A NEW planet has been spotted around a nearby star. A common enough headline, but this planet is different from the hundred or so already discovered. Just a tenth the size of Jupiter, and as far away from its star as Pluto is from the Sun, it is undetectable using conventional methods. Instead, it made itself felt by disturbing the dust disc around its star. The technique will let astronomers look for planets that are smaller and more distant from their stars than ever before.
Virtually all of the planets found orbiting other stars were discovered by watching for the effects of planets鈥� gravity on a star鈥檚 movement. But that technique is only sensitive enough to spot large planets, generally Jupiter-sized or bigger.
Alice Quillen of the University of Rochester in New York wondered how planets affect dust discs around the stars they orbit. After modelling different orbits, she worked out that a planet in an elongated Pluto-like orbit with a tenth of Jupiter鈥檚 mass would create a distinctive pattern in the dust. She then studied high-resolution infrared images of dust discs around various stars, and found a pattern around a star called Epsilon Eridani that matched her prediction, although she still needs to confirm the find by watching how the pattern changes over time.
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Epsilon Eridani is just 10 light years away, yet previous conventional searches of the system only unearthed a planet about twice the mass of Jupiter in a five-year orbit. Quillen鈥檚 planet is much smaller, with an elongated 280-year orbit.
But her technique has its limitations. It鈥檚 not known how many stars have dust discs. And even for those that do, only planets near the disc will be detectable. So if there are any aliens on Epsilon Eridani using the method to peer at our Solar System, they might spot Neptune perturbing dust in the Kuiper Belt but they鈥檇 miss the gas giant Saturn completely.