Finding planets out in the Universe is pretty hard. Neptune wasn’t directly observed until 1846 despite being in our own solar system. We didn’t start discovering planets outside the solar system until 150 years after Neptune. Like Neptune, we find them (though indirectly), through visible light. However an international team of researchers may have just made the first detection of an exoplanet through radio emissions created by the planet’s aurora.
Using the Netherland-based LOFAR (LOw Frequency ARray) Low Band Antenna, radio emission data was captured from three solar systems: 55 Cancri, Upsilon Andromedae, and Tau Boötis. Each of these systems contains known exoplanets. The research wasn’t to discover new exoplanets but test whether the known planets in these systems could be detected by looking for the radio signals. Planets emit radio signals created from interactions between their magnetic fields and the plasma or “solar wind” radiating from their parent stars. When plasma from a star becomes entangled in the magnetic bubble around a planet – the magnetosphere – visible aurora is created just like the Northern/Southern Lights we see on our own planet. Aurora also creates radio emissions that travel light years through space.
Radio emission detection adds a new possible method of exoplanet hunting. Of the three solar systems observed, star system Tau Boötis showed a promising result which the team believes could be a radio emission from a planet. Tau Boötis resides 51 light years from Earth in the constellation of Boötes. The system contains an F-class star (Tau Boötis A) about 50% larger than our own Sun and 3 times more luminous. The star has an M-class red dwarf companion (Tau Boötis B) that orbits at a distance of 220 AU; more than 7 times the distance that Neptune orbits our own Sun. The main F star has a known gas giant exoplanet called Tau Boötis Ab. Tau Boötis Ab was actually one of the earliest discovered exoplanets detected in 1996 using doppler spectroscopy.
There is strong evidence the radio signal from the Tau Boötis system is emanating from the planet itself. Tau Boötis Ab is a “Hot Jupiter” gas giant that orbits one seventh the distance that Mercury orbits our Sun. Its year is just 3 days long. The proximity to the star makes Tau Boötis Ab an ideal candidate for radio emission observation. Entangled so closely in the stellar plasma, the planet’s magnetic field becomes supercharged creating radio emissions a million times stronger Jupiter’s.
The planetary radio emission can be more power than the star’s emission which distinguishes one from the other. The detected signal also showed a degree of polarization expected from auroral planetary radio emission which is also distinct from other astronomical objects. However, stellar flares and outbursts can sometimes be polarized meaning the radio source could originate from Tau Boötis B, the dwarf companion star, as M dwarf stars are known for violent solar flares. As the team notes: “follow-up observations are required to confirm the presence of this faint signal, and subsequently verify its origin.”