Habitable alien planets similar to Earth may not be that rare in the universe, a new study suggests.
About one in five sunlike stars observed by NASA's planet-hunting Kepler spacecraft has an Earth-size planet in the so-called habitable zone, where liquid water — and, potentially life — could exist, according to the new study. If these results apply elsewhere in the galaxy, the nearest such planet could be just 12 light-years away.
"Human beings have been looking at the stars for thousands of years," said study researcher Erik Petigura, a graduate student at the University of California, Berkeley (UC Berkeley). "How many of those stars have planets that are in some way like Earth? We're very excited today to start to answer that question," Petigura told SPACE.com. [9 Exoplanets That Could Host Alien Life]
The findings, detailed today (Nov. 4) in the journal Proceedings of the National Academy of Sciences and in a video describing the frequency of Earth-like planets, say nothing about whether these planets actually support life — only that they meet some of the known criteria for habitability.
An artist's representation of the 'habitable zone,' the range of orbits around a star where liquid water may exist on the surface of a planet. A new study unveiled Nov. 4, 2013 suggests one in five sunlike stars seen by NASA's Kepler spacecraft has potentially habitable Earth-size planets.
Petigura also presented the results today in a briefing at the second Kepler Science Conference at NASA Research Park in Moffett Field, Calif, in which the Kepler team also announced the discovery of hundreds of new exoplanets, including many in the habitable zone.
"I think it's by far the most trustable estimate available, but I don't think it's final," said Francois Fressin, an astronomer at the Harvard-Smithsonian Center for Astrophysics who was not involved with the study.
Taking a planet census
Petigura and his colleagues painstakingly developed software to sift through Kepler's mammoth data set. The spacecraft's field of view includes about 150,000 stars, but most of these fluctuate in brightness too much for a planet to be detectable. The team examined 42,000 of the "quietest" stars, finding 603 planet candidates around these stars, 10 of which were Earth-size and lay in the habitable zone.
The team defined Earth-size planets as ones having a radius one to two times that of Earth. Planets were considered to be in the habitable zone if they received about as much light as the Earth does from the sun (within a factor of four). [7 Ways to Discover Alien Planets]
They used the Keck I telescope in Hawaii to take spectra of the stars, in order to pin down the radii of the planets.
But this wasn't the end of the story. Just as taking a census requires some statistical corrections for the people the survey misses, the researchers had to make corrections for planets Kepler missed.
This image shows field of view of NASA's Kepler space telescope, which stared at a single region of stars in the in the constellation Cygnus, just above the plane of the Milky Way Galaxy.
The transit method of finding planets, by definition, only detects planets orbiting in the same plane of view as their host star, which includes just a fraction of the total number of planets. Study researcher Geoff Marcy of UC Berkeley compared planetary orbits to papers fluttering through the air. Very few are going to be edge-on, he said.
Secondly, the analysis misses some planets simply because the tiny amount of starlight they block makes them tricky to detect. To correct for this, the researchers inserted "fake planets" into the data so they could see how many their software would miss.
The analysis was a "Herculean task," Marcy said.
After making these corrections, the researchers had their result: About 22 percent of sunlike stars observed by Kepler have Earth-size, potentially habitable planets.
Chances for life
The researchers were quick to point out that the fact that these planets are Earth-size and lie in the habitable zone does not mean they could support life. The planets might have scorching-hot atmospheres, or no atmospheres at all, they said. Even if the planets have all the basic ingredients for life, scientists don't know the probability that life would ever get started.
The definition of Earth-size planets in this study was pretty broad, Fressin said. For instance, a planet that has a radius twice the size of Earth's might not even be rocky, he said.
Kepler mission scientist Natalie Batalha, an astronomer at NASA's Ames Research Center who was not involved with the study, agrees it's a generous definition. Rocky planets with a radius about 1 to 1.5 times the size of Earth's have been found, but the fraction of larger planets that are rocky is probably much lower, Batalha told SPACE.com. Still, it's a fair start, she said.
"Kepler's prime objective was to understand the prevalence of habitable planets in the galaxy," Batalha said at a news conference. "This is the first time a team has offered such a number for stars like the sun."
The researchers had to extrapolate the number of planets with orbits longer than 200 days, because these haven't been detected in the Kepler data. "Ideally, we won't rely on extrapolations," Batalha said. "But as a first cut, this is a valid thing to do."
