Journal Article

Habitability of exoplanetary systems with planets observed in transit

Barrie W. Jones and P. Nick Sleep

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 407, issue 2, pages 1259-1267
Published in print September 2010 | ISSN: 0035-8711
Published online September 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.16978.x
Habitability of exoplanetary systems with planets observed in transit

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We have used the measured properties of the stars in the 79 exoplanetary systems with one or more planets that have been observed in transit, to estimate each system's present habitability. Such systems have the advantage that the inclination of the planetary orbits is known, and therefore the actual mass of the planet can be obtained, rather than the minimum mass in the many systems that have been observed only with the radial velocity technique. The measured stellar properties have been used to determine the present location of the classical habitable zone (HZ). To establish habitability we use the estimated distances from the giant planet(s) within which an Earth-like planet would be inside the gravitational reach of the giant. These distances are given by nRH, where RH is the Hill radius of the giant planet and n is a multiplier that depends on the giant's orbital eccentricity eG and on whether the orbit of the Earth-like planet is interior or exterior to the giant planet. We obtained nint(eG) and next(eG) in earlier work and summarize those results here. We then evaluate the present habitability of each exoplanetary system by examining the penetration of the giant planet(s) gravitational reach into the HZ. Of the 79 transiting systems known in 2010 April, only two do not offer safe havens to Earth-like planets in the HZ, and thus could not support life today. We have also estimated whether habitability is possible for 1.7 Gyr into the past, i.e. 0.7 Gyr for a heavy bombardment, plus 1.0 Gyr for life to emerge and thus be present today. We find that, for the best estimate of each stellar age, an additional 28 systems do not offer such sustained habitability. If we reduce 1.7 Gyr to 1.0 Gyr, this number falls to 22. However, if giant planets orbiting closer to the star than the inner boundary of the HZ have got there by migration through the HZ, and if this ruled out the subsequent formation of Earth-like planets, then, of course, none of the presently known transiting exoplanetary systems offers habitability. Fortunately, this bleak conclusion could well be wrong.

As well as obtaining results on the 79 transiting systems, this paper demonstrates a method for determining the habitability of the cornucopia of such systems that will surely be discovered over the next few years.

Keywords: astrobiology; methods: numerical; planets and satellites: dynamical evolution and stability; planetary systems

Journal Article.  5106 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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