Astronomers believe they have discovered Earth's "bigger cousin" orbiting a star 15 light years away.
The planet could be the first rocky, terrestrial world to be found circling a normal star outside the solar system.
Nearly all of the nearly 150 other extrasolar planets identified so far have been larger than Uranus, an icy gas giant in our own system with about 15 times the mass of the Earth.
Finding a rocky planet orbiting a common, ordinary star raises hopes of one day discovering another Earth.
The new planet orbits the star Gliese 876, 15 light years away in the direction of the Aquarius constellation. Like other extrasolar planets, it was not observed directly but detected by measuring the way its gravity causes the parent star to "wobble".
Astronomers have calculated that the planet is about six times bigger than Earth, making it small enough to be rocky.
Unlike the Earth, the planet orbits at a distance of only two million miles from its star, orbiting Gliese 876 in just two days.
It is so close that its day-side temperature could reach between 200C and 400C - far too hot for any kind of Earth-like life. Nevertheless the astronomers who made the discovery from the Keck Observatory in Hawaii are ecstatic.
Team member Dr Paul Butler, from the Carnegie Institution of Washington, said: "This is the smallest extrasolar planet yet detected and the first of a new class of rocky terrestrial planets. It's like Earth's bigger cousin."
Although there is no proof that the planet is rocky, scientists believe its low mass means it could not hold on to huge amounts of gas like Uranus or Jupiter.
Professor Geoffrey Marcy, from the University of California at Berkeley, who led the astronomers, said: "This planet answers an ancient question.
"Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets. Now, for the first time, we have evidence for a rocky planet around a normal star."
Dr Michael Turner, from the US National Science Foundation (NSF), which helped to fund the research, said: "Today's results are an important step toward answering one of the most profound questions that mankind can ask: Are we alone in the universe?"
PA