The Architecture of Planetary Systems
Friday, September 26, 2014
Science Update - A look at CfA discoveries from recent journals

There are 1822 confirmed exoplanets reported so far, and NASA's Kepler satellite has found evidence for more than two thousand others. Many exoplanets are expected to be in systems with multiple planets; indeed, one Kepler system is thought to contain seven or perhaps even more planets. As astronomers amass data on the characteristics of planets of all kinds, the large number of expected planetary systems allows them to study as well the nature of these systems and the stability of the orbits over time.

CfA astronomers Darin Ragozzine, John Geary, and Matt Holman, together with their colleagues, have analyzed 899 transiting planet candidates in 365 systems in an effort to understand the statistical properties of planetary systems and the extent to which our solar system might be unusual. The most complex system in their set has six planets. The sample is dominated by planets between about one and four Earth-radii in size and which orbit their stars in about ten days, making the planets hot and not Earth-like.

The astronomers found one striking feature of these exoplanetary systems: the planets seemed to lie in the same plane, to within an estimated 2.5 degrees (the team also estimates how this value might vary in time). For comparison, the solar system's planets are coplanar to about 3 degrees, with Mercury being an outlier with its angle of seven degrees; Pluto (not a planet) has an orbital angle of seventeen degrees. The team argues that this exoplanet result suggests that the individual planetary orbits are each nearly circular, a significant conclusion because it means the orbits are not likely to overlap, and hence implies (at least for systems of close-in planets) that they have long-term stability. The new paper marks continuing significant progress in unraveling the picture of planets and planetary systems in the universe.


"Architecture of Kepler's Multi-Transiting Systems. II. New Investigations with Twice as Many Candidates," Daniel C. Fabrycky, Jack J. Lissauer, Darin Ragozzine, Jason F. Rowe, Jason H. Steffen, Eric Agol, Thomas Barclay, Natalie Batalha, William Borucki, David R. Ciardi, Eric B. Ford, Thomas N. Gautier, John C. Geary, Matthew J. Holman, Jon M. Jenkins, Jie Li, Robert C. Morehead, Robert L. Morris, Avi Shporer, Jeffrey C. Smith, Martin Still, and Jeffrey Van Cleve, ApJ 790, 146, 2014