Artist's impression of an exoplanet-induced stellar aurora. (Danielle Futselaar/artsource.nl)

Mysterious Radio Signals From Distant Stars Suggest The Presence of Hidden Planets

11 OCTOBER 2021

Astronomers have found a collection of stars doing something unexpected.

Within 160 light-years of the Solar System, four red dwarf stars that should be quiet in radio observations have been caught emitting radio signals. According to an analysis of these signals, the best explanation for this activity is the presence of unseen exoplanets.

 

It's not, to be clear, a technosignature hinting at an alien civilization; rather, it seems to be the result of an interaction between the exoplanet and the star's magnetic field, generating intensely strong auroras that can be detected using the Low Frequency Array (LOFAR) – a powerful radio telescope headquartered in the Netherlands.

Following the report of a similar discovery announced last year, the research suggests a new way for hunting exoplanets in our solar neighborhood.

"We've discovered signals from 19 distant red dwarf stars, four of which are best explained by the existence of planets orbiting them," said physicist Benjamin Pope of the University of Queensland in Australia.

"We've long known that the planets of our own Solar System emit powerful radio waves as their magnetic fields interact with the solar wind, but radio signals from planets outside our Solar System had yet to be picked up. This discovery is an important step for radio astronomy and could potentially lead to the discovery of planets throughout the galaxy."

The inspiration for the search came from our very own Solar System. Here, interactions between gas giant Jupiter and its moon Io result in powerful, permanent auroras at the Jovian poles, loud in the radio spectrum.

 

They're not dissimilar to Earth's auroras, but they are made differently. Here on Earth, auroras are created by particles blowing in from the Sun. When charged particles like protons and electrons collide with Earth's magnetosphere, they're sent whizzing along the magnetic field lines towards the poles, where they rain down on Earth's upper atmosphere and collide with atmospheric molecules. The resulting ionization of these molecules results in auroras.

On Jupiter, the auroras are not only created by solar particles, but particles from the moon Io, the most volcanic world in the Solar System. It's constantly belching out sulfur dioxide, which is immediately stripped via a complex gravitational interaction with the planet, becoming ionized and forming a plasma torus around Jupiter, which constantly feeds the auroras via magnetic field lines.

The Sun's magnetic field isn't strong enough, and the distances are too great, to produce a similar effect from its interaction with the planets in the Solar System, but red dwarfs are different. These very long-lived, small, dim stars have much more powerful magnetic fields than the Sun's, and the exoplanets we've found orbiting them can be much closer than anything in the Solar System.

 

It was expected that a red dwarf star's close-orbiting planet might produce a similar but more powerful emission than that produced by Jupiter and Io, resulting in auroras at the star's poles. The first red dwarf radio emission consistent with this type of interaction was found in a red dwarf star last year. Now scientists have cast a larger net, resulting in three new stars.

"Our model for this radio emission from our stars is a scaled-up version of Jupiter and Io, with a planet enveloped in the magnetic field of a star, feeding material into vast currents that similarly power bright [auroras]," said astronomer Joseph Callingham of the Netherlands Institute for Radio Astronomy (ASTRON), who led the research.

"It's a spectacle that has attracted our attention from light-years away."

The two main current methods for detecting exoplanets work best on large, massive ones. Astronomers look for dips in the star's light as the exoplanet passes between us and the star, called the transit method, or they look for signs that the star is wobbling on the spot, a clue that it is orbiting a mutual center of gravity with an exoplanet, called the radial velocity method. Both these effects are a lot bigger if the exoplanet is huge.

The team hasn't found any signs of the exoplanets hinted at by the new method, apart from the radio emissions, but if the exoplanets are there, future observations using the radial velocity method could help reveal them. And, as more powerful radio telescopes come online in the future, who knows what we'll find.

"We can't be 100 percent sure that the four stars we think have planets are indeed planet hosts, but we can say that a planet-star interaction is the best explanation for what we're seeing," Pope said.

"Follow-up observations have ruled out planets more massive than Earth, but there's nothing to say that a smaller planet wouldn't do this."

The research has been published in Nature.