According to astronomers, you don’t need the Starship Enterprise to find worlds that may be supporting life. Although we haven’t even set foot on Mars, our closest neighbor, scientists are already searching for habitable planets, and they’re doing so without leaving home.
The first step toward identifying another life-sustaining world is, well, to find a planet. But this is easier said than done. Exoplanets, or planets orbiting stars other than the sun, are really far away, and they don’t give off their own light. So one common way to detect them is to use a method called transiting, where astronomers measure changes in the amount of light coming from a star as a planet orbits in front of it. Although the scientists can’t actually see the planet, they can see how much light it blocks out as it passes in front of its star. To date, scientists have found over 400 exoplanets, only one of which has a rocky surface, like Earth.
But it takes more than a big, orbiting rock to make a comfortable home.
“The party line is that it [must be] the right distance from its star for water to be liquid,” said David Latham, a senior astronomer at the Harvard-Smithsonian Center for Astrophysics who has been looking for exoplanets for the past 25 years.
In other words, planets need to be in what Sara Seager, an exoplanet scientist at the Massachusetts Institute of Technology, calls the “Goldilocks zone.” The planet must be far enough from its sun for water to be liquid, but not so far that water turns to ice, and it must be big enough to maintain an atmosphere but not so big that its interior gets very hot. In other words, its size and position in its solar system must be just right.
Yet finding a rocky Goldilocks planet is far more difficult than choosing the perfect bowl of porridge. The larger the planet, the more easily it can be seen transiting in front of its star. So most of the known exoplanets are gas giants, like Jupiter, rather than small, dense, terrestrial ones, like Earth.
Nevertheless, astronomers are hopeful that a new generation of planet-finding telescopes, like NASA’s Kepler satellite launched in March 2009, will soon reveal many more rocky exoplanets, some of which may be in the habitable zone.
According to Seager, finding Earth-like planets with Kepler is “just a matter of time.”
If Kepler finds a Goldilocks planet, scientists would need to do more work to figure out if life could exist there. For starters, they would want to find out which molecules make up the planet’s atmosphere. When starlight is viewed through the atmosphere of an orbiting exoplanet, its spectrum looks different, because molecules in the atmosphere filter out certain wavelengths of light. By analyzing the altered spectrum, scientists can determine which gases they’re looking at, and compare those with the gases in Earth’s atmosphere.
But scientists don’t just want to know if life could exist on the distant world; they want to know if life does exist there. Two substances, oxygen and methane, are important clues to knowing if, say, extraterrestrial livestock are already grazing on the planet’s alien pastures. Both molecules are only present in large quantities on Earth because they are by-products of life. So finding these molecules in the filtered light spectrum could indicate that something, or someone, on the planet is producing them.
Of course, there would be a far easier way to know if aliens existed on other planets in our galaxy. “It would be a lot simpler to have them send you a message,” joked Latham.
But astronomers are an audacious bunch, and they prefer to search the stars than to wait by the mailbox.