In a research paper to appear in the Astronomical Journal (arXiv.org preprint), astronomers from MIT, the Geneva Observatory and elsewhere report that the outer planets in the TRAPPIST-1 planetary system may hold substantial amounts of water. The results are based on observations of the host star made by the NASA/ESA Hubble Space Telescope.
This artist’s concept allows us to imagine what it would be like to stand on the surface of TRAPPIST-1f. Because this planet is thought to be tidally locked to its star, meaning the same face of the planet is always pointed at the star, there would be a region called the terminator that perpetually divides day and night. If the night side is icy, the day side might give way to liquid water in the area where sufficient starlight hits the surface. One of the unusual features of TRAPPIST-1 planets is how close they are to each other — so close that other planets could be visible in the sky from the surface of each one. In this view, the planets in the sky correspond to TRAPPIST1e (top left crescent), d (middle crescent) and c (bright dot to the lower right of the crescents). TRAPPIST-1e would appear about the same size as the Moon and TRAPPIST1-c is on the far side of the star. The star itself, an ultra-cool dwarf, would appear about three times larger than our own Sun does in Earth’s skies. Image credit: NASA / JPL-Catlech.
TRAPPIST-1 is an ultracool dwarf star in the constellation Aquarius, 38.8 light-years away. It is barely larger than Jupiter and has just 8% of our Sun’s mass. It is rapidly spinning and generates energetic flares of ultraviolet (UV) radiation.
This dwarf star is host to seven transiting planets, named TRAPPIST-1b, c, d, e, f, g and h. All these planets are similar in size to Earth and Venus, or slightly smaller, and have very short orbital periods.
They are likely all tidally locked, meaning the same face of the planet is always pointed at the star, as the same side of the Moon is always pointed at Earth. This creates a perpetual night side and perpetual day side on each TRAPPIST-1 planet.
TRAPPIST-1e, f and g lay in the star’s habitable zone, meaning they may harbor suitable conditions for life.
Following up on the discovery, Dr. Vincent Bourrier of the Geneva Observatory and colleagues used Hubble’s Space Telescope Imaging Spectrograph to study the amount of UV radiation received by TRAPPIST-1 planets.
“UV radiation is an important factor in the atmospheric evolution of planets. As in Earth’s atmosphere, where UV sunlight breaks molecules apart, UV starlight can break water vapor in the atmospheres of exoplanets into hydrogen and oxygen,” Dr. Bourrier said.
“While lower-energy UV radiation breaks up water molecules — a process called photodissociation — UV 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 UV radiation emitted by the TRAPPIST-1 star indeed suggests that the planets could have lost gigantic amounts of water over the course of their history.
This is especially true for the innermost two planets of the system, TRAPPIST-1b and TRAPPIST-1c, which receive the largest amount of UV energy.
“Our results indicate that atmospheric escape may play an important role in the evolution of these planets,” said co-author Dr. Julien de Wit, of MIT.
The astronomers estimate that the innermost planets lost more than 20 times Earth’s current oceanic water stores over their 8-billion-year journey toward their star.
The outer planets in the system — including TRAPPIST-1e, f and g which are in the habitable zone — lost much less, equivalent to around 3 times the ocean stores on Earth.
“Earth-sized planets can capture hundreds of Earth-oceans’ worth of water when they form, but it’s highly dependent on so many factors, and difficult to say,” Dr. de Wit said.
“We can say the inner ones probably lost a huge amount of water, and the outer ones way less, allowing them to actually still have some water, if they captured it when they first formed.”
“It depends a lot on their initial water content,” Dr. Bourrier added.
“If they formed as ocean planets, even the inner ones would likely still harbor a lot of water. We are still a long way to determining the habitability of these planets, but our results suggest that the outer ones might be the best targets to focus our future observations.”
“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,” Dr. Bourrier concluded.
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V. Bourrier et al. 2017. Temporal evolution of the high-energy irradiation and water content of TRAPPIST-1 exoplanets. AJ, in press; arXiv: 1708.09484
