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A planet’s chance of hosting life depends on more than just its proximity to its parent star, and the amount of radiation it receives. New research looks at how a star’s magnetic field affects exoplanet behavior.
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 that is neither too hot nor too cold for a liquid water planet to hosting. But, despite the alternative name for this region – “The Habitable Zone” – it does not guarantee that a planet will be suitable for life but is in this band.
Well, life as we know it, at least.
For example, in our solar system, Venus, Earth and Mars are all within the sun’s habitable zone, but our planet only currently has the right conditions for life (as far as we currently know). This encouraged scientists to investigate the conditions around other stars and their respective lives.
This new work redefines Goldilocks’ zone to also include the magnetic field of her star. By adding such additional criteria, the team provides 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?
“Interest in 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 the formation and habitability of the Earth are the main drivers behind our study of distant worlds.”
Stellar magnets, how do they work?
The presence of a magnetic field around a planet is one of the main factors in its ability to host life. For example, we know that without the Earth’s magnetosphere, complex molecules needed for life on our planet would be torn apart by intense radiation and high-energy charged particles drifting from the sun in the solar wind.
Furthermore, the reason why Mars is dry and arid today, despite having flowed with liquid water in the past, is thought to be due to the lack of a planetary magnetic field. This allowed charged solar particles to gradually remove its atmosphere. This forced the Red Planet to lose most of its liquid water into space, depleting its habitability.
However, another way in which the planet’s magnetic field is important to its behavior is through its interaction with the magnetic field of its star. A planet’s magnetic field must be strong enough to shield it from the smell of charged particles coming from its star, yes, but it must also be far enough from this star’s magnetic field to avoid direct contact and prevent a powerful event is called “magnetic reconnection” from happening.
Alexander and colleagues assessed magnetic interactions between exoplanets and their host stars, taking into account “space weather,” which is the effect of stellar wind bombardment on planetary magnetic fields and atmospheres. To do this, they defined a star’s activity using a value called the “Rossby number,” which is the ratio of a star’s rotational period to the time it takes for its layers to rise and fall due to a phenomenon called “convection,” or the star of “convective turnover.”
Once this is done, the team could estimate another important value: the “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 the Alfvén radius of each respective stellar body.
The team found only two planets outside the Alfvén radius of their star, which 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 that is 1.5 times as wide as our planet with 2.2 times its mass; it is 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.
“Although these conditions are necessary for a planet to host life, they do not guarantee it,” said research lead author Anthony Atkinson, a graduate of Rice University. “Our work shows the importance of considering a wide range of factors when searching for habitable planets.”
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Scientists will follow up on these findings by exploring and investigating exoplanets and the planetary systems they inhabit — while applying their knowledge of our own solar system to these discoveries.
It is hoped that this will help develop a framework that will ultimately help 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.
