenhouse limit may not apply for evolved M dwarf planets. If Venus-like, these planets could form sulfuric acid hazes, though we find that TRAPPIST-1 b would be too hot to condense H2SO4 aerosols.

What a prize TRAPPIST-1 has turned out to be. The authors call these worlds “…a natural laboratory to study planetary atmospheric evolution and the associated impact on habitability.”

Consider: We have seven transiting planets that range from well inside the putative habitable zone to well past its outer boundaries. The paper points out how useful this is in examining planet evolution as a function of distance from a star. Moreover, because of their orbital configuration, these planets make frequent transits and offer small star-to-planet ratios, which provide optimum values of signal to noise in the transit signature. We’ll see continued modeling of possible outcomes here as we gear up for next generation observations.

Co-author Victoria Meadows, principal investigator for the NASA Astrobiology Institute’s Virtual Planetary Laboratory at the University of Washington, adds:

“The processes that shape the evolution of a terrestrial planet are critical to whether or not it can be habitable, as well as our ability to interpret possible signs of life, This paper suggests that we may soon be able to search for potentially detectable signs of these processes on alien worlds.”

The paper is Lincowski et al., “Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System,” Astrophysical Journal Vol. 867, No. 1 (1 November 2018). Abstract / Preprint.


HR 8799c: Water Detection Moves Spectroscopy Forward

Some relatives of a friend recently made me realize how routine exoplanet discoveries have become to the public. These are anything but astronomy buffs, but they know that planets can be found without ever being seen. My acquaintances may not understand radial velocity or transits to any high degree, but they accept that the methods are there and have proven reliable. “Someday,” said one, “I guess we’ll actually see one of these planets.”

The image below came as a surprise when I showed it to them. Here we do see a planetary system, four actual planets around the star HR 8799 and not just jiggles in Doppler signals or dips in a lightcurve. For me, what’s astonishing here is not only that we can see planets despite their proximity to the host, but that we’ve accomplished this with telescopes on the ground. Adaptive optics — correcting for turbulence in the atmosphere that would distort an astronomical image, using a guide star as a reference — is the tool that is opening a new era in astronomy from the surface of the Earth. Couple it with spectroscopy and a world of possibilities emerges.

Image: The HR 8799 planetary system is the first stellar system beyond our own that astronomers directly imaged. Captured in 2008 using Keck Observatory’s near-infrared adaptive optics, the picture revealed three planets (labeled ‘b’, ‘c’, and ‘d’) orbiting a dusty young star named HR 8799 (center). In 2010, the team announced they detected a fourth planet in the system (labeled ‘e’). The HR 8799 system is located 129 light-years away from Earth. Credit: NRC-HIA/C. Marois/W. M. Keck Observatory.

We have over a dozen directly imaged exoplanets at this point, but HR 8799 gives us the first multiple planet system to be so viewed. A newly published study reports on the use of a high-resolution spectrometer called NIRSPEC (near-infrared cryogenic echelle spectrograph) that works at infrared wavelengths. Built at the UCLA Infrared Laboratory, it has detected water in the atmosphere of HR 8799c, a gas giant of 7 Jupiter masses in a 200 year orbit. The work also demonstrates a lack of methane in data from an instrument sensitive enough to find it.

Image: Artist’s impression based on published scientific data on the HR 8799 solar system. The magenta, HR 8799c planet is in the foreground. Compared to Jupiter, this gas giant is about seven times more massive and has a radius that is 20 percent larger. HR 8799c’s planetary companions, d and b are in the background, orbiting their host star. Credit: W. M. Keck Observatory/Adam Makarenko/C. Alvarez.

The Keck II study combines the high spatial resolution of adaptive optics with the high spectral resolution of NIRSPEC. Lead author Ji Wang (Ohio State University) and team explain that this is the first time a directly imaged planet has been investigated with spectrometer and adaptive optics using the L-band, a wavelength around 3.5 micrometers. Although challenging for astronomers, this wavelength is rich in markers for chemicals in the target atmosphere.

The lack of methane in HR 8799c’s atmosphere does not come as a surprise, as it confirms earlier analyses:

“We are now more certain about the lack of methane in this planet,” says Wang. “This may be due to mixing in the planet’s atmosphere. The methane, which we would expect to be there on the surface, could be diluted if the process of convection is bringing up deeper layers of the planet that don’t have methane.”

Future work in the L-band, which can make measurements of a planet’s carbon-to-oxygen ratio, will be useful in determining the formation history of directly imaged planets. Protoplanetary constituents including hydrogen, oxygen, water, carbon monoxide and methane each have their own ‘snowline’ where they freeze out from the early planet-forming disk. A planet’s carbon-to-oxygen ratio, then, can be a window into where it formed and its later dynamics.

As we move adaptive optics and high-resolution spectroscopy forward, a new instrument called the Keck Planet Imager and Characterizer will soon see planets that are fainter than the HR 8799 worlds and orbit closer to their stars. Keck astronomers consider it a bridge to the Thirty Meter Telescope planned for the late 2020s. These technologies should be able to analyze the chemical makeup of Earth-like planets in their stars’ habitable zones, looking for potential biosignatures like water, oxygen, and methane. The astrobiology investigation intensifies.

The paper is Wang et al., “Detecting Water in the Atmosphere of HR 8799 c with L-band High-dispersion Spectroscopy Aided by Adaptive Optics,” Astronomical Journal Vol. 156, No. 6 (20 November 2018). Abstract / Preprint.