Our list of exoplanets grew by 700 last week – but how many might be habitable?
A generation ago, we thought Earth was unique: a singular occurrence that allowed for life in the cold, infinite chasm of space. But technological advances such as the Kepler spacecraft have shown us that there could possibly be other Earths out there and planet hunters all over the world are looking in the tracks of stars for clues to their whereabouts.
About two decades ago, we thought that our solar system was the only one of its kind in the universe. In fact, to this day, the official definition of “planet” includes only planets in our solar system. The first exoplanet – a planet not in our solar system, but orbiting another star – was detected and confirmed in 1992. As of last week, there are now about 1 700 verified, although not necessarily habitable, exoplanets, with many more possible candidates.
Seven hundred and fifteen of these were announced on February 26 this year, thanks to data obtained by the United States National Aeronautics and Space Administration’s (Nasa's) Kepler spacecraft.
"These newly verified worlds orbit 305 stars, revealing multiple planet systems, much like our own solar system," Nasa said.
The leap in the number of known exoplanets is largely thanks to Kepler. The spacecraft, which was launched in 2009 and went out of commission last year, was the first mission to search for Earth-sized habitable planets. It has discovered more than 3 600 exoplanet candidates, of which 961 have been verified as "bona fide worlds", Nasa said.
"With Kepler, we have so many [possible] planets to look at, it's embarrassing," said South Africa-based astronomer Dr John Menzies. "We don't know where to look first."
Menzies, in conjunction with 42 international collaborators, published findings in 2012 that planets in the Milky Way were the rule, rather than the exception.
"Wherever you look, you're likely to see a planet," he said at the time. Menzies is an astronomer at the South African Astronomical Observatory, and this country's contribution to the research was made on a 1m optical telescope in Sutherland in the Northern Cape.
They used a technique called microlensing. When a planet moves in front of a background star, it causes the brightness of the star to dim slightly, very slightly if it is a small planet. After careful observation, astronomers can divine whether this dimming is caused by a planet, or another star or object. But it is difficult to find Earth-like planets using this technique because Earth is relatively small and close to its parent star.
Despite the latest glut of exoplanet discoveries, scientists know of only 10 exoplanets that could support life, five of which were discovered last year.
Exoplanets are just planets, not necessarily like Earth. Our blue and green globe has a cushioning atmosphere of nitrogen, with a soupçon of oxygen so we can breathe, and a magnetic core creating an invisible field that protects us from unrelenting cosmic radiation. It is situated in an area called the "Goldilocks zone", the perfect distance from the sun, so that it is neither too hot nor too cold and can have liquid water. If the Earth was closer to the sun, water would evaporate; if it was further away, water would freeze.
But how does one know whether a planet can support life when studying it dozens of light years away? By analysing the light, astronomers can detect the signatures of different elements present on the planet.
"The subject of astrobiology is burgeoning," said Menzies. "[These] people are studying the nature and composition of planets, [which could be conducive to] life forms, how life would survive [on that planet]."
In an interview with National Public Radio, the Massachusetts Institute of Technology's Dr Sara Seager explained it thus: "We will look for life by peering into the atmospheres of exoplanets in an effort to find gases that don’t belong. For example, on Earth, oxygen is 20% of our atmosphere.
"If we didn't have photosynthetic processes [used by plants and other organisms to convert sunlight into energy], then the level of oxygen in our atmosphere would be almost zero. There are a variety of gases the scientific community is considering as important in the search for extra-terrestrial life."
Many planet-hunters are tackling the problem from a statistical angle, estimating that the 1 700 exoplanets discovered so far are just the tip of the iceberg. Late last year, astronomers from the University of California, Berkeley, and the University of Hawaii estimated there were more than two billion habitable planets in our galaxy alone, following a statistical analysis of Kepler’s data. This would mean that one in five stars in our galaxy might have a planet that could support life.
Asked why planet-hunting was necessary, Menzies said: "When we thought it was only the solar system, we had one example [of how planets and solar systems are formed]. We could say: 'Here's an idea of how it started', and no one could say whether it was a good idea.
"Now that we have more planetary systems, we can develop a theory of how planets are formed, where everything came from, and how it got there."