As it orbits the small red dwarf Proxima Centauri, an exoplanet called Proxima b is being pummelled by nearly constant stellar flares and cosmic rays. Until now, it was thought these might be damaging to potential life, but new research suggests the flares could actually help make conditions on the planet more habitable.
Markus Scheucher at the Technical University of Berlin in Germany and his colleagues modelled how incoming energetic particles from stellar flares and cosmic rays would impact the temperature of Proxima b.
Because it orbits such a small star, Proxima Centauri b only receives 65 per cent of the energy we get from our sun, despite orbiting at a twentieth of the distance that we do.
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“An otherwise likely too cold planet could actually experience habitable surface temperatures if [it] is actually Earth-like and is frequently bombarded by stellar energetic particles” says Scheucher.
The stellar flares, along with cosmic rays, batter the planet with hydrogen nuclei. This causes the production of hydroxide in the atmosphere, which reacts with methane to create carbon dioxide and water. In turn, that process limits the amount of methane in the atmosphere, which could otherwise absorb solar radiation and lead to an anti-greenhouse effect, cooling the planet as it is unable to trap heat from the star.
That type of cooling happens in our own solar system. For example, Saturn’s moon Titan has a methane-rich atmosphere, where 90 per cent of the energy from the sun is absorbed in the upper atmosphere, so it doesn’t reach the surface.
The study provides a new way to think about planets orbiting active stars like Proxima Centauri. Red dwarfs are the most common stars in the universe, making up 85 per cent of those in the Milky Way, and many of the Earth-sized exoplanets discovered in recent years are orbiting them.
“This is good for planets such as Proxima Centauri b, but not necessarily for those closer to their star,” says Abel Méndez at the University of Puerto Rico at Arecibo. Yet for those in the sweet spot, stellar flares might not be a bad thing after all.
“There are other possible good things about flares,” says Méndez. “For example, they might provide enough energy to create basic molecules necessary for the origin of life.”
A study in 2018 found that UV light from stars could help form precursors to RNA, a process called abiogenesis.
But detecting life on a planet four light years away, like Proxima b, would be tricky to do. Still, it is possible that life could adapt to these conditions, and any life in the exoplanet’s potential oceans would be more protected.
Here on Earth, life moved from the oceans to land about 450 to 400 million years ago. “This step might be impossible for planets around M-dwarfs due to the hostile surface environment maintained by flares,” says Méndez. “If life is limited to the oceans, this might also exclude intelligent life capable of technology.” For example, they wouldn’t be able to make fire, he says.
“The study also suggests that biosignatures will be harder to detect and interpret on these planets,” says Méndez. The results showed that when looking at the wavelengths of light absorbed by molecules in the planet’s atmosphere, methane and carbon dioxide might mask signs of water, making it potentially harder to detect signs of life.
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