Earth-like exoplanets near smaller stars are likely to have magnetic fields which protect their environments from radiation. This magnetic protection could allow for conditions to exist on those planets to make them habitable.
According to Phys.Org, a group of astronomers from the University of Washington combined models of orbital interactions between planets and small stars with planetary heating patterns to study the likelihood of life-supporting conditions existing on close-orbit planets with small star hosts.
A planet’s magnetic field serves as a shield against solar radiation and helps keep the planet’s atmosphere intact. As the magnetic field protects a planet’s surface, it also protects any life forms on the planet from excessive radiation exposure.
It is believed that magnetic fields get their start when a planet’s interior cools.
Stars of a low mass are common in the universe, and planets orbiting those stars are easier to detect and study than planets of large stars, since planets in transit of smaller stars block out a larger percentage of the star’s light and are more visible to Earth observers.
Smaller stars also offer a tighter orbit for planets to exist in the habitable zone, or the distance from a star at which liquid water could exist.
The closer orbital distance also involves a greater gravitational pull on the planet, which could cause such planets to become tidally locked with their host star, with one side always facing the star as the planet orbits. Tidal locking creates tidal heating in the planet’s interior, which drives volcanic activity on bodies such as Jupiter’s moon Io.
“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?” lead author Peter Driscoll said.
The team also looked at whether tidal heating would affect a planet’s magnetic field over time. There had been a general thought that tidally locked planets would lose their magnetic fields, but the new research showed that not to be the case.
In fact, the team found that tidal heating can actually increase the effectiveness of a planet’s magnetic field. When a planet’s mantle is heated, it becomes better at dissipating heat, thereby ultimately cooling the core.
“I was excited to see that tidal heating can actually save a planet in the sense that it allows cooling of the core. That’s the dominant way to form magnetic fields,” model creator Rory Barnes said.
The team also found that planets that begin with noncircular orbits and tidal heating eventually settle into circular orbits with no tidal heating.
“These preliminary results are promising, but we still don’t know how they would change for a planet like Venus, where slow planetary cooling is already hindering magnetic field generation,” Driscoll said. “In the future, exoplanetary magnetic fields could be observable, so we expect there to be a growing interest in this field going forward.”
The findings were published in the journal Astrobiology.