Analysis of four years of precision measurements from NASA's planet-hunting Kepler spacecraft shows that about one in five sunlike stars may have Earth-sized planets in the habitable zone.
Last week, Marcy and his colleagues reported the discovery of the alien planet Kepler-78b, a rocky world nearly the same size and density as the Earth. But Kepler-78b hugs its star at a distance far too close and hot to be habitable, with surface temperatures of about 3,680 degrees Fahrenheit (2,027 degrees Celsius).
Kepler went out of commission in May, after the loss of a wheel used for pointing the spacecraft. Nevertheless, scientists will mine Kepler data for decades to look for potentially habitable planets.
"Maybe with future instruments, we could actually image these planets," Petigura said.
- Gallery: A World of Kepler Planets
- The Search For Another Earth | Video
- 7 Greatest Alien Planet Discoveries by NASA's Kepler Spacecraft (So Far)
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NASA's Kepler Mission Discovers Planet
In this handout illustration made available on December 5, 2011 by NASA, the Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star is digitally illustrated. For the first time NASA's Kepler mission has confirmed a planet to orbit in a star's habitable zone; the region around a star, where liquid water, a requirement for life on Earth, could persist. The planet is 2.4 times the size of Earth, making it the smallest yet found to orbit in the middle of the habit. Clouds could exist in this earth's atmosphere, as the artist's interpretive illustration depicts. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images)
NASA's Kepler Mission Discovers Planet
In this handout illustration made available on December 5, 2011 by NASA, a diagram compares our own solar system to Kepler-22, a star system containing the first 'habitable zone' planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth. The diagram displays an artist's rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun. Kepler-22b has a yearly orbit of 289 days. The planet is the smallest known to orbit in the middle of the habitable zone of a sun-like star and is about 2.4 times the size of Earth. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images)
Extrasolar Planet HD 209458 b, Osiris
Artist's conception released by NASA of extrasolar planet HD 209458 b, also known as Osiris, orbiting its star in the constellation Pegasus, some 150 light years from Earth's solar system. Scientists have used an infrared spectrum -- the first ever obtained for an extrasolar planet -- to analyze Osiris' atmosphere, which is said to contain dust but no water. The planet's surface temperature is more than 700 Celsius (1330 Fahrenheit).'
Planet & Its Parent Star
Picture released 04 October 2006 by the European Space Agency shows an artist's impression of a Jupiter-sized planet passing in front of its parent star. Such events are called transits. When the planet transits the star, the star's apparent brightness drops by a few percent for a short period. Through this technique, astronomers can use the Hubble Space Telescope to search for planets across the galaxy by measuring periodic changes in a star's luminosity. The first class of exoplanets found by this technique are the so-called 'hot Jupiters,' which are so close to their stars they complete an orbit within days, or even hours. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM
Picture released 04 October 2006 by the European Space Agency shows an artist's impression of a unique type of exoplanet discovered with the Hubble Space Telescope. This image presents a purely speculative view of what such a 'hot Jupiter' (word dedicated to planets so close to their stars with such short orbital periods) might look like. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM
The Goldilocks Planet: Glises 581 G
Scientist have found a new potentially habitable planet.
Imagining Extrasolar Planets
From the Spitzer Science Center. While astronomers have identified over 500 planets around other stars, they're all too small and distant to fill even a single pixel in our most powerful telescopes. That's why science must rely on art to help us imagine these strange new worlds. From Spitzer Space Telescope. Even without pictures of these exoplanets, astronomers have learned many things that can be illustrated in artwork. For instance, measurements of the temperatures of many "Hot Jupiters," massive worlds orbiting very close to their stars, hint that their atmospheres may be as dark as soot, glowing only from their own heat. While "Hot Jupiters" would be relatively dark in visible light, compared to their stars, their brightness is proportionally much greater in the infrared. Illustrating this dramatic contrast change helps explain why the infrared eye of NASA's Spitzer Space Telescope plays a key role in studying exoplanets. As our understanding evolves, so must the artwork. Astronomers found a blazing hot spot on the exoplanet Upsilon Andromedae b that at first, appeared to face towards its star. More data has revealed that the hottest area is actually strangely rotated almost 90 degrees away, near the day/night terminator. WASP 12b is as hot as the filament in a light bulb, and would be blazing bright to our eyes. Most interestingly, if it proves to have a strongly elliptical orbit, as first thought, calculations show it would be shedding some of its outer atmosphere <b>...</b>