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Astronomers have discovered a small planet around a white dwarf, which in this artist’s conception is plowing through a disk of dust and leaving a trail of gas in its wake.

University of Warwick/Mark Garlick

Astronomers spy an iron planet stripped of its crust around a burned-out star

In a glimpse of what may be in store for our own solar system, astronomers have discovered what appear to be the shattered remains of a planet orbiting a white dwarf, the burned-out ember of a star like our sun. If the team’s calculations are correct, the orbiting object may be the iron core of a small planet that had its outer layers ripped off by the white dwarf’s intense gravity.

Although astronomers know of thousands of exoplanets in the Milky Way, they struggle to see anything much smaller than Earth. The new object is by far the smallest, more of an asteroid than a planet. Its discovery also provides a clue into the fate of planets as their stars age. When sunlike stars run out of hydrogen fuel and start to burn elements like helium and carbon, they swell up into red giants and consume any planets that orbit too close. Those that survive witness what can happen next when the red giant’s fuel is exhausted: It collapses into a small and dense white dwarf, which cools over trillions of years. Its intense gravity can rip apart any surviving planets that stray too close, consuming some material and leaving the rest in a swirling disk of dust.

Finding the planetesimal, 400 light-years from Earth, wasn’t easy. A team of astronomers, led by Christopher Manser of the University of Warwick in Coventry, U.K., had been watching this particular white dwarf for 15 years. They gained some observing time on the world’s largest optical telescope, the 10.4-meter Gran Telescopio Canarias on La Palma in Spain’s Canary Islands, in 2017 and 2018. The white dwarf, known as SDSS J122859.93+104032.9, or SDSS J1228+1040 to its friends, is one of only a handful of white dwarfs with a surrounding disk of both gas and debris, and the team wanted to study minute-by-minute changes in the gas.

Most exoplanets can’t be seen directly, but are found when they cast a shadow crossing the face of their star or when they tug their star back and forth with the force of their gravity. Manser’s team used a similarly indirect method. They picked apart the light coming from the disk to see its spectrum of frequencies and zoomed in on three bright spectral lines produced by calcium ions, which act as a flag for the gas circulating in the disk.  

As the gas—including the calcium ions—zips around the white dwarf, its light gets Doppler shifted to slightly higher frequencies when moving toward Earth and lower frequencies when moving away. The effect also spreads out the normally narrow calcium emission lines into broad bands with peaks at each end—shaped like a hammock slung between two poles.

Manser says his team had expected to see such broadened lines with perhaps some random fluctuations in the peaks, caused by pieces of debris colliding and producing flares of gas. Instead they saw that the two peaks in each calcium line rose and fell in opposition to each other every 2 hours like clockwork. “It was a really exciting discovery,” Manser says.

The researchers give several possible explanations for the metronomic peaks, including a large planet in orbit and vortices in the dust disk. But writing in Science today, they reject all but one: that this is the signature of a planetesimal orbiting the star. They argue that the calcium lines are not from the planetesimal itself, but from a cloud of gas that surrounds it, either because it is being battered by disk debris or because radiation from the star causes it to emit gas. As that gas cloud follows the planetesimal in its orbit, it boosts one emission peak while moving toward Earth and, an hour later, the other peak while moving away.

“It’s amazing to me that they can deduce the existence of an object so small,” says astronomer Ben Zuckerman of the University of California, Los Angeles, who was not involved in the work. But he and astronomer Mukremin Kilic of the University of Oklahoma in Norman agree that the team’s explanation is the likeliest one. “Is it a planetesimal?” Kilic asks. “Given the information available, that’s probably the best conclusion.”

The result is also surprising because the object is so close to its Earth-size star. If it was in our solar system, it would be orbiting inside the surface of the sun. Any object that close to a white dwarf would normally be torn apart. The researchers calculate that if the planetesimal were simply held together by its own gravity, the entire thing would need to be the density of iron, making it similar to the metallic asteroids found in our solar system. If it had differentiated layers to give it strength, it could be less dense and as large as 720 kilometers across, on a par with the dwarf planet Ceres. Whatever the object was originally like, the researchers say, it must have had its outer rocky layers ripped away by the white dwarf, leaving only its metallic core.

The fact that this object was found around one of the very few white dwarfs that has both dust and gas in its disk suggests gas could be “a smoking gun for planetesimals,” Manser says. So the team is hoping to look at other white dwarfs that have gassy disks in search of more orbiting survivors.

Meanwhile, the fate of SDSS J1228+1040 and its companion gives us a sobering picture of our solar system’s future. It is thought that when the sun swells into a red giant, it will consume Mercury, Venus, and Earth. The other planets may move outward and survive, but those movements could cause gravitational jostling that ejects planets entirely or sends them spiraling inward to their doom. Not a pretty thought, but we do have about 6 billion years to contemplate our fate.