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.
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 hundreds of confirmed exoplanets, and thousands more candidates are still awaiting confirmation.*
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).