Back in June of last year, astronomers working with the Kepler space telescope made the bold proclamation that all stars have planets. While this is all fine-and-well (if not intuitively obvious), what was not known is how many Earth-sized planets are out there. But now, a new analysis of Kepler data may be providing the answer — and it's potentially huge. According to research just presented at the 221st meeting of the American Astronomical Society in California, the Milky Way hosts no less than 17 billion planets roughly the size of earth.


The study, which was conducted by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics (CfA), indicates that about 17 percent of stars have an Earth-sized planet in an orbit closer than Mercury. That's about one in every six star systems. Given that the Milky Way has about 100 billion stars, that adds up to the figure of 17 billion.


Not Earth-Like, and Not Necessarily Habitable

Now, it's important to note that this number only describes those planets which are in close proximity to their parent stars — a distance that places them outside the solar system's habitable zone (orbits that are about 85 days or less).

The reason for the distinction is that it is currently very difficult to detect small planets further out because of the limitation of current telescopic techniques (namely the transit method, which detects minute dips in a star's light when a planet passes in front of it).


At the same time, however, the number is revealing. If there are 17 billion Earth-sized planets in Mercury-like orbits, it's probably safe to assume that there's a substantial number residing further out in the habitable areas. This is an unsubstantiated assumption, of course, but at least there's more data available now to make such conjectures.

To conduct their investigation, the astronomers surveyed about 2,400 candidate planets spotted by the Kepler satellite over the first 16 months of its operation.


Fressin's figures had to take into account an obvious selectional effect: the only planets that can be detected are the ones that pass along the same plane as the Earth. This required the astronomers to do some extrapolating.

Looking at the Numbers

The researchers found that nearly 50 percent of stars have a planet that's about the size of the Earth or larger in a close orbit. When including larger planets, like close-proximity super Jupiters, that number creeps up to 70 percent. That said, the astronomers believe that all stars have planets on account of data coming in from other observations and detection techniques.


Fressin's team categorized the planets into five types.

As already noted, 17 percent of stars have Earth-sized planets (which fall within 0.8 to 1.25 times the size of Earth). About 25 percent of stars feature a super-Earth (planets 1.25 to 2 times the size of Earth, and orbit in 150 days or less). Similarly, a quarter of stars have mini-Neptunes (2 to 4 times the size of Earth, in orbits 250 days long). Only 3 percent of stars have large Neptunes (4 to 6 times Earth), and 5% have Jupiter-like gas giants (6 to 22 times Earth, in orbits of 400 days or less).


Interestingly, the researchers also discovered that, except for gas giants, all types of planets are proportionately represented around different types of stars, whether they be red dwarfs or main sequence stars like our own.

Moving forward, a challenge for the astronomers will be to detect Earth-sized and Earth-like planets that sit farther out. But because they orbit less frequently, they are less susceptible to the transit method of detection. It's a problem that will likely be solved by due diligence — and extreme amounts of patience.


And in addition to this news, Fressin's team also announced 461 new planet candidates — bringing Kepler's total number of planets discovered to 2,740.

The study has been accepted for publication in The Astrophysical Journal.

Source: Harvard-Smithsonian Center for Astrophysics.

All images via Harvard-Smithsonian Center for Astrophysics.