At least five Earth-sized planets appear to be orbiting their stars at just the right distance to host liquid water, NASA said Wednesday, announcing a significant step in the agency's search for extraterrestrial life.
The Kepler telescope gleaned an extraordinary amount of new information in less than two years of Earth orbit, observing a small slice of the Milky Way galaxy.
The discoveries include whole solar systems and planets close enough to each other to give scientists clues about what they are made of — thus how hospitable to life they may be.
"Not only is Kepler telling us about planetary systems of a type that we had no idea existed, but right now it's providing our best clues on the composition of these planets as individual worlds," Kepler scientist Jack Lissauer said Wednesday at a briefing to announce the discoveries.
Several hundred new "planet candidates" — they are, for now, just known to be celestial bodies — bring the Kepler mission discoveries to 1,235 possible planets, potentially tripling the count of extra-solar planets from the current 519.
Of those bodies, 68 are about the size of Earth, and five of those Earth-sized bodies were observed in what scientists call the "habitable zone," where liquid water could exist on a planet's surface.
"If we find that Earth-sized planets are common in habitable zones of stars, it's very likely that life is common around these stars," Kepler scientist William Borucki said Wednesday.
Before Wednesday, there were only thought to be at most two extra-solar planets in their system's habitable zone.
Another 49 planet candidates were found by Kepler in the habitable zone, but their enormous sizes suggest big, gassy planets more like Saturn, which, regardless of their distance from their star, cannot sustain life.
The Kepler mission has also turned up 170 stars that appear to be orbited by multiple planets. Such systems are crucial because the planets' effects on one another's orbits allow scientists to glean more information about their environments.
"We're studying systems of planets," Mr. Borucki said, describing multiplanet systems as "the most valuable things that we can find."
The largest such system so far has six planets orbiting a star dubbed Kepler-11.
"The five inner planets are essentially close together, something that we didn't think would happen for worlds of this size," Mr. Lissauer said. The planets' proximity means they are disturbing one another's orbits enough to determine what they might be made of, he said.
The inner two possible planets of this system could be mixtures of rock and water or rock and gas, while the outer planets are large for their mass and must be made of large parts hydrogen and helium, Mr. Lissauer said.
Such a pattern would be similar to our solar system, which has four small, rocky "inner planets" (Mercury, Venus, Earth and Mars) and four large, gassy "outer planets" (Jupiter, Saturn, Uranus and Neptune).
However, our own solar system also suggests limits to Kepler's discoveries. For example, Mars and Venus are on the edge of our solar system's habitable zone and even have water (if not in liquid form), but neither has known living beings. Also, being hospitable to life doesn't necessarily mean hospitable to intelligent life, but possibly just to simple forms such as bacteria.
Kepler cannot determine whether these planets do have the kinds of atmospheres and carbon contents necessary for advanced life forms.
Still, Yale University astronomy professor Debra Fischer said the system orbiting Kepler-11 is an enormous discovery.
"It shows that planetary systems with several small planets like our own seem to be common," she said. "The adjacent neighborhoods in the galaxy look a lot like our own neighborhood. I think that's encouraging and important if we're trying to make extrapolations about the formation of planets elsewhere and perhaps life."
Ms. Fischer said she is amazed at the number of possible planets — especially Earth-sized planets — the mission has discovered since the orbiting observatory was launched in March 2009. She had expected the discoveries to come in slower, over the course of years.
"Kepler's blown the lid off of everything we know about extra-solar planets," she said.
The Kepler observatory uses a telescope that measures light to monitor the brightness of more than 156,000 stars. Orbiting planets block some of that light, causing stars to appear to dim at regular intervals.
The telescope remains constant over the same small patch of sky, about a four-hundredth of the total sky.
This batch of data has undergone only preliminarily analysis and, now that the entire astronomical community has access to it, it is expected to turn up many more discoveries, especially some refining our understanding of Wednesday's data.
"The stars that surround us have a huge number of planets and candidates for us to look at," Mr. Borucki said. "Kepler is a step in the exploration of the surrounding galaxy to find life and the extent of life in our galaxy."
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