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A sky full of earths

An artist’s conception of Kepler-22b, a planet in the habitable zone of its star. It is the first planet that NASA’s Kepler mission has confirmed to orbit in a star’s habitable zone, but scientists are now finding potentially habitable planets by the dozens. photo: NASA/Ames/JPL-Caltech

Stroll along Crissy Field at night and look up into the sky, and you will see lots of stars. But because of the interference of other lights in the metro area, you won’t see nearly as many stars as someone in the countryside would see.

For scientists who have been studying stars for years – even decades – the analogous situation for finding planets around those stars is more dire. For millennia, there was widespread doubt that planets even existed near other stars, much less that we could find them. With better understanding of the universe and the perfection of telescopes, people learned that there were countless billions of stars and logically concluded that at least some of them had to have planets. Then in the 21st century, the discovery of extra-solar planets began to be announced – initially just one here or there, but soon by the dozens and hundreds. These were large gas giants, big enough to be “seen” (actually, for their existence and mass to be interpolated from observing their gravitational effects on their stars as they orbited them) but clearly not capable of supporting any sort of life as we know it. And just in the past couple years, our search for planets that might in some important ways resemble our own has succeeded with multiple discoveries.

In October 2011, for example, scientists announced the discovery of a planet with two suns. Just two months later, astronomers using the Kepler space telescope confirmed a planet (called Kepler 22-b) that is considered Earth-like and a possible candidate for holding life. That’s because it lies in the so-called “habitable zone” around a star; not too far away from its star to be warmed by it, and not so close that it is baked to a crisp. As the BBC noted, the planet “lies 15 percent closer to its sun than the Earth is to our Sun, and its year takes about 290 days. However, the planet’s host star puts out about 25 percent less light, keeping the planet at its balmy temperature that would support the existence of liquid water.”

On the other hand, as the BBC reported, it is not yet known if Kepler 22-b is made of rock, gas or liquid. If Kepler 22-b turns out not to pass all of the tests that would make it a likely candidate, there are dozens of other planets that have been discovered in habitable zones around their stars, and at least 10 of them, like Kepler 22-b, are Earth-sized.

“This discovery supports the growing belief that we live in a universe crowded with life,” said Carnegie Institution for Science astronomer Alan Boss, part of the team that announced the discovery of Kepler 22-b. “Kepler is on the verge of determining the actual abundance of habitable, Earth-like planets in our galaxy.”

The discoveries have continued. In February 2012, scientists at the Harvard-Smithsonian Center for Astrophysics identified a large, watery planet about 2.7 times the diameter of Earth. Dubbed GJ1214b, the planet drew attention because it was not primarily made up of rock, gas or the other expected materials. Instead, “a huge fraction of its mass is made up of water,” said Zachory Berta, an astronomer on the project. In April, planet Gliese 667Cc was announced. It lies within the habitable zone around its red dwarf star. Located 22 light years from Earth, the planet is about 4.5 times the mass of our planet and receives about the same amount of light energy from its star as Earth does from ours, which means water could be liquid. And where there’s liquid water, there’s a possibility of finding life – maybe even cheap water parks built by species intelligent enough to create commercial real estate.

Other discoveries have followed, and they will certainly continue.

Astronomer Jill Tarter, who retired in 2012 as director for the SETI Institute in Mountain View, has said that scientists are dramatically expanding the number of stars they examine for possible signs of intelligent life. “In the last 10 years, we’ve looked at 1,000 stars, out to 150 light years,” she told a San Francisco audience in 2010. “The Milky Way galaxy is 100,000 light years, so [we’ve studied] 1 percent of the galaxy.” With the institute’s Allen Telescope Array and other new tools, “Over the next couple of decades, we can look at somewhere between 10 and 100 million individual stars.”

The march of research continues to bring us closer to the point where there will be so many planets with the right components found within habitable zones that it would take the greatest stretch of the imagination to believe absolutely none of them could support life.

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