Giant misalignment in multi-planet system

Two University of Sydney astronomers are part of an international team, led by their former PhD student, that has discovered the first multiplanet system in which the equator of the host star is misaligned with the orbital plane of its planets. Published in today's edition of Science, the finding puts a new twist on one of the longest standing puzzles in exoplanet theory: the formation of "hot Jupiters", giant planets in close-in orbits around their host stars.

Graphical sketch of the Kepler-56 system. Image credit: Daniel Huber, NASA/AMES

To explain the short orbital periods of hot Jupiters, it has been suggested that they form in distant orbits and then quiescently migrate through the protoplanetary disc to their present position. This theory was challenged when the orbits of hot-Jupiters were discovered to be frequently misaligned with the equator of their host stars, which was interpreted as evidence that hot Jupiters form through dynamical perturbations by other bodies. A decisive test between the two theories are multiplanet systems: if misalignments are indeed caused by the process that creates hot Jupiters, multiplanet systems without hot Jupiters should be not be misaligned.

The team used data from NASA's Kepler space telescope to uncover the first misaligned multiplanet system. Kepler-56, a red-giant star four times larger than the Sun, is located at a distance of about 3000 light years from Earth. "This star is remarkable in many ways" says Dr Daniel Huber, lead-author on the paper. Huber obtained his PhD from the Univeristy of Sydney in 2011 before joining NASA Ames Research Center as a Postdoctoral Fellow. By analyzing the oscillation frequencies of Kepler-56, Huber's team discovered that the spin-axis of the star is tilted by about 45 degrees to our line of sight. "This was surprising because we already knew about the existence of two planets transiting in front of Kepler-56; therefore, the host star must be misaligned with the orbits of both planets", explains Huber. "What we found is quite literally a giant misalignment in an exoplanet system."

"This is a very puzzling result that is sure to challenge our understanding of how solar systems form," says co-author Tim Bedding, professor of physics at the University of Sydney and Huber's former PhD supervisor. The team, which also includes Dr Dennis Stello at the University of Sydney, measured oscillations in the host star and used these to measure the angle between the star's rotation axis and the orbital plane of its planets.

Nearly 20 years after the discovery of the first hot Jupiter, the "giant misalignment" in the Kepler-56 system marks an important step towards a unified explanation for the formation of hot Jupiters. As more multiplanet systems are discovered, observations are expected to reveal whether the tilting mechanism in Kepler-56 could also be responsible for misalignments observed in hot-Jupiter systems.

Contact: Tom Gordon

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