Star-exoplanet interaction detected at radio frequencies

Using the Low Frequency Array (LOFAR) radio telescope, a team led from the Netherlands have discovered low-frequecncy radio emission associated with the M4.5V star GJ 1151. The radio waves bear the tell-tale signature of aurorae caused by an interaction between a star and its planet. The radio emission from a star-planet interaction has been predicted for over thirty-years, but this is the first time astronomers have been able to discern its signature. This discovery paves the way for a novel and unique way to probe the environment around exoplanets and to determine their habitability.

Notably, follow-up observations with the HARPS-N instrument on the Telescopio Nazionale Galileo ruled out the alternate possibility that the interacting companion is another star as opposed to an exoplanet.

"The motion of the planet through a M dwarf’s strong magnetic field acts like an electric engine. This generates a huge current that powers aurorae and radio emission on the star." says Dr. Harish Vedantham, the lead author of the Nature Astronomy paper and a Netherlands Institute for Radio Astronomy (ASTRON) staff scientist. Such a system is modelled on the interaction of Io with Jupiter’s magnetic field, which generates such bright radio emission that it can outshine the Sun at low frequencies.

"We adapted the knowledge from decades of radio observations of Jupiter to the case of this star" said Dr. Joe Callingham, Veni fellow at Leiden Observatory/ASTRON and co-author of the study. “A scaled up version of Jupiter-Io has long been predicted to exist in the form of a star-planet system, and the emission we observed fits the theory very well.”

To be sure, the astronomers had to rule out an alternate possibility—that the interacting bodies are two stars in a close binary system instead of a star and its exoplanet. For this reason, an intensive radial-velocity monitoring was performed with HARPS-N, using the Director Discretionary Time (DDT) procedure.

“Interacting binary stars can also emit radio waves,” notes Benjamin Pope, NASA Sagan Fellow at New York University and lead author of the companion ApJL paper. “Using optical observations to follow up, we searched for evidence of a stellar companion masquerading as an exoplanet in the radio data. We ruled this scenario out very strongly, so we think the most likely possibility is an Earth-sized planet too small to detect with our current dataset. We place a constraint on the presence of a planet of a Msini < 5.3 Earth mass, consistent with the fact the radio emission can be generated by anything as small as a mercury-like planet in a less than 10 day orbit.”

The group is now concentrating on finding similar emission from other stars. We now know that nearly every red-dwarf hosts terrestrial planets, so there must be other stars showing similar emission. The long-term aim is to determine what impact the star’s magnetic activity has on an exoplanet’s habitability, and radio emissions are a big piece of that puzzle. The case of GJ 1151 has shown that this is viable with the new generation of radio telescopes and put us on an exciting path.

Artistic impression of a red-dwarf star’s magnetic interaction with its exoplanet. Image credit: Danielle Futselaar.

Figure 3 from Pope et al. 2020. HARPS-N radial-velocities time-series of GJ 1151. We see no clear Keplerian fit and put 99th-percentile upper limits on the mass of any close companion to GJ 1151 at Msini < 5.6 Earth masses.

Link to the papers:
“Coherent radio emission from a quiescent red dwarf indicative of star–planet interaction” , Vedantham H. , Callingham, J. et al., 2020, Nature Astronomy.
“No Massive Companion to the Coherent Radio-Emitting M Dwarf GJ 1151”, Pope al., 2020, ApJ Letters, 890, L19.