Astronomers have analyzed the orbits of 365 observed systems of suspected, multiple exoplanets. This figure shows the relative placements and sizes of the planets in systems with three or more planets. The horizontal axis shows the orbital period in days for these systems, in all of which the planets are very to their stars and so complete their annual orbit in under about 100 days (a few suspected planets orbit in under one day!). The planetary radii are colored with the largest in each system being red; most of the planets in this study are between about one and four Earth-radii.
NASA/Kepler; Fabrycky
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.
Reference(s):
"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
