Radio emissions from an exoplanet have been detected

Invisible glow

Finding planets in the Universe is quite difficult. I say this despite the fact that two planets in Earth’s skies will align tomorrow to form one of the brightest objects seen in hundreds of years. But while the bright Jupiter and Saturn are always visible to the naked eye, Neptune wasn’t directly observed until 1846 despite being in our own solar system. We don’t start discovering planets outside the solar system up to 150 years after Neptune. Like Neptune, we find them (albeit indirectly), through visible light. However, it is possible that an international team of researchers made the first detection of an exoplanet through the radio emissions created by the planet’s aurora.

Led by Cornell postdoctoral researcher Jake D. Turner, Philippe Zakara of the Observatoire de Paris, and Jean-Mathias Greissmeier of the Université d’Orléans, the team has an upcoming article in Astronomy & Astrophysics based on the theorized detection of exoauroa. Using the Netherlands-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 investigation was not to discover new exoplanets, but test whether known planets in these systems could be detected by looking for radio signals. Planets emit radio signals created from interactions between their magnetic fields and the plasma or “solar wind” radiated by their parent stars. When a star’s plasma becomes entangled in the magnetic bubble around a planet, the magnetosphere, a 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.

Tried and tested

Currently, we only have a handful of methods for detecting distant worlds outside of our own solar system. The two most successful are Doppler spectroscopy (or the radial velocity method) and the transit method. You are familiar with the Doppler effect in sound waves. The siren of a passing ambulance sounds higher when it approaches, but lower when it moves away. Light waves also experience a Doppler effect. When an object approaches us, its light changes to a bluer part of the visible spectrum. As an object moves away from us, its light shifts towards the reddest part of the spectrum. Stars with planets show displaced light in both blue and red because they literally wobble back and forth as they are pulled by the gravity of their orbiting planets. Wobble is measured as the “radial velocity” of the star or the speed at which it travels towards or away from us during the course of the wobble.

The second is the transit method used by planet search missions like TESS and Kepler. These missions see the silhouettes of distant exoplanets. As these planets orbit their host stars, they block some of the starlight from our perspective by casting a measurable shadow across space – a transit. The transit tells us about the size of the planet, the distance from its parent star, and the length of its year. Using both methods, thousands of exoplanets have been discovered.

A loud and hot Jupiter

The detection of radio emissions adds a new possible method of searching for exoplanets. Of the three observed solar systems, the Tau Boötis star system showed a promising result that the team believes could be a radio emission from a planet. Tau Boötis resides 51 light years from Earth in the constellation Boötes. The system contains a class F star (Tau Boötis A) approximately 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 star F has a known gas giant exoplanet called Tau Boötis Ab. Tau Boötis Ab was actually one of the first discovered exoplanets detected in 1996 using Doppler spectroscopy.

There is strong evidence that the radio signal of the Tau Boötis system emanates from the planet itself. Tau Boötis Ab is a “hot Jupiter” gas giant that orbits a seventh the distance that Mercury orbits our Sun. Its year lasts only 3 days. The proximity to the star makes Tau Boötis Ab an ideal candidate for observing radio emissions. So tightly entangled in stellar plasma, the planet’s magnetic field becomes overloaded creating radio emissions to million times Stronger Jupiter.

The planetary radio emission can be more powerful than the emission from the star that distinguishes one from the other. The detected signal also showed an expected degree of polarization of the auroral planetary radio emission that is also distinct from other astronomical objects. However, eruptions and stellar flares can sometimes become polarized, which means that the radio source could originate from Tau Boötis B, the dwarf companion star, as M-dwarf stars are known for their violent solar flares. As the team points out: “Follow-up observations are required to confirm the presence of this weak signal and subsequently verify its origin.”

Habitable magnetospheres

If the signal did indeed originate from Tau Boötis Ab, we may be seeing a new era of exoplanet detection. It is fitting that this new age is ushered in by Tau Boötis Ab. Hot Jupiters were some of the first planets discovered by Doppler spectroscopy because their mass and close orbit to their stars made the “wobble” of those parent stars more pronounced. I also have a personal affinity for this planet as the observatory where I began my science outreach career, the Trottier Observatory at Simon Fraser University, which replicated Doppler observations showing the presence of the Hot Jupiter at Tau Boötis A.

What was once the realm of professional observatories decades ago can be more widely reproduced by oscilloscopes around the world like the one at SFU. Perhaps in a few more decades, smaller centers will also experience the same trickle into LOFAR-based technology, where we are hearing the aurora from distant worlds at public observatories or even at home. In addition to a new detection tool, the implication of this discovery is that we have a way to determine the strength of the magnetosphere of a distant world, relevant to habitability. Earth’s atmosphere is protected by our magnetic field that prevents solar winds from blowing our atmosphere into space, literally blown away by the Sun, as happened with the once denser atmosphere of Mars.

In the meantime, as we perfect new methods for finding planets in the cosmos, be sure to try and capture the Jupiter / Saturn conjunction here in our own solar system on December 21.^{S t}. Universe Today is hosting a Virtual Star party to (hopefully) convey a clear view from somewhere on the planet that we can then indirectly experience given our climate here in the Pacific Northwest. You can connect to the Virtual Star party using the link below. (The start time is still running out. Check the link in the stream for updates.)

More to explore

Cornell Postdoc Detects Possible Radio Emission from an Exoplanet | Cornell Chronicle

Searching for radio emissions from exoplanetary systems 55 Cancri, upsilon Andromedae, and tau Boötis using LOFAR beamform observations (aanda.org)

Detecting exoplanets through their exoauroras – Universe Today

https://carlsaganinstitute.cornell.edu/

Radio broadcast from an exoplanet – Jake Turner – YouTube

LOFAR – ASTRON – The observatory used for the discovery

[1210.1864] Origin of Radio Emissions Induced by the Maser Electron Cyclotron in Ultra Cold Dwarfs: Magnetosphere-Ionosphere Coupling Currents (arxiv.org) (free access article)

Trottier Observatory able to see the planet outside our solar system | The Peak (the-peak.ca) Jack Madden Art (artstation.com) – Main Image Artist