The announcement was handed out today: We've found a planet like Earth around a sunlike star. This particular planet, Kepler-452b, is likely rocky and has a year slightly longer than our own. While earth-like planets have been found, this is the first that seems to be an Earth twin in a few important respects.

Over the past several years, as NASA's Kepler space mission has released troves of new exoplanets and candidate exoplanets, we've heard headlines similar to today's a number of times. With this world adding to the tally of Earth-like planets discovered by Kepler, how do we make sense of all these Earths?

The Earth 2.0s so far

If you want to get technical about it, Kepler-452b is not the most Earth-like planet Kepler has found. It's 60 percent bigger than Earth, and while it's believed to be rocky, there's still a 40 percent chance it could be an ocean planet or a mini-Neptune—something like a small gas or ice giant. What's important about Kepler-452b is that it has the most Earth-like orbit found of any exoplanet that orbits a Sun-like star. That's why NASA called it Earth's older cousin. The super-Earth had a past a lot like our planet's, though it is now 1.5 billion years older and hotter.

The most Earth-like planet is probably Kepler-186f, announced in April 2014, which is about 490 light years from Earth. It's just 11 percent larger and lies at the outer edge of its solar system's habitable zone. But while the planet itself looks like the Earth, everything else about it is not quite right. Its sun is smaller, and the planet is the outermost of five known planets. Its year is just 129.9 days long, putting it somewhere between Venus and Mercury in that department. There's a 50 percent chance the world is tidally locked to its star, meaning the same side of it always faces its sun, much in the same way the same side of the Moon faces the Earth at all times.

Other candidates that have been hailed as Earth-like have been found around less than sun-like stars. Partly this owes to the Kepler technology, which does a broad sweep across a particular part of the sky. Many of the worlds it finds orbit the many red dwarf stars in the galaxy, which are cooler and smaller than the sun. This means the planets have shorter years and receive less light. Even if the planet is habitable, it may not resemble Earth.

This is the case with another "most Earth-like" contender, Kepler-438b, announced earlier this year. It is ever so slightly bigger than Kepler-186f, but has a year just 35 days long, putting it even closer to its star than Mercury. Kepler-438b was announced alongside Kepler-442b and 440b. Kepler-440b is larger than the just-announced 452b with a 101-day year. Kepler-442b is about 26 percent larger than Earth and orbits in 111 days. Neither of them are around sun-like stars, though.

So while there are other Earth-like planets out there in curious configurations—(including Kepler-62e and 62f in the exact same solar system, in two different regions of the habitable zone)—none are around sunlike stars, which is what makes Kepler-452b special.

The Earth 2.0s to come

There probably are other amazing planets in the Kepler data sets just awaiting confirmation, as was hinted at in the announcements today. KOI-7235.01, for example, is not yet confirmed as a real exoplanet, but there are strong hints at that it could be a world just 20 percent bigger than Earth orbiting a sun-like star, and within the range of Earth's year. If confirmed, it would almost surely be "The most Earth-like planet yet."

But be warned: The pace of confirming exoplanets is frustratingly slow. Kepler ran flawlessly for four years before the malfunction of one of its reaction wheels hobbled the spacecraft in 2013, forcing it balance on solar particles, effectively ending its first mission (it continues on still today, if diminished). Yet two years later, scientists are still sorting through Kepler data. The telescope used its optics to look at 100,000 stars, waiting for a dip in light. That dip indicates a planet transiting across the light from the star. You can see what that means in the video below:

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The changes in light are very, very tiny. This makes planets hard to find and even harder to confirm. In fact, only around one-sixth of the candidate planets found by Kepler are confirmed to exist. The Kepler catalog has 4,661 planet candidates. Of those, 1,030 are confirmed planets. Of those planets, only 12 are potentially habitable.

And remember, Kepler is a sampling of only one region of sky. As you can see in the below NASA illustration, we're looking at a small fraction of the Milky Way, and at a series of stars that are small enough essentially for Kepler to detect their transits. There aren't 12 Earth-analogues out there in the Milky Way. There are 12 we've been able to find with our present detection methods that have years short enough to capture and confirm multiple transits across their star. A habitable planet around a larger star with a five-year orbital period isn't as likely to be detected.

Fight exoplanet fatigue

Despite these steep odds, each discovery is a step closer to finding a planet like ours. We've found many Earth-sized planets, but this was the first around a Sun-like star. We shouldn't get headline fatigue when a new one comes out. It simply means we're adding to and refining the catalog of possible Earth-like worlds out there.

Now we have more ideas of where to look. The closest Earth-like world discovered (not by Kepler) is 13 light years away. Others, like Kepler 452b, are 1,400 light years away, too distant for a human lifetime even if we could approach the speed of light. Even if we placed a SETI "call" to the planet, a 3,000-year reply will come to our descendants, should they even still be alive or on Earth.

Each new planet inches closer to telling us we may not be alone in the universe—that another Earth can arise. Some may just have bacteria. Others—who knows! It's a big galaxy.

It's okay to have multiple "most Earth-like planet yet" headlines. It means we're getting closer.

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