Findings Suggest that Red Dwarf Stars May Not Sterilize As Many Exoplanets As Feared

Red dwarf suns are the most common in the universe, and many of the exoplanets officially discovered so far orbit this type of “cool” star.  Red dwarfs are much smaller and less powerful than the G type stars such as our own sun, and it is easier to detect exoplanets orbiting them because of their reduced size and energy.

As a result, a number of relatively nearby red dwarf stars — in the Trappist-1 system, Proxima Centauri and Barnard’s star, for instance — are avidly studied for their potential habitability.  The exoplanets of red dwarfs tend to orbit much closer than around other larger stars, but the suns have that lower radiative power and so some are considered habitable candidates.  And if they are indeed habitable, they could be for a very long time because red dwarfs live much longer than most other stars.

But there have been two (at least) problems with the habitable red dwarf exoplanet scenario.  The first is that many of the planets so close to their star are tidally locked, meaning that only one side ever faces the sun.  Some have argued a tidally locked planet can still be habitable, but it would not be easy.

More crucial, however, is that red dwarf stars are known for sending out many, many powerful solar flares, especially during their solar infancy and childhood.  These high radiation and particle flares could and would potentially kill any life emerging on a dwarf exoplanet, and the stellar flares could even sterilize the planets’ atmosphere for all time.  Although direct observations have not shown this deadly scenario to be inevitable or even present, the red dwarf flaring is well documented.  And so potentially the flares have seemed to rule out, or make improbable, life on an estimated 75 percent of the stars in our galaxy.

This is why there is interest in the astrobiology world about a new paper that addresses a particular kind of stellar flare that would hit red dwarf exoplanets.  Such studies of how the behavior of a star effects orbiting planets is one of the less well studied aspects of the exoplanet field, and so the paper is especially welcomed.

And the results suggests that the red dwarf flares would strike orbiting exoplanets from an angle rather than straight on, and therefore would land in a way that would theoretically minimize damage to potential atmospheres and life.

 

Based on the intensive study of four red dwarf stars, the researchers found that much of the white-light flaring (filled with damaging high-energy particles and a broad range of of powerful electromagnetic radiation) emanated from the high latitudes of the star and as a result would miss many exoplanets or hit them with a diminished force.  The implication, the researchers wrote, is that “the impact of flares on the habitability of exoplanets around small stars could be weaker than previously thought.”

So the conclusion from the paper, in the Monthly Notices of the Royal Astronomical Society, is encouraging, if preliminary.   Lead author Ekaterina Ilin of the Leibniz Institute for Astrophysics in Potsdam, Germany (AIP)  put it this way:  “In the end, our results are a hint that exoplanets around those very small stars may be more habitable” than many astrobiologists, astrophysicists and planetary scientists had imagined.

The finding, she said, “also tells us that we need to understand the space weather conditions in these systems in perhaps much greater detail before we can really tell whether a given planet is suited for life as we know it or not, or how habitable our solar system has been in its early days.”

The team used data from NASA’s Transiting Exoplanet Survey Satellite (TESS) to analyze white-light flares on fast-rotating red dwarfs and the researchers worked out the solar latitude where the flares had originated.   They studied four stars out of the more han 3,000 red dwarfs where TESS had collected data..

On all four stars, the flares occurred above a 55 degree latitude—much closer to the poles of the stars than to their equators.

Ilin said in an email that the white-light flares do contain ultraviolet radiation, often discussed as the most worrisome kind of electromagnetic radiation in terms of potential exoplanet habitability.  But the high-latitude origins of the UV light does not necessarily help much since that kind of light is emitted in all directions.

But there are many dangers coming from flares, including very high-energy particles

“What the high latitude is more likely to protect the planets from is particles emitted alongside the electromagnetic emission,” Ilin wrote. “These would be expelled poleward and miss the planet if the latter revolves around the stellar rotational equator.”

“And we have evidence that these particles can evaporate exoplanetary atmospheres, destroying the basis for habitability.”

So the finding is encouraging regarding high-energy particles but less so in terms of ultraviolet light.

The term “red dwarf” does not refer to a single kind of star. It is frequently applied to the coolest stellar objects, including K and M dwarfs — which are true stars — and brown dwarfs, often referred to as “failed stars” because they were never able to sustain hydrogen fusion in their cores.  Red dwarf stars generally burn in the range of 5,000 degrees Fahrenheit compared with our sun at approximately 10,000 degrees Fahrenheit.

These smaller red dwarf stars often lack the internal layers that sun-like stars have, and their internal churning and speedy rotations result in more extreme magnetic activity, and therefore more flares. Flares occur when the star’s magnetic fields get twisted up and then snap back into alignment, sending out high-energy radiation and particles in the process.

Flares produced by our own sun—from what are called  coronal mass ejections or CMEs– can have harmful effects on orbiting satellites and even result in the loss of power over large regions.

Red dwarf stars are known to have periods of very frequent flare activity in their early stages — sending out many more highly-energetic flares than larger stars such as our own. These flares could potentially strip the stars’ planets of any existing atmospheres, making them uninhabitable. These red dwarf exoplanets are particularly vulnerable because to ever be even remotely habitable, these exoplanets have to orbit close to the star to receive enough warmth to be in its “habitable zone” — the distance from the sun where any potential water could remain liquid on a rocky surface.

It is the post-flare period of red dwarfs that intrigues many scientists. After those tumultuous earlier times, red dwarfs become more stable and can last much longer than larger stars. If the exoplanets survive unsterilized by an early bombardment of flares, they potentially can have many billions of years for life to emerge and evolve.

Astrobiologist Shawn Domogal-Goldman, supervisor for the Planetary Environments Laboratory at NASA’s Goddard Space Flight Center, who was not involved in the paper, says a robust finding that red dwarf stars may spare exoplanets some of the worst of their early flaring activity would be welcomed news.

“This paper finds that the impact of the flares will be less than expected before, but will it be enough to protect habitability in the stars’ habitable zones? It’s too early to know much about that.”

“This kind of research into the behavior of stars and how they affect orbiting planets is very important to our field. The relationships between stars and their planets is crucial, and we don’t know enough about them.”