Scientists today announced the discovery of Kepler-186f, a faraway planet that's perhaps the most Earth-like yet discovered. It's the same size as our home world, and at the right distance from its parent star to have liquid water. So, have we at last discovered Earth 2?
"The ultimate goal of all this searching for exoplanets – the real reason we're doing this – is to answer the question 'are we alone?'" So says Tom Barclay, a research scientist working with NASA's planet-hunting Kepler mission, and co-author of the paper recounting the discovery of Kepler-186f, published in today's issue of Science.
Barclay says that the answer to that big, ultimate question is almost certainly contained in the answers to a host of smaller ones, starting with: Are there other places out there like Earth? Today, Barclay tells us, it's clearer than ever that "the answer to that question is 'Yes.'"
Kepler-186f shares a number of key characteristics with our home planet. For starters, it's roughly the same size. Size is important when it comes to planets. Astronomers suspect that smaller bodies tend to be more rocky, and less gaseous, than larger worlds. How does Kepler-186f rank relative to the exoplanets we've discovered to date? When Kepler scientists announced a year ago the discovery of Kepler-62f, a planet roughly 40% bigger than Earth, they called it one of the most similar objects to Earth yet discovered. Kepler-186f, by comparison, is a mere 10% bigger than Earth. In fact, of the five planets that make up the Kepler-186 system, not a single one of them possesses a radius more than 1.5-times that of our home planet.
But planets that are Earth-sized (and smaller) have been detected before. What really sets Kepler-186f apart is its distance from its parent star. The outermost planet in its solar neighborhood, Kepler-186f orbits at the edges of what astronomers call the "habitable zone" of its star, i.e. the region around a star within which planets can potentially host liquid water and, scientists believe, life.
For a planet to be habitable, it must engage in something of a balancing act. It needs enough solar radiation to keep its water in a liquid state, while still remaining distant enough to keep that water from vaporizing outright. There are other things that can dictate whether a planet can host water – how much radiation its atmosphere lets through, for example – but it's this not-too-much, not-too-little business that astronomers see as the biggest key to habitability (and why the habitable zone is known colloquially as the "Goldilocks Zone").
Barclay says there's one major characteristic Kepler-186f doesn't share with Earth. In Kepler-186's size and orbital distance, he says, "we have two things that we would need to call it an Earth twin," but a true twin, Barclay says, would orbit a Sun-like star. Kepler-186f orbits an M-dwarf, a class of star cooler and dimmer than our own. If you want to get technical, Barclay says, Kepler-186f "isn't so much an Earth-twin as it is an Earth cousin."
Above: Artist Danielle Futsellar's conception of Kepler-186f
But these two cousins could still look an awful lot alike. Barclay says that because Kepler-186f receives roughly one-third the energy that we do on Earth, the light it receives would appear redder, its sun a few shades oranger than our own. We don't know if the planet has an atmosphere, but, assuming the gases surrounding it are similar to those enveloping Earth, its skies would appear slightly duller than what we're used to here at home. A sunny day on Kepler-186f, he says, would look similar to a day here on Earth about an hour before sunset.
Kepler's mission is to find planets that meet three criteria: they must be rocky, Earth-like worlds; they must be within habitable zones; and they must have stars like our own Sun. A find like Kepler-186f, which meets two of those three criteria, suggests the search for Earth 2.0 could be nearing its end. "What we're seeing more and more is that there are places that do look like Earth out there, that remind us of home," says Barclay.
So what's the holdup on that third criterion? According to Barclay, Earth-like planets orbiting Sun-like stars take longer to identify and confirm than those orbiting relatively wimpy stars like M-dwarfs. There are two big reasons for this. The first is that Kepler detects exoplanets by measuring how much light they block when they orbit in front of their parent stars. Astronomers call this a "transit."
When a planet transits its parent star, Kepler detects a brief dip in the star's light – but the ratio of planet size to star size affects how obvious that signal is. Imagine a tennis ball flying across the face of one of those big, honking prison spotlights. Now imagine that same tennis ball flying across the face of a cheapo, handheld flashlight. Bigger, brighter stars are like the prison light, while smaller, cooler ones (like M-dwarfs) are more like the handheld; if you fix the size of the planet and shrink the the size of the star, the signal goes up, making its orbiting planets easier for Kepler to detect.
Above: The five planets of the Kepler-186 system orbit at a range of periods and distances, which Kepler observes by monitoring dips in the brightness of their parent star. Credit: Sean Raymond
The second reason is that cooler stars tend to have planets with smaller orbits. A smaller orbit means you can spot more transits in a smaller window of time, and say with greater certainty that the signals you're picking up are, in fact, attributable to orbiting planets. Kepler might expect to see an Earth-like planet transit a Sun-like star roughly once every 365 days. The scientists observing Kepler-186f saw it pass before its parent star at more than twice that frequency. Remember: Kepler's only been in orbit since 2009. In a few years, we could be up to our ears in planets that meet all three of the criteria laid out above. In fact, astronomers have made it clear that they expect this. It's really just a matter of time.
All that being said, it's unlikely anyone reading this will ever set foot on Kepler-186f. At 500 light years away, it's not exactly in our backyard, cosmically speaking. But it is a landmark discovery, nonetheless – and there's no telling what we'll find tomorrow.
Read the full details on Kepler-186f, and the rest of the Kepler-186 system, in today's issue of Science.