In the hunt for extraterrestrial life, it makes sense to look for places that have similar conditions to Earth. But the majority of stars in our Milky Way aren't Sun-like – they're the smaller and cooler red dwarfs. To determine how well these stars might be able to support life, A NASA program is using Hubble to monitor their flare activity, and the first results don't paint a positive picture.
The program is known as HAbitable Zones and M dwarf Activity across Time, clumsily forming the acronym HAZMAT. As the name suggests, the study is looking at the stellar activity (i.e. flares) of M (red) dwarf stars and how that may affect the Habitable Zone, which is the comfortable distance from a star where a planet's surface could host liquid water and maybe life.
Earth, of course, orbits exactly within the Sun's "Goldilocks" zone, where things are not too hot and not too cold. Since red dwarfs are cooler, the Habitable Zone is much closer, meaning planets need to tuck up tighter to stay warm. But that could bring new problems – red dwarfs are also more active than the Sun, and their intense radiation and regular flares could sterilize exoplanets before life can get a foothold.
To get a better understanding, the HAZMAT project examined the flare frequency of 12 red dwarf stars that were relatively young – about 40 million years old. Hubble looked at spikes in ultraviolet light from these stars, and found that they regularly gave off flares that were far more energetic than anything the Sun is capable of. These flares were between 100 and 1,000 times more powerful than those given off by older stars, with one flare in particular (dubbed the "Hazflare") increasing the star's brightness in ultraviolet light by a factor of 193.
"With the Sun, we have a hundred years of good observations," says Parke Loyd, first author of the study. "And in that time, we've seen one, maybe two, flares that have an energy approaching that of the Hazflare. In a little less than a day's worth of Hubble observations of these young stars, we caught the Hazflare, which means that we're looking at superflares happening every day or even a few times a day."
That extreme space weather might be bad news for any potential life, which would be most likely trying to get started on these planets while the host stars are young. But, the researchers say, it's not a guaranteed death sentence. Life doesn't necessarily have to resemble what we know of here on Earth, but even if it does, we've found organisms thriving in extreme environments that were previously thought of as deadly.
"Flares like we observed have the capacity to strip away the atmosphere from a planet," says Loyd. "But that doesn't necessarily mean doom and gloom for life on the planet. It just might be different life than we imagine. Or there might be other processes that could replenish the atmosphere of the planet. It's certainly a harsh environment, but I would hesitate to say that it is a sterile environment."
Space weather is just one factor at play in determining how habitable a red dwarf exoplanet may be. It's been found that red dwarfs could actually be dripping with water worlds, but unfortunately these would be so wet that they can't support stable climates. Other studies suggest their atmospheres would be too dense for life.
On the bright side, red dwarfs are among the longest-lived stars, potentially burning for trillions of years. With such a mind-bogglingly long time to work with, life could arise after the rowdy young stars start to settle down. The next steps of the HAZMAT program are to look at older red dwarfs of about 650 million years, before looking at even older ones, to better understand the evolution of these common stars.
The research was published in The Astrophysical Journal.
Sources: Hubble, Arizona State University