Artwork depicting a planet and star interacting magnetically, creating an aurora on the star. Credit: Danielle Futselaar (artsource.nl)
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Artwork depicting a planet and star interacting magnetically, creating an aurora on the star. Credit: Danielle Futselaar (artsource.nl)

An Earth-sized planet may be igniting an aurora around a nearby red dwarf star

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Feb 19, 2020, 9:00 AM EST (Updated)

Over 4,100 exoplanets have been found, worlds orbiting other stars. A variety of methods has been used to discover them, including transits, radial velocity measurements, and even direct imaging (see the planets themselves in images).

A team of astronomers has just announced they may have found a planet orbiting the red dwarf star GJ 1151, but the technique is entirely novel: The planet is interacting with the star magnetically, creating an aurora not on the planet but on the star itself!

That is so cool.

An aurora on Earth is caused when subatomic particles streaming away from the Sun slam into our atmosphere, guided by the Earth's magnetic field. These particles slam into molecules in our atmosphere, knocking electrons off them. When the electrons recombine, the molecule gives off a characteristic glow, and we call that the aurora.

This happens on other planets too, but in different ways as well. For example, Jupiter has an aurora caused by its moon Io. The moon is volcanic, sending sulfur atoms up off the surface. As Jupiter spins, its strong magnetic field strips those atoms away, and they get trapped in the planet's magnetic field and fall down to Jupiter's poles. As they do, they spiral around the magnetic field lines, emitting radio waves (called synchrotron emission). The radio emission we see from Jupiter strengthens when Jupiter and Io align in a certain way with respect to us observing from Earth, so we see radio emission going up and down, with its strength tied to Io's orbit.

Not only that, but the magnetic field of the planet affects the way the light is emitted, polarizing it. This means the waves are to degree aligned, and that can be measured. In fact, this type of polarization is an excellent way to confirm that the light you see from a planet is actually from an aurora.

This light is very faint, though, so what the astronomers did was clever: Using the Gaia database, they cross-checked a radio wave all-sky survey recently done by the Low Frequency Array (or LOFAR) against the locations of stars known to be within 65 light years of the Sun. This radio aurora glow is relatively weak, so they constrained their efforts to nearby stars to maximize their chance of seeing it.

They found quite a few matches, most of which were red dwarf stars, dim bulbs that are faint and cool. The first viable target they got was the red dwarf GJ 1151, which is a mere 26 light years from Earth. It has a mass 1/6th the Sun's, and a diameter 1/5th of the Sun, so it's indeed pretty dinky.

Then things got interesting. The star was observed by LOFAR four times, and only once was it seen emitting radio waves out of those observations. Many red dwarfs are very magnetically active, blasting out stellar storms (like flares and such), but as it happens GJ 1151 is quiescent. In other words, it doesn't do that. Not only that, but the polarization of the radio waves is exactly what you'd expect from an aurora.

LOFAR observations of the star GJ 1151 show that sometimes it emits low-frequency radio waves (left) and sometimes it doesn’t (right). This may indicate auroral activity generated by an orbiting planet. Credit: Vedantham et al.

If this is the case, then the best fit to their data is a planet orbiting the star, interacting with the star's magnetic field. Perhaps the planet is volcanic, or has an atmosphere that's getting stripped by the star. But one way or another, atoms are "leaking" into space, following the star's magnetic field lines, and emitting polarized radio waves as they slam into the gas above the star's surface, creating an aurora there.

The reason it appears in one observation but not the other three makes sense if that's the case, too, because the planet and star have to be aligned just the right way with respect to Earth for us to see these waves. Again, if this is true, the planet would have to have an orbit of 1–5 days to match the observations.

As it happens, GJ 1511 is a target of planet hunters already, and (very) recently announced observations show that no planet is detected there of more than 5.6 times the mass of the Earth. All that means is that no gas giants orbit the star, but there's lots of wiggle room for smaller ones. And in fact we know that red dwarfs tend to make smaller planets more readily than big ones… and many such systems have their planets all huddled close to the star, so a 1–5 day orbit is no problem.

Hopefully, even more follow-up observations of this star will reveal the existence of this planet (or these planets). If confirmed, this would be a brand new way to find planets around nearby stars: by their aurorae.

Artwork depicting a solitary brown dwarf with an aurora. Credit: Chuck Carter, Caltech, NRAO/AUI/NSF

Incidentally, this method might work for brown dwarfs, too; objects intermediate in mass between planets and stars. In 2018 one was found that appeared to have an aurora, and while it's hard to know for sure it may be low mass enough to be considered a rogue planet, a planet without a star. A massive one for sure, more massive than Jupiter! But still, the auroral emission may indicate it has a moon orbiting it, and it's creating an aurora like Io does with Jupiter. So not only is this a good way to find exoplanets, it might even help us find exomoons.

In the early 1990s we didn't know if other planets even existed. Now we know of thousands, and there are more than a dozen methods used to find them. It's funny what you can do once you know it's actually possible.

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