TRAPPIST-1 exoplanets may host water

Planets orbiting a nearby star are showing signs that they may host water on their surfaces, according to a study using the Hubble Space Telescope.

An artist’s impression imagining the view from the surface of one of the planets in the TRAPPIST-1 system. Might the outer planets host water and therefore be able to support life?
Credit: ESO/N. Bartmann/


Some of the exoplanets surrounding a nearby dwarf star may contain liquid water on their surface, according to observations made with the Hubble Space Telescope.

If true, this would be a major breakthrough for investigations into whether or not the exoplanets could host life as we know it.

Seven Earth-sized exoplanets were discovered around dwarf star TRAPPIST-1, located just 40 lightyears away, and announced in February 2017. It is the planetary system with the largest number of Earth-sized planets discovered so far.


Read more about the TRAPPIST-1 system from BBC Sky at Night Magazine:


Following this revelation, a team of astronomers began using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope to study the amount of ultraviolet radiation from the host star that hits each of the planets.

“Ultraviolet radiation is an important factor in the atmospheric evolution of planets,” says Vincent Bourrier from the Observatoire de l’Université de Genève, who led the study.

“As in our own atmosphere, where ultraviolet sunlight breaks molecules apart, ultraviolet starlight can break water vapour in the atmospheres of exoplanets into hydrogen and oxygen.”

Hydrogen gas created on the exoplanets through this process is very light and can escape the atmosphere and be detected by the Hubble Space Telescope.

This means that the detection of hydrogen could be a sign of water on the surface of the planets.

An animation depicting the seven exoplanets orbiting TRAPPIST-1.
Credit: ESO/L. Calçada/


Observations of the amount of ultraviolet radiation emitted by the TRAPPIST-1 star suggests the planets could have lost a lot of water over the course of their history.

The innermost two planets of the system, TRAPPIST-1b and TRAPPIST-1c, receive the largest amount of ultraviolet energy. They could have lost an amount of water equivalent to 20 times that of Earth’s oceans over the last billion years, according to the study.

The outer planets of the system, however, are already known to orbit the star in the 'habitable zone'. These exoplanets, named e, f and g, probably have lost much less water than the inner planets, and could potentially have retained some on their surface. In theory, this could make them potentially habitable.


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