The Hubble Space Telescope has spotted possible signs of water on the outer planets of TRAPPIST-1, the system with the most exoplanets in a star’s habitable zone.
The TRAPPIST-1 system – named after the TRAnsiting Planets and PlanetesImals Small Telescope in Chile – was discovered last year. It has been described as a mini solar-system, complete with seven Earth-sized planets orbiting a dwarf star over 40 light years away.
Three out of the seven planets are in the habitable zone, the magical region where liquid water can theoretically exist. Now, an international team of scientists believes those planets may harbor a considerable amount of water.
A paper published Thursday in The Astrophysical Journal describes the results gathered from the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The researchers probed the ultraviolet radiation received by each planet.
Vincent Bourrier, lead author of the paper and a researcher from the Observatoire de l’Université de Genève, Switzerland, said: “Ultraviolet radiation is an important factor in the atmospheric evolution of planets.”
“Ultraviolet sunlight breaks molecules apart, ultraviolet starlight can break water vapor in the atmospheres of exoplanets into hydrogen and oxygen.”
The level of ultraviolet energy projected onto each planet is important. At low energies, UV light destroys the bonds in water molecules – a process known as photodissociation. But at higher doses, extreme UV and X-rays heating the upper atmosphere of a planet pass on enough energy for the hydrogen and oxygen, which have been broken up by photodissociation, to escape.
The hydrogen and oxygen gas molecules leaving the planet’s atmosphere can be detected as a water vapor. The results show that the planets could have lost a considerable amount of water over time.
The two planets – TRAPPIST-1b and TRAPPIST-1c – closest to their parent star receive the most UV radiation and have probably lost the most water – a volume of up to 20 Earth-oceans worth of water in the last eight billion years.
The rate of water loss for the three planets in the habitable zone – e, f and g – is much lower, which may mean there could still be some liquid left on their surfaces.
It shows that “atmospheric escape may play an important role in the evolution of these planets,” said Julien de Wit, co-author of the study and a researcher from the Massachusetts Institute of Technology.
No conclusive results can be drawn from the Hubble Space Telescope, and more observations need to be made with the James Webb Space Telescope, which is expected to be launched next year.
“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,” said Bourrier. ®