La evolución de la zona habitable

The evolution of habitable zones during stellar lifetimes and its implications on the search for extraterrestrial life
David R. Underwood, Barrie W. Jones, P. Nick Sleep
Dept of Physics & Astronomy, The Open University, Milton Keynes, MK7 6AA, UK e-mail: d.r.underwood@open.ac.uk Abstract: A stellar evolution computer model has been used to determine changes in the luminosity L and effectivetemperature Te of single stars during their time on the main sequence. The range of stellar masses investigated was from 0.5 to 1.5 times that of the Sun, each with a mass fraction of metals (metallicity, Z) from 0.008 to 0.05. The extent of each star’s habitable zone (HZ) has been determined from its values of L and Te. These stars form a reference framework for other main sequence stars. All of the104 main sequence stars known to have one or more giant planets have been matched to their nearest stellar counterpart in the framework, in terms of mass and metallicity, hence closely approximating their HZ limits. The limits of HZ, for each of these stars, have been compared to its giant planet(s)’s range of strong gravitational influence. This allows a quick assessment as to whether Earth-massplanets could exist in stable orbits within the HZ of such systems, both presently and at any time during the star’s main sequence lifetime. A determination can also be made as to the possible existence of life-bearing satellites of giant planets, which orbit within HZs. Results show that about half of the 104 known extrasolar planetary systems could possibly have been housing an Earth-massplanet in HZs during at least the past billion years, and about three-quarters of the 104 could do so for at least a billion years at some time during their main sequence lives. Whether such Earth-mass planets could have formed is an urgent question now being investigated by others, with encouraging results. Introduction The search for the signs of carbon-based extraterrestrial life outside our SolarSystem is expected to focus on extrasolar planets within the habitable zones of exoplanetary systems around stars similar to the Sun. The habitable zone (HZ) around a star is defined as the range of distances over which liquid water could exist on the surface of a terrestrial planet, given a dense enough atmosphere (Jones et al, 2001). The planet would be rocky, be of order of the Earth’s mass, andwould have had to lie in an orbit within the HZ for at least the order of a billion years (= 1 Gy) for life to have had a detectable effect on its atmosphere. This duration is based on the effect of Earth’s biosphere on our atmosphere (Chyba, 1993). The detection of Earth-mass extrasolar planets is, at the moment, beyond most current technology. This is shortly due to be redressed, however, withthe launch of planet detecting satellites such as the transit-detecting Kepler mission (2008) and the direct imaging missions Darwin (ESA) and TPF (NASA) (2015), plus imaging by extremely large 30-100 m telescopes with high performance adaptive optics (e.g. ESO’s OWL). We can, meanwhile, use computer models to predict in which of the currently known exoplanetary systems Earth-mass planets couldexist, where 119 extrasolar planets of the order of Jupiter’s mass have been found in orbit around 104 stars (catalogued by Jean Schneider at http://www.obspm.fr/encycl/encycl.html). Existence requires that the giant planets would allow terrestrial bodies to remain in stable orbits around their

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parent star i.e. in orbits that do not result in collision or ejection. To determine this, amixed variable symplectic computer integrator has been used previously (Jones & Sleep, 2002; Jones et al., 2001), to simulate the orbital motion of Earth-mass planets over one billion years, in a sample of such systems. If a terrestrial planet could exist in a stable orbit for this length of time it is likely to be able to exist there for the duration of that star’s main sequence lifetime. A...