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Discovery of Kepler-18b, c, and d

Bill Cochran,
MacDonald Observatory and
University of Texas, Austin.

Bill CochranSee also University of Texas news release.

NANTES, France — A team of researchers led by Bill Cochran of The University of Texas at Austin has used NASA’s Kepler spacecraft to discover three planets: a super-Earth and two Neptune-sized planets orbiting in resonance with each other around the star Kepler-18 which is just 10 percent larger than the Sun and contains 97 percent of the Sun’s mass. They announced the find at a joint meeting of the American Astronomical Society’s Division of Planetary Science and the European Planetary Science Conference in Nantes, France. The research will be published in a special Kepler issue of The Astrophysical Journal Supplement Series in November.

Orbits of Kepler-18 planets
All the Kepler-18 planets discovered orbit much closer to Kepler-18 than Mercury does to the Sun.
The planets are designated b, c, and d. Planet c orbits the star twice for every one orbit d makes, but the timing varies slightly. Cochran says "they’re gravitationally interacting with each other. When they are close to each other they exchange energy, pull and tug on each other. One is slightly early when the other one is slightly late, [then] both are on time at the same time, and then vice-versa.” This timing variation made it easier to verify their discovery. But for planet b, the super-Earth, Cochran's team used a technique called “validation,” instead of verification. They set out to figure out the probability that it could be something other than a planet. First, they used the Palomar 5-meter (200-inch) Hale Telescope with adaptive optics to take an extremely high-resolution look at the space around Kepler-18. They wanted to see if anything close to the star could be positively identified as a background object that would cause the transit signal they had attributed to a super-Earth.

Sizes of Kepler-18 planets
Planet b at 6.9 times the mass of Earth and twice Earth’s size is considered a “super-Earth.” Planet c (17 Earth masses and 5.5 times Earth’s size and planet d (16 Earth masses and 7 times Earth’s size) both are low-density “Neptune-class” planets.
“We successively went through every possible type of object that could be there,” Cochran says. “There are limits on the sort of objects that can be there at different distances from the star.” Astronomers know how many of different types of objects (various kinds of stars, background galaxies, and more) are seen on average in the sky. They didn’t find anything in the Palomar image. “There’s a small possibility that [planet b] is due to a background object, but we’re very confident that it’s probably a planet,” Cochran says. His team calculated that the likelihood the object is a planet is 700 times more likely than the likelihood that it’s a background object. The process is called “planet validation,” rather than the usual “planet verification.” Cochran says it’s important to understand the difference — not just for this system, but for future discoveries from Kepler and other missions.

“We’re trying to prepare the astronomical community and the public for the concept of validation,” he says. “The goal of Kepler is to find an Earth-sized planet in the habitable zone [where life could arise], with a one-year orbit. Proving that such an object really is a planet is very difficult [with current technology]. When we find what looks to be a habitable Earth, we’ll have to use a validation process, rather than a confirmation process.”

Media Contacts:
Rebecca Johnson, McDonald Observatory Press Officer, 512-475-6763
Michele Johnson, Kepler Press Officer, NASA Ames Research Center, 650-604-4789
Vishnu Reddy, AAS Division of Planetary Sciences Press Officer, +49 555-15787579623
Anita Heward, European Planerary Science Congress Press Officer, +44 (0) 7756 034243
Science Contact: Dr. William Cochran, 512-471-6474

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