"ith nearly 300 planets now known beyond the solar system, astronomers are honing their hunting skills, trying to determine which stars are most likely to harbor planets that resemble Earth. A four-year study of 400 stars, most of them similar to the sun, have yielded what astronomers are calling a ÒremarkableÓ and unprecedented statistic.

As many as 30 percent of sunlike stars possess close-in, relatively small planets Ñ only four to 30 times as heavy as Earth. These are small enough to have a solid, rocky surface like EarthÕs or an icy one like NeptuneÕs. The planets all lie near their parent star and take less than 50 days to complete an orbit.

Christophe Lovis of Geneva Observatory in Switzerland and his colleagues base their findings on a search for tiny wobbles, induced by the tug of small, unseen planets, in the motion of several hundred F, G and K stars. Such stars range from 0.7 to 1.2 times the mass of the sun. (The sun is a G type star.) The researchers measured the wobbles, indicated by periodic shifts in the wavelength of starlight, using a sensitive spectrometer called HARPS, for High Accuracy Radial velocity Planetary Search, on the European Southern ObservatoryÕs 3.6-meter telescope in La Silla, Chile.

Lovis and his collaborators found signs of 45 low-mass planets among a subset of the stars. These include at least eight superEarths Ñ objects that are likely to have rocky surfaces and have between about four and 10 times EarthÕs mass. Heavier objects, up to 30 times EarthÕs mass, are classified as ice giants or Neptune-like planets, which would have an icy surface and an atmosphere composed mainly of hydrogen and helium."