Unexpected radio signals detected from distant stars

Using the world’s most powerful radio antenna, scientists have discovered stars unexpectedly blasting out radio waves, possibly indicating the existence of hidden planets. Their research has been published in Nature Astronomy and The Astrophysical Journal Letters.

Leiden University’s Dr Joseph Callingham has been leading an international research team in a search for aurorae from exoplanets using the Low Frequency Array (LOFAR) radio telescope, operated by the Netherlands Institute for Radio Astronomy (ASTRON). According to Dr Callingham, “We’ve discovered signals from 19 distant red dwarf stars, four of which are best explained by the existence of planets orbiting them.

“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. This same process drives the beautiful aurorae we see at the poles of Earth,” Dr Callingham continued.

“However, it is only with LOFAR have we had the sensitivity to find auroral emission outside our solar system. This is an incredibly powerful tool to help find planets outside our solar system and to determine their magnetic fields.”

Previously, astronomers were only able to detect the very nearest stars in steady radio emission, and everything else in the radio sky was interstellar gas or exotica such as black holes. Now, radio astronomers are able to see plain old stars when they make their observations; with that information, we can search for any planets surrounding those stars.

The team focused on red dwarf stars, which are much smaller than the Sun and known to have intense magnetic activity that drives stellar flares and radio emission. But some old, magnetically inactive stars also showed up, challenging conventional understanding.

ASTRON’s Dr Harish Vedantham, co-author of the study, said the team is confident these signals are coming from the magnetic connection of the stars and unseen orbiting planets, similar to the interaction between Jupiter and its moon Io.

“Our own Earth has aurorae, commonly recognised here as the northern and southern lights,” he said. “These beautiful aurorae also emit powerful radio waves — this is from the interaction of the planet’s magnetic field with the solar wind. But in the case of aurorae from Jupiter, they’re much stronger as its volcanic moon Io is blasting material out into space, filling Jupiter’s environment with particles that drive unusually powerful aurorae.

“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 aurorae.

“It’s a spectacle that has attracted our attention from light-years away.”

The research team are now investigating the direct presence of the planets around the star using optical telescopes and searching for periodicity in the radio light. According to Dr Callingham, “The radio light should turn on and off like a lighthouse, and we hope to see that periodicity in new LOFAR data.”

While the LOFAR telescope only has the capacity to monitor stars that are relatively nearby (up to 165 light-years away), Australia and South Africa’s Square Kilometre Array radio telescope is now under construction, hopefully switching on in 2029 — at which point the team predict they will be able to see hundreds of relevant stars out to much greater distances.

Image caption: Artist’s impression of a red dwarf star’s magnetic interaction with its exoplanet. Image credit: Danielle Futselaar (artsource.nl).

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