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A decade ago, a group of astronomers set out to investigate one of the oldest questions provoking philosophers, scientists, priests, astronomers, mystics, and the rest of the human race: how many other Earths are there out there, if any? How many distant planets are there that could host life as we know it?
Their instrument was the Kepler space probe, launched in March 2009 with a 3.5-year mission to monitor 150,000 stars in a patch of sky in the Milky Way. He looked for small dips in starlight caused by an exoplanet passing in front of its natal star.
“It’s not ET, but it’s ET’s home,” William Borucki said when the mission was launched in March 2009. It was Borucki, a retired astronomer from NASA’s Ames Research Center, who devised the project and he spent two decades convincing NASA to do it.
Before the spacecraft finally collapsed in 2018, it had discovered more than 4,000 candidate worlds among those stars. So far, no one has shown any sign of life or habitation. (Of course, they are very far away and difficult to study.) Extrapolated, this figure suggests that there are billions of exoplanets in the Milky Way galaxy. But how many of these are potentially habitable?
After analyzing Kepler’s data for two years, a team of 44 astronomers led by Steve Bryson of NASA Ames landed on what they say is the definitive answer, at least for now. Their article was accepted for publication in the Astronomical Journal.
Kepler’s formal goal was to measure a number called eta-Earth: the fraction of sun-like stars that have an Earth-sized object orbiting them in the “goldilocks” or habitable zone, where it is warm enough. to allow the surface to retain liquid water.
The team calculated that at least a third, and possibly as much as 90%, of stars similar in mass and luminosity to our Sun have rocks like Earth in their habitable zones, with the range reflecting researchers’ confidence in their various methods and hypothesis. It is not a gold mine, however you look at it.
According to NASA estimates, there are at least 100 billion stars in the Milky Way, of which about 4 billion are similar to the sun. If only 7% of those stars had habitable planets – a seriously conservative estimate – there could be up to 300 million potentially habitable Earths out there in the Milky Way alone.
“We want to be very cautious in case nature has any surprises when it comes to habitability,” said Ravi Kumar Kopparapu, a researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and one of the report’s authors. “So we are intentionally limiting the estimates.”
On average, astronomers calculate, the closest planet should be about 20 light-years away, and there should be four within about 30 light-years of the sun.
“It took 11 years from launch to publication, but that’s about it,” said Natalie Batalha, an astronomer at the University of California, Santa Cruz, who led the Kepler mission for most of her life and was a of the authors of the new paper, in a triumphant e-mail. “This is the scientific result we have all been waiting for, which is why Kepler was selected for flight in December 2001”.
The new result means the galaxy is at least twice as fertile as estimated in one of the first Kepler data analyzes in 2013. That discovery, by Andrew Howard, Erik Petigura and Geoffrey Marcy, who were not part of Kepler’s team, concluded that about one fifth of the sun-like stars harbored planets in their habitable zones.
Batalha said an improvement this time around was the addition of data from the European satellite GAIA, which measured the position and brightness of 1 billion stars. This knowledge allowed Kepler’s scientists to more accurately map the habitable zones of their stars.
Another improvement was better statistics management, although, as Batalha noted, “polls are inherently incomplete. You can’t call all the citizens; you can’t look at all the stars. “
In Kepler’s case, this limitation was serious. The spacecraft’s orientation system failed before Kepler could complete his first survey, which limited him to detecting planets with orbital periods of less than about 700 days, roughly double the duration of an Earth year.
In an email, David Charbonneau, of the Harvard-Smithsonian Center for Astrophysics, said he was slightly skeptical of the results: “The Kepler mission did not detect many (presumably, none) true Earth analogs, i.e. planets with the same The Earth’s beam is orbiting at the same time, and therefore receives the same amount of light, and orbiting sun-like stars. “
As Batalha said at the time, “We still have no candidate planets that are exact analogs of Earth in terms of size, orbit or star type.” We still don’t. As a result, the astronomers had to extrapolate data from the planets they saw.
Although these Kepler planets are Earth-sized – half to 1 1/2 the Earth’s radius – and presumably rocky, no one knows how they are in detail, nor if anything can or can live on them. They are too far away for further study. So far we know of only one planet, ours, which hosts life.
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