Observing magnetic star-planet interactions offers promise for determining the magnetic fields of exoplanets. Models of star-planet interactions predict that rocky exoplanets in close-in orbits around red dwarf stars can induce detectable stellar radio emission, manifesting as bursts of strongly polarized coherent radiation observable at specific planet orbital positions. Using the Karl G. Jansky Very Large Array, astronomers detected coherent radio bursts from the slowly-rotating red dwarf YZ Ceti, which hosts a compact system of terrestrial planets, the innermost of which — called YZ Ceti b — orbits with a two-day period.
An artist’s conceptual rendering of interactions between a prospective exoplanet and its star. Image credit: NSF / Alice Kitterman.
YZ Ceti is a red dwarf located just 12 light-years away in the constellation of Cetus.
Also known as Gliese 54.1, IRAS F01100-1716, LTT 670 or TIC 439403362, the star is about 13% the mass of the Sun and 17% of its radius.
YZ Ceti and one of its three known planets, YZ Ceti b, provide an ideal pair because the planet is so close to the star that it completes a full orbit in only two days.
As plasma from YZ Ceti careens off the planet’s magnetic ‘plow,’ it then interacts with the magnetic field of the star itself, which generates radio waves strong enough to be observed on Earth.
The strength of those radio waves can then be measured, allowing astronomers to determine how strong the magnetic field of the planet might be.
“We saw the initial burst and it looked beautiful,” said Dr. Sebastian Pineda, an astronomer in the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder.
“When we saw it again, it was very indicative that, OK, maybe we really have something here.”
Dr. Pineda and his colleague, Dr. Jackie Villadsen from Bucknell University and Vassar College, theorize that the stellar radio waves they detected are generated by the interactions between the magnetic field of the exoplanet and the star it orbits.
However, for such radio waves to be detectable over long distances, they must be very strong.
While magnetic fields have previously been detected on massive Jupiter-size exoplanets, doing so for a comparatively tiny Earth-sized exoplanet requires a different technique.
“Because magnetic fields are invisible, it’s challenging to determine if a distant planet actually has one,” Dr. Villadsen said.
“What we’re doing is looking for a way to see them. 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 interactions between YZ Ceti b and its star also produce an aurora, but with a significant difference: the aurora is on the star itself.
“We’re actually seeing the aurora on the star — that’s what this radio emission is,” Dr. Pineda said.
“There should also be aurora on the planet if it has its own atmosphere.”
Both researchers agree that while YZ Ceti b is the best candidate yet for a rocky exoplanet with a magnetic field, it’s not a closed case.
“This could really plausibly be it. 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,” Dr. Villadsen said.
“There are a lot of new radio facilities coming online and planned for the future,” Dr. Pineda said.
“Once we show that this is really happening, we’ll be able to do it more systematically. We’re at the beginning of it.”
A paper on the findings appears in the journal Nature Astronomy.
_____
J.S. Pineda & J. Villadsen. Coherent radio bursts from known M-dwarf planet-host YZ Ceti. Nat Astron, published online April 3, 2023; doi: 10.1038/s41550-023-01914-0
