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A planet’s chances of hosting life depend not only on its proximity to its parent star and the amount of radiation it receives. New research examines the impact of a star’s magnetic field on exoplanet habitability.
You may be familiar with the so-called “Goldilocks zone” around stars. This is the region around a star that is neither too hot nor too cold for a planet to host liquid water. However, despite the alternative name for this region, the “Habitable Zone,” being in this band does not guarantee that a planet will be suitable for life.
At least, that’s life as we know it.
For example, in our solar system, Venus, Earth, and Mars are all in the sun’s habitable zone, but only our planet currently has the right conditions for life (as far as we know). This has led scientists to investigate the conditions around other stars and their associated worlds.
This new study redefines the Goldilocks zone to take into account the star’s magnetic field. By adding such additional criteria, the team is providing a more detailed picture of life in the universe.
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“Our fascination with exoplanets stems from our desire to better understand our own planet,” team leader David Alexander, director of the Rice Space Institute and professor of physics and astronomy, said in a statement. “Questions about Earth’s formation and habitability are the primary drivers behind our study of these distant worlds.”
How do star magnets work?
The existence of a magnetic field around a planet is one of the key factors in its ability to host life. For example, we know that without Earth’s magnetosphere, the complex molecules necessary for life on our world would be torn apart by harsh radiation and high-energy charged particles from the sun in the solar wind.
Additionally, it is thought that the reason Mars is dry and barren today, despite having been flooded with liquid water in its distant past, is because of the lack of a planetary magnetic field. This allowed charged solar particles to strip its atmosphere over time. This caused the Red Planet to lose much of its liquid water to space, thus reducing its habitability.
There’s another way a planet’s magnetic field is important to its habitability, though, and that’s through its interaction with its star’s magnetic field. A planet’s magnetic field must be strong enough to protect it from bombardment by charged particles from its star, yes, but it must also be far enough away from that star’s magnetic field to avoid direct contact and prevent a powerful phenomenon called “magnetic reconnection.”
Alexander and his colleagues assessed the magnetic interactions between the exoplanets and their parent stars by accounting for “space weather,” the effect of stellar wind bombardment on planetary magnetic fields and atmospheres. To do this, they defined stellar activity using a value called the “Rossby number,” which is the ratio between a star’s rotation period and the time it takes for its layers to rise and sink due to a phenomenon called “convection,” or the star’s “convective transformation.”
Once that was done, the team could estimate another important value: the star’s “Alfvén radius,” which describes the point at which the stellar wind from that star breaks away from, or “separates” from, the star and its magnetic field. The researchers then evaluated 1,546 exoplanets to see if any worlds orbited their stars within the Alfvén radius of each stellar body.
The team found only two planets outside the Alfvén radius of their stars; these planets were far enough away to host liquid water and exhibited magnetic fields strong enough to withstand stellar wind bombardment.
The first was K2-3 d, a super-Earth 1.5 times the size and 2.2 times the mass of our planet, located 144 light-years from the solar system. The other was Kepler-186 f, slightly smaller than K2-3 d but still larger than Earth and 1.7 times the mass of our planet. This second planet is located 579 light-years from the solar system.
“While these conditions are necessary for a planet to host life, they do not guarantee it,” said Anthony Atkinson, lead author of the study and a graduate of Rice University. “Our work highlights the importance of considering a wide range of factors when searching for habitable planets.”
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Following these findings, scientists will continue to explore and study exoplanets and the planetary systems they reside in, while also applying their knowledge of our own solar system to these discoveries.
We hope this helps develop a framework that will help us answer the most fundamental question: Are we alone in the universe?
The team’s research was published July 9 in The Astrophysical Journal.