UC San Diego Physicist Contributes to Insight on TRAPPIST-1 Planets’ Water Content

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An international team of astronomers, including University of California San Diego physicist Adam Burgasser, have used the NASA/ESA Hubble Space Telescope to estimate whether there might be water on the seven Earth-sized planets orbiting the nearby dwarf star TRAPPIST-1. The results suggest that the outer planets of the system might still harbor substantial amounts of water—enough to increase their habitability potential.

Earlier this year astronomers announced the discovery of seven Earth-sized planets orbiting the ultracool dwarf star TRAPPIST-1, 40 light-years away. This makes TRAPPIST-1 the planetary system with the largest number of Earth-sized planets discovered to date.  

Following up on that earlier discovery, the team, under the leadership of Swiss astronomer Vincent Bourrier from the Observatoire de l’Université de Genève, used the Hubble’s Space Telescope Imaging Spectrograph (STIS) to examine the amount of ultraviolet radiation received by the individual planets of the system. 

“Ultraviolet radiation is an important factor for the atmospheric evolution of planets,” explains Bourrier. “As in our own atmosphere, where ultraviolet sunlight breaks molecules apart, ultraviolet starlight can break water vapor in the atmospheres of exoplanets into hydrogen and oxygen.”

While lower-energy ultraviolet radiation breaks up water molecules — a process called photodissociation — higher-energy ultraviolet rays can also heat the upper atmosphere of a planet, causing the hydrogen and oxygen to escape into space.

Hydrogen gas escaping from an exoplanet can be detected with Hubble, and this detection acts as a “smoking gun” for the presence of atmospheric water vapor. The observed amount of ultraviolet radiation emitted by TRAPPIST-1 in the study suggests that the planets could have lost gigantic amounts of water over the course of their history.

This is especially true for the two innermost planets of the system, TRAPPIST-1b and TRAPPIST-1c, which receive the highest amount of ultraviolet energy.

“Our own atmosphere loses some hydrogen and helium to high energy radiation from the Sun and cosmic rays from space,” said Burgasser, a co-author of the study, “but the UV radiation from TRAPPIST-1 is so intense that it has likely boiled oceans off of the planets.”

According to the team’s determinations, the inner planets could have lost more than 20 Earth oceans’ worth of water during the roughly eight-billion-year-old lifetime of the system, making them most likely bone-dry. The outer planets of the system, including the planets e, f and g which are in TRAPPIST-1’s habitable zone, should have lost much less water—less than three Earth oceans’ worth. This suggests that the outer planets could indeed be habitable. With the data currently available, however, no firm conclusion can be drawn on the current 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,” concludes Bourrier.

The Hubble Space Telescope is a project of international cooperation between the European Space Agency and NASA. The study team includes:

• Vincent Bourrier (Observatoire de l’Université de Genève, Switzerland)
• J. de Witt (Massachusetts Institute of Technology, USA)
• E. Bolmont (Laboratoire AIM Paris-Saclay)
• V. Stamenkovic (Jet Propulsion Laboratory, USA; California Institute of Technology, USA)
• P. J. Wheatley (University of Warwick, UK)
• Adam J. Burgasser (UC San Diego, USA)
• L. Dekerez (Cavendish Laboratory, UK)
• B.-O. Demory (University of Bern, Switzerland)
• D. Ehrenreich (Observatoire de l’Université de Genève, Switzerland)
• M. Gillon (Université de Liège, Belgium)
• E. Jehin (Université de Liège, Belgium)
• J. Leconte (Université Bordeaux, France)
• S. M. Lederer (NASA Johnson Space Center, USA)
• N. Lewis (Space Telescope Science Institute, USA)
• A. H. M. J. Triaud (Institute of Astronomy, Cambridge, UK)