Two planets of the TRAPPIST-1 system could be habitable according to a study of their tidal warming

The possible look of TRAPPIST-1's planets (Image NASA/R. Hurt/T. Pyle)
The possible look of TRAPPIST-1’s planets (Image NASA/R. Hurt/T. Pyle)

An article published in the journal “Astronomy & Astrophysics” describes a study of the internal structure and tidal warming of the 7 planets of the ultra-cool dwarf star TRAPPIST-1 system. Amy C. Barr, Vera Dobos, and László L. Kiss created models of those 7 planets, concluding that two of them are most likely to be habitable because the temperatures on their surface could allow the presence of liquid water.

Almost a year passed since the announcement of the confirmation that the TRAPPIST-1 system hosts 7 rocky planets and subsequently the number of studies about it increased considerably. A lot of data were collected using various telescopes but for now it’s impossible to understand only through them if among those 7 planets there are some habitable ones. For this reason, many researchers have already created various models to try to calculate the probabilities that they’re habitable.

In this study, the researchers focused on the characteristics of the star TRAPPIST-1, close to the minimum size and mass limits for a star, and the known ones of its 7 planets. However, they needed to include some hypotheses where the collected data were not enough. For example, in the creation of the models used, the researchers hypothesized that the planets are composed of iron, rock and water.

The planets’s mass and size have been measured in a still approximate way so the researchers presented several possible internal structures and compositions within the margins of error of the measurements. Their artistic representation in the image has also been drawn trying to imagine what they can look like with the characteristics known and even more the estimated ones.

With those starting parameters, the researchers calculated the effects of tidal warming generated by the star TRAPPIST-1 on its planets. Even the outermost planets are very close to their star so, despite its low mass, its gravity is sufficient to generate friction and therefore heat inside them. The calculations concerned the balance between tidal warming and convection heat transfer in the planets’ mantles.

According to the calculations, the planets TRAPPIST-1b and TRAPPIST-1c, the innermost ones, probably have at least partly melted rock mantles. The surface of TRAPPIST-1c may be of solid rock but there may be outbreaks of silicate magmas due to tidal warming. It’s a phenomenon similar to what happens on Io, one of the great moons of Jupiter.

Heat flows are lower in the other planets and among them TRAPPIST-1d and TRAPPIST-1e are those that are most likely to be habitable. An important factor is that heat fluxes must be reduced enough to prevent a runaway greenhouse effect. On TRAPPIST-1d there could be a large water ocean covering its surface.

The presence of liquid water on a planet also depends on the presence of an atmosphere. An article published in the journal “Proceedings of the National Academy of Sciences” in December 2017 described a research that tried to provide an estimate of the possibilities for the 7 planets of the TRAPPIST-1 system to maintain a stable atmosphere. According to the results, the outermost planets are those that are more likely to have it with less optimism regarding TRAPPIST-1d and TRAPPIST-1e.

The different and sometimes at least apparently contradictory results of the various researches on the TRAPPIST-1 system show how many unknowns there may be in the study of exoplanets that influence models that are evolving all the time. Together with the ongoing collection of data on a very interesting system, this will lead to increasingly precise results.

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