New information on the atmospheres and water of the TRAPPIST-1 system’s planets

Artist's concept of TRAPPIST-1's planets with the Earth (Image ESO/M. Kornmesser)
Artist’s concept of TRAPPIST-1’s planets with the Earth (Image ESO/M. Kornmesser)

Two articles, one published in the journal “Astronomy & Astrophysics” and one in the journal “Nature Astronomy”, describe two researches on the TRAPPIST-1 system’s rocky planets. Various telescopes, both space and ground-based, allowed to gather more information on those 7 exoplanets and to improve the measurements of their characteristics. An important result is the estimate of their density, which suggests that some of them are composed up to 5% of water. Spectroscopic detections suggest that at least 3 of them have a compact atmosphere that could allow liquid water to exist on their surface.

The confirmation that 7 rocky planets orbit around the ultra-cool dwarf star TRAPPIST-1, announced in February 2017, stimulated a number of new researches. This system is in the neighborhood from an astronomical point of view, less than 40 light years from Earth, but the star is really tiny having a size similar to the planet Jupiter’s, and the rocky planets have sizes similar to those of the Earth, making precise measurements difficult.

Despite the problems, many astronomers started studying the TRAPPIST-1 system and those who were already studying it conducted follow-up researches with the consequence that recently the number of articles published on it has increased. There are still unknowns but slowly the accumulated data are providing increasingly precise indications and increasingly accurate models that one day could tell us with reasonable certainty if at least some of the planets of that system are potentially habitable by life forms similar to the Earth’s ones.

A team led by Simon Grimm of the Swiss University of Bern conducted the research published in “Astronomy & Astrophysics”. By pooling data collected with the SPECULOOS project at ESO’s Paranal Observatory and with NASA’s Spitzer and Kepler space telescopes, complex modeling methods have been applied to calculate the density of the TRAPPIST-1 planets with much greater accuracy than to previous ones.

According to the results, the planets TRAPPIST-1b and TRAPPIST-1c, the innermost ones, probably have a rocky core and could have a much denser atmosphere than the Earth’s. The planet TRAPPIST-1d has a mass that is only 30% of that Earth and could have a vast atmosphere but also an ocean or an ice crust. TRAPPIST-1e is a little denser than the Earth so it could have an iron core but there are only hypotheses about the possible atmosphere and water on the surface. The other planets are further from their star and even if they had an atmosphere it’s difficult for water to be liquid on their surface.

The atmospheres of TRAPPIST-1’s planets are the subject of research published in “Nature Astronomy”. In this case, the astronomers used the Hubble Space Telescope’s Wide Field Camera 3 instrument in a study that follows the previous one conducted in May 2016 of this system’s innermost planets.

These planets are very close to their star with the consequence that their year lasts only a few Earth’s days. For astronomers this means that they pass between their star and the Earth very often and they can also examine the star’s light emissions filtered by each planet’s possible atmosphere. The results are limited but still provide some information.

At least the planets TRAPPIST-1d, TRAPPIST-1e and TRAPPIST-1f have compact atmospheres, not like those of Neptune-like gas planets. The problem is that these could be atmospheres like the very dense one of Venus, like the Earth’s or like the very thin one of Mars. It’s possible that originally those planets’ atmospheres contained a lot of hydrogen that was then dispersed in space under the action of the star’s radiation.

Michaël Gillon, one of the authors of both articles and already the protagonist of various studies on TRAPPIST-1’s system, pointed out that these results highlight the interest in exploring ultra-cool dwarfs in the vicinity. It’s the ESO’s SPECULOOS project, the successor of the TRAPPIST project that discovered the extraordinary system at the center of these new researches.

Jupiter's system compared to the TRAPPIST-1's and the inner solar system (Image NASA/JPL-Caltech)
Jupiter’s system compared to the TRAPPIST-1’s and the inner solar system (Image NASA/JPL-Caltech)

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