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In the past year alone, a supposed “Earth twin” was found orbiting a star 1400 light years away. But if we dig deeper, the true hidden allure in the search for Earth-like planets is the tacit desire to find extraterrestrial life.

Another Earth

We all know there’s just one planet with an Eiffel Tower and a MacDonalds quarter pounder. So why does “another Earth” have a familiar ring?

The public adores the notion of “Earth-like planets,” and the hope of finding ET life is the engine driving that train. It’s the same way that finding “Canals on Mars’ was the headliner a century ago.

The honest-to-goodness major discoveries in finding Earth twins are:

1. The first exoplanet was found orbiting a star in Pegasus two decades ago. 
2. Discovering thousands more of all kinds, letting us know our galaxy has at least 20 billion Earth-sized planets in comfortable regions.

Those are the important takeaways. 

The next potentially big news would be discovering either radio signals signifying intelligence or else free oxygen in a planet’s atmosphere, indicating plant life. Nothing else matters very much. Having Earth-size and a comfortable orbit isn’t enough to draw any conclusions about life.

As for life, perhaps we needn’t travel very far. We might want to fast-track a mission to Jupiter’s moons. At least two of them have warm, life-friendly, saltwater oceans!

Earth-Like Planets

In our present hunt for exoplanets—bodies circling other stars—some researchers consider a planet Earth-like if it orbits at the correct distance from its parent star to permit liquid surface water. After all, life begins in our own oceans, so water has stirring implications.

But when you hear or read that term, be aware that other astronomers call a planet “Earth-like” if it merely possesses a size similar to ours. Or it is smallish and has a moderate temperature, like Mars. 

Our first point, then, is that we ought to obtain lots more information before we truly get excited.

What About Kepler 452B?

In truth, this exoplanet, named Kepler 452b, has five times the mass of Earth, a 50% larger diameter, and twice our gravity. Not remotely an Earth twin. It’s a coin toss whether it even has a rocky surface instead of being a gas world like Neptune. It probably didn’t deserve the front page.

An artist compares Earth to this new planet (called Kepler 452b). Source: NASA

How Many Earth-Like Planets Are There?

Since we started finding planets in the 1990s, we know there are millions of Earth-mass planets in our galaxy’s comfort zones. However, back then, our technology could only detect huge bodies orbiting low-mass stars because we could only discover something if it were sufficiently tugging on its parent star to make it reliably wiggle.

So, we spotted lots of these giant planets and used simple logic. Nature always creates more small things than big ones. More minnows than whales. More viruses than linebackers. So, finding 500 Jupiters meant that many unseen Earths must be there too, and probably a fifth of them would lie at comfortable-temperature distances from their parent star.

Newer technologies soon let us find smaller bodies, and sure enough, Earth-mass planets have abundantly popped up. We’ve already cataloged over a thousand.

Now that we’ve discovered 5,616 confirmed exoplanets around other stars, we need to find one that’s Earth-like enough to have life. And finally, with the , we have a realistic chance of accomplishing this.

But given all the possible curveballs of false Earths, what should you be on the lookout for? We’ve now reached the practical heart of the matter, which is: how to recognize the real deal.

What Makes Exoplanets Habitable?

You want the exoplanet to be not too hot nor too cold. But you shouldn’t care whether it matches Earth’s size.

You’re really looking for the announcement of air around that world that contains a very specific substance. That’s because life always changes its environment. A few types of molecules simply wouldn’t be there if life didn’t have a hand in the process.

Here on Earth, and probably everywhere else, the main one is oxygen. It’s an element that promiscuously likes to combine with almost anything. So, all our surface silicon has become a silicon-oxygen compound called silica, which fills our vast deserts. Our carbon shows itself mostly as carbon dioxide. With dozens of elements eagerly grabbing free oxygen, we might legitimately wonder how there’s any left to float around by itself to make up .

The only explanation is that something is continually replenishing it. Something’s creating and releasing oxygen in an ongoing process. That “something” is the enormous terrestrial plant and tree community including oceanic plankton, which takes in carbon dioxide, uses the carbon to create their crunchy bodies including tree trunks, and releases the oxygen nonstop.

If we detect oxygen in an exoplanet’s atmosphere, we can be pretty sure it’s a place with life or at least plant life. Other molecules would be good news, too, like methane and water vapor. But oxygen would sit atop anyone’s list.

Is There Another Planet Like Earth?

“Another Earth” cannot be something nearby. We learned that over a century ago. We know a lot about our solar system, and nothing around here even has a surface where anyone can breathe.

Most people assume life will be found on the surface of an Earth-mass planet because that’s where we live, and we’re not very imaginative. But the likeliest known place for life right now is on Jupiter’s moon Europa, simply because it’s the nearest place with subsurface oceans. Its seas are even warm saltwater. Yet Europa isn’t anywhere near our solar system’s “comfort zone.” So, our long-held opening-act assumptions already seem unduly limited in scope.

Much revolves around the issue of how common life might be.

Chromosomes and DNA are so complex, and the assembly of amino acids into proteins so intricate, the standard assumption that life is created by random chemical activity may seem as silly as it did to physicist Fred Hoyle in 1982 when he compared it with a tornado sweeping through a junkyard and assembling a functioning jetliner. 

On the other hand, the suggests that perhaps microbial life roams the universe hiding in meteoroid cracks and rains down on countless young planets, ready to propagate wherever conditions are favorable. In which case, life may be abundant.

The bottom line is an interesting disconnect: Most laypeople find the Earth-like planet search riveting, while some astrophysicists have told me: “Just in our own galaxy, millions of planets share our mass and temperature. If someone wants to seek out and create long lists of them, go right ahead.” It’s valid science.

But let’s jump ahead and do something here that, bewilderingly, no one seems to have thought of. Namely, anticipate the next move.

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After finding atmospheric signs of life, the immediate problem becomes a very big one. Simply put, we’d be unable to obtain any additional knowledge. Through our greatest telescopes in space or on Earth, the clearest possible exoplanet image would show it as a single pixel, a dot.

No future instrument would let us get any sort of hint about what form the life there has taken. All plants? A mix like here? Any intelligent creatures? We’d need a space probe to go there to orbit or land and send back images and information. 

Alas, even a relatively nearby exoplanet would require thousands of years to get there. Until then, we’d know there’s life but all else would be speculative. Would that be fun? Or would it be impossibly frustrating?

This is why finding evidence of life – which will almost surely happen in our lifetimes – will predictably result in several thousand years of nerve-wracking inconclusiveness. I may be the only one here who’s somewhat dreading that first announcement and probably should see a therapist.