Trappist-1c: a bare rock?
Observations of Trappist-1c using the James Webb Space Telescope point to the planet having no significant atmosphere
If you can only name one exoplanet, chances are the word Trappist comes to your mind. Technically that gives you seven planets: Trappist-1b, c, d, e, f, g, and h are seven exoplanets that all orbit the star Trappist-1. A recent study used data from the James Webb Space Telescope to investigate the atmosphere of one of these, Trappist-1c. Their observations suggest that the planet may be a bare rock.
Trappist-1 has been known to host planets since 2016 when the discovery of the first three planets orbiting the star was announced. A year later, it became apparent the star hosts at least seven small planets. Both discovery papers were led by Professor Michaël Gillon, a Belgian astronomer who used the TRAnsiting Planets and PlanetesImals Small Telescope or TRAPPIST, together with other telescopes, to detect the planets.
The discovery rocked exoplanet astronomers, not only because of the large number of planets orbiting a single star, but because of the properties of the star itself. Compared to our own Sun, Trappist-1 is nearly ten times smaller — roughly the size of Jupiter — and much cooler and fainter. The planets orbit very close to the star and take between 1.5 days and 19 days to complete an orbit, and the planetary system can be thought of as a miniature solar system — or similar to Jupiter and its moons.
Since the discovery of the seven planets, they have become some of the most studied exoplanets — the discovery paper has now been cited over 1,000 times by other studies. The planets are all small, with sizes similar to that of Earth. Previous estimates of their mass and radius suggest they are likely rocky, similar to our own Earth.
More recently, astronomers have attempted to study the atmospheres of these planets. Earlier this year, a study led by Dr. Thomas Greene, Astrophysicist in the Space Science and Astrobiology Division at NASA’s Ames Research Center, suggested Trappist-1b likely contains no or no significant atmosphere. In June a study led by Sebastian Zieba, a PhD student at the Max-Planck-Institut für Astronomie in Heidelberg, reaches a largely similar conclusion for planet c. Both of these studies make use of the James Webb Space Telescope (JWST), which was launched on Christmas day of 2021.
Professor Laura Kreidberg, director of the Atmospheric Physics of Exoplanets (APEx) department at the Max Planck Institute for Astronomy and coauthor of the study, told the Extrasolar Times:
“Astronomers have been speculating about whether the TRAPPIST-1 planets have atmospheres since the moment they were discovered. It’s incredible that with the power of JWST we’re finally able to search for realistic atmospheres! Our observations showed that even though TRAPPIST-1c has a similar size and irradiation as Venus, it does not have a Venus-like atmosphere. The planet could still have a thin atmosphere, with very little carbon dioxide. With future JWST observations at additional wavelengths, we can test whether one of these more tenuous atmospheres is present, or if instead the planet is a bare rock.”
While the lack of atmospheres for TRAPPIST-1b and c may come as disappointing news to some, these new studies show the impressive capabilities in constraining the properties of even the smallest known exoplanets. The other planets in the system — which have longer orbits and therefore aren’t blasted by quite as much radiation from the star — may yet have significant atmospheres.