For centuries, humans have wondered if planets exist around other stars and dreamed of what these alien worlds might be like.
Eventually technology began catching up with our imagination. Despite many false starts and dead ends over all that time, astronomers announced the first discovery of exoplanets in 1992. These planets were not like Earth, nor were they orbiting a star like the sun, but they were confirmed to exist. It was the first evidence ever found of worlds beyond those of our solar system.
But there is an argument that the first evidence was actually obtained years before, in 1988, when astronomers announced the detection of a planet around the nearby star Gamma Cephei A, 45 light-years from Earth. That claim was later retracted because of uncertainty in the data. In 2003, however, further observations showed that the planet did indeed exist. Surely this was the first evidence of alien worlds—or so many textbooks and articles would have you believe.
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But that’s not quite true either. In a paper published in the proceedings of an astronomical conference, American astronomer Ben Zuckerman showed that the first evidence was arguably found much earlier than that.
In 1917 Dutch-American astronomer Adriaan van Maanen was looking for stars that happened to be near the sun in space. To do this, he searched for stars that had high proper motion, meaning they moved more rapidly across the sky than the stars around them. The idea is that stars closer to Earth will appear to move faster than ones farther away, just as trees alongside a road appear to zip past you faster then distant hills when you’re in a car.
He found one, and it turned out to be weird. Observations at the time indicated it was decently close to us, about 13 light-years away. (Modern measurements put it at 14.1.) But that didn’t make sense: these observations also indicated it was a hot star, hotter than the sun, and those kinds of stars are very luminous. Yet this one, later named van Maanen 2, was much too dim. Based on temperature alone, it should’ve been among the brightest stars visible in the sky, but instead it was so faint that it couldn’t be seen without a telescope!
Over the next couple of decades, this mystery was solved. Van Maanen 2 is an example of a white dwarf, the remnant of a relatively low-mass star like the sun after it exhausts all the nuclear fuel in its core, then expands into a swollen red giant and blows away its outer layers. What’s left is the hot stellar core exposed to space. From a distance it looks like an ordinary star, and its temperature mimics that of one with much higher mass. Such a white dwarf is small but extraordinarily dense, squeezing about as much mass as our sun into an orb only about the size of Earth! A cubic centimeter of white dwarf material roughly the size of a six-sided die can weigh as much as a metric ton—that’s about 100,000 times as dense as lead.
Van Maanen 2 is the closest solitary white dwarf to the sun, so it’s studied quite intensely. (Two others are closer but orbit other stars.) White dwarfs are so faint that they become difficult to study at greater distances, and having one so nearby is a boon to astrophysics.
But such studies revealed another mystery about this star. Astronomers can examine many characteristics of a cosmic object by creating a spectrum—breaking its incoming light into many individual wavelengths (or colors). Different elements and molecules absorb light at very specific wavelengths, so it’s possible to measure the presence of such material in stars by looking for areas of the spectrum where there is less light. (These features are called absorption lines.) At the time Van Maanen 2 was found, such spectra were recorded in a similar fashion as images; the light from the telescope was projected onto a glass plate and sprayed with photographically sensitive chemicals.
When astronomers examined the spectrum of Van Maanen 2 on one of these plates, they were astonished to see spectral features usually associated with much more massive stars—namely, a strong calcium absorption line in the star’s atmosphere. To a modern astronomer’s eye, that’s very odd indeed.
Calcium simply shouldn’t be present in a white dwarf’s atmosphere; the stellar corpse has monstrously strong surface gravity. Standing on the surface, a typical human would weigh about 7,000 metric tons—more than 20 times the weight of the Statue of Liberty! Despite that intense gravity, a white dwarf can have an atmosphere almost entirely composed of extremely hot hydrogen and helium above its surface. Any heavier, less buoyant elements, such as iron or calcium, should fall to the surface rapidly on astronomical timescales, scrubbing the atmosphere clean. Indeed, most white dwarfs are seen to have extremely pure atmospheres of just those two lighter elements.
But not all—some are “polluted” with heavier elements, with van Maanen 2 being the first example known to science. The next piece of this exoplanetary puzzle fell into place in the early 2000s, when astronomers found an “infrared excess” for some white dwarfs; that is, these white dwarfs give off more infrared light than expected. Such things have been seen in normal stars and indicate the presence of a starlight-warmed debris disk emitting an infrared glow. For white dwarfs, this result was unexpected but was quickly attributed to the remains of asteroids torn apart by the dead star’s fierce gravity. The asteroids themselves were likely part of the planet-forming process that occurred billions of years earlier, when the white dwarf was still a young, vigorous star like the sun. After the star died, asteroids could have been gravitationally deflected by any lingering gas giant planets into close-in orbits and torn apart. Over time, that heavy element–enriched debris rained down on the white dwarf, polluting the atmosphere.
Many of these white dwarfs are old but still show this signature of debris in their spectra. This means the rain of asteroidal debris is an ongoing process! Otherwise that heavier material would long ago have been cleaned from the atmosphere.
And this is why the calcium-spiked spectrum of van Maanen 2 was so bizarre: besides mimicking a hotter star, it was also contaminated with debris from an ancient planetary system.
While World War I raged on Earth, a very different war of the worlds was occurring 14 light-years away. A tiny spark of light collected by a telescope and recorded on a small glass plate showed that planets existed around other stars.
So in some sense, alien worlds were not first discovered in the 1990s or even the 1980s. It took nearly a century to fully understand it, but the first evidence for exoplanets was found in 1917.
