Life Outside the "Zone."

As I've stated in earlier posts, I support the idea that extrasolar life may be readily found outside the traditional "Habitable Zone" or the "Goldilocks" zone around a star - the band of space around a star that is neither too cold nor too warm for liquid water to exist. This is too simplistic. Liquid water can be found outside this zone - mainly on moons circling large planets. The tidal forces of the planet on the moon can cause the moon to heat up through internal friction. This is especially true if the moon is in an elongated orbit.

How does this work? This is due to the fact that gravity decreases with distance and the gravitational pull on the near side of the moon is greater than the gravitational pull on the far side. For a moon in a circular orbit, the moon will adjust its shape to adapt to this gravitational differential, and no tidal heating will occur. But for a moon in an eccentric orbit, the gravitational differential will change rhythmically, and the moon will be kneaded like a lump of bread dough (OK, a bit of an exaggeration). This will heat a moon even if it is outside of the solar system's main habitable zone. This increases the areas in a solar system where life can form.

Recent research by Brian Jackson, Rory Barnes, and Richard Greenberg of Arizona's Lunar and Planetary Laboratory extends this idea to planets (this research will be published in an upcoming issue of Monthly Notices of the Royal Astronomical Society). Most extrasolar planets found to date circle their stars in elongated orbits. Like a moon circling a large planet, these planets circling a large star in elongated orbits will experience tidal stress, which will cause internal heating and possibly tectonic activity. This internal heating may be enough to warm the planet to where liquid water can exist even when the planet's orbit takes it outside of its star's traditionally defined Habitable Zone.

However, because the tidal heating scales with the size of the planet, for "super-Earths," terrestrial planets 2 to 10 times the size of the Earth, the tidal heating would be too great to make the planet habitable - the planet may become too hot, with many large active volcanoes.

But for Earth-sized or smaller terrestrial planets that would otherwise be too small or too cold to support life, this type of tidal heating may help them become habitable by not only warming them up so that liquid water can exist but also by causing tectonic activity which may help life to arise. Some scientists feel that the Moon was essential to the origin of life on the Earth due to the tidal mixing which helped to mix, mainly from erosion caused by the tides, chemicals from the soil with the oceans, creating the chemical soup from which life arose. The tidal forces of a star on planet in an elongated orbit may have the same result. In addition, tectonic activity helps regulate carbon dioxide.

Therefore, I believe that the famous Drake Equation may be a bit too conservative. The number of planets (or moons!) that potentially can support life may be higher than first thought.

Reference:

"Tides have major impact on planet habitability." Astronomy. Kalmbach Publishing Co. 14 Oct. 2008. Web. 31 Oct. 2008. Provided by the Div. for Planetary Sciences of the American Astronomical Society. <http://www.astronomy.com/asy/default.aspx?c=a&id=7505>.