Centauri Dreams

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.

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.

Even today, I can well understand the reaction that Dennis Conti had when confronted with the prospect of finding a planet around another star with nothing more than an amateur instrument. Conti, who founded and now chairs the Exoplanet Section of the American Association of Variable Star Observers, was a newcomer to the transit method just a few years ago. “I thought, there’s no way for someone with a backyard telescope to detect a planet going around a distant star,” he says, looking back from the vantage of one now immersed in such observations.

My boyhood 3-inch reflector was not a backyard instrument — too many trees back there. So it became a front-yard telescope. Absent the technological innovations of the past five decades, I could only imagine vast instruments for studying objects around other stars. The transit method in exoplanet detection was a long way off, but the idea of seeing not a planet itself but a change in starlight as the planet crossed the face of its host now seems intuitively obvious. It takes a good deal more than a 3-inch reflector to get into the game, but today’s more sophisticated amateur telescopes can definitely make a contribution, as Conti has demonstrated.

Consider: In 2016, Conti coordinated more than 40 amateur astronomers who would assist a professional scientist in characterizing the atmospheres of 15 different exoplanets. A year later, he would begin teaching a course in observing exoplanets through the AAVSO, one that has taught more than 120 students ranging from professional scientists to high-school teachers. His Practical Guide to Exoplanet Observing, now in its 4th revision, is in use in many countries.

The AAVSO is now building its own Exoplanet Database, a useful entry into the field because the increasing amount of data gathered by amateurs should have a unified home. Many existing databases are built around specific space missions or ground-based surveys. The AAVSO’s entry will be a place where amateur astronomers can archive their exoplanet transit observations to make them available to the broader community. Long-term archiving of observations may turn up interesting features in lightcurves like transit timing variations, that could potentially identify the presence of a second planet in the same observed system.

Image: This artist’s concept shows the Kepler-444 planetary system, in which five small planets orbit a distant star. These five planets were detected by the transit method, which involves recording the periodic dimming of a star as a planet transits across its face. Amateur astronomers have been using the same technique to successfully and accurately detect exoplanets for more than a decade, and their observations can now be recorded in the AAVSO’s Exoplanet Database. There, they can be archived long-term and used by professionals and other amateurs to build scientific knowledge of interesting planets. Credit: NASA/JPL-Caltech/AMES/Univ. of Birmingham.

I never followed up with larger home telescopes of my own, but I admire the dedicated amateurs who have plunged into this work. The synergy between amateur and professional can be productive. With TESS (Transiting Exoplanet Survey Satellite) now in operation, we’re reminded that thousands of exoplanet candidates are going to turn up in the next two years. Follow-up observations on the TESS candidates are to be submitted to NASA. The TESS Follow-Up Observing Program is coordinating its efforts with the AAVSO and has adopted its guidelines for best practices, as noted in this AAVSO news release.

“This emphasizes the value that nonprofessionals bring to the field of science,” says Stella Kafka, Ph.D., AAVSO Executive Officer. “People with moderate means can contribute from the ground to the knowledge base of the community. In principle, one can see the AAVSO as an international collaboration between professional and non-professional astronomers, working together to understand some of the most exciting phenomena in the universe.”

Image: The light curve shown here records the dimming of the exoplanet WASP-12b, taken Jan. 5, 2016, by Dennis Conti, Ph.D., founder and chair of the AAVSO’s Exoplanet Section. Conti used equipment available to amateur astronomers and compared his results to published data to show that he was able to successfully and accurately detect the exoplanet. The AAVSO’s Exoplanet Database will provide a place for amateur astronomers following established procedures to make their exoplanet transit observations available to the broader community of researchers, and to have their data archived long-term. Credit: Dennis Conti.

Many eyes on target with a wide range of instruments are better than a few, and bear in mind that observing an exoplanet transit from different locations and times can help astronomers assemble data on a complete transit that might otherwise be lacking. I’ll also remind non-astronomers with a passion to contribute of the Planet Hunters site, where participants can look at exoplanet data and sort through lightcurves from the Kepler mission. The ways for amateurs to make a contribution to exoplanet science are multiplying.