The Kepler field of view, located between two bright stars in the summer triangle, rising over the WIYN telescope in southern Arizona.
Imagine living on an exoplanet with two suns. One, you orbit and the other is a very bright, nearby neighbor looming large in your sky. With this “second sun” in the sky, nightfall might be a rare event, perhaps only coming seasonally to your planet. A new study suggests that this could be far more common than we realized.
The NASA Kepler Space Telescope has confirmed about 1000 exoplanets,
as well as thousands more stars considered “Kepler objects of interest”,
dubbed KOIs – stars that could possibly host planets. Until now, there
has been an unanswered question about exoplanet host stars; how many
host stars are binaries? Binary stars have long been known to be
commonplace – about half the stars in the sky are believed to consist of
two stars orbiting each other. So, are stars with planets equally
likely to have a companion star, or do companion stars affect the
formation of planets? A team of astronomers, led by Dr. Elliott Horch,
Southern Connecticut State University, have shown that stars with
exoplanets are just as likely to have a binary companion: that is, 40%
to 50% of the host stars are actually binary stars. As Dr. Horch said,
“It’s interesting and exciting that exoplanet systems with stellar
companions turn out to be much more common than was believed even just a
few years ago.”
Their study makes use of very high spatial resolution observations
that were carried out on the WIYN telescope located on Kitt Peak in
southern Arizona and the Gemini North telescope located on Mauna Kea in
Hawaii. The technique used by the team is called speckle imaging and
consists of obtaining digital images of a small portion of the sky
surrounding a star of interest, 15 to 25 times a second. The images are
then combined in software using a complex set of algorithms, yielding a
final picture of the star with a resolution better that that of the
Hubble space telescope. By using this technique, the team can detect
companion stars that are up to 125 times fainter than the target, but
only 0.05 arcseconds away. For the majority of the Kepler stars, this
means companion stars with a true separation of a few to about 100 times
the Sun-Earth distance. By noting the occurrence rate of these true
binary companion stars, the discoveries can be extended to show that
half of the stars that host exoplanets are probably binaries.
Co-author of the study, Dr. Steve B. Howell (NASA Ames Research
Center), commented, “An interesting consequence of this finding is that
in the half of the exoplanet host stars that are binary we can not, in
general, say which star in the system the planet actually orbits.”
Kepler has discovered a number of circumbinary planets, that is, a
planet that orbits both stars in very close binary systems. There also
exist exoplanets that are known to orbit one of the stars in very wide
binary systems. If the two stars are very close to each other and the
planet far away, a circumbinary planet will be reminiscent of Tatooine
in Star Wars. If instead the exoplanet orbits one of the stars in a very
wide pair, the companion star might appear simply as a bright star
among others in the night sky. “Somewhere there will be a transition
between these two scenarios,” Howell said,” but we are far from knowing
where.”
The accompanying figure shows the Kepler field of view, located
between two bright stars in the summer triangle, rising over the WIYN
telescope in southern Arizona.
In a study like this, it is critical to rule out faint companions
that are only in the line of sight with the KOI star. To allow for these
possibilities, the team performed a model simulation that relied on
known statistical properties of binary star systems and line of sight
companions. The results suggest that the large majority of the stellar
companions to KOIs are true bound companions, not line of sight stars
unconnected with the system.
This work has been accepted for publication in the Astrophysical
Journal. The additional authors are Dr. Mark E. Everett, National
Optical Astronomy Observatory and Dr. David R. Ciardi, NASA Exoplanet
Science Institute, California Institute of Technology.
The WIYN telescope is operated by the WIYN Consortium, which consists
of the University of Wisconsin, Indiana University and the National
Optical Astronomy Observatory (NOAO). Kitt Peak National Observatory is a
division of NOAO, which is operated by the Association of Universities
for Research in Astronomy Inc. under a cooperative agreement with the
National Science Foundation.
Science Contact
Dr. Steve B. Howell
NASA Ames Research Center
PO Box 1
M/S 244-30
Moffett Field, CA 94035
steve.b.howell@nasa.gov
650.604.4238
Dr. Steve B. Howell
NASA Ames Research Center
PO Box 1
M/S 244-30
Moffett Field, CA 94035
steve.b.howell@nasa.gov
650.604.4238
