A Star’s Auroras Light the Way to a New Exoplanet

In doing so, they found GJ 1151, a faint star with a shockingly long-lived emission. GJ 1151 belongs to a class of stars called M dwarfs, which are small, dim, and extremely common; they make up roughly 70% of all stars in the Milky Way. M dwarfs are often extremely magnetically active. Many spin rapidly, sometimes rotating all the way around in just a few hours. This rotation can generate flares.

But GJ 1151 is a quieter star, less prone to outbursts than its siblings. And the bright radio activity that Vedantham’s team observed lasted at least eight hours—the total extent of their observation time. Such an extended flare could not have come from inside the star itself.

Charged particles from Jupiter’s moons streak down the planet’s magnetic field lines, triggering auroras at its poles.Courtesy of NASA

The radio flare had another curious property. Its light appeared to be created by electrons that were moving in a circle. That’s not what’s expected for an ordinary solar flare. It would, however, make sense if the bursts were coming from a planet’s charged particles looping through the star’s magnetic field.

As a result, the team concluded that the source of the strong radio emissions is a hidden Earth-size planet. “I think this group has done an exceptionally good job of teasing out, by process of elimination, the best remaining scenario that could explain what they see—an orbiting planet,” said Gregg Hallinan, an astronomer at the California Institute of Technology who is not part of the research.

Not everyone is completely convinced, however. Evgenya Shkolnik, an astrophysicist who studies star-planet interactions at Arizona State University, points out that there aren’t a lot of studies of M dwarfs at the low frequencies mapped by LOFAR. “The reality is that we just don’t know what the stars are doing at these frequencies, on these timescales,” she said. “Yes, it makes it unlikely that it would be a generic flare, but it doesn’t mean it couldn’t be some giant super-flare that is really rare.”

There are a few possible ways to confirm that the flare comes from an exoplanet. Researchers could continue to monitor GJ 1151’s radio waves. If they find three or four more bursts that happen on a regular schedule—perhaps a burst for each revolution of the planet—that would be “the gold standard,” said Hallinan.

Or they could use one of the established planet-hunting methods, though each has limitations. The radial velocity method watches for the gravitational tug of a planet on its host star, but this technique works best for massive Jupiter-size planets. Alternatively, the transit method watches for a dip in the light of the star that occurs when a planet passes between the star and Earth. In this case, the planet and star have to be directly aligned with our line of sight, and estimates suggest that fewer than 1% of planets are so perfectly oriented.

Harish Vedantham, an astronomer at the Netherlands Institute for Radio Astronomy, led a team that used a star’s radio emissions to discover an exoplanet.Photograph: Astron

So far, confirmation from these complementary techniques has proved elusive. In a related paper, published yesterday in The Astrophysical Journal Letters, Vedantham and colleagues report that they couldn’t find any planet around GJ 1151 using the radial velocity method with a dedicated planet-hunting instrument in the Canary Islands. The result implies that any such planet has to be smaller than five Earth masses.

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