As astronomers scan the skies for Earth’s twin, they are assembling a collection of wild worlds large and small.

Most are too hot, cold or massive to support life as we know it.

Some might harbor some of the more exotic life forms found on this planet, such as the crabs and tube worms that congregate at volcanic vents on the seafloor. Those creatures, far from the sunlight on the surface, survive on the basis of chemosynthesis, turning chemicals into energy, as opposed to the photosynthesis-based life up top.

And it’s only a matter of time, astronomers say, before an Earth-size rocky planet is found in its star’s "Goldilocks zone," an orbit that is neither too cold nor too hot for the existence of liquid water.

In November, scientists from the University of Hawaii and the University of California, Berkeley, calculated that 1 in 5 sunlike stars in our galaxy have Earth-size planets that could host life. The findings were based on observations from NASA’s Kepler spacecraft and the Keck Observatory on Mauna Kea.

"What this means is, when you look up at the thousands of stars in the night sky, the nearest sunlike star with an Earth-size planet in its habitable zone is probably only 12 light-years away and can be seen with the naked eye," said UC-Berkeley graduate student Erik Peti­gura, who led the analysis of the data. "That is amazing."

Just a week earlier, astronomer Andrew Howard of UH-Manoa and colleagues reported finding the first-ever Earth-size rocky planet around another star.

As it happens, Kepler 78b, as the planet is known, is scorchingly close to its host star, making life impossible. But its size — only 20 percent larger than Earth — and density offer hope that similar worlds exist with more comfortable climes.

On Jan. 6 a team of astronomers announced the discovery of an exoplanet with the same mass of Earth but with a large, gassy atmosphere. Orbiting a red dwarf star about 200 light-years away, KOI-314c is so far the lightest planet to have both its mass and physical size measured.

It weighs the same as Earth but is 60 percent larger in diameter. The team, led by David Kipping of the Harvard-Smithsonian Center for Astrophysics, estimated the surface temperature at 220 degrees Fahrenheit, too hot for life as we know it.

There is another unfavorable factor.

"The crushing pressure of the atmosphere at the surface of the planet means you don’t have a proper surface and probably can’t have normal organic chemistry that life on the Earth depends upon," Howard said in an interview.

NASA launched the Kepler space telescope in 2009 to look for planets that cross in front of, or transit, their stars, which causes a slight dimming — about one-hundredth of 1 percent — in the star’s brightness. From among the 150,000 stars photographed every 30 minutes for four years, the Kepler team reported more than 3,000 planet "candidates."

The mass of the planet is then calculated from Earth-based telescopes like Keck, which look for the planet to wobble under the gravitational influence of the star.

Many of Kepler’s candidates are large planets with thick atmospheres like Neptune or in orbits so close to their stars that they are hellishly hot.

In December 2011, astronomers reported the first planet in the Goldilocks zone of a sunlike star. Kepler 22b is about 2.4 times the radius of Earth but may have liquid water on its surface.

"When you detect the planet, you get its radius," said Howard. "You can use ground-based telescopes to do the Doppler measurements to measure the mass. Mass plus radius tells you density. And from the density you can make some inferences about composition. But the really important information comes from studying the planet’s atmosphere."

That will become easier with a planned NASA mission called the Transiting Exoplanet Survey Satellite, scheduled for launch in 2017. TESS will conduct an all-sky survey to discover transiting exoplanets, ranging from Earth-size to gas giants, in orbit around the nearest and brightest stars in the sky.