Dear Science: I just saw the news that scientists found a potential planet orbiting the Sun's nearest neighbor, Proxima Centauri. So cool! But if astronomers can't just see the planet through a telescope, how can they tell it's even there?
Here's what science has to say:
The night sky is full of possible planets — each of the 1 billion trillion stars in the observable universe is the potential sun to some distant world. In 1584, the philosopher scientist Giordano Bruno wrote, "God glorified in not one, but in countless suns; not in a single Earth, a single world, but a thousand thousand, I say, in an infinity of worlds. " He was burned at the stake for it, but now scientists know that most stars host planets. In fact, most stars in our galaxy are thought to have planets with the potential to hold liquid water.
But finding the planets that Bruno predicted is still an incredible challenge. As you've noted, astronomers can't simply look through a telescope and spot them. Stars are millions of times bigger and brighter than their planets, and even they appear only as blotches of light when viewed through Earth's best telescopes. Their glow entirely obscures the dim, tiny planets that surround them.
It wasn't until nearly 400 years after Bruno's death that scientists developed a method for proving his hypothesis. One of the earliest confirmed exoplanet discoveries was reported in 1995 by scientists using the Doppler method, a way of detecting the faint "wobble" of a star caused by its planet's presence. This is the same technique astronomers used to detect Proxima b — the potentially habitable planet orbiting the sun's nearest stellar neighbor that was reported Wednesday.
"Planets don't actually orbit their star," said Paul Butler, an astronomer at the Carnegie Institution for Science who was part of the Proxima b team. "The planet and the star both orbit their common center of mass."
This common center of mass is kind of like a fulcrum on a see-saw, Butler said. If two kids of the same weight are on a see-saw, they should be able to keep the board balanced, because the fulcrum is at their center of mass. But if one is twice as heavy as the other, the fulcrum must be twice as close to the heavier person in order to keep the board even. The common center of mass, between two children or two celestial bodies, is the point at which the gravitational forces they exert on one another are at equilibrium.
This is true in every solar system, including our own. But because suns are so much more massive than their planets, that center of mass is extremely close to the sun. So, rather than appearing to orbit, the way planets do, stars just show a small "wobble." Scientists can detect this wobble via tiny shifts in the light the star emits, and use it to calculate the mass and distance of its planet.
As their telescopes got better, astronomers developed another method for finding planets – the transit technique. This involves detecting tiny dips in the light emitted by a star as a planet transits, or crosses in front of it from the perspective of our telescopes. These variations are very small — even the largest planet in our solar system, Jupiter, is only 1 percent of the size of the sun and blocks only 1 percent of the sun's light, and Earth is 1,000 times smaller than that. If far-off aliens are looking for evidence of us using the transit technique, they're going to need incredibly sensitive instruments, and incredibly good luck to catch us right as we pass in front of the sun.
Likewise, earthly astronomers need a lot of luck to catch a planet transiting. But there are so many stars in the universe that they do spot some; in May, NASA scientists working with the Kepler Space Telescope announced they'd found a record-breaking 1,284 new exoplanets using the transit method.
The vast majority of planet candidates are found using one of these two methods (scientists say "planet candidates" because they can't be 100 percent sure it's a planet until they've seen it directly). A few more have been detected by measuring the way a planet's gravity distorts the light of a distant star — a method called gravitational microlensing.
But "the holy grail looming in this field," Butler says, "is to be able to directly detect these planets."
Scientists have directly imaged a few dozen exoplanets by blocking the glare of stars so they can see their planets' fainter glow. But this is hard to do, and requires incredibly sensitive telescopes capable of capturing tiny blips of light. Right now, three ground telescopes are under construction that will be larger — and more sensitive — than anything else in the world. Butler hopes that these will help launch a new era of exoplanet detection, when astronomers can see new planets directly.
This is a big deal, because actually capturing light from a distant planet (even though it'll look like just a few pixels on a screen) will allow scientists to figure out what those planets are like. All we know about Proxima b is that it's roughly Earth-sized and sits in its sun's habitable zone. But if astronomers could see light from that planet, they could use a technique called spectroscopy to sort that light into its component parts and figure out what kinds of molecules are in the planet's atmosphere. Then they could start looking for things such as water, oxygen, and gases that indicate biological process — in other words, signs of life.
"In the case of Proxima b right now, what we don’t know is more important than what we do know," Butler said. The fact that it sits in its sun's habitable zone raises the question, "Could there be life?" but doesn't come anywhere close to answering it.
"But if you had spectral signatures of things like water vapor or oxygen, then you would flip the question," Butler said. "It wouldn't be 'Prove there's life.' It would be closer to 'Prove there isn't.' "