Stars’ weather could amplify signs of alien life

If you’re someone hoping aliens exist, the discovery of rocky, potentially habitable planets circling distant stars counts among the most exciting developments of the last few decades. To better understand these worlds and look for signs of life, scientists largely rely on spectroscopy to determine the chemicals in the planets’ atmospheres. New research shows that flares and other stellar weather can have serious impacts on some exoplanets’ atmospheric chemistry—and in some cases that could make evidence of life easier to spot (Nature Astron. 2020, DOI: 10.1038/s41550-020-01264-1).

Stellar flares, coronal mass ejections, and proton events—activities that blast out energy and particles from stars—bombard stars’ orbiting planets with radiation that can induce chemical reactions. Earth’s magnetic field and ozone layer largely protect our atmosphere from these effects, but the events can have larger effects on planets without magnetic fields. Previous work has predicted the effects of such stellar weather on atmospheric chemistry, but Howard Chen of Northwestern University and colleagues wanted to understand these events in the full context of complex physical atmospheric processes like wind, cloud dynamics, and mixing of air masses. The team modified a computer model designed to study Earth’s climate, and used it to simulate the effects on exoplanets’ atmospheres of varying the magnetic field strengths, the size of the stars the planets orbited, and the amount of stellar activity the planets experienced, based on flares observed by astronomers.

Their predictions could provide important information to astronomers hoping to identify chemical biosignatures. The group reports that the spectroscopic signals of nitrogen dioxide, nitrous oxide, and nitric acid—all of which are thought to be possible indicators of biological activity—should be enhanced after stellar flares. That could make these species easier to spot, although it may still be difficult; analysis of the group’s simulated peak signals for all three species is predicted to be complicated by instrument noise even on the cutting-edge James Webb Space Telescope set to launch in October 2021.

Although the researchers did not simulate them, they suggest that signatures of methane, dimethyl sulfide, and carbonyl sulfide—other potential signs of life—could be lessened by stellar flares on planets with reducing atmospheres.

This research “highlights the crucial need for us to understand the star in order to understand the planet,” exoplanet atmosphere expert Sarah Rugheimer of Oxford University says in an email. She also notes that the work relies on UV spectroscopy, which only the aging Hubble Space Telescope is currently capable of. “We will need to learn as much as we can while Hubble is still active, and consider prioritizing a UV mission in the future.”