On Monday the University of California, Berkeley released the results of the kind of awe-inspiring study that makes for excellent space journalism. Using data from NASA’s Kepler spacecraft, a team of scientists deduced that there are numerous “Earth-like” planets in the Milky Way, with temperatures in the “Goldilocks zone”—meaning not too warm, or too cold, to host liquid water (and potentially sustain life!).
But none of the dozens of articles covering the discovery of these “earth-like planets” can agree on just how many of doppelgangers are out there.
After the embargo dropped Monday afternoon, Bloomberg wrote that “about 4.4 billion planets are similar to Earth in size and temperature, suggesting they may be able to host life,” while Reuters weighed in with “10 billion potentially habitable worlds.” The San Francisco Chronicle announced that the galaxy “may have 11 billion Earth-like planets,” and The New York Times’ Dennis Overbye came in with a figure almost 10 times that of Bloomberg: “Astronomers reported that there could be as many as 40 billion habitable Earth-size planets in the galaxy.”
Space enthusiasts are a fanatical bunch, and the discrepancies between the figures were dramatic enough to cause a bit of a rumble on Twitter this morning:
The numbers conundrum doesn’t get any easier if you refer back to the media briefing and press releases that prompted the coverage. Berkeley’s original press release says that the discovery means that the Milky Way Galaxy is host to “several tens of billions” of Earth-like planets. But in a recording of the media briefing proceeding the embargo lift, the paper’s author Erik Petigura does the math and comes up with a wildly different estimation: 11 billion.
By Petigura’s calculations, of the 200 billion stars in the galaxy, a quarter are sun-like and 22 percent of that quarter will be planets approximating the earth.
Even trickier, Bloomberg’s Nicole Ostrow came to her 4.4 billion figure using similar calculations, given to her by researcher Geoff Marcy in a pre-embargo interview. “He said if you go with 100 billion stars in the Milky Way (some people say 200 billion, some say higher but he said it was accurate to go with 100 bln) about 20 percent of those are sun-like stars,” she wrote in an email. “So 20 billion. Then you take 22 percent of that and you get 4.4 billion.”
As it turns out, there’s a simple explanation to the wildly discrepant number of earths floating around these articles—and it’s not a math problem. “The confusion here is because nobody knows how many stars there are in the Milky Way Galaxy,” says Bob Sanders, manager of science communications at UC Berkeley. “It’s between 100 and 400 billion, so the team’s estimates are based on what percentage of the suns have earth sized planets, which varies based on which figure you’re using.”
Sanders even has an explanation for the Old Gray Lady’s 40 billion figure—”[it] was reached by throwing in the earth-like planets around red dwarf stars”—which, though not included in this study, have been estimated in past research.
Though most of the outlets caveated the figure as an “approximation” or “estimate,” none of them illuminated the number by unpacking the fact that the number of stars in the Milky Way is relatively uncertain. Coverage was confusing, but Sanders says that the desire for a nice round number “is legitimate.”
“They may not realize that astronomers are used to rounding off,” says Sanders. “For an astronomer, a factor of four doesn’t phase them—the public may not feel that way. But it does give you a sense of the size of them, there’s a whole heck of a lot of em out there not just a few.”
“They probably should’ve just gone with ‘billions and billions,” he says. “That would’ve been less confusing.”