Scientists have announced the discovery of three planets, in two planetary systems, that are in or on the edge of the so-called habitable zone, the range that is just the right distance from their stars so they wouldn't be too hot or too cold to have liquid water.
They made these discoveries with NASA's Kepler space telescope.
William Borucki, the Kepler science principal investigator at NASA's Ames Research Center in California, described the Kepler-62 planetary system, which he says is 1,200 light years away. One light year is 10 trillion kilometers.
Much like our solar system, he said, Kepler-62 is home to two habitable-zone worlds, called Kepler-62e and Kepler-62f. Those planets are about one-and-a-half times the size of Earth.
"In fact, these two planets are our best candidates for planets that might be habitable, not just in the habitable zone," he told reporters at a news conference at Ames. "They're part of a planetary system of five planets that we've discovered so far, but these are the two that are most important."
Borucki said 62e might be a water world, but scientists aren't certain as they only know its radius, not its mass. He says planet 62f, the smaller of the two, might very well be a rocky planet, and possibly have polar caps, significant land masses and water.
Planets 62e and 62f orbit a star about two-thirds the size of our sun and one-fifth as luminous. Borucki says if you imagined yourself standing on 62f, it would be as bright as a cloudy day on Earth.
Why Size Matters
These are the smallest exoplanets found in a habitable zone.
Lisa Kaltenegger of the Max Planck Institute for Astronomy in Heidelberg, Germany, says scientific models indicate that planets about this size could have water.
"The fascinating idea is that maybe we've actually found the first ocean planets, the first water worlds out there, and what it just shows you is the diversity that we're discovering out there," she said. "And let me say that we only have the radius, so what we infer from the models is very exciting but always has to be taken a little bit with a grain of salt."
Thomas Barclay, a Kepler scientist at the Bay Area Environmental Research Institute in California, is most excited about the discovery of a planet in another system -- Kepler 69. He says Kepler-69c is a planet 70-percent bigger than Earth that orbits a sun-like star, and it's just on the edge of the habitable zone.
"The habitable zone is a region between fire and ice. Well, this is orbiting closer to the fire than the ice," he said. "We consider this perhaps to be more of a super-Venus than a super-Earth, perhaps."
Kepler Space Telescope
The Kepler space telescope explores the structure and diversity of planetary systems. It finds planets by looking for tiny dips in the brightness of a star that occur when a planet crosses in front of it.
Kepler was launched in 2009, and it has found more than 100 planets.