The Kepler spacecraft has discovered the first-known instance of a planetary system where two planets orbit the same star, the space agency NASA announced.
NASA announced its Kepler spacecraft
launched in March 2009, has discovered the first confirmed planetary
system with more than one planet crossing in front of, or transiting,
the same star. The transit signatures of two distinct planets, named
Kepler-9b and 9c, were seen in the data for the sun-like star
designated Kepler-9, the space agency confirmed.
Scientists refined the estimates of the masses of the planets using
observations from the W.M. Keck Observatory in Hawaii. The observations
show Kepler-9b is the larger of the two planets, and both have masses
similar to but less than Saturn. Kepler-9b lies closest to the star
with an orbit of about 19 days, while Kepler-9c has an orbit of about
38 days. By observing several transits by each planet over the seven
months of data, the time between successive transits could be analyzed,
NASA documents explained.
"This discovery is the first clear detection of significant changes in
the intervals from one planetary transit to the next, what we call
transit timing variations," said Matthew Holman, a Kepler mission
scientist from the Harvard-Smithsonian Center for Astrophysics. "This
is evidence of the gravitational interaction between the two planets as
seen by the Kepler spacecraft."
The distance of the planet from the star can be calculated by measuring
the time between successive dips as the planet orbits the star. Small
variations in the regularity of these dips can be used to determine the
masses of planets and detect other non-transiting planets in the
system. NASA said Kepler's "ultra-precise" camera measured tiny
decreases in the stars' brightness that occur when a planet transits
them: The size of the planet can be derived from these temporary dips.
"Kepler's high quality data and round-the-clock coverage of transiting
objects enable a whole host of unique measurements to be made of the
parent stars and their planetary systems," said Doug Hudgins, the
Kepler program scientist at NASA's headquarters in Washington.
In June, mission scientists submitted findings for peer review that
identified more than 700 planet candidates in the first 43 days of
Kepler data. The data included five additional candidate systems that
appear to exhibit more than one transiting planet. The space agency
noted the Kepler team recently identified a sixth target exhibiting
multiple transits and accumulated enough follow-up data to confirm this
In addition to the two confirmed giant planets, Kepler scientists also
have identified what appears to be a third, much smaller transit
signature in the observations of Kepler-9, according to NASA. "That
signature is consistent with the transits of a super-Earth-size planet
about 1.5 times the radius of Earth in a scorching, near-sun 1.6
day-orbit," an agency press release explained. "Additional observations
are required to determine whether this signal is indeed a planet or an
astronomical phenomenon that mimics the appearance of a transit."