Red Dwarf Superflare Shows Why These Stars’ Planets Are Bad Places For Life

When red dwarf stars flare, they can really go off, particularly when young. An artist's impression of what that would do to a nearby planet. NASA, ESA, and D. Player (STScI)

Astronomers using the Hubble telescope have witnessed an enormous flare on an M-type star, commonly known as a red dwarf, in ultraviolet light. Red dwarves are well known for putting out much larger flares than stable stars like our Sun, but this is the first time we have had this kind of view of such a large flare.

Dr Parke Loyd of Arizona State University is part of a team investigating red dwarfs and the so-called habitable zones where temperatures are right for liquid water to exist around them. The project was awarded 10 hours observing time on the Hubble Space Telescope to view 12 red dwarfs 35-45 million years old at wavelengths that don't pass well through our atmosphere.

In that short time, they observed 18 flares. One of these, on an M2.0 star named J02365, was dubbed a superflare and 30 times more powerful than any previously seen by the Hubble.

"When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, 'Whoa.'" Loyd said in a statement.

"With the Sun, we have a hundred years of good observations," said Loyd. "And in that time, we've seen one, maybe two, flares that have an energy approaching that of the superflare."

Loyd describes the superflare, and the 17 others observed, in The Astrophysical Journal (pre-print on arXiv).

The flare was so big no one even bothered trying to fit it on the same scale as normal. P.Lloyd/ASU

Most of the stars in the galaxy are red dwarves. Numerically, they dominate our near neighborhood and are relatively easy to study, so the prospects of finding life depend heavily on whether planets that circle these stars are suitable.

Unfortunately, many astronomers have argued that these flares' intense radiation would likely sterilize nearby planets. This is particularly the case because a planet needs to orbit a red dwarf very closely if it is to be warm enough to support liquid water, which is 10-20 percent of the distance from the Earth to the Sun in the case of J02365.

Red dwarfs do settle down with time, putting out fewer and smaller flares. The next stage of Loyd's HAbitable Zones and M dwarf Activity across Time (HAZMAT) project will be to look at red dwarfs around 650 million years old. These are known to flare 20-100 times less frequently than their younger equivalents, but the details of the flares they do produce still require investigation. Finally, the project will investigate old red dwarfs, many of which are almost as old as the galaxy (yet still a small part of their way through their lives).

By the time red dwarfs simmer down, it is feared they have stripped the atmosphere from planets in close orbit. Loyd hopes such planetary atmospheres may be replenished, but how this might occur is unknown.

 

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