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Trappist Planets Wet?

An international team of astronomers used the NASA/ESA Hubble Space Telescope (HST) to estimate whether there might be water on the seven earth-sized planets orbiting the nearby dwarf star TRAPPIST-1. The team found that the outer planets of the system might still harbor substantial amounts of water. This includes the three planets within the habitable zone of the star, suggesting that they may be congenial for some form of life.

The team led by the Swiss astronomer Vincent Bourrier, from the Observatoire de l’Université de Genève, used the HST Imaging Spectrograph (STIS)  to study the ultraviolet radiation received by individual planetsin the system. “Ultraviolet radiation is an important factor in the atmospheric evolution of planets,” explained Bourrier. As in our own atmosphere, it can break water molecules apart in the atmosphere of exoplanets into hydrogen and oxygen.

While lower-energy ultraviolet radiation breaks up water molecules — a process called photodissociation — ultraviolet rays with more energy (XUV radiation) and X-rays heat the upper atmosphere of a planet, which allows the products of photodissociation, hydrogen and oxygen, to escape.

As it is very light, hydrogen gas can escape the exoplanets’ atmospheres and be detected around the exoplanets with Hubble, acting as a possible indicator of atmospheric water vapor. The observed amount of ultraviolet radiation emitted by TRAPPIST-1  suggests that the planets have lost gigantic amounts of water.

This is especially true for the innermost two planets of the system, TRAPPIST-1b and TRAPPIST-1c, which receive the largest amount of ultraviolet energy. “Our results indicate that atmospheric escape may play an important role in the evolution of these planets,” said Julien de Wit, from MIT, USA, co-author of the study.

The inner planets could have lost more than 20 Earth-oceans-worth of water during the last eight billion years. However, the outer planets of the system — including the planets e, f and g which are in the habitable zone — should have lost much less water, suggesting some remains on their surfaces. The  water loss rates calculated and estimated geophysical water release rates support the idea that the outermost, more massive planets retain their water. However, with the currently available data and telescopes no final conclusion can be drawn on the water content of the planets orbiting TRAPPIST-1.

“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,” concluded Bourrier. [Source: NASA/JPL-Caltech]