Magnetospheric Radio Emission from Extrasolar Planets

We expect that some extrasolar planets will be brighter at radio wavelengths than their host star. This offers the possibility of using radio emission to detect extrasolar planets, and also to study their magnetospheres, and just possibly any orbiting moons of the extrasolar planets.

The analogy is with Jupiter in our Solar System. Jupiter is extremely bright at MHz frequencies. This radio emission is a consequence of electrons spiralling within the magnetic field of Jupiter, many of which are injected by the satellite Io. We would expect extrasolar planets to have large magnetospheres as well, and potentially to be detectable at radio wavelengths.

For planets within the Solar System, the level of radio emission is controlled by the solar wind (principally, the amount of kinetic energy intercepted by the planetary magnetosphere).

By using some scaling laws we have been able to estimate how strong some the radio emission will be from the nearby extrasolar planets. Some of the host stars of these extrasolar planets have wind much stronger than that of the Sun, and many of the extrasolar planets are in much closer orbits than say Jupiter. Both of these effects mean that these planets are expected to be much brighter than Jupiter, and hence detectable at reasonable distance with upcoming instruments such as LOFAR.

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A schematic view of the magnetospheres of planets within our own Solar System. We would expect extrasolar planets to have similarly complex magnetospheres, particularly as many of them orbit much closer to their host stars, and some of the host stars have much stronger winds that our Sun, both effects leading to stronger radio emission. (Image by D. Williams at the APL).

With the Giant Metrewave Radio Telescope (GMRT) we have observed two extrasolar planetary systems (Epsilon Eridani and HD128311) at low radio frequencies (150 MHz). We did not detect either system, but are able to place tight upper limits on their low frequency radio emission, at levels comparable to the theoretical predictions for these systems. From these observations we have a 2.5-sigma limit of 7.8 mJy for Epsilon Eri and 15.5 mJy for HD128311. For more about these observations see, George & Stevens, 2007, MNRAS, 382, 455.

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Above is a radio contour map of the region around Epsilon Eridani, the X marks the location of Epsilon Eridani. This shows that no emission can be seen from the planet but there are a number of other interesting other sources in the field.