Planets spotted in changing orbits

NASA's Kepler planet-hunting probe has spotted a system where two giant planets are locked in constantly changing orbits — with a super-Earth potentially pinned down in the crossfire.

Astronomers like to think of planets as a kind of celestial clockwork, keeping regular time. For example, the time it takes for the planets in our own solar system to complete their orbits can be calculated to within fractions of a second, and unless something huge happens, they'll stick to that timetable for billions of years.

In contrast, the two Saturn-size planets circling a sunlike star now known as Kepler-9, more than 2,000 light-years from Earth, shift their timetable with every go-round. Kepler-9b has an orbit lasting approximately 19.24 Earth days, while Kepler-9c has an orbit lasting a little more than twice as long, 38.91 days. But on average, Kepler-9b's orbit got about 4 minutes longer every time the Kepler astronomers checked, while Kepler-9c's averaged about 39 minutes shorter.

That suggests the planets are in the midst of a gravitational push-pull that keeps the orbits close to a 2-to-1 ratio, in what's known as a planetary resonance. In our own solar system, Pluto and Neptune are in a similar resonance (2-to-3), which is why little Pluto can't be kicked out of its orbit. The same thing applies to the Kepler-9 system.

"The system is stable in the sense that no planet will be ejected," said Matthew Holman, an astronomer at the Harvard Smithsonian Center for Astrophysics who is the principal author of a Kepler paper being published today on the journal Science's website.

"The orbits of the planets are changing, but these variations are oscillatory," Holman told me in an e-mail. "On average, the period ratio will be very close to 2-to-1. However, at any given instant that ratio may be bigger than 2-to-1 or smaller than 2-to-1."

Orbital variations has long been known to be theoretically possible, but Kepler-9 is the first confirmed planetary system where astronomers have been able to register this type of off-schedule behavior. It's actually quite a lucky break for the Kepler team. "The variations in what we call the transit times are large enough that we can use those transit timing variations to estimate the masses of these bodies," Holman said in a Science podcast. ...

via Cosmic Log - Planets spotted in changing orbits.