2002-09-11 | SCIENCE
Tuning in to Other Worlds

Solar wind particles that strike Earth's magnetosphere erupt not only in brilliant auroras, but also in bursts of low-frequency radio waves. Other planets in our solar system with magnetospheres—Jupiter, Saturn, Uranus and Neptune—also produce auroras and radio waves. Some scientists hope to detect extrasolar planets by listening for similar radio emissions, though they may have to strain to hear them. Earth's ionosphere screens out low-frequency radio waves, and radio noise from other sources may drown out distant planets. But radio detection might identify planets from a single radio blast; most optical planet detection methods require an entire orbit's worth of observations, which can take years.

Radio signals might even tell us about a planet's magnetic field, the rotation rate, if there are any moons, and whether the planet is terrestrial or a gas giant—all of which have a bearing on the planet's ability to support life. The strength of radio emissions depends not only on the size of the planet, but on a variety of factors such as its spin and its distance from its parent star. Planets without a magnetosphere, such as Venus and Mars, would not be detectable. But they might also be less likely to harbor life, since a magnetosphere holds back dangerous solar radiation and may help preserve a planet's atmosphere. Planets that are tidally locked to the parent star (that is, the same side of the planet always faces its sun) spin slowly, and therefore emit only faint radio waves. But such planets may be lifeless anyhow, since they might suffer extremes of temperature.

Because Earth is close to the Sun, it "broadcasts" more strongly than the much larger planets Uranus and Neptune; tuning in an Earth-like planet is a real possibility. But the planets likely to blare the loudest are gas giants many times larger than Jupiter, and gas giants that orbit their parent star extremely closely (the "hot jupiters "). Not only are hot jupiters unlikely to harbor life, they might make life impossible in the rest of their planetary system.

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