The problem of flares in red dwarf planetary systems is stark. With their habitable zones relatively near to the star, planets that might support life are exposed to huge outbursts of particles and radiation that can strip their atmospheres. We can see that in nearby M-dwarfs like Proxima Centauri, which is extremely active not only in visible light but also in radio and millimeter wavelengths. New work at the Atacama Large Millimeter/submillimeter Array (ALMA) digs into the millimeter-wavelength activity. The results do nothing to ease the concern that systems like this may be barren of life.

Small M-dwarf stars are a problem because they operate through convection as energy from fusion at the core is transferred to the surface. A convective structure is one in which hot material from below moves constantly upward, a process that can be likened to what we see in a boiling cauldron of water. Larger stars like the Sun show a mix of radiative transfer – photons being absorbed and reabsorbed as they make their way to the surface – and convection. That enhances M-dwarf flare activity as their plasma is twisted and rotated, producing magnetic fields that snap open only to reconnect. Powerful flares and outbursts of particles are the result.

For a world in an otherwise habitable region around the star, that spells danger. Meredith MacGregor (Johns Hopkins University), who worked with Kiana Burton on the flaring at Proxima Centauri, explains:

“Our Sun’s activity doesn’t remove Earth’s atmosphere and instead causes beautiful auroras, because we have a thick atmosphere and a strong magnetic field to protect our planet. But Proxima Centauri’s flares are much more powerful, and we know it has rocky planets in the habitable zone. What are these flares doing to their atmospheres? Is there such a large flux of radiation and particles that the atmosphere is getting chemically modified, or perhaps completely eroded?”

Image: Artist’s concept of a stellar flare from Proxima Centauri. Credit: NSF/AUI/NSF NRAO/S. Dagnello.

MacGregor and Burton have been working on what they describe as the first multi-wavelength study using millimeter observations to probe into the physics of these flares. At their disposal are 50 hours of ALMA observations, covering some 463 flare events at energies between 10²⁴ to 10²⁷ erg. Most of these flares end quickly, ranging in duration from 3 to 16 seconds. The operative term in the study is flare frequency distribution (FFD), which maps the number of flares against energy levels. A power law function as at optical wavelengths would mean that lower-energy flares would be expected to occur more frequently than flares of higher energy, but the team found many flares within each energy range because of the high flare activity at Proxima.

Adds MacGregor:

“The millimeter flaring seems to be much more frequent–it’s a different power law than we see at the optical wavelengths. So if we only look in optical wavelengths, we’re missing critical information. ALMA is the only millimeter interferometer sensitive enough for these measurements.”

The point is significant, and I want to dig into the paper on this:

Proxima Cen has been observed frequently at optical wavelengths, with a much shallower FFD power-law index of 1.88 ± 0.06. This significant difference could indicate a disconnect between sources of optical and millimeter emission during flares. Since optical observations of stellar flares are more readily available and often used to infer the flaring flux at other wavelengths, this result underlines the need for further multiwavelength campaigns to constrain scaling relations. In particular, the higher rate of millimeter flares compared to optical flares and the tight correlation between FUV and millimeter emission observed by M. A. MacGregor et al. (2021) may suggest that the extreme-UV radiation environment of Proxima b due to small flares is also higher than predicted from the optical flare rate.

So the flare activity at Proxima Centauri is more complicated and perhaps more dangerous than we thought. As we learn more about flaring at this star, we have to hope that Proxima Centauri b has a strong magnetic field that can mitigate the effects of this incoming stream of energy and particles. The prospect of an atmosphere being stripped of ozone and water, for example, makes modification or erosion of its gases a strong possibility. Instruments like the Square Kilometer Array may one day be capable of detecting the interactions between such a magnetic field and the star’s stellar wind. But for now, we can only wait for further data.

The paper is Burton et al., “The Proxima Centauri Campaign — First Constraints On Millimeter Flare Rates from ALMA,” Astrophysical Journal Vol 982, Number 1 (17 March 2025), 43. Preprint / Abstract.