70 Viriginis is a G-type Yellow Dwarf Star about 59 light years from Earth in the constellation of Virgo. It is
calculated to have a mass 109% of that of the Sun, but radius 194% of the
Sun’s, and a lower temperature, 5393K, compared to 5778K for the Sun, from
which it is calculated to be somewhat older, approximately 7.77 billion years
(compared to about 5.0 for the Sun). This star hosts one of the first
discovered exoplanets, 70 Virginis b, asuperjovian planet in a short (116 days)
but highly eccentric orbit discovered in 1996.
In a paper published on the arXiv database at Cornell University
Library on 15 April 2015 and submitted for publication in The Astrophysical Journal, Stephen Kane of the Department of Physics & Astronomy at San Francisco State University, Tabetha Boyejian of the Departmentof Astronomy at Yale University, Gregory Henry of the Center of Excellence in
Information Systems at Tennessee State University, Katherina Feng of the Departmentof Astronomy and Astrophysics and Center for Exoplanets & Habitable Worlds
at Pennsylvania State University and the Department of Astronomy & Astrophysics at the University of California, Santa Cruz, Natalie Hinkal, also
of the Department of Physics & Astronomy at San Francisco State University,
Debra Fischer, also of the Department of Astronomy at Yale University, Kaspar
von Braun of Lowell Observatory, Andrew Howard of the Institute for Astronomy
at the University of Hawaii and Jason Wright, also of the Department
of Astronomy and Astrophysics and Center for Exoplanets & Habitable Worlds
at Pennsylvania State University, present a fresh study of the 70 Virginis
system using new data from the Cente rfor High Angular Resolution Astronomy
(CHARA array) at Georgia State University and the HIRES echelle spectrometer on
the 10.0m Keck I telescope, which they combine with previously acquired data on
the system from the Hamilton Echelle Spectrograph on the 3.0m Shane Telescope at Lick Observatory and the ELODIE spectrograph on the 1.93m telescope at Observatoirede Haute-Provence, which they use to build a model of the Habitable Zone of the
system, and calculate the possibility of an Earth-sized planet remaining in a
stable orbit within it.
Kane et al. derive a
‘conservative’ habitable zone for the 70 Virginis system with an inner boundary
at 1.63 AU from the star (i.e. 1.63 times the average distance at which the
Earth orbits the Sun) and an outer boundary at 2.92 AU from the star, and an
‘optimistic’ habitable zone with an inner boundary at 1.29 AU and an outer
boundary at 3.08 AU.
A top-down view of the 70 Virginissystem showing the
extent of the Habitable Zone calculated using the stellar parameters established
with the CHARA, HIRES, Hamilton Echelle and ELODIE data. The conservative Habitable
Zone is shown as light-gray and optimistic extension to the Habitable Zone is
shown as dark-gray. The revised Keplerian orbit of the known planet is overlaid
as a continuous dark line. Kane et al. (2015).
Next Kane et al. attempted
to calculate the possibility of an Earth-sized planet remaining in a stable
orbit within this habitable zone. In order to do this they calculated the
stability of planets at the inner and outer margins of the conservative and
optimistic Habitable Zones (i.e. 1.29 AU, 1.63 AU, 2.92 AU and 3.08 AU), since
if these orbits are stable then intermediate orbits, fully within the Habitable
Zone, ought to be available.
These calculations revealed that while it was possible for an
Earth-sized planet to remain in a stable orbit within the habitable zone, the
gravitational influence of the known planet, 70 Virginis b, would make it impossible
for such a planet to remain in a stable orbit in the same orbital plane as the
larger body. This is problematic if we consider the Solar System to be a
typical planetary system, as all the planets in the Solar System orbit in
approximately the same plane, and models of Solar System formation suggest that
this was the way in which they formed, apparently ruling out other
configurations. However other stellar systems have been discovered in which not
all the planets orbit in the same plane, indicating that such an outcome is not
impossible.
Kane et al. calculate that
an Earth-sized planet orbiting 70 Virginis at a distance of 1.29 AU would need
to have an orbit tilted at an angle of at least 24˚ to that of 70 Virginis b to
remain stable. Such a planet at 1.63 AU would need to be tilted at 25˚ to
remain stable, one at 2.92 at 10˚ and one at 3.08 AU at 3˚. This is roughly
linear, with hypothetical planets further from 70 Virginis b able to adopt less
inclined orbits due to the reduced influence of its gravity, though the planet
at 1.63 was more affected than that at 1.29 AU, due to its being closer to
being in a resonant orbit (planets in resonant orbits pass one-another
regularly on their orbital cycle, typically with the inner planet completing
two orbits for one of the outer planet or some similar arrangement; such
resonant orbital arrangements are extremely stable, but orbits close to
resonant arrangements are highly unstable, with the smaller body typically
being either pushed into the stable arrangement or ejected from the system
completely).
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