5 Intriguing Earth-Sized Planets
The reigning champ of most Earth-sized planets yet is a world called Gliese 581 e, which circles a star that has four planets total. It is called the smallest because of its mass, which is just 1.9 times the mass of Earth, making it the lightest known alien planet to date. The other planets range in mass from 5 to 16 times the mass of Earth.
Credit: ESO/L. Calcada

The announcement today of a newfound group of alien planets orbiting a distant star has added another candidate for the growing list of potential Earth-sized worlds beyond our own solar system.

Astronomers still have to confirm the presence of the possible Earth-sized world. But they suspect it has a radius just 1.5 times that of Earth, which would make it the smallest alien planet yet found, if confirmed.

Here's a look at this world and four other possible Earth-sized planets that show the diversity of alien planets:

Smallest alien planet yet?

Name: Not yet named
Distance: 2,300 light-years from Earth
Mass: Unconfirmed, but radius estimated at 1.5 times that of Earth.
Announced: Today

Researchers still need to confirm that this planet exists, yet they do have a general idea of its characteristics.

This postulated planet circles the sun-like star Kepler-9 with two much larger, Saturn-sized planets, and it orbits closer to its star than its big companions do. All three planets could fit inside the orbit of Mercury if they were inside our own solar system.

Observations from NASA's planet-hunting spacecraft suggest this planet takes only 1.6 Earth days to revolve around the star.

It is likely not habitable because of its close orbit. The two outer planets are estimated to see daytime temperatures of 740 degrees Kelvin (872 degrees Fahrenheit) and 540 degrees Kelvin (512 degrees Fahrenheit), scientists have said. [The Strangest Alien Planets]

Planet orbiting star HD 10180

Name: HD 10180b (if confirmed)
Distance: 127 light-years from Earth
Mass: 1.4 times that of Earth
Announced: August 2010

This is another potential planet that remains to be confirmed, but astronomers using the European Southern Observatory (ESO) in Chile found hints of its existence around the sun-like star HD 10180, which may have up to seven planets in all.

Five Neptune-sized planets have been confirmed, with the Earth-sized world ? which ESO calls HD 10180b ? and another planet still pending. If all are confirmed, it would be the most planet-populated alien solar system yet found.

This planet, if it exists, could be among the least massive ? and thus closest to Earth-sized ? ever found outside our solar system. But the similarities with our home planet end there.

Astronomers think HD 10180b orbits extremely close to its parent star, just 2 million miles (3.2 million km) or so. Compare that to Earth, which stays about 93 million miles (150 million km) from the sun.

Observations suggest the planet completes its orbit in only 1.18 Earth days.

Rocky World

Name: Gliese 581e
Distance: 20.5 light-years away
Mass: 1.9 times that of Earth
Announced: April 2009

The alien world Gliese 581e definitely exists. It orbits the red dwarf star Gliese 581 along with at least three other planets.

Gliese 581e is nearly twice the mass of Earth, and it's likely a rocky planet. But life probably has no hope of taking root there. The planet is so close to its star ? it completes an orbit every 3.15 days ? that its surface is almost certainly far too hot.

A Candidate for Liquid Water

Name: Gliese 581d
Distance: 20.5 light-years away
Mass: 7-8 times that of Earth
Announced: April 2007

Gliese 581e's companion planet, Gliese 581d, is quite a bit bigger than Earth. But Gliese 581d intrigues astronomers, who think it might just be in the habitable zone of its star ? somewhere in that just-right range of distances that allow liquid water to exist on a planet's surface.

Another planet in the system, Gliese 581c, was also a contender for liquid water, but astronomers later found that it was likely too hot.

Another point in the Gliese 581 planet system's favor: its star appears to be old and stable.

Based on the stability of its light, astronomers think Gliese 581 has been around for a few billion years, and that it likely hasn't belched out huge, planet-scorching bursts of radiation too often.

Scorched World

Name: CoRot-7B
Where: 500 light-years away

Mass: 5 times that of Earth
Announced: February 2009

CoRoT-7B (named after the French telescope that discovered it) shares some important characteristics with Earth. It is, for example, the first confirmed rocky world outside our solar system. But it would not be a particularly pleasant place to live.

CoRot-7B orbits just 1.6 million miles (2.5 million km) from its parent star, and the planet is likely tidally locked: the same side is always facing the star, just as the moon only presents one face to Earth.

So one side of CoRot-7b is probably a hellishly hot 4,000 degrees Fahrenheit (2,200 degrees Celsius) while the other, in perpetual shadow, dips down to about minus 350 F (minus 210 C).