From New Scientist
How to hunt an alien Earth WHILE space agencies spend hundreds of millions of dollars developing a host of space-based missions to discover distant terrestrial planets, researchers are making ingenious use of existing technology back here on Earth. They're confident that expensive space missions will be pipped to the post before they even get off the ground.
The ultimate prize would be the discovery of a rocky Earth-like planet orbiting its star at a suitable distance for liquid water, and perhaps life, to exist. So far, the best candidate system for harbouring such a planet is 47 Ursae Majoris (New Scientist, 12 January, p 32). 47UM is 51 light years away and has a Sun-like star, along with two gas giants. The planets move in comfortable circular orbits like Saturn and Jupiter do, unlike the bizarre eccentric orbits of most of the 74 extrasolar planets found so far.
Gregory Laughlin of the University of California at Berkeley is using computer simulations to work out whether 47UM could contain an Earth-like planet. Rocky planets form by the conglomeration of asteroid-sized rocks. However, the gravity of gas giants creates a zone where small rocks are constantly jostled from one orbit to another and can't stick together. In our Solar System this is the asteroid belt between Mars and Jupiter, but in 47UM, the asteroid belt would be slap-bang in the habitable zone, Laughlin calculated.
Although 47UM is not a possible home from home, Laughlin told New Scientist that with gas giants being found every few months, he expects to find a system that does have terrestrial planets before NASA's planet-finding Kepler mission is launched in 2006 (New Scientist, 12 January, p 10).
Laughlin's colleague Andrea Lommen announced her discovery of a system in which a planet the size of Mars orbits a pulsar. She analysed radio waves emitted by the pulsar and found disruptions in the signals caused by the gravity of the orbiting planet. She's using a similar technique to look for gravitational waves (see opposite). Although a planet orbiting a pulsar couldn't support life, Laughlin says the example "shows terrestrial planet formation is pretty easy".
Even if the major prize is scooped soon, there will still be plenty of work for Kepler and similar missions. Once astronomers know we're not alone in the Universe, they'll want to know just how many friends we have.
Author: Eugenie Samuel reports from the American Astronomical Society's annual meeting in Washington DC
New Scientist issue: 19th January 2002
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