An artist's view of HR 8799b, one of extrasolar planets orbiting the young star HR 8799, which lies 130 light-years away. The planetary trio was originally discovered in images taken with the Keck and Gemini North telescopes in 2007 and 2008. But scientists also found the telltale glow of the planet hidden in the mass of Hubble archival data, in data collected in 1998.
Credit: NASA/ESA/G. Bacon
SYDNEY: In space at least, the real estate business is booming: the number of planets discovered outside of our Solar System has now passed 500 - and that's just the beginning, astronomers say.
While many of these distant planets are the size of Jupiter or even larger, planets are continually being found that are looking more and more like the Earth.
Astronomers last week announced the first planet found outside the galaxy, a gas giant one-and-a-quarter times Jupiter's mass orbiting a dying star 2000 light years away. The star's galaxy was disrupted by the larger Milky Way.
Earth-like planets
The exoplanet count now stands at 502, with most Earth-like yet discovered probably Gliese 581 g, just three times Earth's mass. It speeds around its host star, 20.3 light-years (194 trillion km) distant in just 37 days, yet because of its star's low mass sits firmly within the 'habitable zone'.
About once a month astronomers claim the discovery of yet another planet that is Earth-like in one way or another, says astronomer Chris Tinney, from the University of New South Wales Exoplanetary Science Group in Sydney.
Tinney is part of the Anglo-Australian Planet Search (AAPS), which has found 34 of the almost 400 planets that were discovered with the 'Doppler' or 'radial velocity' technique. This technique picks up the tiny gravitational 'tug' exerted by a planet on its host star as the star and planet revolved around a common centre of mass.
Kepler scanning 156,000 stars
The Doppler technique can't see Earth-like planets around stars like the sun, but can be used to find potentially habitable planets around low-mass, cooler stars because the star's habitable zone "shrinks in" closer to the star, says Tinney.
One of exoplanet research's most promising missions is NASA's space-based Kepler mission, which is scanning 156,000 faint stars in search of transiting planets. The transit method pinpoints the miniscule dip in the star's brightness as a planet passes in front of the star as viewed from Earth.
Kepler team member Geoffrey Marcy from the University of California, Berkeley, estimates the mission will find 120 to 260 Earth-mass planets orbiting 10,000 nearby dwarf stars in orbits of 50 days or less.
"They'll find thousands"
A third technique, 'microlensing' is complementary to the other techniques and is geared at finding lower-mass planets and solar system equivalents. By looking at the way a planet-star system 'bends' the light of a distant source star, astronomers can spot planets as small as three times Earth size.
University of Tasmania astronomer John Greenhill leads a team that has so far found six planets using the microlensing method.
"I think many people recognise that it's the technique that's going to characterise planetary formation better than any other technique," says Greenhill. And 500 is just the tip of the iceberg, he says. "I suspect they'll find thousands."
