A new study of data archived from the Galaxy Evolution Explorer (GALEX) spacecraft is revealing just how hard life might be on exoplanets like those in the TRAPPIST-1 system.
Artist’s concept of what the sky might look like from one of the seven known terrestrial planets in the TRAPPIST-1 system.
ESO / M. Kornmesser
Roughly 39 light-years away toward the constellation Aquarius is a planetary system unlike any other. Standing on one world at twilight, you would be able to gaze at three neighboring exoplanets: The first is a black blemish slowly gliding across the setting star’s disk, the second is a thin crescent trailing the star, and the third is a giant orb — nearly twice the size of our full Moon — rising on the opposite horizon and raising tides that are four times as strong as those on Earth.
It’s no wonder the TRAPPIST-1 system captured our imagination last February. Not only are these worlds so tightly packed that they routinely dance in each other’s skies, they’re also close enough that three of the worlds orbit in the star’s habitable zone, defined as the region where liquid water could exist on a rocky surface. What’s more, all seven exoplanets have been hailed as being “temperate,” meaning that they might be equally warm if they have the right internal temperatures or cozy atmospheres.
But there’s one problem: their host star. At first glance that salmon-colored star — nearly 10 times the size of our Sun yet 200 times dimmer — seems fairly innocent, gently washing each world in a warm glow. But at 8% of the Sun’s mass, TRAPPIST-1 is an M dwarf, a type of star that’s known to throw tantrums in its youth. Also known as red dwarfs, these stars throw off intense flares and winds that easily erode the atmospheres of nearby exoplanets and sterilize their surfaces.
With this in mind, Chase Million (Million Concepts) combed through archived data from the now-retired Galaxy Evolution Explorer (GALEX) spacecraft, initially designed to study galaxy evolution, to hunt for flaring red dwarfs instead. He and his colleagues found dozens of smaller flares that had evaded detection until now. This result, announced at the summer meeting of the American Astronomical Society, will help astronomers determine whether the TRAPPIST-1 exoplanets and others like it are truly habitable.
Breathing New Life into Archival Data
The Galaxy Evolution Explorer (GALEX) launched on April 28, 2003.
“You can think of flares as all-frequency radiation bombs,” Million says. These stellar eruptions release radiation across a wide range of wavelengths, from X-rays to the radio. Although this makes them visible in nearly any telescope, they peak in the ultraviolet, with a significant fraction of the energy released in the same bands that GALEX once observed. When not flaring, though, red dwarfs are relatively dim in the ultraviolet — a contrast that allows a satellite like GALEX to easily pick out flares across the galaxy.
Yet, observers couldn’t easily access GALEX data. Throughout the mission’s lifetime (which ran from 2003 to 2012), astronomers could only download images. To study galaxies, this was all they needed. But the satellite itself collected the data one photon at a time, which would easily allow astronomers to see the flares form and fly off the surface of a star.
So Million created software, published in the Astrophysical Journal (full text here) and called gPhoton, that finally enables astronomers to access the individual photons within each image. The code gives new life to data that has been sitting on hard drives for up to 15 years, says James Davenport (Western Washington University).
“Because the photons are time-tagged, you can actually make a little tiny slice — like a movie — of the photons coming in,” says Don Neill (Caltech), who wasn't involved in this study.
Already, Million and his colleagues have searched several hundred M dwarfs and detected dozens of flares. The goal is to constrain the flares: to see how often they occur and at what energies, so that astronomers can better estimate what the probability is for life on a circling exoplanet.
Early research shows that there are far more low-energy flares than high-energy ones. Although that might sound promising in that low-energy flares seem less hostile to life, their high frequency might add up over time to produce inhospitable environments in the long run.
An artist's illustration shows an exoplanet orbiting a red dwarf star. Red dwarfs, or M dwarfs, tend to be magnetically active, displaying gigantic arcing prominences, a wealth of dark sunspots, and (especially early in the star's life) violent flares that could strip a nearby planet’s atmosphere over time, or make the surface inhospitable to life as we know it.
NASA / ESA / G. Bacon (STScI)
The research will help astronomers better understand far more exoplanets than just those in the TRAPPIST-1 system. Red dwarfs are the most common type of star in the universe. They’re also hot spots in our search for life because their habitable zones are so close in, making any exoplanets therein — especially Earth-size planets — easier to detect with current and near-future instruments.
But most astronomers warn that these stars are less likely to host life than you might suspect, in part because the definition of a habitable zone is oversimplified. Not only does an exoplanet need to be in the right spot, but quite possibly orbiting the right star as well.
It doesn’t take much imagination, for example, to realize the kind of havoc that a violent episode from a red dwarf could wreak on nearby objects. Just think about a nascent planet, where life is just starting to get a foothold. “If you irradiate any of those organisms with high-energy photons like X-rays or ultraviolet light, it can damage the DNA, it can damage some of the cellular structures,” Neill says. “And that would be a major hurdle for life and evolution.”
But these high-energy flares aren't the only ones that matter. Rachel Osten (Space Telescope Science Institute), who worked with Million in analyzing the preliminary data from GALEX, points out that most astronomers have solely focused on these high-energy flares because our instruments thus far have only been able to spot the biggest and brightest events. “We tend to think about a quiescent state and a flaring state,” she says. But the latest work reveals that there might never be a truly quiescent state.
How these results might affect habitability is an open question. Most astronomers speculate that these quieter flares are still harmful — perhaps even more so — given how often they occur. Remember, too, that flares lose energy as they travel through space. So planets that circle a red dwarf within the habitable zone are likely to be hit harder than planets, like Earth, that orbit farther away.
The picture of habitability around red dwarfs is far from complete. But understanding the number and power of flares that occur will help astronomers better understand how likely life might be found on these easy-to-observe planets. So, Million and his colleagues will continue searching through GALEX in the hope of studying hundreds of thousands of these stellar explosions.
In the meantime, Neill is particularly glad we find ourselves orbiting a quiet star. Although the GALEX data suggests that we might need to worry about the quiet flares from M dwarfs, some of the more energetic ones can be much brighter than your typical solar flare.
“If the sun emitted something like that, that might be strong enough to actually disrupt our magnetic field and blow some of our atmosphere off — it could be devastating,” Neill says. “It’s a good thing we have a nice G star to orbit and not an M star.”