A new study suggests the galaxy we inhabit probably contains around 17 billion planets similar to our own.
Francois Fressin of the Harvard-Smithsonian Centre for Astrophysics says 17 percent of stars have a planet around the same size as ours in a tight orbit. A quarter have worlds 1.25 to twice the size of Earth in orbits up to 150 days.
The estimates are based on data collected by NASA's Kepler telescope, which has discovered more than 800 exoplanets, with thousands more likely to be confirmed in the near future.
Two independent teams studied the data. Fressin's team came up with an estimate of at least one in six stars having an Earth-size planet orbiting it.
While researchers at the University of California, Berkeley and University of Hawaii, using different methods of analysis, reported 17 percent of stars host planets that are one to two times the diameter of Earth.
As Kepler's specialty is spotting planets that orbit their star quite closely, it's still not known how many Earth-like planets there may be orbiting in the 'Goldilocks zone', where conditions are right for life as we know it.
Around 25 percent of stars have a "mini-Neptune" orbiting in 250 days or less, but gas giants are much more rare. Only 3 percent have Neptune-class planets four to six times bigger than Earth, and only 5 percent have gas giants six to 22 times bigger than Earth, orbiting in 400 days or less.
The study also showed where there are stars, there are planets – planets have been seen around red dwarf stars, which are smaller and cooler than the Sun, as well as binary systems, where two stars orbit one another. Around 50 percent of all stars in the Milky Way have a planet at least as large as Earth in a tight orbit, according to data from Kepler.
"When you get into the subject of habitability, which is a very sticky subject, then it might matter that you're around a red dwarf instead of a sun-like star," astronomer John Johnson told Discovery News.
"Earths and super-Earths aren’t picky," said study co-author Guillermo Torres. "We’re finding them in all kinds of neighbourhoods."
But whether we'd be able to live on any of them is another matter.
"It could be that the 20 or 30 factors that go into habitability that we know of on the Earth might not be met on other planets at all," said Johnson.
"For example, it might be very rare to have a planet like Earth that is only partially covered with water, instead of completely submerged or with no water at all.
"That's something that we can't even begin to address right now," Johnson said.