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The Mystery of The Giant Misalignment

My collaborators and I, led by NASA postdoctoral fellow Daniel Huber, just published an exciting new paper in Science magazine. Here's the press release!

NASA Ames - An international group of scientists has announced the discovery of the first multiplanet system in which the equator of the host star is misaligned with the orbital planes 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.

To explain their short orbital periods, it was suggested that hot Jupiters 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 preferentially aligned.

Now, scientists have used data from the 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 Daniel Huber, lead-author and NASA Postdoctoral Program fellow at Ames Research Center. By analyzing the oscillation frequencies of Kepler-56, Huber and collaborators 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."

The culprit for the misalignment was revealed through radial-velocity observations taken with the Keck telescope on Mauna Kea, Hawaii. "Combining transit-timing variations caused by the interaction of the planets with the Keck data and dynamical simulations showed that the planet orbits are well aligned with each other." explains Josh Carter, co-author and Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics. "However, the Keck data also revealed an additional third, massive companion in a wide orbit." Calculations confirmed that the torque exerted by this third companion causes the orbital planes of the transiting planets to precess in concert, periodically misaligning them with the equator of the host star. Such a dynamical tilting scenario had been recently suggested theoretically, and has now been observed for the first time.

By combining Kepler and Keck data, the team was also able to precisely measure the radii, masses and densities of the host star and both transiting planets. "The study is a great demonstration of the wide expertise of scientists working together in the Kepler community." says John Johnson, co-author and Professor of Astronomy at the Harvard-Smithsonian Center for Astrophysics. "Each expertise was absolutely essential for explaining the architecture and formation history of this intriguing system."

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.

Graphical sketch of the Kepler-56 system. The line of sight from Earth is illustrated by the dashed line, and dotted lines show the orbits of three detected companions in the system. The solid arrow marks the rotation axis of the host star, and the thin solid line marks the host star equator. Note that the sizes of the orbits and bodies are not to scale. Original images used to depict the host star and its companions are courtesy of NASA/GSFC.

Credit: Daniel Huber, NASA/Ames


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