The only known life in the universe lives on a mid-size rocky planet that orbits a mid-size yellow star. That makes our planet a bit unusual. While small rocky planets are common in the galaxy, yellow stars are not. Small red dwarf stars are much more typical, making up about 75% of the stars in the Milky Way. This is why most of the potentially habitable exoplanets we’ve discovered orbit red dwarfs.
All things being even, you would expect then that red dwarf planets are the ones most likely to harbor life. But all things aren’t equal. Red dwarfs can be much more active than Sun-like yellow stars. They can emit enormous solar flares and strong x-rays. And since red dwarfs are much cooler than the Sun, planets must orbit very close to them to be potentially habitable. All of this paints a grim picture for life on red dwarf planets. A red dwarf would likely strip the atmospheres of close planets, and fry any life those worlds might harbor. But a new study finds that things might not be as bad as we thought.
The team used data from the Transiting Exoplanet Survey Satellite (TESS). While the primary goal of the TESS mission is to study exoplanets that transit their stars, the TESS survey also contains data on stellar flares. So the team looked for the stellar flares of red dwarfs. From this, they could determine the latitude of solar flares on the star. They found that the distribution of flares on red dwarfs is very different from that of our Sun.
Solar flares generally occur within the equatorial region. Because of this, the energy and particles from these flares can strike planets in the inner solar system. This most recently happened in 1859 with the Carrington Event. But Earth’s strong magnetic field does a good job protecting us. If such an event happened today it would disrupt our electronic infrastructure, but it wouldn’t threaten Earth-life as a whole. If Earth orbited the Sun much closer than Mercury, such a flare would be much more dangerous.
It’s been generally thought that red dwarfs also emit flares from their equatorial regions, but this new study found that the largest flares tend to appear close to the star’s poles. The red dwarf flares they observed all appeared above the 60-degree latitude. Their sample size was small, so they couldn’t entirely rule this out as a fluke, but if further observations support the trend that’s good news for red dwarf planets. It means that most flares will be directed out of the orbital plane, and potentially habitable worlds will be spared from an apocalypse.
Reference: Ilin, Ekaterina, et al. “Giant white-light flares on fully convective stars occur at high latitudes.” Monthly Notices of the Royal Astronomical Society 2021.