Misaligned planetary system

Using NASA’s Kepler Space Telescope, astronomers have discovered the first multi-planet system on highly tilted orbits compared to the spin axis of their host star.

Kepler 56 is an evolved red giant star about 4 times the size of the Sun, which hosts two exoplanets that were discovered by Kepler with the transit method. By monitoring the brightness of stars, Kepler looks for periodic dimming of the stellar light as planets pass in front – or transit – their host stars. In the case of Kepler 56, both planets, Kepler 56b and Kepler 56c, are closer to their host star that Mercury is to our Sun, with orbital periods of  10.5 and 21.4 days.

Transit observations provide information on planet properties relative to their host star, so to learn more about the planets, a team led by Daniel Huber of NASA Ames Research Centre used Kepler astroseismology data to study the host star. They were surprised to discovered that the spin axis of the star, which is about 1.3 times that mass of  the Sun, was tilted by 45 degrees to the orbital plane of the planets. This is in contrast to all the planets in our Solar System which orbit the Sun within about 7 degrees of the Sun’s equatorial plane, which means the rotation of the Sun and the orbits of the planets are aligned.

Schematic of the Kepler 56 planetary system. The spin axis of the host star is tilted by about 45 degrees to the orbital plane of two transiting planets that orbit the star. The stellar tilt is likely caused by a third massive planet on a more distant tilted orbit. (Credit: Daniel Huber/NASA Ames Research Center)

Combining the transit lightcurve data with observations made using the High-Resolution Echelle Spectrometer (HIRES) on the Keck 10-m telescope, Huber et al. have found signatures of a third, more massive companion on a wide orbit beyond the two known transiting exoplanets.  This previously unseen planet does not transit the star, meaning it too – like the host star – is tilted to the orbit of the two transiting planets. The authors suggest that the gravitational effects of this third planet might explain the system’s misalignment.

Previously, only single ‘hot Jupiters’ (massive planets which orbit very close to their host star, with periods on the order of a few days) were found to have their orbits misaligned to their host star’s spin axis.  All astronomers agree that massive hot Jupiters must have initially formed further from their host stars (since most of the material in planetary disks is found far from the star) and migrated inwards.  But exactly how is still debated.  The spin-orbit misalignment of hot Jupiters has been used to suggest that their inward migration resulted from dramatic planet-planet interactions rather than more gentle planetary migration through the disk out of which the planets formed.

Kepler 56 is the first system without a hot Jupiter to have tilted planetary orbits.  In this case it is thought that the massive outer planet produced the tilted orbits of the two inner transiting planets.

For more detail; see

[Sarah Maddison & Sheridan Lacey]

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