How Will We Know if Planets Are Inhabited?

The recent and scientifically controversial announcement of arsenic-eating microbes in the eastern California desert has ratcheted up the expectation of finding life among the stars.

Add this to the building anticipation of the NASA Kepler mission’s promise to find “Earth-sized” planets in its survey of over 100,000 stars near the Summer Triangle.

Another Kepler data release is scheduled for February and once again there will probably be a flurry of blogs speculating if the mission has found the interstellar Holy Grail -- an Earth-sized planet in the balmy habitable zone about a sunlike star.

A term that's often kicked around is finding an Earth-analog. But what does that really mean? Does it imply something is living there? If so, the answer is very likely to be decades away, and full of uncertainty.

What Makes Earth Habitable?

The habitability of Earth depends on a lot more than just orbit location and its mass. Proponents of the Rare Earth hypothesis say that a long chain of unlikely events led to the emergence of complex life here. The Gaia hypothesis proposes Earth is nurtured and reshaped by life into a single mega-organism.

The flip side is the Medea hypothesis that says life is self-destructive and poisons a planet. For example, over 2 billion years ago blue green algae pumped out oxygen that was toxic to ancient bacteria. Microbes may have sucked enough greehouse gasses out of the atmosphere to trigger two "mother of all ice ages" at 2.3 billion years ago and again at 700 million years ago.

Based on current theories of planetary evolution, any extraterrestrials observing the solar system 4 billion years ago would have seen oceans on Venus, Earth, and Mars. Alien scientists would have cataloged all of them as potentially habitable.

Earth’s biggest advantage comes from having plate tectonics that stabilize the amount of carbon dioxide in the atmosphere that keeps Earth warm. Carbonic acid (essentially soda pop) dissolved in silicate rocks is transported to our oceans by rain. Sea life makes carbonate shells from this runoff. This makes carbonate sediments on the floor of the ocean that are subducted back into the Earth though plate tectonics. Volcanism recycles the carbonates into the atmosphere.

Read more at Discovery News