There’s an interesting problem in exoplanet studies: how to tell if a planet has a magnetosphere. It’s not like we can visibly see it unless we find a different way of looking. A pair of scientists may have found one. They used radio telescopes to track emissions given off by magnetic star-planet interactions. These happen when a planet with a magnetic field plows through star stuff caught its star’s magnetic field.
Sebastian Pineda (the University of Colorado Laboratory for Atmospheric and Space Physics) and Jackie Villadsen (Bucknell University) used the Karl G. Jansky Very Large Array to search for those emissions at a star called YZ Ceti and its planet, YZ Ceti b. Over the course of several observation runs, they found a repeating radio signal from the star. It was a first.
“I’m seeing this thing that no one has seen happen before,” said Villadsen, describing the moment she first isolated the radio signal while pouring over data at her home on a weekend.
“We saw the initial burst and it looked beautiful,” said Pineda. “When we saw it again, it was very indicative that, OK, maybe we really have something here.”
That something looks like proof of a magnetosphere (the region around a planet dominated by the planet’s magnetic field) around YZ Ceti b. The search method could give astronomers another way to confirm the existence of exoplanetary magnetic fields. And, finding one at this planet has interesting implications in the search for habitable worlds.
Radio Waves Give Away the Existence of a Planetary Magnetosphere
YZ Ceti is a small red dwarf star with a known rocky exoplanet orbiting around it in a two-day orbit. It’s a good target to determine if a planetary magnetic field collides with its star’s magnetic field to provide outbursts. That’s because the exoplanet is very close to the star and can easily encounter material given off during a flare or other outburst. As plasma from YZ Ceti bounces off the planet’s magnetosphere it interacts with the star’s magnetic field. That action generates radio waves strong enough to be observed on Earth. The researchers then use the intensity of the radio waves to figure out the strength of the planetary magnetic field.
If the method works out well, it will provide a way to detect magnetic fields at other stars, according to Villadsen. “What we’re doing is looking for a way to see them,” she said. “We’re looking for planets that are really close to their stars and are a similar size to Earth. These planets are way too close to their stars to be somewhere you could live, but because they are so close the planet is kind of plowing through a bunch of stuff coming off the star. If the planet has a magnetic field and it plows through enough star stuff, it will cause the star to emit bright radio waves.”
The study also provides a look at the environment around stars and how they interact with planetary magnetic fields. “This idea is what we’re calling “extrasolar space weather'” said Pineda. “We have been doing this radio monitoring of stellar systems like these in the hope of gathering evidence of the magnetic interaction of the star and planet.”
Extrasolar Space Weather and Magnetospheres
Using extrasolar space weather to look for magnetospheres around exoplanets is quite new, said Pineda in a follow-up email. “That interaction hasn’t really been demonstrated in the literature before, so discovering it would also be significant. However, the reason to pursue this sort of physics is precisely because it is a unique observable that has a clear dependence on the exoplanetary magnetic field. If we can measure these bursts, prove that they are planet-induced, and know the star sufficiently well, then we have real concrete constraints on an exoplanet’s magnetic field.”
Interestingly, during the interactions between YZ Ceti b and its star, aurorae get produced. But, as Pineda points out, they’re not planetary aurorae as we see on Earth. “We’re actually seeing the aurora on the star — that’s what this radio emission is,” he said. “There should also be aurora on the planet if it has its own atmosphere.”
This discovery will need to be confirmed by a great many other observations in the coming years, according to Pineda and Villadsen. “This could really plausibly be it,” says Villadsen. “But I think it’s going to be a lot of follow-up work before a really strong confirmation of radio waves caused by a planet comes out.”
Why is a Magnetosphere Important?
On Earth, our magnetic field protects us from the solar wind and some aspects of flares and coronal mass ejections. Of course, we are affected by some space weather. We see auroral displays during milder events. If a solar storm is particularly strong, our magnetosphere shields us from the worst effects. But, even then, space weather can still pose a threat to our satellites, power grids, and other technology.
However, we can thank our magnetosphere for our very existence. It’s very likely that life on Earth wouldn’t be here if our planet didn’t have its protective magnetic field. So, in the hunt for habitable worlds in the galaxy, the presence of a magnetosphere could be a good sign. It not only tells scientists something about a world’s physical characteristics, but it might be a clue to that world’s habitability.
If a distant world doesn’t have a magnetosphere, it’s probably not hospitable to life. If it does have one, then it—and any life it harbors—would have an umbrella of protection from outbursts by its star. That adds a new dimension to the search for exoplanets, particularly those that might be harboring life.
For More Information
Do Earth-like exoplanets have magnetic fields? Far-off radio signal is promising sign
Coherent radio bursts from known M-dwarf planet-host YZ Ceti
It is not a clear mechanism, though they do match the burst flux density with a model where the planet magnetic field cuts the stars radial wind flux. And the two events has some promising features:
“Two coherent bursts occur
at similar orbital phases of YZ Ceti b, suggestive of an enhanced probability
of bursts near that orbital phase. We model the system’s magnetospheric
environment in the context of sub-Alfvénic SPIs and determine that
YZ Ceti b can plausibly power the observed fux densities of the radio
detections. However, we cannot rule out stellar magnetic activity without
a well-characterized rate of non-planet-induced coherent radio bursts on
slow rotators.”
Re the nature and amount of atmospheric escape and the effects of a magnetic field, it is a generally open question.
“A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) escape, and impact erosion. The relative importance of each loss process depends on the planet’s escape velocity, its atmosphere composition, and its distance from its star. ” [Wikipedia]
E.g. apart from hydrogen and helium, Earth loses scant atmosphere even at the magnetically open pole areas where wind erosion is concentrated. And Venus, having no geodynamo, retains a dense atmosphere still.