A Crowded Planetary System

Kepler-11 is an example of a Sun-like star around which six planets orbit tightly. At times, two or more planets pass in front of the star at once, as shown in this artist’s conception of a simultaneous transit of three planets observed by NASA’s Kepler spacecraft on Aug. 26, 2010. Image credit: NASA/Tim Pyle

An extraordinarily crowded planetary system is providing critical clues for understanding why most known planetary systems appear different from our own solar system. Using data from NASA’s Kepler space mission, scientists are investigating the properties of KOI-500, a planetary system that crams five planets into a region less than one twelfth the size of the Earth’s orbit. Dr. Darin Ragozzine, a postdoctoral researcher at the University of Florida, presented recent findings about this system this Tuesday at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Reno, NV.

KOI-500 is an especially compact planetary system, hosting five planets whose "years" are only 1.0, 3.1, 4.6, 7.1, and 9.5 days. "All five planets zip around their star within a region 150 times smaller in area than the Earth’s orbit, despite containing more material than several Earths (the planets range from 1.3 to 2.6 times the size of the Earth). At this rate, you could easily pack in 10 more planets, and they would still all fit comfortably inside the Earth’s orbit," Ragozzine notes. KOI-500 is approximately 1,100 light-years away in the constellation Lyra, the harp.

NASA’s Kepler mission searches for exoplanets — planets around other stars — by observing over 160,000 stars simultaneously and identifying small dips in a star’s brightness due to the shadow of a distant planet. Kepler has opened a whole new chapter in the study of exoplanets by discovering hundreds of planetary systems containing multiple closely-spaced planets. These discoveries include a surprising new population of planetary systems that contain several planets packed in a tiny space around their host stars. KOI-500 is the most compact of them all.

Artist impression of NASA’s Kepler telescope. Credit: NASA

"From the architecture of this planetary system, we infer that these planets did not form at their current locations. The planets were originally more spread out and have ‘migrated’ into the ultra-compact configuration we see today," said Ragozzine. Although recent theories for the formation of the large planets of the outer solar system also involve planets moving during the formation process, it is still unclear how the inner planets in the solar system, including Earth, avoided this fate.

Using Kepler data, astronomers can measure the sizes and orbits of planets orbiting Sun-like stars more precisely than ever before, giving birth to a new subfield of study. In the case of KOI-500, the planets are so close together that their mutual gravity pushes and pulls on their orbits, causing slight changes in the times that the planets pass in front of their host star. By detecting this effect, Dr. Ji-Wei Xie, a postdoctoral researcher at Nanjing University and the University of Toronto, recently confirmed that the two candidates orbiting farthest from KOI-500 were actually planets.

Ragozzine’s work, still unpublished, goes farther, confirming additional planets and characterizing their masses and orbits. Additionally, four of the planets orbiting KOI-500 follow synchronized orbits around their host star in a completely unique way — no other known system contains a similar configuration. Work by Ragozzine and his colleagues suggests that planetary migration helped to synchronize the planets. "By precisely characterizing the delicate arrangement of planets in this extraordinarily crowded system, Kepler is providing insights into the formation of KOI-500 and other compact planetary systems," said Eric Ford, an associate professor of astronomy at the University of Florida and a contributor to the study.

"As the most compact system of a new compact population of planets, KOI-500 will become a touchstone for future theories that will attempt to describe how compact planetary systems form," said Ragozzine. "Learning about these systems will inspire a new generation of theories to explain why our solar system turned out so differently."