Illustration of a big fiery red sun with planets and moons in the foreground.

Stellar signals point to hidden planets

Australian and Dutch researchers have unexpectedly found radio waves blasting out from far-flung red dwarf stars. These emissions might indicate unseen planets – potentially marking a new way of discovering exoplanets.

“We’ve discovered signals from 19 distant red dwarf stars, four of which are best explained by the existence of planets orbiting them,” says Joseph Callingham, lead researcher from Leiden University and Dutch national observatory ASTRON.

These weird emissions were spotted by an extremely sensitive radio telescope, the Low Frequency Array (LOFAR) in the Netherlands.

Artist impression of star-exoplanet interaction. Credit: ASTRON/Danielle Futselaar

The findings were published in a paper in Nature Astronomy. Then, in a second paper in the Astrophysical Journal Letters, the team used data from NASA’s Transiting Exoplanet Survey Satellite (TESS) to confirm that the radio emissions weren’t generated by an unknown process in the stars – instead, they were likely generated by the interaction of the stars with hidden planets.

“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,” says Benjamin Pope from the University of Queensland, who was lead author on the second paper.

This process – of magnetic fields interacting with solar wind – drives the aurora seen at the poles of Earth, Saturn, Jupiter and other planets. Fast-moving charged particles from the Sun are drawn down to the poles by a planet’s magnetic field and slam into gas particles in the atmosphere, producing a glow of both optical light and extremely bright radio waves.

“If I could turn your eyes into radio receivers, you’d be able to see that the aurora on Jupiter even outshine the Sun at 40 MHz,” Callingham says.

Now, LOFAR has picked up these signals from planets in other star systems. Callingham says that the team is confident that the radio signals are a result of planets.

“It is very hard for this type of emission to come from a star,” he says. “This is because the emission is highly circularly polarised. While it is possible for some of the more active stars to have crazy coronal processes we do not understand, we have four stars in our sample that are probably the most boring stars you can imagine – very slow rotators, and not active at all.

“For these stars, it only makes sense if the emission is being driven by the presence of an exoplanet.”

The emissions would be created by interactions similar to those seen between Jupiter and Io, its volcanic moon. Io is blasting material out into space, feeding Jupiter particles that drive aurora more powerful than those seen on Earth.

The team used the Jupiter-Io system to understand their distant observations, Callingham says.

“Our model for this radio emission from our [red dwarf] 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 aurorae.”

This new research builds on results from last year, when the same team found an exoplanet GJ 1151 orbiting a red dwarf 30 light-years away, via the telltale radio signatures of its aurora.

They have now added observations of 18 more red dwarfs, with four potentially harbouring planets.

“We can’t be 100% 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,” says Pope.

“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 team is now working on confirming the exoplanets using other detection techniques – namely, the radial velocity method using the Hobby-Eberly Telescope in Texas, US, and the Calar Alto Observatory in Spain.

This radio emissions technique may also be a novel way to discover exoplanets.

“This method has very different biases than transits or radial velocity techniques,” Callingham says. “In particular, we are more sensitive to lower mass objects.”

It could also help astronomers better understand exoplanets’ magnetic fields.

“From our Solar System we know that having a planetary magnetic field is likely very important for a planet to be habitable,” Callingham adds. “For example, the Earth’s magnetic field protects our atmosphere in the face of Solar activity. In contrast, Mars does not have a magnetic field and its atmosphere has been severely stripped.”

This is also an important step for radio astronomy in general. Until now, astronomers could only see nearby stars using radio telescopes. Now, they’re able to spot stars much further away; currently, LOFAR has the capacity to monitor stars up to 165 light-years away.

“We are just seeing the tip of the iceberg with LOFAR,” Callingham says. “The Square Kilometre Array [SKA] is going to be nearly an order of magnitude more sensitive, so we will be able to find many, many more of these systems – those that are fainter and closer to Earth, and those far away.”

The SKA – an array of radio telescopes in Australia and South Africa – is currently under construction, and slated to start observations in 2029.