By Joel Achenbach
Tuesday, October 20, 2009
Astronomers are finding planets by the dozen. European astronomers announced Monday that they had found 32 new "exoplanets" orbiting distant stars.
That brings to about 400 the number of planets found since the breakthrough discovery in 1995 of the first planet outside our solar system. Most have been quite large, many times the size of Jupiter. This latest batch, however, includes modest-sized worlds that are in the category known as "super Earths." The smallest of the bunch has the mass of about five Earths.
The planets cannot be seen directly because of the glare of the parent star. Astronomers detect them by studying variations in starlight. In this latest study, astronomers from the European Southern Observatory scrutinized about 2,000 relatively nearby stars over five years. They used a telescope in La Silla, Chile, equipped with an instrument known as the High Accuracy Radial Velocity Planet Searcher (HARPS). The instrument measured the slight wobble in starlight caused by the gravitational disturbance from a planet.
The 32 new planets were found around a variety of stars. Some stars were circled by multiple planets. The European team thinks the findings indicate that planets are common in our galaxy. Planets were found, for example, in orbit around "metal-poor" stars -- those lacking elements other than hydrogen and helium -- which until now had been considered unlikely places for planets to form.
Astronomers hope someday to find signs of a rocky, Earth-mass planet in an Earth-like orbit -- moving around the star in the "habitable zone" where water could, in theory, be liquid at the planet's surface. But such hypothetical planets would have such a slight perturbation on starlight that they are difficult to detect with current instruments.
Astronomer Stéphane Udry of Geneva University said by e-mail that a new instrument under development, known as ESPRESSO (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations), "should allow us to detect Earth twins around solar-type stars, within 5 to 10 years."
"Personally, I am convinced that planets are everywhere," Udry said. "Nature does not like void."