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What makes a planet livable? Five things scientists look for.

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3. What are the planet's available energy sources?

Light from the host star is one obvious source. Light supports photosynthesis, the process plants use to capture carbon dioxide and produce oxygen. The team estimates that in our solar system, sunlight could support some level of photosynthesis out to at least 10 astronomical units, or 10 times the distance between Earth and the sun.

Heat is another potential energy source and also affects general habitability, even if it is not used directly as an energy source. Depending on a planet's distance from its star (or a moon's distance from its planet), the gravitational tug of war between the two can generate internal heat through friction as a rocky planet flexes.

Tidal flexing of rocky cores is thought to have generated the heat that allowed subsurface seas to form under the ice covering Jupiter's moon Europa and Saturn's moon Enceladus. In addition, micro-organisms can draw energy from chemical elements and molecules, from sulfur and iron to ammonia and hydrogen sulfide.

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