Researchers searching for life on other planets have always believed that there is at least one necessary requirement for life to exist: There must be water. But a new theory by astrobiologists Nediljko Budisa and Dirk Schulze-Makuch suggests that there are alternatives to water that could make life possible even on desert worlds, reports io9.com.

It's an exciting idea. If the theory is correct, then the number of planets believed capable of supporting life would increase dramatically. 

The reason that water is considered such an essential resource for life is that it is a solvent; it makes most biological chemical reactions possible. Without water or an equivalent solvent, the chemistry of life would simply be non-existent. Budisa and Schulze-Makuch's theory acknowledges this fact, but suggests that there is another substance capable of operating as a viable solvent. Namely, supercritical carbon dioxide.

Most people are familiar with carbon dioxide, an abundant compound. But what turns good, old-fashioned CO2 into a supercritical compound? Turns out, fluids become supercritical when they exceed their temperature and pressure thresholds. Once this critical point is reached, distinct liquid and gas phases no longer exist. They can effuse through solids like a gas, and dissolve materials like a liquid. 

Carbon dioxide's critical point is reached when its temperature exceeds 305 degrees Kelvin and its pressure goes beyond 72.9 atm (a standard measure for atmospheric pressure). This equates to roughly 89 degrees Fahrenheit and pressure that amounts to what you'd find about a half-mile beneath the ocean surface.

Supercritical carbon dioxide happens to act as a solvent, and in some instances it even makes for a better solvent than water. For example, enzymes can be more stable in supercritical carbon dioxide than in water, and they are more specific about the molecules they bind to. This can mean fewer unnecessary side reactions.

One candidate world that qualifies under this model exists right in our planetary backyard: our neighbor, Venus. Venus' atmosphere is about 97 percent carbon dioxide, its average temperature is about 872 degrees Fahrenheit, and the atmospheric pressure there is roughly 90 times greater than Earth's. Perhaps Mars is not the only nearby planet where we should be searching for signs of life.

Several other recently discovered super-Earths — or rocky planets with a mass higher than Earth's — could also be candidates for harboring such life. 

"I always have been interested in possibly exotic life and creative adaptations of organisms to extreme environments," said Schulze-Makuch. "Supercritical CO2 is often overlooked, so I felt that someone had to put together something on its biological potential."

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