In 2007, New Horizons, a NASA probe on its way to Pluto, photographed a volcanic eruption on Io, Jupiter’s innermost large moon. The image showed a plume of sulfur shooting nearly 320 kilometers (200 miles) above the surface of the moon and out into space. It provided further evidence of what scientists had first glimpsed during the Voyager mission nearly 30 years earlier: this small moon, roughly the size of Earth’s Moon, and located about 770 million kilometers (480 million miles) from the Sun, is not a frozen and barren world. Rather, Io is covered by lava flows and other volcanic features driven by an internal heat source. So where does the energy come from? And what significance does it have with respect to the possible presence of life beyond Earth?

The key to understanding Io's volcanic activity is its parent planet, Jupiter. The strong gravitational pull of Jupiter’s enormous mass, coupled with Io’s close proximity, deforms the moon so that it is slightly nonspherical. Gravitational forces of its fellow moons, Europa and Ganymede, also pull on Io, tugging it in different directions and into a noncircular orbit. Based on its irregular shape and elliptical orbital pattern, Io is continually squeezed and stretched as it orbits Jupiter. Its surface rises and falls like the tides do on Earth. Deep in its interior, the immense friction caused by this stretching produces the heat that fuels Io’s many volcanoes. With a self-sustaining source of energy, Io has one of the three ingredients needed to support life.

A habitable environment needs to provide not just energy, but also organic materials from which more complex molecules can be built as well as a liquid medium (water). To be habitable for life, a planet has to be just the right distance from a star so that all three conditions can exist at once. When a planet is in what is called the “habitable zone,” the temperature is not too cold; neither is it hot enough to boil water or break down the organic molecules on which life depends. According to this narrow definition, only Earth and Mars are in the habitable zone of our solar system.

However, discoveries made by missions to the outer solar system demand that we expand our thinking. The discovery of volcanism on Io has inspired the description of another kind of habitable zone that isn’t linked to a planet’s distance from its star. While the chances of life existing on Io are slim—it has an energy source and possibly the right chemical building blocks, but scientists have not yet detected the third key ingredient, water—other discoveries have also broadened definitions of habitability. Scientists have recently discovered deepwater microbes in the frigid waters of Earth’s polar oceans. Instead of using sunlight to trigger vital reactions, microbes like these use sulfur, hydrogen, and methane as chemical sources of energy. Off Earth, life might possibly occur where other liquids substitute for water. This makes Titan, another of Saturn's moons, a potential candidate for further study. Despite its subzero temperatures, Titan contains vast bodies of liquid methane and ethane on its surface.

Today, scientists are learning that places in the solar system that might harbor life are more widespread than ever before realized. Could there be planets or moons in our solar system where life could potentially gain a foothold?

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