But there’s a clement middle ground, what astronomers call the Goldilocks Zone (or more formally the Habitable Zone), where liquid water can exist on the exoplanet’s surface. The zone depends on many factors including how big and bright the star is, and it’s a good place to start.
So the next question is, have we found the right size planets nestled comfortably in the Goldilocks Zone?
Why, yes. Yes, we have.
Some astronomers went through the Kepler data looking just at stars like the Sun (ranging from a bit cooler to a bit warmer), more than 42,000 stars in total. From that list, 600 or so had planets. The astronomers then looked for just those planets in the liquid water zone, where they would receive no less than a fourth and no more than four times the light the Earth does (a reasonable range). Finally, they culled the list to include exoplanets that were at least as big as Earth, but no more than twice its diameter. Bigger planets can have Earth-like gravity, but it gets tougher to support life the bigger the planet is, and a planet like that will probably have a hugely thick atmosphere, making it uninhabitable.
After they went through the list, how many planets did they have left?
Ten. None matched conditions here perfectly, but they were close enough that we can consider these worlds potentially Earth-like.
It’s not a lock, though. These planets could still have a thick poisonous atmosphere as Venus does, or be airless and cold. But still, they found 10 out of 40,000 stars searched.
And it turns out that’s a lower limit. Many of those stars might have planets that orbit in the wrong plane, so we can’t see them. Others may have simply been missed by Kepler for various reasons (or are not yet confirmed). When the astronomers took all that into account, they calculated that roughly 1 in 5 stars like the Sun may have an Earth-like planet circling it.
These types of stars make up about 10 percent of all stars in the Milky Way, so there are roughly 20 billion of them. That means that there could be several billion Earth-like planets in our galaxy alone!
Even that is a lower limit. There are stars far cooler than the Sun, and they can harbor planets with the right conditions to maintain liquid water, and these stars are the most common in the galaxy. We may be severely underestimating how many Earths are out there.
One drawback with all this is that although we have detected these worlds, and we know they’re there, we haven’t actually seen them. And by that I mean actually taken a direct photograph of them.
We do have such images of many exoplanets (more than a dozen now), but those planets tend to be massive, distant from their star, and still glowing with the fire of youth—literally, they are still luminous from the heat left over from their formation. It’s different for an Earth-like exoplanet, one that’s a few billion years old, small, and a hundred million kilometers or so from its star. Separating it from that inferno of stray light is a Herculean task.
But not an impossible one. We already have designs on some pretty advanced telescopes that could do it. They could divvy out the handful of photons from the planet and the star and provide us with the image that could and should change humanity forever: A soft, faint green spark, floating next to a star not too terribly different from our own. And isn’t that why we want to do this? To see if there are other places for us, or places where others may actually be? Alien, for sure, but life.
So, finally, to answer the question posed at the beginning of this article: When will we find another Earth? The answer is: We may have already. And the statistics clearly show we’ll find plenty more in the next few years.
And when will we have a photo of this new Earth? That’s trickier, and I don’t know when that day will come. But it will be soon. The technology is within our grasp, should we choose to fund and build it. But we know how to do it, and in general things tend to happen once we first understand how.
It may not be all that long before we do finally look out into the heavens and find a second home among the stars.
Correction, Feb. 3, 2014: The credit line for the top image erroneously included NASA and JPL-Caltech rather than just the artist, Dan Durda. Also, this article originally misstated how many planets had been found by Kepler. Of the 1,000-plus confirmed exoplanets, Kepler has currently found about 250.