Friday, July 27, 2012

The Cause of Irregular Orbits

The Solar System is remarkably regular, its eight planets orbiting the Sun in the same direction in very nearly circular trajectories. Their orbits lie nearly in the same plane, which is aligned with the Sun's equator. These facts point toward a common origin for the Solar System, where everything collapsed from a single protostellar disk.

However, many exoplanetary systems are very different: exoplanets often orbit in highly elliptical orbits, and some "hot Jupiters" (giant planets in very small orbits) even revolve in the opposite direction from their host stars. A current major challenge in astrophysics is to understand why irregular systems exist.

The exoplanet system Kepler-30 could provide some help. A new analysis by Roberto Sanchis-Ojeda and colleagues showed the three known planets in the system orbit in a regular fashion: nearly circular orbits aligned with the rotation of the host star Kepler-30a. The researchers found the Kepler-30 system to be as orderly as the Solar System, leading them to suggest that misaligned hot Jupiter systems arise from interactions among the planets, rather than a different process of planet formation.
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Using 2.5 years of Kepler telescope data, the researchers found instances where more than one of the exoplanets passed successively across the same starspot group. This verified both the direction and the plane of orbit for each planet in the system within a range of values, which were consistent with a Solar System-like model. Independent measurements of the planets' orbits showed them to be nearly circular as well.

While Kepler-30 is just one star system, its regularity places it in the same category as the Solar System, making it very different from many exoplanet systems containing hot Jupiters. The authors argue that this could be an indication that hot Jupiters got where they are via orbital interactions with other planets. If other multi-planet systems have similarly low obliquities, then it's unlikely that protoplanetary disks formed out of alignment with the equators of the host stars. The starspot transit method laid out in this paper should help resolve whether all multiple-planet systems are similarly regular to Kepler-30 and the Solar System.
(Full story here).

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