Fine-Tuning the Milky Way Radio Dial to Zero in on Alien Planets

Dn18516-2_300 Radio waves from the aurorae's flares of ultraviolet light in the upper atmosphere of planets like Jupiter and Saturn could be used to detect exoplanets that orbit at large distances from their parent star, according to a new study by scientists at the University of Leicester in England.

The Leicester team have shown that emissions from the radio aurorae of planets such as Jupiter and Saturn could be detectable by radio telescopes such as the European Low Frequency Array, or LOFAR. Construction of the LOFAR radio telescope, with stations primarily located in the Netherlands, will be completed later this year.

"This is the first study to predict the radio emissions by exoplanetary systems similar to those we find at Jupiter or Saturn," said Jonathan Nichols, who is presenting the study's results today (April 18) at the Royal Astronomical Society's National Astronomy Meeting in Wales.

"At both planets, we see radio waves associated with aurorae generated by interactions with ionized gas escaping from the volcanic moons, Io and Enceladus," Nichols said. "Our study shows that we could detect emissions from radio aurorae from Jupiter-like systems orbiting at distances as far out as Pluto."

Nichols examined how the radio emissions of Jupiter-like exoplanets would be affected by the rotation rate of the planet, the rate of plasma outflow from a moon, the orbital distance of the planet and the ultraviolet brightness of the parent star.

Nichols found that in many scenarios, exoplanets orbiting stars that emit bright ultraviolet light would generate enough radio power to be detectable from Earth. In fact, for the brightest stars and fastest moving planets, the radio emissions would be detectable from systems up to 150 light-years away from Earth.

"In our solar system, we have a stable system with outer gas giants and inner terrestrial planets, like Earth, where life has been able to evolve," Nichols said. "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."

Most exoplanets have been found using the so-called transit method, which detects the dimming of light as a planet moves — or transits — in front of a star. Another technique looks for a wobble effect as a star is tugged by the gravity of an orbiting planet. With both of these methods, it is easiest to detect planets that closely orbit the star and move very quickly.

"Jupiter and Saturn take 12 and 30 years respectively to orbit the sun, so you would have to be incredibly lucky or look for a very long time to spot them by a transit or a wobble," Nichols said.

Another example of radio emissions are closer to home: the chirps and whistles of our planet's auroral kilometric radiation (AKR) might be the first thing an extraterrestrial civilization is likely to hear from Earth. In reality, they are the sounds that accompany the aurorae. The European Space Agency's Cluster mission is showing scientists how to understand this emission and, in the future, search for alien worlds by listening for their sounds.

AKR is generated high above the Earth, by the same shaft of solar particles that then causes an aurora to light the sky beneath. For decades, astronomers had assumed that these radio waves traveled out into space in an ever-widening cone, rather like light emitted from a torch. Based on Cluster, astronomers now know this is not true.

By analyzing 12,000 separate bursts of AKR, a team of astronomers have determined that the AKR is beamed into space in a narrow plane, similar to placing a mask over the torch with just a small slit in the middle for light to escape.

"Whenever you have aurorae, you get AKR," says Mutel. That includes aurorae on other planets, too. Visiting spacecraft have seen aurorae and detected AKR on Jupiter (image above) and Saturn, the two largest gas giants in our Solar System.

Not only will this new understanding of how the AKR is beamed into space help astronomers understand the magnetic environment of those gas worlds, it will also help them search for similar planets around other stars.

Although looking for AKR from extra solar planets will require much larger radio telescopes than are currently available, these instruments are on the drawing boards. Once these planets have been identified, the AKR can be monitored for how it winks on and off, allowing astronomers to calculate how long the planet takes to rotate.

"We can now determine exactly where the emission is coming from," says Robert Mutel, University of Iowa, who conducted the three-year study with colleagues. For each of the AKR bursts they analyzed, the astronomers pinpointed its point of origin to regions in Earth's magnetic field just a few tens of kilometers in size. These were located a few thousand kilometers above where the light of the aurora is formed.

"This result was only possible because of the Cluster mission's four spacecraft," says Mutel. Consisting of four nearly identical spacecraft flying in formation, Cluster allowed the scientists to precisely time when the AKR washed over each of the satellites. Using this information, the scientists triangulated the points of origin, in a similar way to the way GPS navigation works.

AKR was discovered by satellites in the early 1970s. It is blocked from reaching the ground by the ionosphere, the upper reaches of Earth's atmosphere. This is just as well because otherwise it would overwhelm the transmissions from all our radio stations. It is 10,000 times more intense than even the strongest military radar signal.

It also provides new routes of investigation into the magnetic fields of other stars, many of which have magnetic fields thousands of times stronger than the Sun. They too, will produce radiation similar to AKR and these can be monitored.


Casey Kazan. ESA Cassini Image  shows radio emissions from Jupiter.

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