Astronomers discover pair of planets surprisingly close to each other

June 21, 2012

GAINESVILLE, Fla. — NASA’s Kepler mission, along with astronomers from Harvard, University of Washington and University of Florida, has discovered a rocky planet orbiting very close to a much larger planet around the same star, about 1,200 light years from Earth, according to research being published online today in Science Express.

Orbiting a star in the Cygnus constellation referred to as Kepler-36a, the planets are designated Kepler-36b and Kepler-36c. Planet b is a rocky planet, but 4.5 times more massive and with a radius nearly 50 percent larger than that of the Earth. Planet c is 8.1 times more massive than Earth and has a radius 3.7 times greater than our home planet.

The planets occupy nearly the same orbital plane and on their closest approach come within about 1.2 million miles of each other — just five times the distance from Earth to the moon. But their orbits are timed so that they do not collide.

The larger planet was originally spotted in data from NASA’s Kepler satellite, which is monitoring more than 160,000 stars, measuring their apparent brightness so precisely that it can detect when a distant planet passes in front of its host star, briefly reducing the light detected by Kepler’s instruments.

“The two planets pass so close to each other than Kepler has already observed changes in their orbits,” said Eric Ford, associate astronomy professor at UF. “Initially, those interactions made it difficult to detect the second planet. Once we recognized the rapid changes in the orbit of the first planet, we suspect there was likely another planet.” Eric Agol, an associate professor at the University of Washington, and Joshua Carter, a Hubble fellow at the Harvard-Smithsonian Center for Astrophysics and lead author of the Science paper, developed methods to search for signals that are not perfectly periodic. The rocky planet was discovered once they applied those methods to the Kepler-36 system.

“Once we had identified the planets, the gravitational tug-of-war between the two closely spaced planets made it possible for Kepler to measure their masses with unusual precision,” said Ford. “This is the best-characterized system of small planets orbiting a distant Sun-like star.”

Astronomers estimate that the smaller planet likely contains significant iron and a small fraction of water, but less than 1 percent atmospheric hydrogen and helium. On the other hand, the larger planet could be either a Neptune-like body with a 50 percent of water by mass or a rocky core covered by a hydrogen and helium gas envelope.

The planet’s densities differ by a factor of 8 but their orbits differ by only 10 percent. Explaining the difference in the planet’s compositions is a new challenge for planet formation models.

Ford recognized the Kepler-36 system as particularly interesting in summer 2010. For most of the more than 160,000 stars that the Kepler spacecraft observes, astronomers only receive one measurement every 30 minutes. Because Ford identified the system as a particularly interesting target, Kepler has been sending back one measurement per minute, a capability reserved for just a few hundred stars at a time. The additional data enabled the precise measurements of the planets’ masses and densities.

This research was funded by NASA, the Space Telescope Science Institute and the National Science Foundation. Kepler is NASA’s 10th Discovery Mission and is funded by NASA’s Science Mission Directorate. Searching for exoplanets using real Kepler data is open to everyone by visiting planethunters.org. For more information about the Kepler mission visit: http://www.nasa.gov/kepler.