Of the approximately 1,500 exoplanets found so far, only a few are in the so-called habitable zone around their parent star. It’s also called the “Goldilocks zone,” a coveted sweet-spot where the amount of radiation coming from a star is “just right” for liquid water to exist on the surface of a rocky planet.

For example, Earth is within the present habitable zone around our sun, which is a comparatively narrow swath 18 million miles across.

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Enthusiasm about finding other worlds nestled inside other stars’ habitable zones is so strong that in 2008 a radio message was beamed to a planet known to exist in the habitable zone of the red dwarf star Gliese 581, located 20 light-years away.

The signal contains 501 greetings that were selected through a competition on a social networking site.

The tight noose defined by a habitable zone is one of the important variables in the famous Drake Equation, which tries to estimate the number of extraterrestrial civilizations in our galaxy.

The habitable zone concept was widely popularized in the mid 1970s by astronomer Michael Hart. He used it to argue that Earth was the only home for intelligent life in the galaxy. In fact, Hart’s own calculation from the Drake Equation came up with a probability of .000000000000000000000000000000000001 percent of there being intelligent life elsewhere in the galaxy.

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But we’ve come a long way since then with the discovery of extremeophiles tucked away in every nook and cranny of Earth. Add to that a more pragmatic definition of life as simply being a self-sustaining chemical system that undergoes Darwinian evolution.

Life as we know it couldn’t have started without water. Water provides a nurturing environment for complex molecules to form and organize themselves. But water is a liquid over a temperature range that is present in only a very small number of objects in the universe. Hence, the idea of a narrow habitable zone.

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Perhaps so, but in the cosmological context there’s no obvious reason why life has to be exclusively dependent on a water-based environment. Life could be so opportunistic and adaptable it may simply work with whatever liquid is at hand. Alternative solvents considered by astrobiologists include dihydrogen (a simple molecule composed of two hydrogen atoms), sulfuric acid, dinitrogen, formamide, and methane, among others.

If so, this antiquates the notion of a narrow habitable zone encircling billions of stars in our galaxy. Depending on your flavor of life, there could be multiple habitable zones around the sun and other stars.

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Move out to one billion miles from the sun and you’re in a potential “methane habitable zone.” This is where the giant Saturnian moon Titan is. Titan has rain, rivers, and giant lakes of freestanding liquid. It’s not water, which is rock-hard at Titan’s surface temperature of minus 300 degrees Fahrenheit, but instead it is liquid methane and ethane.

Astrobiologist Chris McKay of NASA’s Ames Research Center has hypothesized the presence of a “cryolife” in Titan. The organisms would breath hydrogen instead of oxygen, use acetylene in place of glucose, and exhale methane. This scenario could certainly explain some of Titans odd chemistry.

Titan may also have an ammonia-water subsurface ocean. Ammonia could be as gentle to organic molecules at least as well as water is.

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McKay has considered the feasibility of methane habitable zones around the most common stars in our galaxy, red dwarfs. These stars are much cooler than the sun and so the habitable zones could be as close to their stars as Earth is our sun, or even closer.

If Titan orbited a red dwarf, more light for powering life would reach the surface because Titan’s atmospheric haze is more transparent to infrared light.

McKay thinks there could be many Titan-like worlds beyond our solar system that have surface methane lakes and oceans.

Therefore, the whole idea of just one type of habitable zone in the galaxy may be terribly geocentric thinking. On the other hand, could intelligent life arise from other exotic biochemistries?

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At cryogenic temperatures the metabolism of such creatures would pass at a glacial rate. So their life activities would be very stretched out relative to ours. A conscious entity’s perception of time’s passage would be markedly different from ours.

Nevertheless, such beings might regard water-worlds as unlikely places for finding evidence of life, just as we would regard a world covered in molten lava. To them we would live in an uninhabitable zone.

Images courtesy ESA and NASA