LOFAR Sees Strange Radio Signals Hinting at Hidden Exoplanets

LOFAR sees ‘exoplanet aurorae’ near distant red dwarf suns.

A powerful new method may help to detect exoplanets, via the aurorae they induce on their host star. The finding was announced recently from ASTRON’s Low Frequency Array radio telescope (LOFAR), based out of Exloo in the Netherlands, and sprawled across sites in Europe.

The survey looked at red dwarf stars near our solar system. The Sun-Earth interaction between the space weather emitted from the Sun and the Earth’s magnetosphere generates powerful aurorae, along with copious amounts of radio signals, and it has long been thought that interactions in other planetary systems should do the same.

But the search for exo-aurora has thus far been spurious at best. This sort of interaction induces low frequency radio waves, something that most radio observatories aren’t suited to detect. We see a similar situation with aurorae on Jupiter, induced by the powerful Io flux tube between the innermost Galilean moon and the planet itself. This radiation source is so powerful, that a home-built amateur radio telescope can pick it up, and spacecraft such as NASA’s Juno mission must avoid passing near the Io flux tube or risk having its electronics fried.

The Lowdown on LOFAR

But LOFAR was perfect for the survey. A series of 20,000 omnidirectional antennae based at 52 sites across nine countries in Europe, LOFAR acts like one large, two thousand kilometer diameter low-frequency radio antenna for astronomy. Commissioned in 2012, LOFAR has already probed the re-ionization era of the early Universe, detected new pulsars in the Milky Way Galaxy, and completed large scale maps of the solar wind.

LOFAR stations across Europe. Credit: Astron/LOFAR.

“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,” says Joseph Callingham (Leiden University) in a recent press release. “However, it is only with LOFAR have we had the sensitivity to find aurora emission outside our Solar System.”

LOFAR is a prime example of a low-cost, basic design for an array, producing maximum science. The array actually ‘slews’ towards targets across the sky using a technique known as electronic beam steering, a virtual method allowing the omnidirectional system to observe several targets simultaneously. The team discovered 19 red dwarfs emitting key radio signatures, indicating a nearby interaction with an unseen exoplanet. Low frequency radio signals from an exoplanet-red dwarf star interaction are like the Io-Jupiter flux tube, only scaled up. They also approach and recede from the LOFAR receiver in a tell-tale fashion, as the planet orbits the host star.

The LOFAR ‘superterp’ core station in Exloo, the Netherlands. Credit: ASTRON/LOFAR.

“The radio light should turn on and off like a lighthouse,” says Callingham in a recent press release. “and we hope to see that periodicity in new LOFAR data.”

NASA’s Transiting Exoplanet Survey Satellite (TESS) even got in on the action, doing follow up observations of the target stars in the study to rule out stellar activity. One target star highlighted in the study suspected of hosting an aurora generating exoplanet is GJ 1151, located 26.2 light-years distant in the constellation of Ursa Major, the Great Bear.

The location of GJ 1151 in the sky. Credit: Stellarium.

But that’s just the beginning for LOFAR. The array should be able to spy red dwarf-exoplanet interactions out to a distance of about 165 light-years encompassing potentially thousands of red dwarf systems. The Square Kilometre Array set to come online in 2029 could extend this capability even farther out, to a radius of hundreds of light-years.

…and to think, there was a time waaaay back before the 1990s when no exoplanets were known of, and many astronomers argued that it could very well stay that way. Fast forward to 2021, and 4,848 worlds beyond our solar system are known of, discovered using radial velocity, transiting, pulsar timing methods and more, a repertoire that now includes low frequency radio wave detections of exo-aurorae… exciting times, indeed.

Lead image credit: An artist’s conception of an exoplanet inducing aurorae on its host star. Credit: ASTRON/LOFAR