One of the reasons life flourishes on Earth is because of our magnetic field. Without it, Earth would be bombarded by harmful radiation. But what about the Earth-like exoplanets discovered by the Kepler mission?
That’s what astronomers at the University of Washington wanted to find out.
Before we dive into what the researchers found, let’s talk about these exoplanets for a second. The best way to spot an exoplanet is to first find a low-mass star. Then you study it for possible transits. A transit occurs when a planet moves in front of their host star. By seeing how much light is blocked and how long the transit lasts, you can determine the size, orbit and more of the planet. Why low-mass stars? They block a larger percentage of light than a planet orbiting a more massive star.
But a smaller star also means the habitable zone (the orbital area around a star where liquid water could exist) is closer to the host star. This also makes it more susceptible to the star’s gravitational pull. Some of these exoplanets may even be tidally locked, where the same side of the planet always faces the host star. This, in turn, would create tidal heating.
Lead author of the recently publsihed study, Peter Driscoll said, “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?”
Using models of orbital interactions and heating along with thermal evolution of planetary interiors, the researchers were able to “produce a more realistic picture of what is happening inside these planets,” according to Rory Barnes, an assistant professor of Astronomy.
Many astronomers believe tidally locked planets are a no go for life because they “are completely at the mercy of their star.” This new research says that assumption could be wrong. In fact, the models suggest tidal heating could help generate the magnetic field.
Here’s how the press release explains it:
The more tidal heating a planetary mantle experiences, the better it is at dissipating its heat, thereby cooling the core, which in turn helps create the magnetic field.
It’s this cooling that is vital for the formation of a magnetic field. In most of the model runs, magnetic fields were generated for the lifetime of these planets. “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,” said Barnes.
The researchers also found the tidal heating process is more drastic for planets in the habitable zone around smaller stars (stars less than half the mass of our sun).
Because small or low mass stars are more active early in their lives (first few billion years), these “magnetic fields can exist precisely when life needs them the most,” says Barnes.
Driscoll is quick to point out that these results are preliminary, and there is still much more research to be done. “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.”