Interest in Earth-like planets orbiting within the habitable zone of their host stars has grown, fueled by the quest to discover life beyond our solar system. But the habitability of such planets, known as exoplanets, is affected by more than just their distance from the star.
A new study by David Alexander and Anthony Atkinson of Rice University expands the definition of a habitable zone for planets to include the magnetic field of their star. This factor, well-studied in our own solar system, could have important 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 shield it from stellar activity, and must orbit far enough from its star to avoid a direct and potentially catastrophic magnetic connection.
“The fascination with exoplanets stems from our desire to better understand our 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 the main drivers behind our study of these distant worlds.”
Magnetic interactions
Traditionally, scientists have focused on the “Goldilocks Zone,” the area around a star where conditions are right for liquid water to exist. By adding the star’s magnetic field to the criteria for habitability, Alexander’s team provides a more nuanced understanding of where life might thrive in the universe.
The investigation focused on the magnetic interactions between planets and their host stars, a concept known as space weather. On Earth, space weather is driven by the sun and affects our planet’s magnetic field and atmosphere. For the study, the researchers simplified the complex modeling typically required to understand these interactions.
The researchers characterized the stellar activity using a measure of a star’s activity known as the Rossby number (Ro): the ratio of the star’s rotation period to its convective circulation time. This helped them estimate the star’s Alfvén radius – the distance at which the stellar wind effectively detaches from the star.
Planets within this radius would not be viable candidates for habitability because they would magnetically re-attach to the star, leading to rapid erosion of their atmospheres.
Applying this approach, the team examined 1,546 exoplanets to determine whether their orbits lay inside or outside the Alfvén radius of their star.
Life Elsewhere in the Galaxy
The study found that only two planets, K2-3 d and Kepler-186 f, out of 1,546 examined met all the conditions for possible habitability. These planets are about the size of Earth, orbit at a distance favorable for the formation of liquid water, lie outside the Alfvén radius of their star, and have magnetic fields strong enough to shield them from stellar activity.
“While these conditions are necessary for a planet to support life, they do not guarantee it,” 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 when looking 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, the researchers provide a framework for future studies and observations to work toward determining whether we are alone in the universe.
Alison Farrish, a postdoctoral researcher at NASA Goddard Space Flight Center and former Rice graduate student, is also an author of this study, which was supported by the Edinburgh-Rice Strategic Collaboration fund.
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