NASA discovers Tatooine-like binary star planet system

A two-sun system similar to Tattooine in the "Star Wars" films has been discovered by the US National Aeronautics and Space Administration (NASA)'s Kepler mission.

 

NASA said Kepler-47 is a transiting circumbinary system - a double-star system that has two planets, one of which orbits in the so-called "habitable zone."

 

"This discovery proves that more than one planet can form and persist in the stressful realm of a binary star and demonstrates the diversity of planetary systems in our galaxy," NASA said.

 

NASA said this was the first time the Kepler mission discovered multiple transiting planets orbiting two suns.  

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One star is similar to our Sun in size, but only 84 percent as bright. The second star is small, measuring only one-third the size of the sun and less than 1 percent as bright.

 

Kepler-47 is estimated at 4,900 light-years from Earth in the constellation Cygnus and contains planets Kepler-47b and Kepler-47c, NASA said.

 

NASA said Kepler-47b, the inner planet, has three times the radius of earth and orbits the pair of stars in less than 50 days.

 

Kepler-47c is believed to be a gaseous giant, slightly larger than Neptune with an orbital period of 303 days.

 

Astronomers said the two planets eclipse each other every 7.5 days from our vantage point on Earth.

 

"Unlike our sun, many stars are part of multiple-star systems where two or more stars orbit one another. The question always has been -- do they have planets and planetary systems? This Kepler discovery proves that they do," said William Borucki, Kepler mission principal investigator at NASA's Ames Research Center in Moffett Field, California.

 

"In our search for habitable planets, we have found more opportunities for life to exist," he added.

 

To search for transiting planets, the research team used data from the Kepler space telescope, which measures dips in the brightness of more than 150,000 stars.

 

Ground-based spectroscopic observations using telescopes at the McDonald Observatory at the University of Texas at Austin helped characterize the stellar properties.

 

The findings are published in the journal Science.

 

Transiting moving target

 

Jerome Orosz, associate professor of astronomy at San Diego State University and lead author of the paper, said the planet in a circumbinary system must transit a "moving target," unlike a single planet orbiting a single star.

 

As such, Orosz said time intervals between the transits and their durations can vary substantially, sometimes short, other times long.

 

"The intervals were the telltale sign these planets are in circumbinary orbits," he said.

 

Sweltering world, habitable zone

 

NASA said Kepler-47b, which at three times the radius of Earth is the smallest known transiting circumbinary planet, orbits the pair of stars in less than 50 days.

 

"While it cannot be directly viewed, it is thought to be a sweltering world, where the destruction of methane in its super-heated atmosphere might lead to a thick haze that could blanket the planet," it said.

 

Kepler-47c, the outer planet, orbits the two suns every 303 days, and may be in the so-called "habitable zone," the region in a planetary system where liquid water might exist on the surface of a planet.

 

NASA said that while Kepler 47-c does not appear hospitable for life, it is thought to be a gaseous giant slightly larger than Neptune, "where an atmosphere of thick bright water-vapor clouds might exist."

 

Searching for transiting planets

 

Greg Laughlin, professor of Astrophysics and Planetary Science at the University of California in Santa Cruz, said the presence of a full-fledged circumbinary planetary system orbiting Kepler-47 is "an amazing discovery."

 

"These planets are very difficult to form using the currently accepted paradigm, and I believe that theorists, myself included, will be going back to the drawing board to try to improve our understanding of how planets are assembled in dusty circumbinary disks," he said.

 

Most systems are binary

 

Orosz also said most of the stars in the galaxy are in binary or higher-order multiple systems, "so the fact that planetary systems can exist in these types of systems is important."

 

"If we were restricted to looking for planets around single stars, we would be missing most of the stars in the galaxy," he told Space.com.

 

Study co-author William Welsh at San Diego State University is excited about the potential for habitability in a circumbinary system.

 

"Kepler-47c is not likely to harbor life, but if it had large moons, those would be very interesting worlds," he said.

 

Wild climate swings

 

Space.com quoted Orosz as saying that while our Sun is a relatively stable source of light, a planet around a binary system can expect wild climate changes.

 

"For a planet around a binary system, there can be changes in the insolation of several percent on the time-scale of days to weeks. In addition, if the planet's rotation axis is tilted, then that also has an effect. Therefore the seasons are rapid and complicated," he said.

 

He also said that with two suns, "you can have more than 12 hours of daylight, depending on the positions of the stars at sunrise or sunset."

 

Also, the circumbinary planets the Kepler space telescope has discovered have orbits closely aligned with the orbits their stars have with each other.

 

"You would see your suns eclipse each other on a regular basis," Orosz said.

 

In the case of Kepler-47, he said that when the secondary star passes in front of the primary, the total light drops by 15 percent.

 

This would happen every 7.5 days or so, he said.

 

Origins

 

The researchers theorize that the planets in Kepler-47 originated much farther out than their present orbits, in areas where the conditions for the formation of giant planets are more favorable.

 

But the planets eventually migrated inward due to interactions with the disk of gas and dust that also encircled the stars.

 

"We think these planets and most other planets formed from a residual disk of debris left over from the star-formation process.k It was not at all obvious that this disk could survive near a newly formed binary star, given the orbital motions of the two stars. However, it now appears that apart from minor differences in the orbital spacings, planetary systems around binary stars can be similar to those around single stars," Orosz said.

 

Meanwhile, the researchers want to look for smaller and smaller alien planets around binary stars.

 

"We are at the limit of what simple visual searches can do, so we need better software to help us automate the process. Given more time and data, I think we can find more circumbinary planetary systems in the Kepler data," Orosz said. — TJD, GMA News