An artist's conception of Kepler-22b, a planet known to be in the habitable zone of a sun-like star.
Kepler-22b is a planet orbiting in the habitable zone of its sun-like star, Kepler-22, which is located roughly 600 light-years from Earth. With a radius of about 2.4 times that of Earth, astronomers noted after its discovery in 2011 that the planet's temperatures about the same. If the greenhouse effect is the same as well, scientists estimated its surface temperature is a life-friendly 72 degrees Fahrenheit (22 degrees Celsius).
Although the planet is larger than Earth, astronomers hailed the discovery as a step to Kepler's goal of discovering Earth-size planets in the habitable zones of their respective stars.
"We're getting closer and closer to discovering the so-called 'Goldilocks planet,'" Pete Worden, director of NASA's Ames Research Center in Moffett Field, Calif., said during the 2011 press conference announcing its discovery.
Habitability tough to define
According to NASA's exoplanet archive, Kepler-22b orbits its star every 290 days and is about 0.85 Earth-sun distances or astronomical units away. It has a density that is similar to that of rock, meaning that its environment could be similar to that of Earth -- assuming other properties are also within the same range as our planet.
Habitability depends not only on the distance from the star, but also on things such as the star's variability and whether the planet has an atmosphere. It's harder for current telescopes to pick out planetary properties for those worlds that are closer to Earth's size, however.
Kepler-22b was discovered using the planet-seeking Kepler space telescope, which launched in March 2009 and concluded its primary planetary mission four years later after two of its four reaction wheels or pointing devices failed. (As of late 2013, NASA is considering other uses for the telescope, which could have it looking for planets again.) [Infographic: NASA's Planet-Hunting Kepler Telescope Explained]
The telescope detected thousands of planets, including Kepler-22b, by monitoring a decrease in light from its host star that occurs when the planet passes in front of the star from the perspective of Earth. This process is called the "transit method," as opposed to the "radial velocity" method that looks at how the star wobbles as the planet orbits around it.
Kepler-22b ended up being a very early discovery for the telescope, which caught the planet in the act of transiting only three days after mission managers said the telescope was ready to start observations. The "defining transit", as managers put it, took place during the holidays in 2010.
Looking for a second Earth
Kepler-22b is an important symbol for the exoplanet hunters that are seeking an Earth 2.0. Finding an Earth-size planet within the habitable region of its star was a goal of the Kepler mission. There are still years of data to plumb through, so it's quite possible that the planet is lurking in there somewhere. [Countdown: 7 Greatest Alien Planet Discoveries by NASA's Kepler Spacecraft (So Far)]
"I believe Kepler will find a 'Goldilocks planet' within the next two years," said Shawn Domagal-Goldman, a researcher at NASA Headquarters in Washington, D.C. who specializes in exoplanet biology, in a 2012 statement.
"We'll be able to point at a specific star in the night sky and say 'There it is — a planet that could support life!'"
Astronomers are getting closer, however. In August 2013, researchers announced they had found a "lava planet" that orbits around its star every 8.5 hours. Although the planet is far too hot to support life as we know it, the size of the planet is practically identical to Earth.
"With a lot of effort and a lot of patience, you could detect the transit from the largest telescopes," Roberto Sanchis-Ojeda, a Ph.D. student at the Massachusetts Institute of Technology who led the research, told SPACE.com in 2013. "We also think it's possible with the Hubble Space Telescope. From space, you should not have any issue [spotting it.]"