A large team led by Harvard astronomers announced today a major milestone in the long-running hunt for other worlds capable of supporting life elsewhere in the cosmos: the detection of a planet the size of Earth.

The rocky planet, and another they found that’s a bit smaller than Earth, are the smallest ever discovered orbiting another star. They provide the powerful proof astronomers have been waiting for that it is possible—using a space-based telescope—to detect planets that fit the profile that has successfully spawned life in our own solar system.

The planets’ unusual solar system, with gas giants interspersed with small rocky planets, also shakes up conventional explanations of how the solar system formed.

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Astronomers are still far from the ultimate dream of finding an inhabited world; these so-called exoplanets sit scorchingly close to their sun and would be too hot for life, at least as we know it. But just weeks ago, scientists reported the discovery of a planet that is bigger than Earth, but otherwise just right—sitting squarely in the “Goldilocks zone” that is not too hot and not too cold for liquid water. Together, the discoveries signal that that techniques are ready to pinpoint the right planets—if they are out there to be found.

“It is not just a milestone; it was the goal we were all headed for—and now, we turn toward repeating this success at cooler environments,” said David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics and a co-author of the paper describing the discovery, published online today by the journal Nature.

The two planets circle a star called Kepler-20, whose light takes 950 years to reach Earth and that has already been found to have three more massive planets in its solar system. Kepler-20e, slightly more petite than Venus (which is smaller than Earth), takes slightly less than a week to whip around its star, and would be too hot for water, at 1,400 degrees. Kepler-20f, on the other hand, which is almost precisely Earth-sized in its diameter, circles the sun in a little less than three weeks, and would be a more temperate 800 degrees.

Francois Fressin, the lead author of the study and a Harvard astronomer, said that it’s possible that the planet could have migrated to its spot from further out, meaning it could possibly have developed a water vapor atmosphere—a tantalizing possibility that the scientists can’t currently prove, but also can’t rule out.

The search for exoplanets has moved forward rapidly over the last 15 years, as astronomers have detected smaller and smaller planets, and in spots that might be habitable for life.

As a graduate student in 1999, Charbonneau first used the technique that’s now become standard for detecting and measuring planets circling other stars. Using a small telescope, he measured the regular dip in a star’s light caused as a large, Jupiter-sized planet circled it, crossing between the telescope and the sun. Think of a person walking in front of the screen as they duck out of a movie to head to the bathroom—you could tell something about their size by how obtrusive their silhouette is.

But it hadn’t been easy to find that first Jupiter-sized planet orbiting another star, and Charbonneau said that from that moment astronomers began wondering how much it would take to find a slighter planet, like Earth, which would block only 1 percent as much light as a Jupiter-sized planet blocks.

Today, the scale of the project has changed. Charbonneau still has the type of small telescope he used to make that initial discovery in his Cambridge office, but the search for habitable planets now involves dozens of scientists at institutions in the United States, Canada, and Europe, and NASA’s $600 million Kepler space-based telescope that is closely watching more than 100,000 stars.

Michael Endl, a research scientist at the McDonald Observatory at the University of Texas at Austin, who was not involved in the finding, said it was a significant achievement since the researchers had to do extensive analysis to exclude the possibility that the minor blip in the light reaching the telescope was caused by something other than a planet.

“The Kepler mission is specifically designed to give us the first estimate of Earth-sized planets in the habitable zone,” Endl said. “This paper is a milestone because it demonstrates that Kepler can detect those planets.”

Finding such planets orbiting their stars at a greater distance may take more time, he said. Because these two planets are so close to their sun, whipping around it rapidly with a “year” that would last less than a month, the scientists have had time to measure the change in the star’s light over many crossings. For planets in the habitable zone, like Earth, which might spend a year circling the sun, it will take more time to accumulate enough data.

Even with these two planets, the team had difficulties to overcome. Fressin said the conventional method to measure the mass of a planet could not be used because these planets were so light. The team toiled to rule out other possible explanations, utilizing a supercomputer, called Pleiades, at NASA’s Ames Research Center in California. The computer, about the size of two basketball courts, ran calculations to show that it was very unlikely to be any other astrophysical phenomenon than another planet.

A next step will be to try to devise a way to measure the mass of the planets. Charbonneau said that a new instrument being built in Cambridge, the HARPS/North, would be used to help answer questions about these small planets, as well as to continue the search for potential other Earths.

“If all it was, was looking for smaller pieces of rock, I wouldn’t do it,” Charbonneau said. “There has to be this promise, we have to eventually look for life. That has to be on the table.”