Will Astronomers Ever Find Another Earth?

By Phil Plait

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.

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