Saturday, March 6, 2010

Alien Earths

Earth-size and probably even Earth-like planets are expected to be common throughout the galaxy; orbiting stars not too different from our Sun. How will such Earth-like worlds differ from our own? A paper by Courtney D. Dressing, et al. (2010) entitled “Habitable Climates: The Influence of Eccentricity” examines how factors such as obliquity, spin rate, orbital eccentricity, orbital distance from host star and the fraction of surface covered by ocean might affect the habitability of Earth-like extrasolar planets. In this paper, regions of a planet that are at temperatures between 273 to 373 degrees Kelvin are considered habitable while regions outside that temperature range are considered uninhabitable.

Obliquity refers to the tilt of a planet’s axis, spin rate refers to the time required for a planet to complete one rotation about its axis, orbital eccentricity refers to how much a planet’s orbit around its star deviates from a perfect circle and orbital semimajor axis refers to the mean distance of a planet from its host star. An orbital eccentricity of zero denotes a perfect circle and an orbital eccentricity of one denotes a parabola. The Earth for example, has an obliquity of 23.4 degrees, a spin rate of 24 hours, an orbital eccentricity of 0.0167 and an orbital semimajor axis of 149.6 million kilometres. In addition, the surface of the Earth is 70 percent ocean and 30 percent land.

Of all the extrasolar planets with measured orbital eccentricities, a large fraction of them have significant orbital eccentricities and this suggests that Earth-like planets in near circular orbits, like ours, probably represent only a small subset of potentially habitable worlds. This paper basically studies the numerous possible types of Earth-like planets and many of the models of Earth-like planets presented are particularly interesting.

Take for example, a desert planet with an obliquity of 90 degrees, an orbital semimajor axis of 1.225 AU and an orbital eccentricity of 0.2. Winter at the southern hemisphere of this planet occurs when the planet is furthest from its star and during this long winter, the southern pole freezes and reaches an incredibly cold temperature of minus 120 degrees Centigrade. For this planet, the southern pole becomes transiently habitable only during northern winter when the planet is closest to its star. The southern pole of this planet experiences the most extreme temperature variations. During southern winter, the planet is furthest from its star and the southern pole experiences perpetual darkness. During southern summer, the planet is closest to its star and the southern pole experiences perpetual daylight.

This paper can be obtained at and it investigates the many types of possible Earth-like worlds that can exist. Notable examples described in this paper include:
- An Earth-like planet whose spin axis is tilted 90 degrees, such that the entire northern hemisphere can be in constant daylight while the entire southern hemisphere can be in constant darkness and vice versa, during specific points of the planet’s orbits around its host star.
- A planet where one day has a length of 8 hours or another where one day has a length of 72 hours.
- An Earth-like planet whose highly eccentric orbits around its host star brings it from a distance where most of its surface is scorching hot out to a distance where most of the planet’s surface plunges into a deep freeze.