NASA/JPL-Caltech

Astronomers Aren't Sure if TRAPPIST-1's Planets Are Habitable After All

But there's still some hope!

MIKE MCRAE
13 MAR 2017
 

In what is becoming a bit of a rollercoaster of emotions, it seems that at least a couple – if not all – of the seven planets in the TRAPPIST-1 solar system could have already been stripped of their atmosphere by the star's radiation, making it unlikely that liquid water could flow on their surfaces after all.

But hold onto your tears; researchers studying TRAPPIST-1's spectral emissions have found evidence that the star might just be young enough to not have had time to blow away their atmospheres quite yet, meaning we can still dream of life on those far distant worlds a little longer.

 

Astronomers from the University of Geneva Observatory in Switzerland have compared the two types of radiation being emitted from the ultra-cool dwarf star TRAPPIST-1, and concluded the star doesn't seem to be "extremely old",

That brings into question just how much atmosphere still clings to the surfaces of the star's beloved family of rocky planets.

For those of you who missed the fuss, TRAPPIST-1 is a star about 39 light-years away that was discovered to have at least three planets last year. At the time, that didn't seem so remarkable, but then last month, NASA made a much-hyped announcement that TRAPPIST-1 actually hosted a seven-planet system

Nicknamed our sister star system, it's believed to consist of a large family of seven small, terrestrial bodies occupying tight orbits relatively close to their star.

Last month, astronomers announced that those planets actually consisted of a large family of seven small, terrestrial bodies occupying tight orbits relatively close to the star.

Early speculation suggested that at least a few of the planets could sit within a 'Goldilocks zone', where liquid water might pool on their rocky surfaces, and life could therefore have hypothetically evolved in its tepid oceans.

 

The exciting news inspired NASA to release fan art and travel posters displaying planets that appeared larger than our Moon in classic sci-fi styles, all to set our imaginations blazing.

Of course, we should have prepared for broken hearts. The signs were all there.

Orbiting so close to their parent, the planets are more than likely tidally locked, meaning one hemisphere constantly faces their sun while the other side is in perpetual darkness.

Only last month, we were somewhat dismayed to learn that the nearest known planet outside our own Solar System, orbiting Proxima Centauri, is probably just a hunk of bare rock polished of any atmosphere by bursts of radiation from its red dwarf star.

In these cases, radiation from the star ionises gases in the planet's atmosphere, allowing the particles to be pushed up and away from the planet's surface on streams of solar wind.

Last year, astronomers considered whether TRAPPIST-1 was another rather temperamental parent, calculating that the inner planets might have lost as much as 15 Earth-oceans of water to this solar scouring effect over the course of their lifetime.

Of course, it would depend on what "lifetime" meant.

In this most recent research, astronomers compared two types of radiation emitted from the dwarf star: X-rays shed by the star's wispy corona, and ultra-violet light called Lyman-alpha radiation, which comes from the hydrogen atoms from the chromosphere layer just beneath the corona.

It seems TRAPPIST-1 emits less than half as much Lyman-alpha radiation as Proxima Centauri, which is to be expected, since it's a cooler star.

But the two stars emit about the same amount of X-rays, which, all things considered, is kind of odd, since the X-ray and ultra-violet radiation output for this category of star both decrease over time, with the X-rays fading a lot faster.

"The fact that TRAPPIST-1 emits nearly three times less flux at Lyman-alpha than in the X-ray would thus suggest it is still relatively young," the researchers write in their paper.

With a fair bit of hand-waving, it seems that "relatively young" could mean anything up from about half a billion years old.

The fact that it spins quite quickly also adds weight to the conclusion that it's not an extremely old star.

Yet it also means the X-ray emissions were stronger in the past, since they have decreased over time.

Since it's predicted that the blasts of radiation would blow away any Earth-like atmosphere from the inner two planets within 1 to 3 billion years, and could take between 5 and 22 billion years to strip the rest of the family, there could still yet be liquid water up on those rocks if TRAPPIST-1 is indeed little older than 500 million years old.

There's further hope in the fact that the spacing of the planets indicates the possibility they migrated in close to their sun from further out in the solar system, subjecting them to intense blasts of radiation for even less time.

"If they migrated within a disk, typical time scales are about 100 million years, but that may not be valid for a system like TRAPPIST-1," researcher Vincent Bourrier told Camille M. Carlisle at Sky and Telescope. "Uncharted territory here!"

TRAPPIST-1 is a bit of an odd duck, however; in spite of seeming young, its motion through space places it within an older crowd of stars, which is either coincidence or a sign that there is more to learn.

If you still feel any love for our distant dwarf star, NASA just released footage of changes in its brightness taken over an hour period on 22 February, providing a snapshot of one of its planets passing in front the star and dimming its light.

TRAPPIST-1 animationNASA Ames/G. Barentsen

While a block of blinking pixels might not seem all that exciting, keep in mind this animated image covers just 44 square arcseconds of the sky, which is about the same area as a grain of sand held at arm's length.

So hold back from ripping down that fabulous TRAPPIST-1e poster ... for now. The Brady Bunch of stars is weird enough that it might still have a few secrets up its sleeve.

This research was published in Astronomy and Astrophysics.

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