The Sun dwarfs its nearest stellar neighbor, but Proxima Centauri’s flares pack a comparatively energetic punch.
Proxima Centauri b may be the closest planet outside our solar system, but it’s probably an unpleasant place to visit. Red dwarf stars like the host star of this system are active in their youth — and not in a good way. High-energy particles and radiation from Proxima Centauri likely stripped the planet of its atmosphere long ago.
But Proxima Centauri defies expectations even for red dwarfs. Even though the star is nearing middle age, it’s still acting the spring chicken, literally radiating with the energy of youth. Multi-wavelength observations of one brief flare, captured on May 1, 2019, showed it was 100 times more powerful than typical solar flares. And such energetic events are not uncommon, the way they are for the Sun.
“A human being on this planet would have a bad time,” comments study lead Meredith MacGregor (University of Colorado, Boulder). She and her colleagues report on the flare in the Astrophysical Journal Letters (preprint available here).
May Day Flare
These observations were part of a longer campaign that combines 40 hours’ worth of images from nine different telescopes of the nearest stellar neighbor to our Sun. For the brief flare at the beginning of the campaign, though, only five of these were watching, including the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the Hubble Space Telescope. Over just 10 seconds, the star to brightened its millimeter-wavelength emissions by a factor of 1,000; the far-ultraviolet light Hubble saw brightened even more, by a factor of 14,000.
What that kind of emission does to the atmosphere around Proxima Centauri b, assuming there’s any left, depends on what accompanies it. “[Powerful] X class flares from the Sun are almost always accompanied by coronal mass ejections (CMEs),” MacGregor says. CMEs are explosions that eject some of the Sun’s plasma, sending out high-energy particles that can ram into a planet’s atmosphere and strip it even more easily than the star’s heat or light could.
“If M dwarf flares are like solar flares,” she adds, “we might expect that large flares like this one should also have an associated CME.” That said, she adds, no one has yet caught a CME coming from a star besides our Sun; whether the correlation holds for red dwarf stars remains an open question.
What Makes a Powerful Flare?
The flare also sheds light on the star’s magnetic activity, something we can’t observe so readily on Proxima Centauri as we do on the Sun. The millimeter-wavelength photons are emitted by electrons gyrating around magnetic field lines. While the Sun emits this sort of radiation too, its electrons are not so energetic and the millimeter emission isn’t as strong.
“Detecting millimeter-wave emission from [Proxima Centauri] means that there are strong magnetic fields and electron acceleration,” says Silja Pohjolainen (University of Turku, Finland), who was not involved in the study.
“Indeed,” she adds, “the given magnetic field values are similar to what we have in sunspots and flare footpoints lower in the solar atmosphere, but to have such values higher in the corona, at the flare site, is strange.”
Puzzling out what’s happening with Proxima Centauri’s magnetic field will take some ingenuity. Even though this star is only four light-years away, astronomers can’t observe it the same way we do our Sun. Using particle detectors and magnetometers, we can measure solar particles and magnetic fields right where they are. For Proxima Centauri, we’re stuck with remote sensing for now — barring a far-future mission to the star.
Nevertheless, MacGregor says, millimeter emission might be a new way to understand the star’s behavior, and the behavior of planet-hosting red dwarf stars in general. All-sky millimeter-wavelength surveys are currently organized around questions in cosmology, but she suggests those same surveys could help study stellar flares and shed light on planets’ habitability.