Finding exoplanets by detecting magnetospheres

Scientists are developing new and more refined ways to find and characterize exoplanets, and it involves a familiar local phenomenon. Magnetospheres of distant planets may help us spot them, and could tell us a lot about their potential for habitability.

Matt Tilley, a University of Washington graduate student working on a doctoral degree in computational space plasma physics and astrobiology, gave a talk last week titled, “The Magnetospheres of Solar System Planets and Beyond.” The lecture was part of the Pacific Science Center’s PubSci series at the Hilliard’s Beer Taproom in Ballard.

Matt Tilley
Photo: Greg Scheiderer

Tilley explained that any planet that has a strong magnetic field will have a magnetosphere generated by the stellar wind from the star it orbits. Earth qualifies.

“The solar wind is actually an electrically charged gas that carries with it a magnetic field,” he said. “It’s an electrically charged magnetic wind blowing off of the Sun at a million miles an hour.”
The magnetosphere is essentially a bubble where the stellar wind is deflected around the planet.

“It literally is the force field for Earth, and it shields the Earth from being blasted by this electrically charged magnetic wind.”
Some recent research suggests that we may be able to spot the magnetospheres of exoplanets. To date we have found some 1,800 confirmed exoplanets, most of them by the Kepler mission which watched for slight dimming of stars which would occur as a distant planet transits the stellar disk. Usually the change in the light curve is pretty uniform, but in some cases it is not. Tilley noted that material from the stellar wind can accumulate in a bow shock at the magnetosphere, and this could be enough to show up in the Kepler data.

“If you have varying amounts of density of this electrically charged magnetic gas, this stellar wind, piled up against the bow shock, it will enter and start blocking some of the light before the planet ever enters the frame of the shot,” Tilley said.

There’s still debate about whether this is actually what is happening, but Tilley said it would be quite a useful discovery.

“It would be our first observation of a remote magnetic field,” he noted. “That tells us something about the composition, it tells us something about the mass, the rotation rate—we can infer multiple planetary characteristics from just the magnetic field, just from this distance, this one measurement of light.”

That data, plus the existence of the magnetic field, could tell us a lot about a planet’s potential habitability.

There’s another possible way to discover exoplanets because of magnetospheres. Tilley noted that the transit method only works for edge-on systems in which the transit of planets can be detected from our vantage point. It’s extremely difficult to spot exoplanets visually because they’re so dim in contrast to their host stars. However, Tilley said that the magnetosphere generates strong signals called auroral radio emissions that shoot out from the planet’s poles. Planets generate much stronger radio waves than do stars, and so for face-on systems looking for these radio waves may well be a way to detect exoplanets.

Tilley said it’s an exciting time to be working in the field.

“Astrobiology is really the study of the conditions on a planet, the stellar conditions and the planetary conditions that make the situation right for life to form and right for it to survive long enough to evolve into something interesting,” he said.