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Astronomers tune in to exoplanets
Posted: 19 April 2011

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It is possible to tune into radio signals from aurorae to hunt down exoplanets, according to research presented at the Royal Astronomical Society’s National Astronomy Meeting held in Llandudno, North Wales.

Dr Jonathan Nichols, who presented his findings at the conference, along with his colleagues at the University of Leicester, has shown that emissions from the radio aurorae of planets, like the Northern Lights on Earth, should be detectable by radio telescopes such as the European LOw Frequency ARray (LOFAR). “We are looking at planets that are at a Jupiter, Saturn or Uranus distance from their host stars,” says Nichols.

An optical image of Saturn by the Hubble Space Telescope with ultraviolet images of its aurora overlaid. Can aurorae on exoplanets be detected by their radio emissions? Image: ESA/NASA/Hubble.

The distant worlds that he speaks of are planets similar to Jupiter, and detecting them at these large distances from their star has always been a challenge when searching for exoplanets. Less than 10 percent of the hundreds of exoplanets currently known are at distances where we would find the outer planets in our own Solar System. Most of these alien worlds have been located either via the transit method, which detects a dip in brightness as a planet moves in front of its host star, or by looking for a wobble in a star as it is tugged by the gravity of an orbiting planet. This new study is the first to predict radio emissions from exoplanetary systems similar to those we find at Jupiter and Saturn, where we see radio waves associated with the generation of aurorae.

Nichols examined how the radio emissions for the Jupiter-like exoplanets would be affected by the rotation rate of the planet as well as the rate of plasma outflow from a moon. “Material from a volcanic moon is released into the magnetosphere - in [Jupiter’s moon] Io’s case, this will be sulphur dioxide,” he says. “This material becomes ionised, which means that it becomes separated into negative and positively charged particles.”

The magnetic field of the planet accelerates the newly charged particles up to the rate at which the planet rotates, believed to be once every 10 hours in Jupiter’s case. “All of a sudden, the material is thrown out from the planet and it slows down due to the conservation of momentum,” he says. The slowing down of the planet then causes the formation of electric fields and electric currents which then flow around the magnetic fields and this results in radio emissions and an aurorae. “The more material thrown out, the stronger the signal,” says Nichols.

The scientist also considered the effect of the orbital distance of the planet and the ultraviolet (UV) brightness of the parent star. “Any star that has an extreme ultraviolet luminosity, shines really brightly in extreme ultraviolet,” explains Nichols. “These are called active stars and they tend to be young. It’s important to note that it’s not the particular colour or temperature of the star that affects the exoplanet, but the activity of the star itself.” The more active a star, the more powerful its stellar wind, which carries charged particles that can act as an electric current that interacts with magnetic field lines in the planet's upper atmosphere. The more extreme ultraviolet light there is coming from the star and incident on the planet, the more electrically conducting the planet's upper atmosphere becomes, leading to brighter aurorae.

Nichols found that in most cases, exoplanets orbiting UV-bright stars between 1 and 50 times the Earth–Sun distance (one astronomical unit) would generate enough radio power to be detectable from Earth. The emissions from the fastest and brightest spinning planets are believed to be detectable from systems 150 light years away from our home planet.

“Being able to detect Jupiter-like planets may help us find planetary systems like our own, with other planets that are capable of supporting life,” says Nichols.

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