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70 Billion Planets

Posted January 11, 2013

Extrapolations from two new analyses of the Kepler spacecraft's ongoing observations, and results from other detection techniques, indicate that practically all Sun-like stars have planets. As far as Earth-size planets are concerned, the analyses indicate that about 17 percent of stars have an Earth-sized planet in an orbit closer than Mercury. Planets closer to their stars are easier to find because they circle their stars more frequently. An estimate of the number in Earth-like orbits will be gained as more data comes in.  However, the number detected is increasing and suggests that 50 per cent of stars in our Galaxy have planets in close-in orbits and 70 per cent (70 Billion) have planets out to Earth orbit. Since the Milky Way has about 100 billion stars, there are at least 70 billion Earth-sized worlds. in orbits as large as the Earth’s or smaller.

"The analysis of increasingly longer time periods of Kepler data uncovers smaller planets in longer period orbits-- orbital periods similar to Earth's," said Steve Howell, Kepler mission project scientist at NASA’s Ames Research Center. "It is no longer a question of will we find a true Earth analogue, but a question of when."

The dramatic increases over the 11 months are in the number of Earth-size and super Earth-size candidates, which grew by 43 and 21 percent. The new data increases the number of stars discovered to have more than one planet candidate from 365 to 467. Today, 43 percent of Kepler's planet candidates are observed to have neighbor planets. "The large number of multi-candidate systems being found by Kepler implies that a substantial fraction of exoplanets reside in flat multi-planet systems," said Jack Lissauer, planetary scientist at NASA Ames. "This is consistent with what we know about our own planetary neighborhood."

Kepler detects the presence of a planet by a slight dimming in its star’s light as the planet passes in front of it. At least three such transits are required to verify a signal as a potential planet. There are however other phenomena that can cause dimming. Known spacecraft instrumentation and astrophysical false positives can masquerade as planetary candidates, so that some 13,000 transit-like signals have been analyzed by the Kepler team to eliminate false candidates. Some new candidates have also emerged as other scientific groups have analyzed the published Kepler data. The complete list of Kepler planet candidates is available in an interactive table at the NASA Exoplanet Archive.

A new Kepler catalog of its planet candidates released in January 2013 shows a 20 percent increase in the total since February 2012. 2,740 potential planets orbiting 2,036 stars have now been found. Four of the potential new planets are less than twice the size of Earth and orbit in their sun's "habitable zone," the region in a planetary system where liquid water might exist on the surface of a planet.

Planet candidates require additional follow-up observations and analyses to be confirmed as planets. At the beginning of 2012, 33 candidates in the Kepler data had been confirmed as planets. By January 2013, 105 had been confirmed.

The prediction that 17 percent of all sun-like stars have planets one to two times the diameter of Earth orbiting close to their host stars came from two independent analyses based on the first three years of data from NASA’s Kepler mission. Both estimates include only planets that circle their stars within a distance of about one-quarter of Earth’s orbital radius – well within the orbit of Mercury. This is the limit for extensive statistical analyses of Kepler’s detection capability for the period analyzed.

According to a team of astronomers from the University of California, Berkeley, and the University of Hawaii at Manoa further evidence suggests that the fraction of stars having planets the size of Earth or slightly bigger orbiting within Earth-like orbits may amount to 50 percent. The estimate is based on a better understanding of the percentage of big Earth-size planets that Kepler misses because of uncertainties in detection. The Berkeley team estimated this to be about one in four, or 25 percent.

The team – UC Berkeley graduate student Erik Petigura, former UC Berkeley post-doctoral fellow Andrew Howard, now at the University of Hawaii, and UC Berkeley professor of astronomy Geoff Marcy – reported their findings in January at the American Astronomical Society meeting in Long Beach, Calif. The key result of their findings is that the frequency of planets increases as you go to smaller sizes, but it doesn’t increase all the way to Earth-size planets – it stays at a constant level below twice the diameter of Earth.

To find planets, the Kepler telescope captures repeated images of 150,000 stars in a region of the sky in the constellation Cygnus. The data are analyzed by computer software – the “pipeline” – in search of stars that dim briefly as a planet passes in front. For planets as large as Jupiter, the star may dim by 1 percent, or one part in 100, which is easily detectable. A planet as small as Earth, however, dims the star by only one part in 10,000, which is likely to be lost in the noise generated by other sources, according to Petigura, who wrote a software program called TERRA very similar to Kepler’s.

Planets one to two times the size of Earth are not necessarily habitable. Observations by Petigura’s team show that planets two or three times the diameter of Earth are typically like Uranus and Neptune, which have a rocky core surrounded by helium and hydrogen gases and perhaps water. Planets close to a star may even be water worlds – planets with oceans hundreds of miles deep above a rocky core. Nevertheless, planets between one and two times the diameter of Earth may well be rocky and, if located within the habitable zone could support life.

The UC Berkeley/Hawaii team fed TERRA simulated planets to test how efficiently the software detects Earth-size planets. This was important as a separate cross-validation. After carefully measuring the fraction of planets missed by TERRA, the team corrected for this and conducted a thorough analysis of 12 of the 13 quarters of Kepler observations freely available on the Internet. They identified 129 Earth-like planets ranging in size from nearly six times the diameter of Earth to the diameter of Mars. Thirty-seven of these planets were not identified in previous Kepler reports.

The analysis confirmed that the frequency of planets increases as the size decreases, which Howard and the Kepler team reported last year. Perhaps 1 percent of stars have planets the size of Jupiter, while 10 percent have planets the size of Neptune. Accounting for planets that Kepler misses still means that “big Earths” are one third as abundant as would be expected if the rising trend continued below twice the diameter of Earth.

Another analysis by a team led by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics also reported at the AAS meeting that one in six stars, or at least 17 billion stars in the Milky Way Galaxy, have an Earth-size planet within an orbit like Mercury’s. By their computer analysis of Kepler results, Fressin and his colleagues were able to correct both the impurity and the incompleteness of this list of candidates to recover the true occurrence of planets orbiting other stars, down to the size of Earth. "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. All the other signals are bona-fide planets," reported Fressin.

Altogether, the HSC researchers estimated that 50 percent of stars have a planet of Earth-size or larger in a close orbit. By adding larger planets, which have been detected in orbits up to the orbital radius of the Earth, this number reaches 70 percent, or 70 billion stars.

The HSC team then grouped planets into five different sizes. They found that 17 percent of stars have a planet 0.8 - 1.25 times the size of Earth in an orbit of 85 days or less. About one-fourth of stars have a super-Earth (1.25 - 2 times the size of Earth) in an orbit of 150 days or less. (Larger planets can be detected at greater distances more easily.) The same fraction of stars has a mini-Neptune (2 - 4 times Earth) in orbits up to 250 days long.

Larger planets are much less common. Only about 3 percent of stars have a large Neptune (4 - 6 times Earth), and only 5 percent of stars have a gas giant (6 - 22 times Earth) in an orbit of 400 days or less.

The researchers also asked whether certain sizes of planets are more or less common around certain types of stars. They found that for every planet size except gas giants, the type of star doesn't matter. Neptunes are found just as frequently around red dwarfs as they are around sun-like stars. The same is true for smaller worlds. This contradicts previous findings.