HATFIELD, England – In the search for Earth-like planets, astronomers zeroed in Tuesday on two places that look awfully familiar to home. One is close to the right size. The other is in the right place. European researchers said they not only found the smallest exoplanet ever, called Gliese 581 e, but realized that a neighboring planet discovered earlier, Gliese 581 d, was in the prime habitable zone for potential life.
"The Holy Grail of current exoplanet research is the detection of a rocky, Earth-like planet in the 'habitable zone,'" said Michel Mayor, an astrophysicist at Geneva University in Switzerland.
An American expert called the discovery of the tiny planet "extraordinary."
Gliese 581 e is only 1.9 times the size of Earth  while previous planets found outside our solar system are closer to the size of massive Jupiter, which NASA says could swallow more than 1,000 Earths.
Gliese 581 e sits close to the nearest star, making it too hot to support life. Still, Mayor said its discovery in a solar system 20 1/2 light years away from Earth is a "good example that we are progressing in the detection of Earth-like planets."
Scientists also discovered that the orbit of planet Gliese 581 d, which was found in 2007, was located within the "habitable zone"  a region around a sun-like star that would allow water to be liquid on the planet's surface, Mayor said.
He spoke at a news conference Tuesday at the University of Hertfordshire during the European Week of Astronomy and Space Science.
Gliese 581 d is probably too large to be made only of rocky material, fellow astronomer and team member Stephane Udry said, adding it was possible the planet had a "large and deep" ocean.
"It is the first serious 'water-world' candidate," Udry said.
Mayor's main planet-hunting competitor, Geoff Marcy of the University of California, Berkeley, praised the find of Gliese 581 e as "the most exciting discovery" so far of exoplanets  planets outside our solar system.
"This discovery is absolutely extraordinary," Marcy told The Associated Press by e-mail, calling the discoveries a significant step in the search for Earth-like planets.
While Gliese 581 e is too hot for life "it shows that nature makes such small planets, probably in large numbers," Marcy commented. "Surely the galaxy contains tens of billions of planets like the small, Earth-mass one announced here."
Nearly 350 planets have been found outside our solar system, but so far nearly every one of them was found to be extremely unlikely to harbor life.
Most were too close or too far from their sun, making them too hot or too cold for life. Others were too big and likely to be uninhabitable gas giants like Jupiter. Those that are too small are highly difficult to detect in the first place.
Both Gliese 581 d and Gliese 581 e are located in constellation Libra and orbit around Gliese 581.
Like other planets circling that star  scientists have discovered four so far  Gliese 581 e was found using the European Southern Observatory's telescope in La Silla, Chile.
The telescope has a special instrument which splits light to find wobbles in different wavelengths. Those wobbles can reveal the existence of other worlds.
"It is great work and shows the potential of this detection method," said Lisa Kaltenegger, an astronomer at the Harvard-Smithsonian Center for Astrophysics.
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Associated Press Science Writer Seth Borenstein contributed to this report from Washington.
Scientists have made two major discoveries in the search for other Earth-like planets. British astronomers say they have discovered the smallest known Earth-like exoplanet orbiting a living star called Gliese 581 e and also discovered that a nearby planet called Gliese 581 d, discovered in 2007, is in the much sought after 'habitable zone'. Astronomers also believe there is a possibility that 581 d could have liquid oceans on its surface, calling the find "extraordinary."
Artist's impression of the planetary system around the red dwarf Gliese 581. 
"The Holy Grail of current exoplanet research is the detection of a rocky, Earth-like planet in the 'habitable zone'," said astrophysicist at Switzerland's Geneva University, Michel Mayor.

Astronomers say 581 e is 20.5 light years away from Earth and that it's roughly 1.9 times larger than Earth, making it the smallest exoplanet discovered to date. It was found using the High Accuracy Radial Velocity Planet Searcher (HARPS). It orbits too close to its parent star, Gliese 581, which would make it too hot, unable to support life as we know it. It takes just over 3 days for it to orbit the star.

Exoplanet 581 d, however, is another story. Astronomers say this makes it possible to not only support life, but have liquid water in the forms of oceans. 581 d is approximately 7.7 times larger than Earth. Astronomers originally thought it was too cold to have liquid water thus would not be able to host Earth-like life.

"It is very exciting that such a promising candidate for an Earthlike planet has been found so close to us. It means there are likely to be many more when we search further," added Mayor.