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Red dwarf planet
This artist’s impression shows how the surface of a planet orbiting a red dwarf star may appear. The planet is in the habitable zone, so liquid water exists. (CFA Illustration / M. Weiss)

Do some of the Earth-sized planets around a dwarf star called TRAPPIST-1, just 40 light-years away, have liquid water? Newly reported findings from the Hubble Space Telescope give astrobiologists continued cause for hope.

The seven TRAPPIST-1 planets created a sensation in February because they’re the biggest assemblage of Earth-scale worlds known to exist in a single planetary system. What’s more, three of the planets – known by the letters e, f and g – are in an orbital region where scientists say water could exist in liquid form.

That’s thought to be a key condition for life as we know it, which is why the region is known as TRAPPIST-1’s “habitable zone.”

But is the water really there? To get at that question, astronomers used Hubble to study the amount of ultraviolet radiation received by the planets, and what that might be doing to their atmospheres.

“Ultraviolet radiation is an important factor in the atmospheric evolution of planets,” Vincent Bourrier, an astronomer from the Observatoire de l’Université de Genève in Switzerland, explained today in a news release. “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.”

After the UV radiation breaks apart the H2O, some of those hydrogen and oxygen molecules should be cooked out of the atmosphere by higher-energy UV light and X-rays.

Bourrier and his colleagues analyzed data from Hubble’s Space Telescope Imaging Spectrograph, looking for signs of hydrogen surrounding the TRAPPIST-1 worlds. The results on that front were inconclusive, but they were able to measure changes in the strength of the star’s UV radiation.

The researchers factored those readings into a computer model that suggested a large amount of water vapor should have been broken down over the past 8 billion years.

‘Hopes are still high’

In a paper published by The Astronomical Journal, the researchers say the innermost planets could have lost 20 times as much water as there is in Earth’s oceans. The farther-out planets should have lost far less water – about than three oceans’ worth each.

Those planets – such as e, f and g – might still retain water on their surfaces and in their interiors.

“In terms of habitability, this is a positive step forward to say that hopes are still high,” MIT planetary scientist Julien de Wit, a co-author of the study, said in a news release. “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 is planning another Hubble observing run to look for clouds of hydrogen that should pass over the TRAPPIST-1 star as the planets make their transits.

For life, let there be ultraviolet light

Coincidentally, a different study published in The Astrophysical Journal suggests that ultraviolet radiation may have played a critical role in the emergence of life on Earth by powering the chemical reactions that created ribonucleic acid, or RNA.

That implies that a certain level of UV light, not too much and not too little, might be needed to kick-start life in other planetary systems as well.

“It would be like having a pile of wood and kindling and wanting to light a fire, but not having a match,” lead author Sukrit Ranjan, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said in a news release. “Our research shows that the right amount of UV light might be one of the matches that gets life as we know it to ignite.”

In addition to Bourrier and de Wit, the authors of “Temporal Evolution of the High-Energy Irradiation and Water Content of TRAPPIST-1 Exoplanets” include E. Bolmont, V. Stamenkovic, P. J. Wheatley, A. J. Burgasser, L. Delrez, B.-O. Demory, D. Ehrenreich, M. Gillon, E. Jehin, J. Leconte, S. M. Lederer, N. Lewis, A.H.M.J. Triaud and V. van Grootel.

In addition to Ranjan, the authors of “The Surface UV Environment on Planets Orbiting M-Dwarfs: Implications for Prebiotic Chemistry and Need for Experimental Follow-up” include Robin Wordsworth and Dimitar Sasselov.

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