BYAfter discovering scores of oddball planets around distant stars in recent years, astronomers have finally detected something that looks familiar: a family of planets including one which appears to be roughly equivalent in size and orbit to Jupiter.

“For the first time, we?ve found a family of planets that has some similarity to our own solar system,” said Geoffrey W. Marcy, a professor of astronomy at the University of California at Berkeley, who announced the discovery Thursday with his colleague R. Paul Butler of the Carnegie Institution of Washington.

“Clearly, finding another system like that of our own begs the question that the ancient Greeks asked: Are there other Earths?” said Mr. Marcy.

At a news conference at the National Aeronautics and Space Administration in Washington, the scientists announced a total of 15 new planets, one of which weighs in as the smallest found thus far, at 15 percent the mass of Jupiter, or half the size of Saturn.

“They are announcing more planets today than we have in our solar system,” said David N. Spergel, a professor of astrophysics at Princeton University. Since the mid-1990s, scientists have found more than 90 planets orbiting distant stars. Astronomers call them extra-solar planets, or “exoplanets” for short.

All of the previously identified exoplanets have seemed strange because they are giant planets with tight orbits, staying much closer to their stars than Earth does to the Sun. That made our solar system appear unique because the gaseous giants around the Sun all keep their distance. Jupiter, the closest, orbits at 5.2 astronomical units (an astronomical unit equals 93 million miles, the average distance between Earth and the Sun).

The planet reported Thursday, however, orbits its star at a distance of 5.9 astronomical units and has a mass of at least 3.5 to 5 times that of Jupiter. Its parent star is 55 Cancri, which is visible to the naked eye in the constellation Cancer.

The same star has two other planets orbiting close in, one of which Mr. Marcy and Mr. Butler reported Thursday and another of which they described in 1996.

The new find has excited scientists because “suddenly our solar system is not special,” said Mr. Spergel. He noted that Jupiter is thought to have played an important role in allowing life to evolve on Earth: The giant planet has functioned as a gargantuan broom, sweeping comets away from the inner solar system where some would have collided with Earth, perhaps sterilizing the planet. With a Jupiter-like planet orbiting such a distance from 55 Cancri, it could have played a similarly protective role in that system.

The technique used by Mr. Marcy and Mr. Butler cannot detect planets as small as Earth, so it remains unknown whether 55 Cancri harbors any. But calculations by Gregory Laughlin, an assistant professor of astronomy and astrophysics at the University of California at Santa Cruz, indicate that an Earth-size planet could safely orbit at 1 astronomical unit from 55 Cancri without getting disrupted by the pull of the giant planets in that system, said Mr. Marcy.

At a distance of 41 light-years from Earth, the new planet is close enough that astronomers could try to obtain a direct image of it, said Mr. Spergel. “This may well be the first planetary system we image, beyond our own.”