Could These Nearby Earth-Sized Exoplanets Harbor Liquid Water?

The astronomy world was rocked last February when astronomers announced the discovery of seven rocky planets orbiting the star TRAPPIST-1 just 39 light years from Earth. Since then, scientists have scrutinized the seven planets to learn more about conditions on the ground and whether they could harbor life.

New research published this week in The Astronomical Journal suggests that five of the TRAPPIST-1 exoplanets could have water on their surfaces, reports George Dvorsky for Gizmodo.

"In terms of habitability, this is a positive step forward to say that hopes are still high," co-author Julien de Wit, a planetary scientist at MIT, says in a statement.

While scientists aren't able to directly observe the surfaces of the TRAPPIST-1 exoplanets, they monitored the ultraviolet radiation emitted by the dim star itself, writes Miriam Kramer for Mashable. "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," lead author Vincent Bourrier, an astronomer at Observatoire de l’Université de Genève, says in statement.

Using the famed Hubble Space Telescope, the team of astronomers measured the UV light emitted by the star over three months, and calculated how it would affect the atmosphere and potential water on the surface of each of the seven planets. The two innermost planets, similar to our own Mercury, have likely been sufficiently scorched, reports Dvorsky. Estimates suggest that vaporization resulted in a massive amount of water released into space over the last 8 billion years—more than 20 times the volume of Earth's oceans.

But the five planets orbiting beyond the first two would have only lost a modest amount of water to ultraviolet light assuming they had built up sufficient atmospheres. Three of those planets fall in the "habitable zone" for the TRAPPIST-1 zone, which means they sit in a sweet spot distance from their star to harbor liquid water on their surface.

These results are only educated guesses at best. Attempts to measure hydrogen, a signature of water vapor, around the planets themselves were largely inconclusive. Researchers hope to make more direct observations in the future with more powerful tools such as the forthcoming James Webb Space Telescope.

"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," de Wit says in a statement. "But we need to gather more information and actually see a hint of water, which we haven’t found yet."

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Ben Panko is a staff writer for Smithsonian.com