After years of combing the heavens, astronomers have found planets that are the same size as Earth as well as planets in a temperature zone where water may be able to remain liquid.
But until now they have never found a planet that is both of those things.
That moment has finally arrived with the announcement of a distant world that has at least the possibility of being a close cousin to our own.
"This is a big milestone," said Jason Rowe, a member of the discovery team and a scientist at the National Aeronautics and Space Administration's Ames Research Center in California. "We know if you have something Earth-sized, you can produce something that's Earth-like."
Astronomers have long been looking for signs of worlds that are like Earth – the only kind of planet in the universe that can demonstrably support life.
But most planets discovered to date have either been too large – and therefore unlikely to have a solid surface – or have been found in orbits that would render them too hot for life.
Dubbed Kepler-186f, the newly found planet is one of five circling a star about 500 light years from our solar system. It is the best Earth analog so far to emerge from roughly 3,600 candidate planets detected by the Kepler spacecraft.
Kepler is unable to observe planets directly but instead infers their existence by measuring how much a star dims when an orbiting planet temporarily crosses in front of it. The change in the star's brightness is related to the size of the planet, while the speed at which the planet moves is related to its distance from the star, and therefore its temperature.
What makes Kepler-186f unique, so far, is that it is only 10 per cent larger than Earth in size and it orbits just inside the "habitable zone," a Goldilocks region where the energy of the star is sufficient to keep water liquid but not boil it away (assuming the planet has an atmosphere). While it receives less energy than Earth does from the sun, it still falls within the range of livable.
"Like a cloudy day outside" here on Earth, Dr. Rowe said.
However, the planet also differs from Earth in a more significant way: The star it orbits is much cooler than the sun. A shorter distance to the star is what keeps the planet at a potentially livable temperature.
Scientists speculate that such planets could be just as habitable as our own, but there would be differences. For example, plant life on Earth relies on the sun's energy, which peaks in the visible part of the spectrum. Kepler-186f circles a star whose energy output is predominantly infrared light. Any plants living on the planet's surface would therefore have to rely on a different form of photosynthesis to survive.
Despite this key difference, the Kepler discovery is significant because the vast majority of stars in our galaxy are much cooler than the sun. Four of the five nearest solar systems to our own circle such stars.
"I think this bodes well for habitable planets all over the place," said Todd Henry, an astronomer at Georgia State University who was not part of the Kepler discovery.
Sara Seager, an astrophysicist at the Massachusetts Institute of Technology who specializes in the study of other worlds, said that while finding a close match to Earth is exciting, it is possible that life could emerge under a diversity of different conditions.
"We wouldn't want to miss the chance to find an inhabited world because we are fixated on Earth-size planets," she said.
Dr. Seager is part of a follow-up mission to Kepler know as the Transiting Exoplanet Survey Satellite (TESS), which will look for planets around nearby stars. That along with James Webb Space Telescope, of which Canada is an international partner, is expected to advance the search for habitable worlds over the next five to 10 years.
Because of its distance, it is unlikely that astronomers will be able to glean much more information about Kepler-186f in the short term, Dr. Rowe said. Kepler is now offline, but the spacecraft produced enough data to keep planet hunters busy for about two more years.
Dr. Rowe, a transplanted Torontonian, was asked which would likely come first: the detection of a true Earth twin orbiting a star like our sun, or his beloved Maple Leafs winning the Stanley Cup.
"That's easy," he replied. "Detecting an Earth."
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