The two newest planets spied by the Kepler space telescope are locked in a forever back-and-forth.
When Kepler’s scientists saw a star 2,000 light years away dim slightly, they knew there was the chance it was the telltale signature of a planet passing in front. But when the calculations were done and the confirmation came in, they found a surprise—what they’d seen was actually two planets transiting in front of the star.
NASA says it’s the first time they’ve ever caught such a sight, and today the scientists officially announced the finding with a study in Science. While other studies have found multiple planets around a single star–in fact, it happened earlier this week–those studies have used different planet-detection techniques like the wobble method.
The two worlds, both gas giants, do more than orbit the same star on the same plane, though. They push and pull each other in a motion that keeps the two exoplanets close to arithmetic celestial perfection. Kepler-9B, the larger, orbits the star in 19.24 days on average, the astronomers saw. Kepler-9c, the smaller, completes a revolution in an average of 38.91 days. But every time the scientists checked, 9b’s orbit was getting 4 minutes longer, while 9c’s shrank by 39 minutes.
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 its orbit. The same thing applies to the Kepler-9 system [MSNBC].
While the ratio at the moment slightly exceeds 2 to 1, Kepler-9b’s growing orbital time and 9c’s shrinking one mean the system is moving back toward 2 to 1. Like a pendulum it will swing to a ratio just smaller than 2 to 1, and then swing back as the two planets’ gravities keep each other constantly in check.
Study leader Matthew Holman says that the larger, inner world has a mass about 80 times that of Earth. Its smaller counterpart tallies about 54 Earth masses.
“The variation in transit times depend upon the masses of the planet,” Holman told reporters in a news conference announcing the findings. “The larger the mass the larger the variations. These variations allows us to determine the mass of the objects and we can confirm that they are planets” [Universe Today].
There’s something else in the Kepler-9 system, too: a candidate for a small planet just about one and a half times the size of Earth. However, while the two large planets are now confirmed—bringing Kepler’s exoplanet count to seven—the possible super-Earth remains with the hundreds of candidates in Kepler’s files that must be confirmed through further studies.
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