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: NASA GSFC/Ames/D Huber
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).
