MOUNTAIN VIEW, Calif. — Are we still alone?
Setting the stage for the next chapter in the quest to end cosmic loneliness, astronomers released a list on Monday of 4,034 objects they are 90 percent sure are planets orbiting other stars.
The new list is the final and most reliable result of a four-year cosmic census of a tiny region of the Milky Way by NASA’s Kepler spacecraft.
“The search for planets is the search for life,” said Natalie Batalha, a Kepler mission scientist from NASA’s Ames Research Center. “These results will form the basis for future searches for life.”
Extrapolated from one small patch to the entire sky, the data will help NASA design a space telescope for the 2030s or thereabouts, big and powerful enough to discern the images of planets around other stars.
The catalog — the eighth in the endeavor — was released at a meeting of exoplanet astronomers here at the Ames Research Center that represents a last hurrah for the survey mission, which will end on Sept. 30. The space telescope itself is doing fine, and it has embarked on a new program of short-term searches called K2.
Among other things, Dr. Batalha said, for the first time there is at least one planet known as KOI 7711 (for Kepler Object of Interest) that almost matches the Earth, a rock that is only 30 percent wider and has an orbit of almost exactly one year.
In all, there are 219 new planet candidates in the catalog. Ten of them, moreover, are in the habitable zones of their stars, the so-called Goldilocks realm, where the heat from their stars is not too cold nor too hot for liquid water.
They are fascinating, but Kepler’s mission is not to pinpoint the next tourist destination — it is to find out on average how far away such places are. Or, as Dr. Batalha said, “We’re not stamp collecting, we’re doing statistics.”
Another result reported on Monday deepened a mystery about how nature goes about making planets. Over the years, Kepler has discovered that nature likes to make small planets, but it makes them in two ways, rocky, like Earth, and gaseous, like Neptune.
A new study, led by Benjamin Fulton of the California Institute of Technology, of 1,305 stars and 2,025 planets that orbit them has found a curious gap in the planet population that seems to mark the boundary between rocky planets, which can be up to one and a half times the size of the Earth, (sometimes called super-Earths) and gaseous planets, so-called mini-Neptunes, which are more than about twice the size of Earth. (Neptune itself is four times the diameter and 17 times the mass of Earth.)
Andrew Howard, a Caltech professor who worked with Mr. Fulton, compared this splitting of small planets into two populations to discovering a major branch point in the tree of life.
All planets seem to start out with about the same amount of rock in their cores, he said. How much gas — mostly hydrogen and helium from the primordial cloud that birthed us — adheres to them makes all the difference. While the Earth, which has hardly any atmosphere at all by weight, is a pleasant place, the pressure on a world with just a little more gas would be toxic.
“It doesn’t take much gas to puff up a planet,” Mr. Fulton said. “This has significance in the search for life.”
Presumably, Mr. Fulton said, the planets that are rocky now, like Earth, had their gassy envelopes stripped away or evaporated by radiation from their stars. But nobody really knows how it works. Adding to the mystery is that our own solar system has no example of a mini-Neptune, and yet they are prevalent in alien planet systems.
In 1984, William Borucki, a NASA physicist and expert on photometry, or measuring light intensity, and a colleague, the late David Koch, had a pretty simple idea: If a distant star blinked or dimmed periodically, it might mean there was a planet going around it. All you had to do was watch, very precisely and steadily.
At the time, nobody knew if any other stars besides the sun harbored planets. NASA turned down Mr. Borucki and Dr. Koch five times before the experiment was finally approved in 2001.
Kepler was launched into an orbit around the sun on March 6, 2009, with a simple mission: to stare at some 160,000 stars in a patch of sky in the constellation Cygnus. If any of those stars dimmed periodically, the size of the dip in light could tell you how big the planet passing in front of it was. The length of time between blinks would tell you how many days long its year was.
In the case of the Earth as seen from space, the amount of dimmed light would be about 0.008 percent of the sun’s light — about as much as a few fleas crossing a car headlight — once a year. Kepler can detect the equivalent of one flea in the headlight. Since the rules of engagement required three transits to verify a planet, that meant it would take that many years on average to discover an exact analog of our own home: Earth 2.0, it was sometimes called.
At the time Kepler was launched, more than 300 exoplanets, planets outside our solar system, had been found, mostly by examining stars one by one to see if they showed signs of being perturbed — “wobbled” — by the gravitational pull of a planet or planets.
Those on the Kepler team did not know what they were going to find. Dr. Batalha recalled that they had argued about how to construct their catalog of interesting objects — whether it would only be able to go to 1,000 or 10,000. In the end, they almost ran out of room on the list, Dr. Batalha recalled, which wound up running to 9,000.
In its first few months of observations, Kepler almost immediately doubled the number of known or suspected exoplanets. The tally kept climbing, to 1,200 by February 2011 and to more than 4,700 a year ago.
Unfortunately, Kepler also discovered that stars are more jittery than astronomers had expected, complicating the problem of discerning planet transits from random fluctuations in the stars. This volatility, or noise, made it especially hard for Kepler’s crew members to see what they most wanted to see — small rocky planets with years as long as the Earth’s.
Citing this interference, Dr. Batalha and her team a received an extension of Kepler’s original mission in 2013, but shortly thereafter one of the reaction wheels that kept the spacecraft pointed failed, ending its ability to keep staring at the same 160,000 stars.
“We had to live with what we got,” Dr. Batalha said.
The final catalog, compiled by Susan Thompson of the SETI Institute, is slightly smaller than the list from a year ago, thanks to a new algorithm known as Robovetter that automatically corrects the Kepler data for the effects of the extra noise. The result is guaranteed to be 90 percent accurate.
“With this catalog, we are turning from individual planets to trying to understand the demographics of these worlds, which are similar to Earth,” Dr. Thompson said.
About four years ago, Erik Petigura, now at Caltech, extrapolated boldly from the Kepler data and estimated that about a fifth of the sunlike stars in the galaxy had habitable planets.
The data suggested that there could be billions of Earth-size planets in the Milky Way basking in lukewarm conditions suitable for liquid water, and so perhaps life as we think we know it. Billions of tickets in the cosmic lottery, billions of petri dishes in which microbes could organize themselves and grow, billions of chances for dust that was once spewed from stars to become conscious and look back on itself.
The Kepler team will refine those estimates with their new data.
In the meantime the baton is being passed to a new satellite, TESS, for Transiting Exoplanet Survey Satellite, led by George Ricker of M.I.T., to be launched next year. It will use the same technique as Kepler to look at broad areas of the sky, searching for planets around the brightest and nearest stars.
Kepler has now joined the roll call of great voyages of discovery, along with the Voyagers and, yes, Apollo. It never went very far itself, trailing along in Earth’s orbit like a puppy on leash. But it took our imaginations hundreds of light years away to places like the twin-sunned world of Tatooine, lava worlds, water worlds and planets packed almost like marbles in orbits around their star.