The Kepler Space Telescope has located many multi-planet systems since its inception,
which combined with discoveries made by other planet-hunting missions
has enabled scientists to begin to construct models of planetary
systems orbiting other stars. This is particularly complicated where
not all planets are visible to the space telescope, which is only
capable of directly detecting fairly large planets. One such system
is Kepler-68, where two large planets orbiting close to the star have
been directly observed by the telescope as they pass in front of it,
and the presence of a third, larger and more distant, planet has been
inferred by the actions of its gravity upon the star and two
observable planets.
In a paper published on
the online arXiv database at Cornell University Library
on 9 November 2015 and
submitted for publication in the Astrophysical Journal Letters
Stephen Kane of the Department of Physics & Astronomy at SanFrancisco State University describes a model of the Kepler-68 system,
which calculates the masses and orbits of the planets, as well as
plotting the system's Habitable Zone (the zone in which an Earth-mass
planet could potentially host liquid water), and the possibility of a
small rocky planer orbiting within that zone.
Kepler-68 A (when naming
bodies in other stellar systems stars are indicated with an upper
case letter while planers are indicated with lower case letters) is a
Sun-like star with a mass equivalent to 1.079 times that of the Sun
and an effective surface temperature of 5793 K (compared to 5778 K
for the Sun). It has a slightly larger radius, 1.243 times that of
the Sun, and is somewhat brighter, with a luminosity 1.564 times the
Sun's.
The two inner planets of
Kepler-68 A, Kepler-68 b and Kepler-68 c, have orbital periods of
5.399 and 9.605 days respectively, indicating that they orbit at
0.061 AU and 0.091 AU (i.e. 6.1 and 9.1% of the distance at which the
Earth orbits the Sun), and are calculated to have masses equivalent
to 8.3 and 4.8 times that of the Earth.
The third, inferred,
planet, Kepler-68 c, is calculated to have a mass equivalent to 0.947
times that of Jupiter, and to orbit Kepler-68 A every 580 days,
giving it an average orbital distance of 1.4 AU (1.4 times the
distance at which the planer Earth orbits the Sun). However the
gravitational influences exerted suggest that its orbit is not
circular, rather has an eccentric orbit that takes it from 1.15 AU
from the star at its closest to 1.65 AU at its furthest.
Kane calculated two
possible ranges for the habitable zone of the Kepler-68 system, a
conservative estimate, in which an Earth-like planet would be
expected to host liquid water, and an optimistic estimate, within
which a small rocky planet could possibly host liquid water. The
conservative estimate ranges from 1.19 AU to 2.09 AU from the star,
while the optimistic estimate ranges from 0.94 AU to 2.21 AU.
A top-down view of the
Kepler-68 system showing the extent of the Habitable Zone and orbits
of the planets. The physical scale depicted is 3.36 AU on a side. The
conservative Habitable Zone is shown as light-gray and optimistic
extension to the Habitable Zone is shown as dark-gray. The inner-most
(unlabeled) orbit is that of planet b. Kane (2015).
This means that the
distinctly un-Earth-like Kepler-68 c orbits entirely within the
conservative habitable-zone of the system, which greatly reduces the
possibility of an Earth-like planet aslo being found; the
gravitational influence of very large planets makes it hard for
smaller planets to occupy nearby orbits without being nudged onto
completely different paths, and most likely being wither thrown out
of the system altogether or falling into the star. However Kane
calculates that a stable zone does exist within the habitable-zone
that could host an Earth-like planet, between 1.8 AU and 1.9 AU from
the star (outside the orbit of Kepler-68 c). This is in addition to
the possibility of an Earth-like moon orbiting the large planet
itself (a popular scenario in science fiction movies, but not one all
planetary scientists are convinced is possible).
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