First planetary system orbiting two suns discovered

A team of astronomers has found two planets orbiting around a pair of stars roughly 5,000 light years away from Earth, declaring it the first multi-planet system centered on a binary star.

Kepler-47, the transiting circumbinary multi-planet system, as it is academically described, has shown scientists that planetary systems can form and survive even in the chaotic environment around a binary star, according to a study published in the journal Science on Wednesday.

"Each planet transits over the primary star, giving unambiguous evidence that the planets are real," said Jerome Orosz, associate professor of astronomy at San Diego State University in the United States and lead author of the study.

The discovery, also announced at the 28th General Assembly of the International Astronomical Union taking place in Beijing from Aug. 20 to 31, demonstrates that such circumbinary planets -- planets that orbit two stars instead of one -- can exist in the habitable zone of their stars.

The system, located in the constellation of Cygnus (the Swan), contains a pair of stars whirling around each other every 7.5 days. One star is similar to the Sun, while the other is a diminutive star only one-third its size and 175 times fainter.

The inner planet, Kepler-47b, is only three times larger in diameter than Earth, making it the smallest known transiting circumbinary planet. It orbits the stellar pair every 49 days.

The outer planet, Kepler-47c, is about 4.5 times the size of Earth -- slightly larger than Uranus -- and orbits the stars every 303 days, which makes it the longest-period transiting planet currently known.

HABITABLE ZONE

Most importantly, the outer planet orbits in places well within what astronomers refer to as the "habitable zone," the region around a star where a terrestrial planet could have liquid water on its surface.

"A habitable zone in a binary star system is not as obvious as in a single star system. Because there are two stars and they continuously move, the habitable zone is a shifting area that moves back and forth," said William Welsh, co-author of the study and another associate professor of astronomy at San Diego State University.

"But the discovery shows that planets could exist in the habitable zone of a binary star, which is exciting as planets in the habitable zone is the real goal of the Kepler mission," Welsh added.

Although Planet C is in what is termed the habitable zone, it can not support life. "It's a gas planet, and thus not suitable for life," Welsh explained, "but large moons, if present, would be interesting worlds to investigate as they could potentially harbor life.

"All the giant planets in our Solar System have moons, so it's reasonable to think that planets around other stars also have them.

"We did search for moons, but didn't find any. The data are noisy, and it does mean that there could be a moon hiding and we wouldn't see it. So we can't say there are no moons there, but there are no gigantic moons, or moons of the Earth's size."

A THIRD PLANET?

The planets were discovered using NASA's Kepler space telescope. They are much too far away to see, so they were found through gauging the drop in brightness when they transit (eclipse) their host stars.

Precise photometric data from the Kepler space telescope allowed the transits and eclipses to be measured, which in turn provided the relative size of the objects. Spectroscopic data from telescopes at the McDonald Observatory in Texas enabled the precise sizes to be determined.

Based on their size, the inner and outer planets probably have masses approximately eight and 20 times that of Earth.

When monitoring the transits of Planet B, scientists found an unexpected extra transit. "We think that is the signature of another planet in the system," Welsh said.

"With only one eclipse, there is very little information we can get, so we are waiting for more data to see if that is the third planet in the system."

The astronomer suggested that if there was really a third planet, it could be between the two planets, or a little bit outside Planet C. "We don't think there is much room for it to be closer to the stars. We just need to wait one more eclipse and we will know."

UNUSUAL BINARY SYSTEM

The discovery of the two new planets means that there are currently six known planets orbiting a binary star system. But each of those systems has only one known planet, except Kepler-47.

"Kepler-47 shows us that typical planetary architectures, with multiple planets in co-planar orbits, can form around two stars," said co-author Joshua Carter, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics. "We've learned that circumbinary planets can be like the planets in our own Solar System, but with two suns."

Welsh further explained why Kepler-47 is unusual: "The two stars go around each other very closely, and the gravity forces change very quickly. So it's very difficult for pieces and glomeration of dust, gas and rock to come together and form a planet, as the binary star keeps stirring the process up.

"It's much more difficult to form planets close to binary stars than single stars."

Since roughly half of the stars in our galaxy are binaries, finding planets in binary star systems has very important implications for estimating the total number of planets that exist. It will also help us better understand how star-planet systems form, he added.

Scientists also find the new planetary system very stable. "We make a computer simulation and let it run for millions of years to see whether it would fall apart, and it does not," Welsh said.

"We also estimate that the age of the brighter star is the same as the Sun, which shows the system is not very young and has been around for billions of years. So the planets may have also been there for billions of years," he explained, referring to this as another sign that the system is very stable.

KEPLER MISSION

Scientists have confirmed 41 new planets from 2,321 candidates by studying the Kepler data from May 2009 to September 2010, according to Natalie M. Batalha, a Kepler Mission scientist with NASA's Space Science and Astrobiology Division.

These new planets will join the group of 78 Kepler planets previously confirmed and help scientists enlarge their search scope for an extra-solar planet that may sustain life.

"Among thousands of the candidates, we find only one that is of the Earth's size and runs around its host star in the habitable zone. But we need to wait one year or two to confirm whether it is a planet or not," Batalha said.

This particular candidate, numbered 2124, has an orbit period of more than 40 days. "Although close to the star, it goes around a cooler star, which makes it in the habitable zone. So if it's confirmed to be a planet, it's quite worth further study," Batalha said.

It's not easy to find extra-solar planets in the habitable zone. Batalha gave Kepler-11 as an example. "It has 6 planets, but five of them are compact within a distance equivalent to that between the Mercury and the Sun," she said.

The Kepler Mission has been approved by NASA to extend its survey for another four to five years. "In one year or two, we will see more candidates. Next year is going to be very exciting, because the most interesting results are going to appear," she said.