NASA’s Kepler Space Telescope Discovers Two Planets Orbiting Binary Star System
August 29, 2012

NASA’s Kepler Space Telescope Discovers Two Planets Orbiting Binary Star System

Lawrence LeBlond for - Your Universe Online

Astronomers working with NASA´s Kepler mission have found the first evidence of multiple planets orbiting a binary star system. The discovery shows that planetary systems can form and survive in chaotic environments around a binary star system. Furthermore, the discovery suggests that earth-like planets can also exist in the habitable zone of their stars.

The discovery was announced this week at the XXVIII General Assembly of the International Astronomical Union meeting in Beijing and published in the journal Science. The research includes astronomers from San Diego State University and the University of Texas at Austin with assistance from two telescopes at the McDonald Observatory in West Texas.

The team said that gravitational disturbances generated from binary stars are believed to be very severe for any planets orbiting them. However, the Kepler Telescope found two small planets orbiting the pair of low-mass stars, confirming that planets can exist in such tumultuous environments. It follows another announcement of a circumbinary planet, Kepler-16b, which was discovered less than a year ago orbiting two stars. The new Kepler-47 system discovery comes from a circumbinary planetary system 4,900 light-years away in the Cygnus constellation.

Kepler-47 has been found to harbor the smallest known transiting circumbinary planets to date. The star system was discovered by focusing Kepler on a faint drop in brightness produced when possible planets eclipse their host stars.

“In contrast to a single planet orbiting a single star, planets whirling around a binary system transit a moving target,” explained Jerome Orosz of San Diego State University, and coauthor of the study. “The time intervals between the transits and their duration can vary substantially, from days to hours, and therefore the extremely precise and almost continuous observations with Kepler space telescope were fundamental.”

“The intervals were the telltale sign these planets are in circumbinary orbits,” added Orosz.

Astronomers were able to characterize the planetary system thanks to the keen telescopic eye of Kepler. The inner planet, Kepler-47b, is three times larger in diameter than Earth and orbits its twin stars about once every 49 days. Kepler-47c, the outer planet, is 4.5 times the size Earth and orbit´s the star system every 303 days.

While astronomers cannot directly view the planets, it is believed Kepler-47b is a very hot world, with methane destruction in the super-heated atmosphere leaving a thick haze blanketing the planet. Kepler47c is most likely unsuitable for life, being labeled as a gas giant slightly larger than Neptune, with an possible atmosphere of thick bright water-vapor clouds.

“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,” remarked William Borucki, principal mission 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.”

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

The team of astronomers believe that because both planets are relatively small, they do not pose a strong gravitational disturbance to their host stars or each other. The team was also able to determine, based on the upper limits of their masses, that these small objects are in fact planets and not brown dwarfs.

“Since about one third of all stars are either binary or multiple star systems, finding planets in binary star systems has very important implications not only for estimating the total numbers of planets that exist, but for how star—planet systems form as well,” noted Orosz.

Astronomers noted that while it is much more difficult to detect planets around binary systems rather than single host stars, the rich dynamics and wild climate changes make these circumbinary planets worth the effort to find. These two planets join the elite group of 4 previously known transiting circumbinary planets, Kepler-16, 34, 35 and 38.

“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.”

This system, and others like it, may be of particular significance for fans of the sci-fi community. In particular, Star Wars fans would be quick to point out that Luke Skywalker´s home planet Tatooine orbit´s binary stars.

“It's Tatooine, right?” quipped McDonald Observatory astronomer Michael Endl. “But this was not shown in Star Wars,” he said, referring to the periodic changes in the amount of daylight falling on a planet with two suns. Measurements of the star's orbits showed that daylight on the planets would vary by a large margin over the 7.4-Earth-day period as the two stars completed their mutual orbits, each moving closer to, then farther from, the planets (which are themselves moving).

Endl used the 9.2-meter Hobby-Eberly Telescope (HET) and the 2.7-meter Harlan J. Smith Telescope at McDonald for his observations. “The challenging thing is that this is a very faint star, about 6,000 times dimmer than can be seen with the naked eye,” he said.

His observations allowed astronomers to calculate the mass of the primary star, yet the secondary star was too faint to measure. Taken into account with the Kepler eclipse and transit images, the team entered the values into a model that calculated the relative sizes of all bodies involved, Endl explained.

“This is the type of research where McDonald Observatory really excels,” Bill Cochran of McDonald said. “We have excellent scientific instruments on our telescopes, and the queue-scheduled operation of the HET allows us to obtain spectra at the optimal times when they will give us the best information about the stars.”

The work was presented at the International Astronomical Union meeting by Dr. William Welsh, Professor of Astronomy at San Diego State University, on behalf of the Kepler Science Team.

“The thing I find most exciting“¦ is the potential for habitability in a circumbinary system,” Welsh told BBC News. “Kepler-47c is not likely to harbor life, but if it had large moons, those would be very interesting worlds.”

The research was funded in part by NASA and the National Science Foundation.