Planetary

Kepler Announces 11 Planetary Systems Hosting 26 Planets

MOFFET FIELD, Calif. — NASA’s Kepler mission has discovered 11 new planetary systems hosting 26 confirmed planets. These discoveries nearly double the number of verified planets and triple the number of stars known to have more than one planet that transits, or passes in front of, the star. Such systems will help astronomers better understand how planets form.

The planets orbit close to their host stars and range in size from 1.5 times the radius of Earth to larger than Jupiter. Fifteen are between Earth and Neptune in size. Further observations will be required to determine which are rocky like Earth and which have thick gaseous atmospheres like Neptune. The planets orbit their host star once every six to 143 days. All are closer to their host star than Venus is to our sun.

The CCD array

The Kepler photometer is a simple single purpose instrument. It is basically a Schmidt telescope design with a 0.95-meter aperture and a 105 deg2 (about 12 degree diameter) field-of-view (FOV). It is pointed at and records data from just a single group of stars for the four year duration of the mission. The photometer is composed of just one "instrument," which is, an array of 42 CCDs (charge coupled devices). Each 50x25 mm CCD has 2200x1024 pixels. The CCDs are read out every 6 seconds to prevent saturation. Only the information from the CCD pixels where there are stars brighter than mv=14 is recorded. (The CCDs are not used to take pictures. The images are intentionally defocused to 10 arc seconds to improve the photometric precision.) The data are integrated for 30 minutes.

“Prior to the Kepler mission, we knew of perhaps 500 exoplanets across the whole sky,” said Doug Hudgins, Kepler program scientist at NASA Headquarters in Washington. “Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits.”

Kepler identifies planet candidates by repeatedly measuring the change in brightness of more than 150,000 stars to detect when a planet passes in front of the star. That passage casts a small shadow toward Earth and the Kepler spacecraft.

The instrument

The instrument has the sensitivity to detect an Earth-size transit of an mv=12 G2V (solar-like) star at 4 sigma in 6.5 hours of integration. The instrument has a spectral bandpass from 400 nm to 850 nm. Data from the individual pixels that make up each star of the 100,000 main-sequence stars brighter than mv=14 are recorded continuously and simultaneously. The data are stored on the spacecraft and transmitted to the ground about once a month.

Each of the new confirmed planetary systems contains two to five closely spaced transiting planets. In tightly packed planetary systems, the gravitational pull of the planets on each other causes some planets to accelerate and some to decelerate along their orbits. The acceleration causes the orbital period of each planet to change. Kepler detects this effect by measuring the changes, or so-called Transit Timing Variations (TTVs).

Planetary systems with TTVs can be verified without requiring extensive ground-based observations, accelerating confirmation of planet candidates. The TTV detection technique also increases Kepler’s ability to confirm planetary systems around fainter and more distant stars.

Five of the systems (Kepler-25, Kepler-27, Kepler-30, Kepler-31 and Kepler-33) contain a pair of planets where the inner planet orbits the star twice during each orbit of the outer planet. Four of the systems (Kepler-23, Kepler-24, Kepler-28 and Kepler-32) contain a pairing where the outer planet circles the star twice for every three times the inner planet orbits its star.

“These configurations help to amplify the gravitational interactions between the planets, similar to how my sons kick their legs on a swing at the right time to go higher,” said Jason Steffen, the Brinson postdoctoral fellow at Fermilab Center for Particle Astrophysics in Batavia, Ill., and lead author of a paper confirming four of the systems.

Kepler-33, a star that is older and more massive than our sun, had the most planets. The system hosts five planets, ranging in size from 1.5 to 5 times that of Earth. All of the planets are located closer to their star than any planet is to our sun.

<The Kepler spacecraft

The spacecraft provides the power, pointing and telemetry for the photometer. Pointing at a single group of stars for the entire mission greatly increases the photometric stability and simplifies the spacecraft design. Other than the small reaction wheels used to maintain the pointing and an ejectable cover, there are no other moving or deployable parts The only liquid is a small amount for the thrusters which is kept from slosh by a pressurized membrane. This design enhances the pointing stability and the overall reliability of the spacecraft.

The properties of a star provide clues for planet detection. The decrease in the star’s brightness and duration of a planet transit, combined with the properties of its host star, present a recognizable signature. When astronomers detect planet candidates that exhibit similar signatures around the same star, the likelihood of any of these planet candidates being a false positive is very low.

“The approach used to verify the Kepler-33 planets shows the overall reliability is quite high,” said Jack Lissauer, planetary scientist at NASA Ames Research Center at Moffett Field, Calif., and lead author of the paper on Kepler-33. “This is a validation by multiplicity.”

These discoveries are published in four different papers in the Astrophysical Journal and the Monthly Notices of the Royal Astronomical Society.

Ames manages Kepler’s ground system development, mission operations and science data analysis. NASA’s Jet Propulsion Laboratory, Pasadena, Calif., managed the Kepler mission’s development.

For more information about the Kepler mission, visit: http://www.nasa.gov/kepler