Space weather can dramatically alter a planet’s fate

We tend to think of habitability in terms of individual planets and their potential to host life. But barring outliers like rogue planets with internal heating or icy moons with subsurface oceans created by tidal heating, it’s exoplanet/star relationships that generate habitability, not individual planets. New research emphasizes that fact.

Red dwarfs are known for their powerful stellar flaring, which could render nearby planets uninhabitable. However, even relatively quiescent stars like our sun create space weather. Solar flares, stellar wind, and coronal mass ejections have different effects on different types of planets. Earth is largely protected from these effects by its magnetosphere.

However, over long periods, space weather can have powerful effects on how an exoplanet’s atmosphere develops. New research to be published by the American Astronomical Society outlines these effects on the climates of tidally locked exoplanets. It’s titled “Effects of transient stellar emissions on planetary climates of tidally-locked exo-Earths,” and the lead author is Howard Chen from the Department of Aerospace, Physics, and Space Sciences at the Florida Institute of Technology. It is currently available on the arXiv preprint server.

“Space weather events in planetary environments sourced from transient host star emissions, including stellar flares, coronal mass ejections, and stellar proton events, can substantially influence an exoplanet’s climate and atmospheric evolution history,” the authors write. “These time-dependent events may also affect our ability to measure and interpret its properties by modulating reservoirs of key chemical compounds and changing the atmosphere’s brightness temperature.”

The photochemistry of exoplanet atmospheres is a well-researched topic, but this work stands apart from most previous research. It utilizes 3D general circulation models, while most prior work relies on single-column models. Single-column models focus on verticality and how moisture, energy, and momentum in columns affect an atmosphere in a single location. 3D models do a better job of simulating an entire atmosphere and include both vertical and horizontal effects. They incorporate large-scale effects like jet streams that single-column models don’t.

This work focuses on stellar flares and the energetic particles they shower exoplanets with. The authors explain, “We examine their effects on synchronously rotating TRAPPIST-1e-like planets on a range of spatiotemporal scales.” TRAPPIST-1e is a well-known and often studied rocky exoplanet in the habitable zone of TRAPPIST-1, an ultracool red dwarf star.

Data from NASA’s Kepler mission shows that stellar flare energy and amplitude don’t vary much between F, G, and K-type stars. However, the frequency of flaring events and their spectral distribution vary a lot depending on the type of star. Flares with the same energy can have different spectral distributions, meaning that some can emit relatively harmless optical light while others can emit X-rays and UV. Spectral distribution is related to underlying processes in the star, like activity in the magnetosphere and chromosphere.

Stars like TRAPPIST-1 are known to have high levels of magnetosphere and chromosphere for billions of years, which can generate superflares. “This can affect atmospheric environments of close-in exoplanets on long timescales, inducing water loss via photolysis and hydrogen escape,” the authors write.

Flares generate energetic particles that can cause sudden cooling in an exoplanet’s thermosphere through the radiative cooling of nitric oxide (NO) and carbon dioxide. These molecules are excited by energetic particles from stellar flares and release that energy in the infrared, which has a net cooling effect.

In the middle and lower atmosphere, molecules like water and nitrous oxide (N20) absorb infrared energy and have a warming effect. Intense stellar flares can have a powerful warming effect in the middle atmosphere, generating 40 meters per second (144 km/h) winds on the star’s surface near the terminator line.

The research shows that not only do stellar flares change the photochemistry of atmospheres, but they can also change the circulation patterns. This can spread molecular species and heat around the planet in ways that aren’t possible without flaring.

“Our 3D modeling and analysis techniques reveal new insights on how planets around active and flaring stars could experience enhanced climate anomalies,” the authors write in their conclusion. “Our results suggest that, in addition to initiating key photochemical reaction pathways and inducing photochemical disequilibrium, large stellar events could affect atmospheric dynamics and even alter the planets’ circulation regime in the most extreme scenarios.”

While much research shows that X-ray and EUV radiation from stars can have dramatic photochemical effects, not all exoplanets are exposed to them. For exoplanets that aren’t, this research shows that “… transient stellar emissions may be the dominant channel through which the dynamics of substellar atmospheres are driven.”

The study shows that the most susceptible exoplanets are those orbiting younger stars with repeated stellar flares. “Our results suggest that successive, more energetic eruptive events from younger stars may be a pivotal factor in determining the atmosphere dynamics of their planets,” the researchers explain in their paper.

“Thus, planet atmospheres around stars with sporadic, rather than consecutive, eruptive events (which permits more time between each event for chemical species such as ozone to recover photochemically) would likely experience the greatest degrees of variability. These candidates will likely be situated around moderately active stars,” the authors conclude.

The effects outlined in the paper could have profound consequences for terrestrial planets over the long term by altering the thermal evolution of their atmospheres. The authors explain that their results have implications for upcoming missions that will directly image exoplanets because these missions may be able to probe the “astrophysically-influenced weather systems on habitable zone planets.” These missions include the proposed Habitable Worlds Observatory and the Large Interferometer For Exoplanets.