An artist's impression of the newly discovered super-Jupiter exoplanet around an evolved star, only the third known example of such a system.
Credit: NASA/JPL-Caltech
There are 565 exoplanets currently known that are as massive as Jupiter or bigger, about one third of the total known, confirmed exoplanet population. About one quarter of the massive population orbits very close to its star, with periods of less than ten days (the Earth takes about 365 days to orbit the Sun). Heated by the nearby star’s radiation, these giants are often called hot Jupiters.
Despite the large and diverse population of known giant exoplanets,
only two of them orbit older, evolved stars. How and why there are so
many giant planets close to their host stars is still a mystery: perhaps
over time they migrate in from more distant parts of their planetary
system, or instead perhaps they are born there? Evolved stars that host
close-in, giant exoplanets provide a valuable wrinkle to the picture,
and some clues: these stars, as they age, cool off and swell in
diameter, could disrupt or even swallow any nearby planets. Finding
examples allows astronomers to refine their models of planet formation
and evolution.
CfA astronomers Dave Latham, David Kipping, Matthew Payne, David
Sliski, Lars Buchhave, Gilbert Esquerdo, Michel Calkins, and Perry
Berlind and their colleagues have discovered two new giant exoplanets
around an evolved star. Kepler-432b is about 5.4 Jupiter-masses in size
and orbits every 52.5 days – it is the third known example of a
close-in giant around an evolved star; Kepler-434c is 2.4 Jupiter-masses
and orbits much farther away, in 406 days. The host star, Kepler-432
has a mass of about 1.35 solar-masses, an age of about 3.5 billion
years, and it has just finished its stable lifetime burning hydrogen and
begun to swell in size, with a current diameter of 4.16
solar-diameters.
The astronomers found that the massive inner planet is strange in at
least three ways. First, it is not highly irradiated or hot, unlike
typical hot Jupiters. Its orbit is highly eccentric (meaning that its
distance from the star varies considerably over an orbit), suggesting
that it may have migrated to this orbit. Finally, its spin axis happens
to be closely aligned to the star's, another curious property,
especially since it is usually not found in planets that have migrated.
The results highlight the remarkable range of exoplanet properties and
possible formation mechanisms, and imply either that Kepler-432b is an
intrinsically rare case, or that it represents a common class of
exoplanets that are usually destroyed as their host star ages, but which
in this case has so far managed to survive - though its days are
probably numbered (perhaps only another few hundred million years).
Reference(s):
"Kepler-432:
A Red Giant Interacting with One of Its Two Long-Period Giant Planets,"
Samuel N. Quinn, Timothy. R. White, David W. Latham, William J.
Chaplin, Rasmus Handberg, Daniel Huber, David M. Kipping, Matthew J.
Payne, Chen Jiang, Victor Silva Aguirre, Dennis Stello, David H. Sliski,
David R. Ciardi, Lars A. Buchhave, Timothy R. Bedding, Guy R. Davies,
Saskia Hekker, Hans Kjeldsen, James S. Kuszlewicz, Mark E. Everett,
Steve B. Howell, Sarbani Basu, Tiago L. Campante, Jørgen
Christensen-Dalsgaard, Yvonne P. Elsworth, Christoffer Karoff, Steven D.
Kawaler, Mikkel N. Lund, Mia Lundkvis, Gilbert A. Esquerdo, Michael L.
Calkins, and Perry Berlind, ApJ 803, 49, 2015
