By The New York Times
Astronomers reported Monday that there could be as many as 40 billion habitable Earth-size planets in the galaxy, based on a new analysis of data from
One of every five sun-like stars in the galaxy has a planet the size of Earth circling it in the Goldilocks zone - not too hot, not too cold - where surface temperatures should be compatible with liquid water, according to a herculean three-year calculation based on data from the Kep-ler spacecraft by Erik Petigura, a graduate student at the
Petigura's analysis represents a major step toward the main goal of the Kepler mission, which was to measure what fraction of sun- like stars in the galaxy have Earth-size planets. Sometimes called eta-Earth, it is an important factor in the so-called
"It seems that the universe produces plentiful real estate for life that somehow resembles life on Earth," Petigura said.
Over the past two decades, astronomers have logged more than 1,000 planets around other stars, so-called exoplanets, and Kepler, in its four years of life before being derailed by a mechanical pointing malfunction last May, has compiled a list of about 3,500 more candidates. The new result could steer plans in the next few years and decades to find a twin of the Earth - Earth 2.0, in the argot - that is close enough to study.
The nearest such planet might be only 12 light-years away.
"Such a star would be visible to the naked eye," Petigura said.
His result builds on a report earlier this year by
At a news conference Friday discussing the results, astronomers praised the Kepler mission and its team.
According to Petigura's new calculation, the fraction of stars with Earth-like planets is 22 percent, plus or minus 8 percent, depending on exactly how you define the habitable zone.
There are several caveats. Although these planets are Earth- size, nobody knows what their masses are and thus whether they are rocky like the Earth, or balls of ice or gas, let alone whether anything can, or does - or ever will - live on them.
There is reason to believe, from recent observations of other worlds, however, that at least some Earth-size planets, if not all of them, are indeed rocky. Last week, two groups of astronomers announced that an Earth-size planet named Kepler 78b that orbits its sun in 8.5 hours has the same density as the Earth, though it is too hot to support life.
"Nature," as Petigura put it, "knows how to make rocky Earth- size planets."
Also, the number is more uncertain than it might have been because Kepler's pointing system failed before it could complete its prime survey. As a result, Petigura and his colleagues had to extrapolate from planets slightly larger than Earth and with slightly smaller, tighter orbits. For the purposes of his analysis "Earth-size" was anything from one to two times the diameter of the Earth, and Earth-like orbits were between 400 and 200 days.
Batalha said, "We don't yet have any planet candidates that are exact analogues of the Earth in terms of size, orbit or star type."
Meanwhile, in an innovation borrowed from other data-intensive fields such as particle physics, Petigura designed a computer pipeline so that he could inject fake planets into the data - 40,000 in all - and see how efficiently his program could detect planets of different sizes and orbits.
"It was a ton of work," he recalled, explaining that he had to try out tens of billions of different periods for each star in order to find planets. "Fortunately, computers are cheap today."
The New York Times