Large Flares from Small Stars

There's more red dwarfs than sun-like stars, and there's speculation that red dwarfs may harbor life. Alas, these stellar lightweights pack a mighty punch, possibly sterilizing life with massive flares.

I used to rather naively think that if we were to find life elsewhere in our galaxy - especially sentient life - it would be on a planet orbiting a sun-like star.

However, when the universe is making "things," it tends to make lots more "little things" than "big things". So, most stars in the galaxy are quite a bit smaller than the sun, red dwarfs making up approximately 70 percent of all Milky Way stars. However, as a new census re-affirms, life around a red dwarf may not be easy.

Red dwarfs, specifically a class called M-dwarfs, are much cooler and smaller than the sun, being only one-third the radius of our sun and "only" 5,000 degrees Fahrenheit (3,000 Kelvin).

As a result, the habitable zones for planets to have liquid water around M-dwarfs are smaller and closer to the star than the distance from the sun that the Earth enjoys. So, to get reasonable living conditions, you do need to get up-close and personal with your parent red dwarf. As we know on Earth, the sun's storms and flares can affect us here, so imagine the effects of a solar flare hitting a closer planet!

The story gets even more dangerous. M-dwarfs have been known to have much more powerful flares than sun-like stars, partly due to their very strong magnetic fields. Without knowing how often these flares go off, or how many M-dwarfs have flaring storms, it is even harder to estimate the likelihood of extraterrestrial life.

Enter, once again, the Hubble Space Telescope.

A survey of M-dwarfs that was originally used as a planet finding expedition found new life in a search for the incidence of stellar flares. 215,000 red dwarfs were imaged over just seven days, and 100 flares were found, several from the same stars, and some from stars billions of years old.

This is crucial in the search for complex life, as it took billions of years to evolve on Earth. Wouldn't it just be awful to come all that way to have a super-flare wipe out your planet's atmosphere?

The team was able to note that stars that already varied in brightness periodically were more vulnerable to having such outbursts. So, that parameter may help in ruling out the most dangerous environments for life.

This is a great example of how archived data in astronomy can have many lives, especially since such initial studies can inform further searches into such weird and wild phenomena.

Image: Artist's conception of the view of a violent M-dwarf from one of its planets. Credit: NASA, ESA, and G. Bacon (STScI)

See also the Hubble press release and AAS abstract.