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.
Image Credit:
Image Credit: NASA GSFC/Ames/D
Huber
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A long-standing puzzle in the study of exoplanets is the formation of hot
Jupiters, gas giant planets that snuggly orbit their host star. To explain their
short orbital periods, theory suggests that hot Jupiters form in long orbits and
then quiescently migrate through the protoplanetary disc, the flat ring of dust
and debris that circles a newly fashioned star and coalesces to form the
planets.
This theory was challenged when the orbital plane of hot Jupiters were
discovered to be frequently misaligned with the equator of their host stars.
Scientists interpreted this as evidence that hot Jupiters are the result of
chaotic close encounters with other planets.
A decisive test between the two theories are systems with more than one
planet: if misalignments are indeed caused by dynamical perturbations which lead
to the creation of hot Jupiters, then multi-planet systems without hot Jupiters
should be preferentially aligned. What new research reveals is quite
different.
Using data from the NASA's Kepler space telescope, an international research
team led by Daniel Huber, a NASA Postdoctoral Program fellow at NASA's Ames
Research Center in Moffett Field, Calif., studied Kepler-56, a red giant star
four times larger than the sun located at a distance of approximately 3,000
light years from Earth. By analyzing the fluctuations in brightness at different
points on the surface of Kepler-56, Huber and his collaborators discovered that
the star's rotation axis is tilted by about 45 degrees to our line of sight.
"This was a surprise because we already knew about the existence of two
planets transiting in front of Kepler-56. This suggested that 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 is suspected to be a third, massive
companion in a long period orbit, revealed by observations obtained with the
Keck telescope on Mauna Kea, Hawaii.
"Computer calculations show the outer companion may have torqued the orbital
planes of the transiting planets in concert, leaving them co-planar but
periodically misaligning them with the equator of the host star," said Daniel
Fabrycky, co-author and professor of astronomy at the University of Chicago.
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.
"We now know that misalignments are not just confined to hot Jupiter
systems," said Huber. "Further observations will reveal whether the tilting
mechanism in Kepler-56 could also be responsible for misalignments observed in
hot Jupiter systems."
The results are published in the
Oct. 18 issue of the journal Science. To download the paper see: Stellar Spin-Orbit Misalignment in a
Multiplanet System (Huber et al, 2013).
NASA
Guillermo Gonzalo Sánchez Achutegui
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