Tuesday, 28 April 2015

Determining the Habitable Zone of 70 Virginis.


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).

See also…

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