Some Of The TRAPPIST-1 Planets Could Hold On To An Atmosphere For Billions Of Years

Artist's impression of the TRAPPIST-1 system from the view of the outermost planet. ESO/N. Bartmann/spaceengine.org

There is a lot we don’t know about how life started on Earth or how it may have started elsewhere in the Universe, but there are certain things we think are crucial. A planet with an atmosphere is one of these requirements, so researchers have worked out the chance of this happening in the famous seven-planet system TRAPPIST-1.

In a new paper, published in Proceedings of the National Academy of Sciences, researchers have estimated that the outermost planets (TRAPPIST-1g and TRAPPIST-1h) would be able to hold onto their respective atmosphere for billions of years. Of the two, TRAPPIST-1g is also inside the habitable zone, which makes this planet the most interesting candidate to date.

The planets orbit a red dwarf that is more active than our Sun. They are also closer to the red dwarf than our Earth is to its star, with TRAPPIST-1g orbiting it in just 12 days. 

The team began their analysis with three assumptions: all planets have the same atmospheric composition (similar to Venus and Mars), the planets have no magnetic field, and they start with enough gas to have one (Earth) atmosphere of pressure on the surface. As times goes by, the stellar wind takes gas away, but the question is how quickly. For the innermost planet, TRAPPIST-1b, the atmosphere disappeared within 100 million years. 

Billions of years for TRAPPIST-1g and TRAPPIST-1h is clearly good news to give life a chance to form on these planets. However, there are limitations to the model. If the planets had magnetic fields, they would be more resilient to stellar wind erosion. On the other hand, if the red dwarf is more active than we think, this could mean more atmospheric loss.

Several important planetary and stellar parameters from the system are still uncertain, so while the results appear to be robust, it is important to understand them in the context of their limitations. This won’t be the last word on the atmospheres of TRAPPIST-1 planets, but it does provide some new tidbits to what the system might truly look like.

TRAPPIST-1 is located 39.6 light-years from us in the constellation Aquarius. The first three exoplanets around this star were discovered in May 2016, with the discovery of the following four announced in February 2017.

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