A Star's Auroras Light the Way to a New Exoplanet

Jupiter’s moon Io—the solar system’s most volcanic world—has inspired a new way to find distant exoplanets. As the moon orbits Jupiter, it tugs on the planet’s magnetic field, generating bright auroras in Jupiter’s atmosphere. Even if we couldn’t see Io itself, the enormous auroras, pulsing to the beat of a hidden orbiting body, would tell us that something was out there.Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.

Scientists have long suspected that a similar process might be at work with distant planets and the stars they orbit. Now, for the first time, astronomers say they have discovered an exoplanet by mapping the auroras of its host star, opening a new chapter in the quest to map the galactic menagerie of unseen worlds.

In the new study, published yesterday in Nature Astronomy, researchers used a collection of roughly 20,000 small radio antennas spread across Europe to detect the star’s telltale flares. They concluded that the flares could only be produced by a rocky planet about the size of Earth that takes between one and five days to orbit the star. Such a planet would be right at the edge of the star’s habitable zone, where temperatures are right for liquid water.
As with so many new techniques, this one promises more discoveries to come. “This could be a way of discovering more exoplanets than you can with the traditional methods,” said Jonathan Nichols, an astrophysicist at the University of Leicester who was not involved in the research. “It could be a way of probing the types of system that we usually find quite difficult to observe.”

The Telltale Flare

The insight that allowed for the new discovery started a lot closer to home. Up around Jupiter, Io’s eruptions spew gas that’s dense with charged particles. As the moon rotates around its host planet, this charged gas swipes across Jupiter’s magnetic field lines “like plucking strings on a guitar,” said Nichols, who studies space-based magnetic fields. The waves created by these plucks travel up through the field lines and into the planet, where they emit bursts of radio emissions that come and go as the moon rotates around Jupiter.

Jupiter’s moon Io undergoes extreme volcanic eruptions, as seen in these two images from the Galileo spacecraft. Lava flows from these eruptions can stretch more than 60 kilometers long (left), while the plumes of gas and dust lift high above the planet (right). Every second, Jupiter’s magnetic field strips about 1,000 kilograms of material off of Io.Courtesy of NASA

The authors of the new paper suspect they are seeing a similar plucking—but this one is a planet plucking the magnetic field lines of a star.

The group began by analyzing a map of the sky made by the Low Frequency Array, or LOFAR, a collection of small radio antennas that can act as a single giant dish 1,500 kilometers across. LOFAR has been scanning the sky for a decade; in this time it has accumulated enough data to see fainter objects than any previous radio survey.“You end up finding new things when you go deeper,” said Harish Vedantham, an astronomer at the Netherlands Institute for Radio Astronomy and lead author on the new paper.
Vedantham and his team mapped all the radio emissions spotted by LOFAR. They then overlaid this map with another map—this one of stars in the Milky Way, made by the Gaia space telescope. They then singled out the sources that came from stars rather than distant objects such as galaxies.