Distant planets in orbit around smaller stars probably possess magnetic fields from which they are shielded from radiation that is stellar, increasing chances of life evolving on them as it has on Earth, researchers say.
Smaller low-mass stars known as M-class stars are a common type throughout the universe, and exoplanets orbiting those stars are simpler to detect and study, astronomers say.
That's because as they transit their star — that is, pass in front of it from Earth's viewpoint — they block a larger amount of the star's light than they would passing in front of a much larger star, they explain.
However, the habitable zone for smaller, dimmer stars — the region in which an orbiting planet would receive the amount of heat that is necessary to keep life-friendly moisture in a liquid state on the surface — lies fairly close in.
That raises a problem, possibly; any planet that close to the star could risk becoming tidally locked by the star's great gravitational attraction, so that one side always faces the star — in the same way as our moon is gravitationally locked to the Earth.
Those same gravitational tidal forces would also heat the interior of the planet, which led to questions about the effect on such planets over time, explains Peter Driscoll, a geophysicist at the Carnegie Institution for Science in Washington, D.C.
"The question I wanted to ask is, around these small stars, where people are going to look for planets, are these planets going to be roasted by gravitational tides?" says Driscoll, a co-author of a study appearing in Astrobiology.
His collaborator, Rory Barnes, an astronomy professor at the University of Washington, says many astronomers have assumed planets that are tidally locked would probably not possess magnetic fields and as a result are "completely at the mercy of their star."
However, he says, computer simulations he and Driscoll performed suggest that far from being damaging to a planet's magnetic field, tidal heating might actually give it a boost — and thus improve the habitability prospects for a planet.
The more tidal heating that a planetary mantle is subject to, the better it becomes at dissipating its heat, he says.
This somewhat counterintuitive effect can help cool the planet's core, which in turn helps create a magnetic field, he explains.
"I was excited to see that tidal heating can actually save a planet in the sense that it allows cooling of the core," he says. "That's the dominant way to form magnetic fields."
The first few billion years or so in the life of small, low-mass stars is when they are at their most active, so their gravitational effects on orbiting planets may mean "magnetic fields can exist precisely when life needs them the most," Barnes says.
