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What Starlight Teaches Us About Space (Pretty Much Everything)

In the southern sky, there is a constellation called Centaurus, its arms outstretched and its flanks straddling the famous Southern Cross. Look between its front legs with a telescope, and you’ll see a dim red star, Proxima Centauri, the nearest star to our sun. The light in your eyepiece left the star more than four years ago, before the London Olympics. Now look more closely, through the Hubble Space Telescope, say. Proxima shines a brilliant peach-white, even though it is just .15 percent as bright as the sun. Look closer still. Every 11.2 days, the star shimmies. To astronomers, this means it is not alone; it has a planet circling it, gently tugging at the star’s atmosphere as it orbits.

You can’t see this alien world, because it is too faint and small for even the most powerful telescopes on Earth and in space. Yet astronomers are convinced that Proxima Centauri b is there. Why? As with nearly everything else we know about the universe, the answer is found in light.

“Light only has so many properties, so we have to really milk it for everything that it contains,” said Jason Wright, an astronomer at Penn State University who studies exoplanets. “Light can be polarized; it can arrive at a certain time; it can arrive from a certain direction; it can arrive at a certain intensity; and it can arrive at a certain color.”

These are all important in one way or another, but color — which corresponds to wavelength, or the light’s energy — is key for studying stars and exoplanets.

When astronomers study starlight, they are generally not looking at the feeble pinpricks most of us can see when we look up at night. Nowadays, they are looking through sophisticated prisms that are attached to powerful telescopes and are called spectrographs. The prisms split up the light, revealing its entire spectrum. A star’s spectrum is like a chemical barcode, allowing astronomers to study what a star is made of, how fast it is moving and in which direction, and other details.

People have long known that a prism — or droplets of water in the atmosphere, which will act as a prism — will split white light into all the colors of the rainbow. In 1814, a young glassmaker named Joseph Fraunhofer noticed that these colorful beams were also interspersed with fine dark lines. Those lines, which are now named for him, correspond to chemical elements.

“As the light streams out of the star, some of the light is going to be absorbed by whatever constituents are in its atmosphere,” explains John Brewer, a postdoctoral researcher at Yale University. Imagine we’re looking at a star that has some iron atoms, such as the sun. A photon, or a particle of light, in a particular wavelength of green light zips past an iron atom, and the atom’s electrons absorb the photon’s energy. This makes those electrons excited, jazzing them up into a higher-energy state. (Iron has 26 electrons, so there are lots of opportunities for this to happen.) The green photon is immediately spat back out, but now it’s moving in a random direction, so it might not hit our prism anymore. “What ends up happening is your spectrum ends up having gaps,” Brewer said.

A century ago, scientists burned pure chemicals in the lab to figure out exactly where these gaps were for each element. Now, astronomers can use the gaps to figure out how much iron a star contains — or how much helium, magnesium, silicon and so on. The ratios of these elements can give some clue about the star’s age, or what its planets might be like.

A star’s spectrum also tells scientists how fast it is moving through space, and where it’s headed. Once we know what the barcode is supposed to look like, we can determine whether the lines are in the right places. Think of it like the numbers on a UPC code. If they are shifted ever so slightly to one side, no longer perfectly matched to their associated barcode lines, that means something is amiss.

If the starlight is moving toward Earth, the light will be shifted to the bluer end of the spectrum. If the starlight is moving away from Earth, the light will be shifted to the redder end. This is a result of the Doppler effect, the same thing that makes the pitch of a racecar engine seem to rise or fall depending on whether it’s moving toward or away from a listener.

Astronomers can use this shift to infer the presence of planets. As the planet orbits, its gravity tugs on gas in the star’s atmosphere and drags it just a tiny bit toward or away from us on Earth. This changes the color of the star’s spectrum to redder or bluer. When this pattern repeats in a regular cycle, astronomers can infer that there’s a planet tugging on the star. At Proxima Centauri, this spectral shimmy repeats every 11.2 Earth days. That’s one year on Proxima b, and the pattern is the smoking gun for a planet.

Before anyone can measure this shift in a star’s light, even one as close as Proxima Centauri, scientists have to know exactly how the star is moving relative to the observer. On Earth, you would be looking through a terrestrial telescope, so you would need to know how the Earth is moving through space. If you’re using a space telescope, you would need to know its speed and trajectory. Scientists can account for that movement and measure the tiniest of star shimmies, down to one one-thousandth of a pixel, Wright said. By repeating these measurements using different spectrometers and different telescopes, astronomers built confidence that they were really seeing evidence of a planet.

Now that astronomers are pretty sure Proxima b exists, they will try to use the star’s spectrum to look for signs of life. Searching for life on Proxima b would require a lot of luck, from the star system’s arrangement in the sky to the presence on the planet of an atmosphere, but it’s possible in principle, said Lucianne Walkowicz, an astronomer at Chicago’s Adler Planetarium. If the planet passes in front the star as viewed from Earth, we could use the starlight to study the planet’s atmosphere.

“We are essentially using the fact that light passes through the atmosphere from the star,” she said. “Of course, the solid part of the planet blocks the sun, but the atmosphere allows light to pass through, which is why you can have something like a sunny day. The same is true of other planets, as well.”

First, we’d need to assume that the star system is set up such that we could see the planet crossing in front of the star. This is called a transit. In June 2012 — just as the light we’re now seeing from Proxima left the star — Americans were able to see a great example of this, when Venus transited the sun. The tiny dot of the second planet briefly crossed the face of our star, dimming its light by a minuscule fraction. But if you looked at our own solar system from above, you wouldn’t have been able to see this. If a transit were the only way you could find a planet like Earth, you might never know the sun has any planets at all.

There is a very small chance — between 1 percent and 2 percent probability, depending on whose calculations you prefer — that we will be able to see Proxima b transit its home star. And that calculation does not consider the odds of Proxima b having an atmosphere at all. But if it does, we could study light passing through it.

Even then, we would be making plenty of assumptions about whether the ingredients of its atmosphere indicate the presence of life, Walkowicz said. For example, if astronomers see oxygen in Proxima b’s atmosphere, that could be a strong signal for life; oxygen reacts easily with other elements, so something would have to replenish it constantly, just like plants do on Earth. But oxygen in an atmosphere isn’t a guarantee that life exists.

“Part of the problem of only having one example of a planet with life on it is you only really know what is bad or good for Earth life,” Walkowicz said. “To some other world, are we an example of an inhospitable planet? We do the best we can with the information that we have, but ultimately, we’re limited by the fact that we only have this one planetary example.”

Brewer, at Yale, said Proxima Centauri is also a tricky star to study. It’s a flare star, which means it regularly spews X-rays and stellar flares, complicating the shimmy signal. It is also a red dwarf, which means it’s cooler, allowing more chemical elements to absorb energy. The result is a “forest of lines” in the spectrum, he said, making the star and the planet harder to study.

Still, in the few days since the Proxima b discovery was announced, astrophysicists have posted a half-dozen papers to an online server that physicists use to share early versions of their research. They will continue bandying about ways to study the planet and its star with current and next-generation telescopes, both those under construction on Earth and those to be launched into space. In their different ways, these telescopes will continue to collect starlight from Proxima Centauri, and they just might be able to look at Proxima b, teasing out more detail about this small world just one star over.

“It’s amazing what we can pull out of light,” Wright said.

Rebecca Boyle is a science journalist covering a variety of topics, from astronomy to zoonoses. She is a contributing writer for The Atlantic, and her work regularly appears in publications including Popular Science and New Scientist.

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