TRAPPIST-1's eternal sunset hides deadly X-ray emissions

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Mar 11, 2017

When seven planets were confirmed to orbit unassuming star TRAPPIST-1, habitability was inevitably going to be a pressing question—but the red dwarf's extreme radiation has put that question under fire.

To review, the TRAPPIST-1 system is an alien star system 29 light-years away with seven potentially rocky exoplanets that could be hiding extraterrestrial life. It even has its own Google Doodle. Something with this much potential to spawn a thousand sci-fi stories warrants further investigation. Recently, a research team led by Vincent Bourrier of Switzerland's University of Geneva studied the star's high-energy radiation emissions and what those findings could mean for its surrounding planets.

TRAPPIST-1 is an ultracool M8 red dwarf whose diameter is a mere tenth of our sun's. The visible light it radiates is less than one thousandth of what illuminates Earth, which means you probably wouldn't need shades. But wait.

When Bourrier's team studied the star's UV output using the Hubble Space Telescope, they specifically zeroed in on its Lyman-alpha emissions, hydrogen atom-emitted wavelengths which originate between the star's photosphere, or surface, and its burning ionized corona in an intermediate layer called the chromosphere. Red dwarf Proxima Centauri was used as a basis for comparison. The team found that TRAPPIST-1 is on the cooler end of the M dwarfs we know of which are orbited by exoplanets. Proxima Centauri blazes with six times more UV light than TRAPPIST-1, whose light is mostly infrared (read: invisible).

Dream on: this artist’s rendering imagines the TRAPPIST-1 system with water in solid, liquid and gaseous forms, though whether there is actually H2O on any of these planets is questionable.

"Understanding the origin of these Lyman-α variations will be crucial in assessing the atmospheric stability and potential habitability of the TRAPPIST-1 planets," say Bourrier and colleagues in a study published in Astronomy & Astrophysics.

Now here is where it gets ironic. Despite needing considerably less SPF if you're planning to take a hypothetical getaway to the TRAPPIST-1 system, there is no sunblock that could possibly protect you, let alone its orbiting planets, from the deadly levels of X-ray radiation that are equal to those of Proxima Centauri. Turns out this slow-burning star, whose planets are thought to appear like they are basking in an eternal sunset (as in the artist's rendering at the top of the page), may not be so mild as its deceptively low UV levels would have it appear.

Throw in those intense X-rays, and its overall radiation is so high it could sear through a hypothetical Earth-like atmosphere in as little as 1-3 billion years for its closer planets and 5-22 billion for those further away.

Whether this comes to pass depends on TRAPPIST-1's age. Red dwarves burn for about 100 billion years, so if the star is young enough, it could be possible that its surrounding planets will suffer something much worse than sunburn.

Seems habitability might have been hyped a little too early.

Besides the fact that no human could currently walk in the atmosphere of one of TRAPPIST-1's planets without some high-tech X-ray-blocking space suit that even science fiction hasn't figured out yet, TRAPPIST-1's lopsided ratio of Lyman-alpha radiation to X-ray radiation interests scientists for another reason. This ratio makes a vague suggestion that it might just be young enough to eventually scorch the atmosphere off all its planets before it ever burns out, though other evidence points to it being much more ancient. Scientists have only been able to theorize a minimum age of 500 million. Besides that, the star's actual age is as much of a mystery as that of some Hollywood actresses.

While no missions from Earth will be visiting the TRAPPIST-1 system anytime soon, book your future fantasy vacation with NASA.

(via Sky and Telescope)