Low-mass stars create a hostile environment for planets that orbit them, probably stripping their atmospheres and preventing life, new modeling of stellar outbursts suggests. This possibility has been raised before, but further evidence has been presented. The findings mean the recent findings of Earth-like planets around nearby red dwarfs might not be as exciting as we thought.
Most stars are M-type, also known as red dwarfs. Consequently, they illuminate the majority of the planets where we might look for life. In our own galactic neighborhood, we have found abundant planets in the so-called “habitable zone” around red dwarfs within about 40 light-years. Existing telescopes can't get a good view of these, but the next generation, being built at this moment, might well be able to detect atmospheres around these worlds.
Nevertheless, there has been one fly in the exoplanet cornucopia ointment: The fear that coronal mass ejections (CME), to which M-type stars are particularly prone, could strip the atmospheres from planets orbiting close enough to these dim stars not to freeze. Without an atmosphere, liquid water can't be sustained – ice sublimes directly to water vapor, which gets blown away by the next CME.
Planetary magnetic fields provide protection, as Earth's does for us, but astronomers have been troubled by the question of how strong a field would need to be to make life possible around a typical red dwarf. Too strong, according to Boston University PhD student Christina Kay.
Kay picked V374 Pegasi, an M-type star 29 light-years away, and not much more than half as hot. It's magnetic field, flares and CMEs have been particularly heavily studied, but Kay told the UK National Astronomy Meeting she'd found something new.
"We figured that the CMEs would be more powerful and more frequent than solar CMEs, but what was unexpected was where the CMEs ended up," Kay said in a statement. She found CMEs get pushed into an area known as the Astrospheric Current Sheet, roughly equivalent to the plane of the solar equator – and where most planets orbit.
Kay reported in the Astrophysical Journal such regular CME exposure would blast the atmospheres from nearby planets with magnetic fields similar to Earth's. Unless a planet orbited so far out it would be covered in ice anyway, it would need a magnetic field at least 10, and often several thousand, times as strong as the Earth's to hold onto its air.
Without exceptionally strong magnetic fields, planets like Proxima b, and the multiple members of the TRAPPIST-1 system, are likely to be blasted wastelands. Efforts to find life elsewhere might need to go back to focusing on rarer mid-mass stars, where CMEs are rarer, and not focussed into the planetary plane.