My book comes out tomorrow!, Exoplanet magnetism

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Kinda.

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Astronomy News

It’s a big Universe. Here’s a thing about it.

Do rocky exoplanets have magnetic fields?

That’s a tough question to answer. We know Earth has a decently strong field, because we happen to live on that particular planet. Earth’s magnetic field a lot like that of a bar magnet, with two poles (what we call the north and south poles, which unfortunately are confused a lot with its spin axis poles), and generated deep inside the Earth in its outer, liquid iron core.

Mars doesn’t have a global field, though it does have pockets of magnetism here and there. Venus, despite being a lot like Earth, doesn’t have a global field either (it does have a weak field, but it’s induced by the Sun’s magnetism as it interacts with the upper atmosphere of the planet). Mercury does have a global magnetic field but it’s only 1% as strong as Earth’s.

So, judging from our local neighborhood, 1 out of 4 terrestrial (that is, small and rocky) planets have a strong magnetic field. But what of planets orbiting other stars?

We can’t measure them directly; it’s not like we can jump on board the Titan and hold a compass after beaming down. But that doesn’t mean we’re out of options.

If a planet has a strong magnetic field, and it orbits close-in to its star, then the planet’s field can interact with the ionized gas in the star, exciting it (that is, pumping energy into it), and creating a “hotspot” on the star’s surface (we see something very similar happening with Jupiter and some of its moons, in fact — basically generating an aurora). The star’s own magnetic field interacts with this plasma as well.

The physics is fairly complicated (it always is with magnetism; the equations for it are a nightmare, literally for grad students trying to take a class in it — ask me how I know) but in the end this process can be a source of powerful radio waves.

And not just any radio waves, but coherent ones. That doesn’t mean they’re from aliens or anything. It just means that the waves are, in a sense, aligned with each other in an orderly way. It’s kinda like the difference between a crowd of people milling about and a marching band all moving in step.

That sort of radio emission is not common in stars, so if we see it that’s a possible sign of a planet’s magnetic field interacting with that star.

And it’s now been seen [here’s a link to the science journal paper]. YZ Ceti is a dinky red dwarf a mere 12 light-years from Earth. It’s known to have three small worlds orbiting it, and maybe a fourth. The innermost one, YZ Ceti b, is so close to the star it orbits once every 2 days — it’s so close it gets cooked by the star, so it’s unlikely to have any life on it.

But it may have a magnetic field. Astronomers detected two bursts of coherent waves from the star, and moreover, they occurred when the planet was in the same relative position in its orbit. That’s important: As the planet moves around the star its magnetic field moves too, so that radio wave “hotspot” moves around the star with it. We see a burst of coherent radio waves when the alignment is just right and the beam of energy is aimed at us.

This isn’t proof that the innermost planet has a strong magnetic field, but it’s promising. The star itself has a strong field, and so that cannot be ruled out as the source itself, however. That makes this planet a candidate for having a magnetic field, and one worthy of followup.

I’ll note that other red dwarf stars have been seen to have similar activity, but (at least at the time of this research) aren’t known to have small, rocky planets. Gas giant planets orbiting other stars, and some brown dwarfs, have also been seen to have strong fields, too. But no specific small exoplanet has been seen to have one.

Yet. As we learn more about planets around other stars there will be other ways, other methods dreamed up to investigate them. I’m amazed at how much we have already learned, especially given that the first exoplanets were only discovered in the 1990s, and I’m very excited to see what’ll come up next.

Et alia

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