The University of Chicago’s Sean Mills (left) and Daniel Fabrycky describe the complex orbital structure of the Kepler-223 expolanetary system in the May 11, 2016 Advance Online edition of Nature. Credit: Nancy Wong
The 4 planets of Kepler-223 star system seem to have little in common with planets of Earth’s own solar system. And yet a new study shows Kepler-223 system is trapped in an orbital configuration that Jupiter, Saturn, Uranus, and Neptune may have broken from in the early history of the solar system. “Exactly how and where planets form is an outstanding question in planetary science,” said Sean Mills. “Our work essentially tests a model for planet formation for a type of planet we don’t have in our solar system.” These puffy, gaseous planets, far more massive than Earth, orbit close to their stars. “That’s why there’s a big debate about how they form, how they got there, and why don’t we have one,” Mills said.
Mills et al used brightness data from NASA’s Kepler telescope to analyze how the 4 planets block the starlight and change each other’s orbits, thus inferring the planets’ sizes and masses. The team performed numerical simulations of planetary migration that generate this system’s current architecture, similar to the migration suspected for the solar system’s gas giants. The orbital configuration of the solar system seems to have evolved since its birth 4.6 billion years ago. The 4 known planets of the older Kepler-223 system, have maintained one orbital configuration for far longer. The planets of Kepler-223 are much larger than Earth, likely consisting of a solid core and an envelope of gas, and they orbit their star in periods ranging from only 7 to 19 days. Astronomers call these planets sub-Neptunes. They are the most common type of planets known in the galaxy.
Kepler-223’s planets also are in resonance. Planets are in resonance when, for example, every time one of them orbits its sun once, the next one goes around twice. Jupiter’s moons, where the phenomenon was discovered, display resonance. Kepler-223’s 2 innermost planets are in a 4:3 resonance. The second and third are in a 3:2 resonance. And the third and fourth are in a 4:3 resonance. Astronomers had seen extrasolar systems with 2-3 planets in resonance, but not 4.
Some stages of planet formation can involve violent processes, but during other stages, planets can evolve from gaseous disks in a smooth, gentle way, which is probably what the sub-Neptune planets of Kepler-223 did. “We think that 2 planets migrate through this disk, get stuck and then keep migrating together; find a third planet, get stuck, migrate together; find a fourth planet and get stuck,” Mills explained.
That process differs completely from the one that scientists believe led to the formation of Earth, Mercury, Venus, and Mars, which likely formed in their current orbital locations. Earth formed from Mars- or moon-sized bodies smacking together, a violent and chaotic process. When planets form this way their final orbital periods are not near a resonance.
But scientists suspect that the solar system’s larger, more distant planets of today -Jupiter, Saturn, Uranus, and Neptune–moved around substantially during their formation. They may have been knocked out of resonances that once resembled those of Kepler-223, possibly after interacting with numerous asteroids and small planets (planetesimals). “These resonances are extremely fragile,” Fabrycky said. “If bodies were flying around and hitting each other, then they would have dislodged the planets from the resonance.” But Kepler-223’s planets somehow managed to dodge this scattering of cosmic bodies. Other processes, including tidal forces that flex the planets, might also cause resonance separation.
Mills and Fabrycky’s UC Berkeley collaborators were able to determine the size and mass of the star by making precise measurements of its light using the high resolution Echelle spectrometer on the 10-meter Keck I telescope atop Mauna Kea in Hawaii. “The spectrum revealed a star very similar in size and mass to the sun but much older–more than 6 billion years old,” UC Berkeley’s Isaacson said. “You need to know the precise size of the star so you can do the dynamical and stability analysis, which involve estimates of the masses of the planets.”
Video: http://news.uchicago.edu/article/2016/05/11/quartet-exoplanets-locked-complex-dance
http://news.uchicago.edu/article/2016/05/11/quartet-exoplanets-locked-complex-dance
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