It’s a big ‘might,’ but the Hubble Space Telescope hints at possible water on at least some of the seven planets orbiting the dwarf star TRAPPIST-1. After the February announcement of seven Earth-sized planets in one system, teams of researchers went to work to see what they could learn.

Swiss astronomer Vincent Bourrier led a team using the Hubble Space Telescope to study how much ultraviolet radiation each planet receives from the nearby dwarf star.

“Ultraviolet radiation is an important factor in the atmospheric evolution of planets,” says 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.”

Lower-energy UV radiation takes water molecules and splits them into hydrogen and oxygen via a process called photodissociation. The more potent UV rays, like XUV radiation and X-rays, heat the upper atmosphere of a planet causing hydrogen and oxygen to escape. A powerful enough telescope could then see it. Especially hydrogen, which can act as a possible signature of atmospheric water vapor.

The Hubble isn’t strong enough to look for water. But what it can do is get a good idea of how much UV radiation is hitting the seven planets, and get a rough estimate of how much water that radiation would have bled away.

TRAPPIST-1b and 1c get the biggest dose of UV energy and could have lost more than 20 Earth-oceans-worth of water over the last eight billion years. The odds of water still being on these planets surfaces are slim to none. But the outer five, including the three within the habitable zone could warrant a second look from the upcoming James Webb Space Telescope. Scientists suggest these planets lost much less water, possibly less than three Earth-oceans worth.

“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,” says Bourrier.

Studying the atmospheres of exoplanets is an understandably new field of science. The James Webb Space Telescope promises to start revealing far flung worlds in greater detail when it launches next October (2018).

Water is just one part of the equation for habitability on the TRAPPIST-1 planets. The biggest obstacle is the star. Yes, it’s cooler. The problem is how close the planets orbit. All of them are tidally locked. One side faces the star, with all the heat and harmful radiation hitting it. The other sits in complete darkness, and freezes. The next generation of telescopes will hopefully give us a better idea of how the atmospheres of these planets work.


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