Why the universe may be teeming with aliens
WANTED: Rocky planet outside of our solar system. Must not be too hot or too cold, but just the right temperature to support life.
It sounds like a simple enough wish list, but finding a planet that fulfils all of these criteria has kept astronomers busy for decades. Until recently, it meant finding a planet in the "Goldilocks zone" - orbiting its star at just the right distance to keep surface water liquid rather than being boiled off or frozen solid.
Now, though, it's becoming increasingly clear that the question of what makes a planet habitable is not as simple as finding it in just the right spot. Many other factors, including a planet's mass, atmosphere, composition and the way it orbits its nearest star, can all influence whether it can sustain liquid water, an essential ingredient for life as we know it. As astronomers explore newly discovered planets and create computer simulations of virtual worlds, they are discovering that water, and life, might exist on all manner of weird worlds where conditions are very different from those on Earth. And that means there could be vastly more habitable planets out there than we thought possible. "It's like science fiction, only better," says Raymond Pierrehumbert, a climate scientist at the University of Chicago, who studies planets inside and outside of our solar system.
Distance from the nearest star is, of course, important. In our own solar system, Venus has long served as an example of what can happen if a planet gets too close to its star. Venus is only 28 per cent closer to the sun than Earth is, but its surface is a sweltering 460 °C, hot enough to melt lead, and it chokes under a thick carbon dioxide atmosphere 90 times the density of Earth's.