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takyon writes:

Hubble Space Telescope observations suggest that the exoplanets orbiting TRAPPIST-1 in the habitable zone could have water on their surfaces, while the planets closer to the star have likely lost any surface water they may have had:

An international team of scientists, led by Swiss astronomer Vincent Bourrier of the Observatoire de l'Université de Genève, used the [Hubble] space telescope to study the amount of ultraviolet light hitting the planets and measure the amount of hydrogen these worlds are venting into space. The results suggest the innermost planets, TRAPPIST-1b and TRAPPIST-1c, could have lost as much as 20 Earth-oceans-worth of water in the last eight billion years. The outer planets, however, including e, f, and g, which orbit in the habitable zone, would have lost less water, and could still retain vast stores of liquid water on the surface.

[...] The researchers used Hubble to measure the amount of ambient hydrogen floating around the TRAPPIST-1 planets as well as the intensity of ultraviolet light coming from the host star, an ultracool dwarf star. The amount of ultraviolet radiation coming from TRAPPIST-1 suggests the inner planets could have lost an enormous amount of water over the eons, something that is supported by the abundant hydrogen surrounding the planets—a possible indicator of water vapor. Most importantly, the radiation hitting the outer planets and the amount of hydrogen surrounding them suggests these worlds, similar to Earth in many ways, might still retain atmospheric water vapor and even liquid water on the surface.

[...] Whether or not these planets could actually support life is still an open question. First of all, the observations from Hubble are not conclusive, and further observations from other observatories as well as computer simulations are required to support or dispute the possibility of water on the TRAPPIST-1 planets.

"While our results suggest that the outer planets are the best candidates to search for water with the upcoming James Webb Space Telescope, they also highlight the need for theoretical studies and complementary observations at all wavelengths to determine the nature of the TRAPPIST-1 planets and their potential habitability," says Bourrier.

Also at Hubble News and Space.com.

Temporal Evolution of the High-Energy Irradiation and Water Content of TRAPPIST-1 Exoplanets

Original Submission