By: SPACE.com Staff
Published: 01/07/2013 01:43 PM EST on SPACE.com
This story was updated at 1:55 p.m. EST.
The Milky Way hosts at least 17 billion Earth-size alien planets, and probably many more, a new study reveals.
Astronomers have determined that about 17 percent of stars in our galaxy harbor a roughly Earth-size exoplanet in a close orbit. Since there are 100 billion or so stars in the Milky Way, that works out to a minimum of 17 billion small, rocky alien worlds, or an Earth-size planet around one of every six stars.
And there are probably many more such planets orbiting at greater distances from their stars, some of which may even be "alien Earths" capable of supporting life as we know it.
"These kind of rocky objects are everywhere," team member Francois Fressin, of the Harvard-Smithsonian Center for Astrophysics (CfA), told reporters today (Jan. 7) during a meeting of the American Astronomical Society in Long Beach, Calif. [The Strangest Alien Planets (Gallery)]
Crunching the numbers
The research team conducted an analysis of data collected by NASA's planet-hunting Kepler Space Telescope.
Kepler detects alien worlds by noting the telltale dips in brightness caused when planets cross the face of — or transit — their parent stars from the instrument's perspective. The telescope, which launched in March 2009, flagged more than 2,700 potential planets in its first 22 months of operation, more than 100 of which have been confirmed to date.
The research team wanted to know how complete and accurate Kepler's survey has been — that is, what percentage of its finds are real, and how many planets is it likely missing? So they came up with a simulation that mimicked the telescope's work, finding that about 90 percent of its detections are probably the real deal.
"There is a list of astrophysical configurations that can mimic planet signals, but altogether, they can only account for one-tenth of the huge number of Kepler candidates," Fressin said in a statement. "All the other signals are bona-fide planets."
The study has been accepted for publication in The Astrophysical Journal.
Planets, planets everywhere
Using information from both the actual and simulated Kepler surveys, the team calculated some estimates about how common different types of planets are thoughout the Milky Way.
They determined, for example, that 17 percent of stars have a planet 0.8 to 1.25 times the size of Earth in tight orbits, with periods of 85 days or less. About 25 percent of stars have a so-called "super-Earth" (worlds 1.25 to 2 times as big as our own) in an orbit of 150 days or less — the same percentage that hosts a "mini-Neptune" (a planet 2 to 4 times Earth's size) with an orbital period up to 250 days.
Big planets such as Saturn or Jupiter are far less common. Only 5 percent of stars harbor a gas giant with an orbital period of 400 days or less, researchers said.
Overall, the team found that about 50 percent of all stars in the Milky Way have a planet the size of Earth or larger in a tight orbit. Extrapolation and incorporation of data from other instruments suggest that virtually all sun-like stars host planets, Fressin said.
Further, stars don't have to be sun-like to host an Earth-size world. The team also determined that small and medium-size exoplanets are commonly found around red dwarfs as well, which are smaller and cooler than our star.
"Earths and super-Earths aren’t picky," said co-author Guillermo Torres, also of the CfA. "We’re finding them in all kinds of neighborhoods."
Kepler generally requires three planetary transits to flag a potential alien world. Since more tightly orbiting planets transit more frequently, the telescope's early findings have been biased toward close-in worlds. But as Kepler continues to operate, it should find more and more planets farther from their host stars — including, perhaps, Earth-size worlds in Earthlike orbits.
- Gallery: The Smallest Alien Planets
- At Last, Earth-Sized Alien Worlds (Infographic)
- Complete Coverage of the 221st AAS Meeting
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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>