NASA's Kepler planet-hunting probe has identified two potentially habitable planets only a little bigger than Earth, circling a star that's 1,200 light-years away. The planets could conceivably be covered by a global ocean, and they may well lead the growing list of alien worlds that can host life as we know it.
"These two planets are our best candidates for planets that might be habitable," said Bill Borucki, a space scientist at NASA's Ames Research Center who is the principal investigator for the $600 million Kepler mission.
The two habitable-zone planets, Kepler-62e and Kepler-62f, are part of a five-planet system that lies in the constellation Lyra, within a patch of sky that's been monitored by the Kepler space telescope over the past four years. The Kepler-62 parent star is about two-thirds the size of our own sun and about a fifth as bright. Three of the star's confirmed planets circle the star in orbits so close that they'd be too hot for life. But the e and f planets are considered to lie in a zone where liquid water could exist, a ring of space that's defined as the habitable zone.
Water worlds?
Two members of the Kepler science team say their modeling suggests the two planets could be "water worlds" — with no land in sight.
"These planets are unlike anything in our solar system. They have endless oceans," Lisa Kaltenegger, an astronomer at the Max Planck Institute for Astronomy and the Harvard-Smithsonian Center for Astrophysics, said in a news release. "There may be life there, but could it be technology-based like ours?"
The report on the Kepler-62 system was published online on Thursday by the journal Science, and was the focus of a NASA news conference timed to coincide with publication. The water-world analysis, authored by Kaltenegger and Harvard's Dimitar Sasselov and Sarah Rugheimer, is contained in a separate paper that has been accepted for publication in The Astrophysical Journal.
The characterization of the two planets' habitability is based on an analysis of their size, plus what's known about the parent star. The Kepler data can show how wide a planet is, and how quickly it makes its orbit, by analyzing the telltale dips in light as the planet passes over its parent star. But Kepler can't make direct observations of a planet's mass. So, in Kepler-62's case, scientists had to make educated guesses about the planets' mass, composition and whether they had atmospheres.
Kepler-62e is 1.6 times as wide as Earth and orbits its star every 122.4 Earth days. Kepler-62f is 1.4 times Earth's width, with an orbital period of 267.3 Earth days. "It's highly likely they're rocky planets," Borucki told NBC News. "They might be water worlds, but they are so different, we just don't know."
What would life be like?
Astrobiologists say the fact that the planets are bigger than Earth wouldn't be an obstacle for life. In fact, some experts argue that a super-Earth is more likely to have life than an Earth-sized planet. "If you and I walked on it, our weight would double," Borucki said. "But my weight has doubled since I was a teenager ... so we could do it."
If the planets had atmospheres like Earth's, Kepler-62e's surface temperature would be 86 degrees Fahrenheit (30 degrees Celsius), while Kepler-62f's temperature would be 19 degrees below zero F (-28 degrees C), Borucki said. "You'd see the sun being substantially larger than our sun, because it's so much closer," he said. "But it'd be darker, like walking around on a cloudy day."
In their research paper, Kaltenegger and Sasselov assume that Kepler-62e has a slightly cloudier atmosphere than Earth's, and that Kepler-62f has a thick carbon-dioxide atmosphere with a strong greenhouse effect. Without a thick atmosphere, Kepler-62f could get chillier than Mars. It might even look more like a Europa-style iceball than a Kevin Costner-style water world.
"Kepler-62e probably has a very cloudy sky, and is warm and humid all the way to the polar regions," Sasselov said. "Kepler-62f would be cooler, but still potentially life-friendly. The good news is, the two would exhibit distinctly different colors and make our search for signatures of life easier on such planets in the near future."
Habitable worlds ahead
Kepler-62e and Kepler-62f aren't the first habitable-zone planets to be identified by the Kepler team, and they won't be the last. A year and a half ago, Kepler-22b came to light as the mission's first potentially habitable planet. It's 2.4 times wider than Earth, which puts it halfway between our planet and Neptune on the size scale. Kepler-47c, unveiled last year, is also a habitable-zone planet — but it's 4.6 times wider than Earth, which makes it Neptune-sized.
This January, the science team discussed the habitability of another candidate planet, then known as KOI 172.02. The existence of that world has now been confirmed under the name Kepler-69c, with a size that's 1.7 times Earth's width. "Today we can announce that this is a bona fide planet," Thomas Barclay, an astronomer at Ames Research Center, said during Thursday's news conference.
Three months ago, Kepler-69c was hailed as potentially the most Earthlike world detected beyond our solar system, but now researchers say Kepler-62e and Kepler-62f could be stronger contenders.
There will be more contenders ahead: Borucki said about four dozen of the more than 2,700 candidate planets being tracked by Kepler lie within their stars' habitable zones, and it takes about a year to confirm each candidate's existence through detailed analysis. "We really wish we were faster," he told NBC News. "I really wish we could knock off one a week."
Boruckin and his colleagues are poring through the oceans of observations coming in from the Kepler telescope, and although the spacecraft has had its problems, he's hoping that the flood of data will continue for years to come.
"When you're born a scientist, they leave out the gene for saying, 'We have enough data,'" Borucki joked.
More about the planet hunt:
- Alien Earths in our backyard?
- 17 billion hot Earths in our galaxy
- NBC News on planets | Cosmic Log on Kepler
Borucki, Kaltenegger, Sasselov and Barclay are among 64 authors of the Science paper, "Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone." Barclay and Borucki are among 31 authors of "A Super-Earth-Sized Planet Orbiting in or near the Habitable Zone around Sun-like Star," published in The Astrophysical Journal. Kaltenegger, Sasselov and Rugheimer are the authors of "Water Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the Case of Kepler-62e and -62f."
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.