Mystery: Is there life out there?
ON THE night of 8 December 1990, a spaceship buzzed by 960 kilometres above Earth. As it skimmed past, it scanned the surface for light, heat and radio waves, looking for signs of life.
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This was no close alien encounter. The ship, Galileo, was one of our own, en route to Jupiter. But one of NASA’s mission scientists, Carl Sagan, had the idea of using a slingshot past Earth for a dry run of life-detection technology. Sure enough, Galileo detected abundant water, oxygen, methane and a pigment that soaked up red light. Taken together, these observations were “strongly suggestive” that Earth was teeming with life.
Water, water!
It remains the only such place we know of. Yet given the number of planets in our own galaxy, let alone the whole universe, it seems staggeringly unlikely to be the only place. To scratch this itch, we need to find life elsewhere. The default assumption is that it will resemble Earth life. That means it will need liquid water.
Within our solar system, the prime candidates are Mars, which almost certainly had a surface ocean in the past and may retain remnants of it underground, and some icy moons of Saturn and Jupiter, especially Enceladus and Europa. “Even though they have an icy surface, they have an ocean of liquid water beneath,” says , an astrobiologist at the University of Lisbon in Portugal. “They also have minerals that could be used to build organic molecules and a source of energy, so they are a perfect target.” Outside the solar system, the prime targets are medium-sized rocky planets orbiting sun-like stars, or possibly in the liquid-water-friendly “habitable zone” around dimmer red dwarfs.
But is this Earth-centrism blinding us to other possibilities? “What is relevant for life to emerge and persist is mostly unknown,” says astronomer at the University of Birmingham, UK. “We only know that our conditions are propitious, which tells us what is sufficient, but not what is necessary for life to start and thrive.”
Triaud says the search should be extended to other types of exoplanet: those orbiting brown dwarfs, white dwarfs and red giants; circumbinary planets orbiting two stars at once; planets with no atmosphere; and even exomoons orbiting gas giants. These may seem unlikely crucibles for life, but that is the point. “Most people think that getting a yes/no answer to whether there is life out there is sufficient,” says Triaud. “I want to find out how frequently and under which conditions life emerges and persists.”
Our own solar system offers similarly left-field targets. Titan, another of Saturn’s moons, makes up for a lack of liquid water with lakes of hydrocarbons that may be home to life based on different chemistry. Even further-fetched possibilities include the clouds of Venus and even Pluto. “The icy moons are our best bet, but I wouldn’t close the door on anything,” says Martins. With missions to Enceladus, Titan and possibly Europa on the drawing board and new techniques for detecting life on  in the works, perhaps our lonely pale blue dot may soon have some companions.
This article appeared in print under the headline “Object: Earth”
