Hardly a month goes by without the reported detection of planets in orbit around stars other than our own Sun. The discovery of these 'extrasolar planets' strains detectors to their very limits. As yet, nobody has actually 'seen' an extrasolar planet – to an Earthbound observer, perhaps tens of light years distant, an extrasolar planet is just too faint, and too close to its parent star, to be visible. In general, astronomers infer the presence of planets from the subtle yet periodic gravitational effects they exert on the parent star.

A possible new detection method is outlined in a speculative paper published in the June issue of the Journal of Geophysical Research: Planets. In the report, William M. Farrell and Mike D. Desch of the NASA Goddard Space Flight Center in Greenbelt, Maryland, and Philippe Zarka of the Observatoire de Paris, Meudon, France suggest that we might be able to detect extrasolar planets by listening to them on the radio.

No, this is not a scheme to eavesdrop on Little Green Men tuning in to the extraterrestrial equivalent of I Love Lucy. Rather, it is a plan to exploit the fact that many planets, even quite small ones such as the Earth, emit radio waves.

This emission is a consequence of two things: first, that many planets, if they are like planets in our own Solar System, can be presumed to have magnetic fields; and second, that their parent stars, if they are even remotely like our Sun, emit streams of energized subatomic particles. This 'stellar wind' interacts with planetary magnetic fields (or 'magnetospheres') to produce various electrical phenomena, such as aurorae – but another consequence is the acceleration of beams of energized particles out into interstellar space, what the researchers call 'cyclotron emission'. Astronomers have long been able to pick up cyclotron emission from planets in our own Solar System: they detect it by picking up characteristic patterns of radio emission. The characteristics of the emission differ in detail from planet to planet, but all depend, substantially, on the force of the stellar wind as it ploughs into the magnetosphere of the planet.

Using the characteristics of cyclotron emission from planets in our own Solar System as 'models', the researchers worked out whether it would be possible to detect cyclotron emission from an extrasolar planet at a distance of around 30 light years. The answer, sadly, is 'no'. Even the most optimistic forecast places the most powerful cyclotron emission from hopeful candidate extrasolar planets well below the detection threshold of the most powerful present-day radio receivers.

[The researchers estimate that the candidate planet in orbit around the star Tau Boötis would have a median power flux at a frequency of 28 megahertz of just 2 janskys. It hardly matters that a jansky is a standard measure of radio emission: only that the world's largest radio telescope capable of receiving signals of around that frequency – located in Kharkov, in the Ukraine – can detect signals down to around 10 janskys.]

However, there is still hope. The flux of radio emission from a planet is not a fixed quantity, but depends on the interaction of the planet's magnetosphere with the stellar wind. Experience with our own Sun tells us that stars undergo episodes of increased activity, and produce more powerful stellar winds than their long-term average. This results in stronger interactions with magnetospheres, and, therefore, greater cyclotron emission – about a hundred times the usual flux. This means that cyclotron emission from a distant planet circling a very active star might – just might – poke its head above the threshold of detection.