Listening Out for the Magnetospheres of Habitable Exoplanets

Is there a new way to hunt for habitable Earth-like exoplanets? According to a US Naval Research Laboratory researcher there is an obvious, yet ingenious, way of listening for these worlds. Like most Earth-like exoplanet searches, we are looking for characteristics of our own planet. So what do we need to survive on Earth? Obviously we need water and the correct mix of oxygen with other atmospheric gases, but what about the magnetic bubble we live in? The Earth’s magnetosphere protects us from the worst the Sun can throw at us, preventing the atmosphere from being eroded into space and deflecting life-hindering radiation.

Although we have yet to develop sensitive enough radio telescopes, it may be possible in the future to detect the radio waves generated as charged particles in stellar winds interact with Earth-like exoplanetary magnetospheres. If there’s a magnetosphere, there may be a protected atmosphere. If there’s an atmosphere, perhaps there’s life being nurtured below…

*This image is copyright Mark A. Garlick and has been used with permission. Please do not use this image in any way whatsoever without first contacting the artist.

Earth holds the only blueprint of life that we know of, so its only logical that we look for Earth-like planets orbiting Sun-like stars. In fact, the recently launched Kepler space telescope will be doing both of these things. However, it’s not just Sun-like stars that could harbour rocky Earth-like planets within their habitable zones. Stars come in many flavours, and depending on the amount of energy they radiate, the distance of the stars habitable zones will differ.

The habitable zone of a star is often referred to the “Goldilocks zone”, as the energy received by a planet in this zone will receive just the right amount of energy from their parent star for Earth-like life to thrive (i.e. it’s not too hot, it’s not too cold, it’s just right). In fact, in a recent article by Paul Gilster at Centauri Dreams, the possibilities of finding habitable Earth-like exoplanets orbiting small red dwarf stars are examined. Red dwarfs have previously been discounted as stars that can have habitable planets in orbit around them (as the quantity proto-planetary material would be low, reducing the probability of forming large, Earth-like worlds), but it would appear there may be a chance that a handful of Earth-mass planets might form. Red dwarfs make up 70% of all observed stars, so any odds of Earth-mass exoplanets orbiting red dwarfs is a possibility worth studying.

OK, so now we can look for planets that show an orbital period that reflects its mass and therefore its distance from the parent star. If they orbit within their stars habitable zone, perhaps we’ll find some Earth-like candidates and therefore carry out a detailed search for life. In this case, we could possibly do some spectroscopic analysis of the Earth-like exoplanet as it passes in front of (transits) its parent star. This may reveal the constituents of the exoplanet’s atmosphere. If there’s oxygen and water vapour, perhaps we’ve found a candidate for an Earth-like exoplanet capable of sustaining life. But is there another indicator as to whether the observed exoplanet might be able to sustain life? How about trying to observe radio emissions from exoplanetary magnetospheres?

Although we’d need a radio space telescope 100 times more sensitive than what we already have, Joseph Lazio at the Naval Research Laboratory in Washington DC suggests in a recent paper that we should at least look into the possibility of detecting aurorae on exoplanets. “This is something we think is worth studying at a modest level,” says Lazio, “the payoff could be immense.”

On Earth, we observe aurorae in the upper-polar atmosphere (the Aurora Borealis usually over Arctic regions, and the Aurora Australis over the Antarctic regions). As charged particles from the solar wind interact with the the terrestrial magnetosphere, they spiral along the magnetic flux, and are injected into the polar ionosphere. The solar particles interact with atmospheric gases, generating light as aurorae. As electrons interact with the Earth’s magnetic field, they generate radio waves that can be received here on Earth (at frequencies between 400 and 1000 Hz). However, most of the radio wave energy is released to space, making our magnetosphere the Earth’s most powerful radio transmitter.

Therefore, if the Earth’s magnetosphere can generate radio waves and if exoplanets possess magnetospheres, then perhaps we can receive their radio emissions during exoplanet auroral activity. Although the signal is likely to be very weak, scientists are excited by this possibility, even pointing out that this method may be able to detect exoplanets before more traditional exoplanet hunting techniques. However, key to this research is the fact that our magnetosphere protects life on Earth, perhaps if we can detect exoplanetary magnetospheres, it may be an indicator for a habitable alien world…

Source: New Scientist