The effects of stellar magnetism could broaden the criteria for exoplanet habitability

Interest in Earth-like planets orbiting in the habitable zone of their host stars has soared, fueled by the quest to discover life outside our solar system. However, the habitability of such planets, known as exoplanets, is not only affected by their distance from the star.

A new study by David Alexander and Anthony Atkinson of Rice University expands the definition of the habitable zone for planets to include the magnetic field of their star. This factor, well-studied in our solar system, may have significant implications for life on other planets, according to research published in The Astrophysical Journal on July 9.

The presence and strength of a planet’s magnetic field and its interaction with the host star’s magnetic field are key factors in a planet’s ability to support life. An exoplanet needs a strong magnetic field to protect it from stellar activity, and it must orbit far enough from its star to avoid direct and potentially catastrophic magnetic coupling.

“The fascination with exoplanets stems from our desire to better understand our own planet,” said Alexander, professor of physics and astronomy, director of the Rice Space Institute and member of the Texas Aerospace Research and Space Economy Consortium. “Questions about the formation and habitability of Earth are key drivers of our study of these distant worlds.”

Magnetic interactions

Traditionally, scientists have focused on the “Goldilocks Zone”, the region around the star where the conditions are just right for liquid water to exist. By adding a star’s magnetic field to the criteria for habitability, Alexander’s team offers a more nuanced understanding of where life might develop in the universe.

The research focused on the magnetic interactions between planets and their host stars, a concept known as space weather. On Earth, space weather is controlled by the sun and affects our planet’s magnetic field and atmosphere. For the study, the researchers simplified the complex modeling usually required to understand these interactions.

The researchers characterized the stars’ activity using a measure of the star’s activity known as the Rossby number (Ro): the ratio of the star’s rotation period to its convective turnover time. This helped them estimate the star’s Alfvén radius – the distance at which the stellar wind effectively separates from the star.

Planets within this radius would not be viable candidates for habitability because they would be magnetically coupled back to the star, leading to rapid erosion of their atmospheres.

Using this approach, the team examined 1,546 exoplanets to determine whether their orbits lie inside or outside the Alfvén radius of their star.

Life elsewhere in the galaxy

The study found that only two planets, K2-3 and Kepler-186 f, out of 1,546 examined met all the conditions for potential habitability. These planets are Earth-sized, orbit at a distance conducive to the formation of liquid water, lie outside the radius of their Alfvén star, and have magnetic fields strong enough to shield them from stellar activity.

“While these conditions are necessary for a planet to host life, they do not guarantee that” said Atkinson, a graduate student in physics and astronomy and lead author of the study. “Our work highlights the importance of considering a wide range of factors in the search for habitable planets.”

The study also highlights the need for continued exploration and observation of exoplanetary systems, drawing lessons from the Sun-Earth system. By expanding the criteria for habitability, scientists provide a framework for future studies and observations to determine whether we are alone in space.

Alison Farrish, a postdoctoral fellow at NASA’s Goddard Space Flight Center and a former graduate student of Rice’s, is also an author on the study.