The Earth-size planet seen by the European astronomers appears to be one of seven circling the star, HD 10180, located in the constellation Hydrus. Christophe Lovis of the University of Geneva, who led the observations, said the group was certain about the existence of five of the planets, all about the mass of Neptune, but squeezed into orbits closer to the star than Mars is to the Sun.

They are less certain about the smallest planet. “For this one, we have about 1 percent false alarm possibility,” Dr. Lovis said. “For us, 99 percent is just not enough to be completely sure.”

The team also tentatively detected a larger, Saturn-size planet farther from the star.

Neither of the slightly-bigger-than-Earth planets is Earth-like or has much chance for anything to live there. Both have orbits very close to their stars that would sear the surfaces. The small Kepler-9 planet completes an orbit in just over a day and a half at a distance of 2.5 million miles from the star. The small HD 10180 planet is even closer and faster, less than two million miles from the star and completing an orbit in about 28 hours. Earth, by contrast, is 93 million miles from the Sun, and its orbit takes 365 days to complete.

“If one particular word can describe planetary systems today, it’s ‘diverse,’ ” said Douglas N. C. Lin, a professor of astronomy and astrophysics at the University of California, Santa Cruz, who was not involved with either team. “Planets are common, and their properties are diverse.”

To date, most of the extrasolar planets have been found using the technique of the European team, by looking for wobbles in the wavelength of light from the star caused by the back-and-forth gravitational tugging of unseen planets. The discovery of the HD 10180 planets results from six years of observations at the European Southern Observatory’s 3.6-meter telescope in Chile.

If the orbits of planets are not edge-on to observers on Earth, the technique underestimates the masses of the planets. Dr. Lovis said that for HD 10180, computer simulations show that for the orbits to be stable, the planets cannot be more than three times the minimum masses calculated.

Some planets have been detected when a star dims momentarily as a planet passes in front. The duration of the dip tells the size of the planet, and the time between the light dips tells the length of the planet’s orbit.