One of the most exciting space stories so far this year came in February with the announcement that astronomers had discovered seven Earth-sized planets orbiting a relatively nearby dwarf star. The system, called TRAPPIST-1, is especially appealing because it has three planets in the habitable zone, meaning these worlds could potentially support liquid water and perhaps even life.
Because of this intriguing possibility, TRAPPIST-1 has become one of the most studied exo-systems in the relatively short history of exoplanets. But each subsequent study seems to alternately raise or dash our hopes about the potential habitability of these worlds.
One study said solar flares from the host star would cause massive geomagnetic storms on the planets and life as we know it could not survive, while another hinted at possible vegetation on at least one world. Another said the non-detection of large moons around the planets raised questions about habitability.
But now the Hubble Space Telescope has offered a new glimmer of hope about the possible habitability of the TRAPPIST-1 planets.
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Astronomers used Hubble to try and estimate whether there might be water on any of the seven TRAPPIST-1 planets. Their results suggest that the outer planets of the system — including the three worlds in the habitable zone — might be capable of harboring substantial amounts of water.
An international team of astronomers, led by Vincent Bourrier from the Observatoire de l’Université de Genève, used Hubble’s Space Telescope Imaging Spectrograph (STIS) to look at the amount of ultraviolet radiation the planets might receive from their star, and how that might affect the planets’ atmospheres. They also looked for any hydrogen “escaping” from the atmospheres, since that detection would be an indication of water vapor.
“Ultraviolet radiation is an important factor in the atmospheric evolution of planets,” said Bourrier in a statement from NASA and ESA. “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.”
From the STIS date, researchers were able to measure changes in the strength of the star’s ultraviolet radiation. They found wide variations in the amount of UV radiation emitted by TRAPPIST-1. By inputting the STIS data into computer models, the team determined a large amount of water vapor should have been broken down over the past eight billion years.
The models indicated the innermost planets, TRAPPIST-1b and TRAPPIST-1c, could have lost 20 times as much water as there is in Earth’s oceans, since they receive the largest amount of ultraviolet energy. The planets farther out should have lost far less water, approximately three Earth’s oceans’ worth each. Therefore, those planets — particularly e, f and g — might still retain water on their surfaces and in their interiors.
They cautioned that their estimates remain limited by the large uncertainty on the planet masses, but said their estimates likely represent upper limits on the actual water loss.
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However, as far as finding signs of hydrogen surrounding the TRAPPIST-1 worlds, the results on that were inconclusive. They hold out hope that subsequent studies will be able to make this detection. As they wrote in their paper, published in the Astronomical Journal, “ultraviolet transit spectroscopy is a powerful way to search for signatures of atmospheric escape from exoplanets.”
Why do planetary astronomers continue to think these worlds may have a lot of water? From all the studies done on this system, astronomers think the seven planets may have originally formed much farther out from their star, in a cold region that contains crystals of water ice. The planets likely captured the ice, and therefore these worlds could potentially have tremendous stores of water, both in the planets’ interiors and on their surfaces.
“In terms of habitability, this is a positive step forward to say that hopes are still high,” said study co-author Julien de Wit, in a statement from MIT. “This concludes that a few of these outer planets could have been able to hold onto some water, if they accumulated enough during their formation. But we need to gather more information and actually see a hint of water, which we haven’t found yet.”
The team wrote in their paper that observing the TRAPPIST-1 planets over a broad wavelength range from the ultraviolet to the infrared would provide insights into the current state and the dominant physical processes shaping these planets’ atmospheres. Therefore, they are looking forward to upcoming observations with even more powerful telescopes.
“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.
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