The chances of a planet hosting life depends on more than just its proximity to its parent star, and the amount of radiation it receives. New research looks at the impact a star's magnetic field has on exoplanet habitability.
You may be familiar with the so-called "Goldilocks zone" around stars, which gets its name from the fact that it is the region around a star where it's neither too hot nor too cold for a planet to host liquid water. Yet, despite the alternate name for this region — "The Habitable Zone" — merely existing in this band still doesn't guarantee that a planet will be suitable for life.
Well, life as we know it, at least.
For instance, in our solar system, Venus, Earth and Mars are all within the sun's habitable zone, yet only our planet currently has the right conditions for life (as far as we currently know). That has prompted scientists to investigate the conditions around other stars and their respective worlds.
This new work redefines the Goldilocks zone to also factor in the magnetic field of its star. By adding such extra criteria, the team offers a more nuanced picture of life in the universe.
Related: 25 years of exoplanet hunting hasn't revealed Earth 2.0 — but is that what we're looking for?
"The fascination with exoplanets stems from our desire to understand our own planet better," team leader David Alexander, director of the Rice Space Institute and a professor of physics and astronomy, said in a statement. "Questions about the Earth's formation and habitability are the key drivers behind our study of these distant worlds."
Stellar magnets, how do they work?
The presence 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, complex molecules needed for life on our world would be broken apart due to harsh radiation and high-energy charged particles flowing from the sun in the solar wind.
Additionally, it is thought that the reason Mars is dry and arid today, despite having flowed with liquid water in its distant past, is because of the lack of a planetary magnetic field. This allowed charged solar particles to strip away its atmosphere over time. That forced the Red Planet to lose most of its liquid water to space, thus depleting its habitability.
Yet, there is another way in which a planet's magnetic field is important to its habitability, and that is through its interaction with the magnetic field of its star. The magnetic field of a planet must be strong enough to shield it from the bombardment of charged particles coming from its star, yes, but it must also be far enough away from this stellar magnetic field to avoid direct contact and prevent a powerful event called "magnetic reconnection" from occurring.
Alexander and colleagues assessed magnetic interactions between exoplanets and their host stars, factoring in "space weather," which is 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 rotational period to the time it takes for its layers to rise and sink due to a phenomenon called "convection," or the star's "convective turnover."
Once this was done, the team could estimate another important value: the star's "Alfvén radius," which defines the point at which the stellar wind from that star becomes disconnected or "decoupled" from the star and its magnetic field. The researchers then assessed 1,546 exoplanets to see if the worlds orbited their stars within each respective stellar body's Alfvén radius.
The team found just two planets outside their stars' Alfvén radius, which were thus distant enough to host liquid water and exhibited strong enough magnetic fields to withstand stellar wind bombardment.
The first was K2-3 d, a super-Earth that's 1.5 times as wide as our planet with 2.2 times its mass; it's located 144 light-years from the solar system. The other was Kepler-186 f, which is slightly smaller than K2-3 d but still larger than Earth with a mass 1.7 times that 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 research lead author Anthony Atkinson, a Rice University graduate. "Our work highlights the importance of considering a wide range of factors when searching for habitable planets."
The scientists will follow up on these findings by continuing to explore and investigate exoplanets and the planetary systems in which they dwell — while simultaneously applying their knowledge of our own solar system to these discoveries.
The hope is that this will help develop a framework that finally helps us answer the most fundamental question: Are we alone in the cosmos?
The team's research was published on July 9 in The Astrophysical Journal.
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