When will we find another Earth?

By Phil Plait

SLATE -- 3 February 2014 

A Jovian moon

Artwork courtesy of Dan Durda*

One of the biggest questions in astronomical research right now is quite simple to ask but extremely difficult to answer: In the depths of space, is there an Earth-like planet somewhere orbiting a Sun-like star?

The answer is rather surprising: almost certainly yes. We haven’t found a precise twin of Earth yet, but we’ve come mighty close. In fact, it’s likely that there are millions, perhaps billions, of planets like ours in the Milky Way alone. But right now, at this moment, we only know of one for sure: ours.

Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death from the Skies! 

So when will we actually see that blue-green dot in our telescopes?

The search for alien worlds orbiting other stars—exoplanets—has gone on a long time. Quite a few were thought to have been seen, but they were on the thin, hairy edge of what the technology could do and were later shown to be false positives.

Things changed in 1992. Using sophisticated timing techniques, scientists found the very first confirmed planets, which were orbiting a pulsar, the ultradense core of an exploded supernova. That turns out not to be the most hospitable place in the Universe, what with the pulsar spewing out enough X-rays to thoroughly fry surrounding space. Planets they are, Earth-like they are not.

But then in 1995 came the big announcement: A planet had been found orbiting the star 51 Pegasi. The star is similar to the Sun, but the planet was a shock: It had 0.4 times the mass of Jupiter (150 times the Earth’s mass), but it orbited the star a mere 8 million kilometers (5 million miles) from the star! It screamed around the star in just 4.2 days, a far smaller and shorter orbit than had been thought possible.

The method used to find this planet is called the Doppler technique. When a planet orbits a star, its gravity tugs on the star. The planet makes a big circle while the star makes a smaller one. As the star approaches us in that cycle, its light gets compressed a bit, shifting it to shorter wavelengths. When it recedes from us, the opposite happens. This is essentially the same physics that makes a motorcycle make that “EEEEeeeeeeooooooooowwwwwww” sound as it passes you, what scientists call the Doppler effect.

Astronomers had been very carefully looking at many stars for the Doppler effect, but they’d been looking at timescales of months, not days. Once the planet 51 Peg b (as it’s called; a planet is given its star’s name followed by a lower case letter starting at b, then c for the second one discovered, and so on) was found, astronomers looked back at their data and quickly found many more.

This method tends to find huge planets orbiting their stars close in—the effect is larger for that type of world—and so they are not Earth-like at all. These “hot Jupiters” are fascinating in their own right, but they would never be mistaken for home.

Many more of these planets have been found this way, but the real revolution was to come just a few years later.

Kepler is an observatory launched into space in 2009. It was designed to stare at 150,000 stars simultaneously, carefully measuring their starlight. If a planet orbits the star, and we see that orbit edge-on, then the planet will cross the face of its star. The starlight will dim periodically, revealing the presence of the exoplanet.

A few planets had been found this way before, but Kepler opened the floodgates: It has found more than 1,000 confirmed exoplanets and thousands more candidates still awaiting confirmation.*

As a planet orbits its star, the star moves, too.

llustration by NASA/Spaceplace

This technique, called the transit method, makes it easier (though by no means actually easy) to find smaller planets. Kepler has found quite a few Earth-sized planets, and more excitingly, quite a few others orbiting their stars at the right distance.

But, hey, wait a sec. What does it mean to be at “the right distance”?

We don’t know what varied forms life can take out in the cosmos. But it’s not a bad idea to look here at home for hints. All life on Earth needs liquid water, so that’s a pretty good criterion to start with.
That means a planet can’t be too close to its star or else all the water will boil away. And if it’s too far, the water will be frozen (though there can be exceptions—some icy moons in the outer solar system like Europa and Enceladus are heated by their parent planets enough that they have interior oceans).

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.

OGLE-2005-BLG-390L b is an uninhabitable ice ball stuck in a perpetual winter freeze just a few times bigger than Earth.

Artwork courtesy of NASA

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. This article also stated that Kepler has found more than 100 confirmed exoplanets; it has found more than 1,000.