19 Jan. 2011

 

Exoplanets: inclined orbits dominant

 

Research by a team of astronomers at the University of Tokyo and the National Astronomical Observatory of Japan (NAOJ) has found that orbits which are strongly inclined with respect to the spin axis of the star may be the most common scenario for exoplanets.
The research was based on a long series of spectroscopic observations of two stars with giant planets, made with the High Dispersion Spectrograph on the Subaru Telescope in Hawaii. The first, with a mass 1.3 times that of Jupiter, orbits the star XO-4 every 4.13 days, and is 960 light years away in the constellation of Lynx; the second, with a mass similar to Neptune, has an orbital period of 4.89 days about the star HAT-P-11, and is 130 light years away in Cygnus.
Both planets periodically transit the stellar disks, and it is this phenomenon that permits the best spectrographs to measure an asymmetry in the spectral lines known as the Rossiter-McLaughlin (RM) effect.
For a star that rotates, a spectral line has a redshifted and a blueshifted half, coming from the receding and approaching hemispheres respectively. The passage of a planet will slightly dim first one half, then the other, so that the spectral line changes shape during the transit. The net effect of the transit is therefore to cause the mean redshift of the spectral line to vary slightly during the passage of the planet, and this variation can be used to derive the inclination between the planet's orbit and the spin axis of the star. In the case of HAT-P-11 (shown above) an inclination of 103° has been derived.
Despite the obviously limited sample size, these results favour the so-called "planet-planet" interaction models of planetary system formation. In these models, immediately after the formation of the planets, each body begins to affect gravitationally the orbits of the others, causing the phenomenon of migration, and can result in final orbits very close to the central star. One of the results characterising these models is the wide range of orbital inclinations. Although so-called "disk-planet" interaction models are also capable of producing migration to small orbits, they do not result in a wide range in the inclinations of the planetary orbits.

 

by Michele Ferrara & Marcel Clemens

credit: Subaru Telescope Facility, University of Tokyo, NAOJ