Hailed as the most Earth-like alien planet when it was discovered, Kepler-69c (also known as KOI-172.02) is an exoplanet about 2,700 light-years from Earth. Its radius is about 1.5 times that of Earth's, which puts it in the "super-Earth" classification of alien planets — those that are about two to 10 times the size of Earth.
Researchers are especially intrigued because the planet orbits in the habitable zone of its star, which is just slightly smaller than that of Earth's sun. While the possible presence of water is not a guarantee of life, it does show that life-friendly environments are possible for planets that are about the same size as that of Earth.
"This was very exciting because it's our first habitable-zone super-Earth around a sun-type star," astronomer Natalie Batalha, a Kepler co-investigator at NASA's Ames Research Center in Moffett Field, Calif., said at the 221st meeting of the American Astronomical Society in January 2013, when the planet candidate's discovery was made public.
"It's orbiting a star that’s very much like our sun," Batalha added. "Previously the ones we saw were orbiting other types of stars."
Possibility of water
Compared with our own solar system, Kepler-69c's orbit around its star is slightly smaller than that of Earth — it's at a distance of about 70 million miles (112 million kilometers) compared to Earth's 93 million miles (150 million km). A year on Kepler-69c takes about 242 Earth-days to complete.
The planet candidate was spotted by the Kepler space telescope, which found thousands of other candidates during its primary mission. The telescope's finds included Kepler-62e and Kepler-62f, also "super-Earth" planets orbiting in the habitable region around their two host stars. Kepler worked into an extended mission between 2009 and 2013 before science operations ceased due to problems with two of its four reaction wheels or pointing devices. (NASA is considering other uses for the telescope in future years.) [Infographic: NASA's Planet-Hunting Kepler Telescope Explained]
At the time of its discovery in 2013, scientists said they were excited by the possibility of water on its surface. While its prospects of being a real planet were a little uncertain at the time of its discovery — it was designated a Kepler Object of Interest (KOI) in a paper in January — by April another paper based on more data was published saying that the probability of it being a planet was 99.1 percent.
"It's a big deal," astrophysicist Mario Livio, of the Space Telescope Science Institute in Baltimore, told SPACE.com at the time its discovery was announced in January. "It's definitely a good candidate for life."
It is unclear if the planet is rocky, but there could be water on its surface. "Maybe there's no land life, but perhaps very clever dolphins," Livio joked.
More planet candidates
Since Kepler-69c's discovery, several other planets were found in Kepler data that also show habitable planets, and in one case, a planet that is Earth's size but too close to its host star to have life as we know it. Astronomers have many months of Kepler data to analyze, and they expect to find even more planets. [Related: 7 Greatest Alien Planet Discoveries by NASA's Kepler Spacecraft (So Far)]
In 2013, astronomers revealed the presence of Kepler-78b, a "lava planet" with an 8.5-hour year, but which is about the same size as Earth. The researchers were on the hunt for planets with short orbital periods (which usually include "hot Jupiters," or large gas giants that are close to their star). They then discovered this much smaller one among their data. "It turned out to be a really rewarding session because we ended up finding this planet," Roberto Sanchis-Ojeda, a Ph.D. student at the Massachusetts Institute of Technology who led the research, told SPACE.com in 2013.