Milky Way may be brimming with Earth-sized, possibly habitable planets

Somewhere in this galaxy brimming with planets, there should be at least one with someone or something on it that calls it home.

Astronomers announced this week that there could be some 11 billion possibly habitable, Earth-sized planets in our galaxy, according to calculations based on data from the now hobbled Kepler spacecraft.

The research, published Nov. 4 in Proceedings of the National Academy of Sciences, advances the bold, overarching goal of the Kepler mission: to assign a number to the roster of possible habitable planets in our galaxy, pinpointing just how rare – or, perhaps not rare at all – wet, temperate, and life-hosting Earths might be.

An estimated 22 percent of the sun-like stars in the galaxy have an Earth-sized planet in what is called the habitable zone, that plum orbital region where, hypothetically, the planet is not too hot, not too cold, and where liquid water is able to puddle up, according to the researchers.

The research does not mean that any of these billions of planets are, in fact, habitable, since a prodigious roster of factors, some of which are yet unknown, determine if a planet will develop life. So far, the number of known life-hosting planets remains a dreary party of one: Earth. But with the number of promising planets now in the billions, the odds that one of them – just one is all planet hunters ask for – has all the right stuff to be home to life are better than ever.

“With 11 billion planets, that’s a lot of spins of the roulette wheel,” says Erik Petigura, a graduate student at the University of California, Berkeley, and the lead author on the paper. “It’s an open question, but I would imagine a lot of the planets are like Earth in other respects.”

What’s more is that, based on the team’s calculations, the nearest of these Earth-sized, well-positioned planets should be about 12 light-years away, in what Mr. Petigura calls our “cosmic backyard.” In fact, this star should be “visible to the naked eye,” he says, a single twinkle in star-salted night skies that could, maybe, refute Earth’s perceived uniqueness among a slew of lifeless planets.

“It’s quite humbling,” says Petigura.

The news comes just three months after Kepler telescope’s hunt for exoplanets met its end, when one of the wheels used to point the telescope was deemed unfixable. But the telescope has left astronomers a parting gift: heaps and heaps of data from its four-year-long census of the sometimes baffling, often enchanting collection of planets outside our solar system.

One of the telescope’s greatest gifts has been an end to the belief that most alien planets are preposterously large and piping hot gas giants called “hot Jupiters,” so named for their resemblance to our solar system’s fifth planet but situated closer to the sun, where the temperatures range from hellish to more hellish. Since the launch of the Kepler telescope, scientists have discovered that the boiling, Brobdingnagian planets are in fact rarities and are just simpler to spot than cold, rocky planets. Why? Because astronomers detect possible planets based on how much they dim a star as the planet slips in front of it during its orbit. The smaller, darker planets, proverbial wallflowers, had tended to recede into space’s black canvas.

Kepler’s demonstration that small, cooler planets are more common than big, hotter ones was a boon to scientists, since hot Jupiters had met neither of the two criteria that astronomers have established as critical for life. One of them is size: Calculations have shown that planets more than about twice the size of Earth begin to swaddle themselves in a thick, gaseous envelope, ruling out the chances of water accumulating there, says Geoffrey Marcy, an astrophysicist at the University of California, Berkeley, and a co-author on the paper.

The other is distance from the sun. One planet announced last week, Kepler 78b, was about the size of Earth and appealingly rocky – but also so close to its star that its surface is suspected to be a nightmarish sea of liquid lava.

“That poor planet is being blow-torched by its star to temperatures of 3000 C,” says Dr. Marcy. In contrast, planets in the habitable zone sport a temperature comparable to “that at a Caribbean beach,” he said.  

It was these Earth-sized, vacation-destination planets for which the team, which also includes Andrew Howard, of the University of Hawaii, went hunting. Out of the 150,000 stars in the Cygnus and Lyra constellations on which Kepler fixed its gaze, the team chose 42,000 stars that are similar to the sun in size and brightness. Trundling though Kepler’s data, the team found 603 planet candidates orbiting this subset of stars.

Just 10 of those candidates are Earth-sized and in their stars’ habitable zones, the authors said. But, as with any census, “not everyone is answering the door,” says Petigura. To take an accurate census of the number of “Earth-like” planets around sun-like stars, the team had to tally up the planets that are either too small or have orbits too tilted to dim their stars enough to be detected.

Before he could add in these hiding planets, Petigura tested his computer program’s planet-hunting abilities. First, he inserted some 40,000 fake planets into the data to find out whether his program could detect the forgeries. In these tests, “the answer was in the back of the book,” he says, and the program correctly identified the 10 real alien planets in the sea of data.

After proving his program’s capabilities, Petigura then extrapolated that about 22 percent of sun-like stars in the Milky Way should have at least one habitable planet (with an error margin of 8 percent). Since there are about 200 billion stars in the galaxy and about a quarter of them, or 50 billion stars, are sun-like stars, that’s about 11 billion “Earth-like” planets, if each star has just one planet.

Of course, 11 billion is still a extrapolation – from 10 planets, no less – not a pinned-down number, says Petigura. Still, Petigura contextualized the research within the broad, historical effort to count the planets far-flung from our own, as a “first step” in answering a question so grand it was once thought unanswerable, little more than fodder for the space-trundling imagination: how alone are we?

 “20 years ago, if you asked someone how many possibly habitable planets are out there, one person would have said one in a billion and another would have said 100 percent,” he says.

“The number is still uncertain,” Petigura says, “but you have to start somewhere.”

“This announced finding is very exciting to me,” says Steven Kawaler, the leader of the Kepler Asteroseismic Investigation, who was not involved the latest analysis. “It isn't so much about the actual number (and uncertainty) quoted, but the fact that there is an answer at all to the fundamental question of how common are planets like the earth.”

Stephen Kane, a professor at San Francisco State University who was also not involved with the research, said that while the 22 percent figure “needs to be treated cautiously as a statistical argument,” the research nevertheless “makes significant headway in determining the frequency of these Earth-sized planets in the habitable zone.”

Kane emphasized that there is still much work to be done in fine-tuning the results to determine which planets might be genuinely habitable, as opposed to just small and well-located. Venus, which is about the same size as Earth, is also snugly in the middle of our sun’s so-called habitable zone, he noted. But Venus, of course, is “the epitome of the inhospitable planet,” he says, and an array of factors has whipped the planet into a scorching, noxious one where nothing can survive.

“We need to manage our expectations of what this research means,” he says. “What I suspect is that a whole lot of these planets are just like Venus.”