Scientists searching for planets with the potential to sustain life like Earth may have hundreds of millions of options.
The Kepler spacecraft, which was launched in March 2009, monitored 150,000 stars in an area in the Milky Way.
It looked for tiny dips in starlight caused by an exoplanet passing in front of its home star during the three-and-a-half year mission.
When it was launched 11 years ago, William Borucki, an astronomer now retired from NASA's Ames Research Centre said: "It's not ET, but it's ET's home."
He spent two decades convincing NASA to take on the project.
Before the spacecraft gave out in 2018, it discovered more than 4,000 candidate worlds among those stars.
So far, none have shown life or habitation – but they are very far away – but the figure suggests there are billions of exoplanets in the galaxy, reports The New York Times.
A team of 44 astronomers led by Steve Bryson, of NASA Ames, has concluded there is an answer after looking at the data for two years.
The goal was to measure the number of eta-Earth: the fraction of sunlike stars that have an Earth-size object orbiting them in the 'Goldilocks' or habitable zone where it's warm enough for the surface to retain liquid water.
The team calculated at least one-third, and as many as 90 percent, of stars similar in mass and brightness to our sun have planets like Earth.
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There are at least 100 billion stars in the Milky Way, according to NASA estimates, and 4 billion are sunlike.
If just 7% of those sunlike stars have habitable planets, there could be as many as 300 million potentially habitable Earth-like planets in the galaxy alone.
“We want to be very conservative in case nature has any surprises regarding habitability,” said Ravi Kumar Kopparapu, a researcher at NASA’s Goddard Space Flight Center in Greenbelt, Md., and one of the authors of the report.
“So we are lowballing the estimates intentionally.”
Asteroid almost as tall as Blackpool Tower hurtling towards Earth's orbit at speed
The nearest planet could be 20 light-years away, with four being within 30 light-years or so of the sun.
“It took 11 years from launch to publication, but this is it,” said Natalie Batalha, an astronomer at the University of California, Santa Cruz, who directed the Kepler mission during most of its life and was one of the authors of the new paper.
“This is the science result we’ve all been waiting for — the reason that Kepler was selected for flight in December 2001.”
The mission didn't detect many true Earth-like planets and astronomers haven't been able to observe the potential rocks the spacecraft picked up, but instead looked at data from the planets they did see.
They are Earth-size but nobody knows what they are like in detail or if anything could live on them.
But there are plenty of opportunities to find one and to establish if these rocks can sustain life.
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