This artist's concept illustrates the two Saturn-sized planets discovered by NASA's Kepler mission around a star called Kepler-9. A 3rd planet, just 1.5 times the size of Earth, may also be orbiting the star. This is the first star system found to have multiple transiting planets.
A newfound alien solar system with planets the size of Saturn circling close to their star is helping astronomers learn how some giant worlds snuggle up to their stellar parents like moths to a flame.
NASA's Kepler space observatory recently confirmed the presence of the two Saturn-sized planets that orbit a star about 2,300 light-years away from Earth. A third, much smaller planet may also orbit the star, circling so close that one year on the alien world would last just 1.6 Earth days.
While the discovery of the Kepler-9 planetary system is a major find, it is also a starting point for astronomers to learn how the planet arrangement formed in the first place. [Gallery - Strangest Alien Planets]
Scientists think these planets originated much farther from their parent star and gradually migrated inward over time. All three objects could fit inside the orbit of Mercury today.
"It's safe to say that they did migrate because they ended up in this very special set of orbits," said Alycia Weinberger, an astronomer in the department of terrestrial magnetism at the Carnegie Institution of Washington in Washington, D.C., when the planets were announced in late August."
"The likely candidate for how that migration happened is interaction between these planets and the original disk of material ? the gas out of which they formed. It will now take some work to try to figure out exactly how that was likely to happen in this system," Weinberger said.
Timing planet paths
Understanding the process of planetary migration will help astronomers understand the initial conditions that led to the final configuration of the Kepler-9 system, and other planetary systems discovered in the future.
As a planetary system is being formed, "planets can change locations or migrate due to interactions with the raw materials with which they are built," Weinberger said.
A study led by Matthew Holman, associate director of the theoretical astrophysics division at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., determined that the two Saturn-sized planets, named Kepler-9b and Kepler-9c, have somewhat atypical orbits.
It takes Kepler-9b about 19.2 days to complete one orbit. The other Saturn-sized world, Kepler-9c, makes one orbit every 38.9 days, taking almost twice as long to complete the circuit.
"There is a near 2-to-1 orbital resonance, which means the planets have orbital periods in a 2-to-1 ratio," Holman told SPACE.com. "At this configuration, we can see that they strongly interact and we can see large variations in the orbits of the planets."
What planetary migration tells us
Studying how planetary migration occurs, and how much a planet has moved, can tell researchers a lot about the history of how such worlds and their local solar system formed.
Kepler-9b and 9c were found to have a lot of gravitational interaction, and they are located so close to their parent star that their orbits would fit inside the orbit of Mercury in our own solar system, said Holman.?
The two planets most likely migrated to their observed locations, said Weinberger, because being so close to the star would have made it extremely difficult to develop and survive in the first place.
Observations combined with theoretical work will then be able to pinpoint how far apart the planets originated, how long it took them to form and how long their migration lasted.
"Kepler was designed and built to answer fundamental questions," Weinberger said. "We want to know what types of planetary systems there are; what is common amongst the various systems; whether there are any special conditions that result in Earth-like planets; whether the whole system of planet formation is robust and common."
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