Exoplanet Core Orbiting a Dying Star May Help Astronomers Understand What Lies in Store for Our Solar System

It’s likely the planetesimal orbiting a white dwarf 410 light years away was the core of a minor planet caught in its immense gravity

University of Warwick/Mark Garlick

Astronomers have confirmed the existence of nearly 4,000 exoplanets, most of which are in the prime of life. Researchers recently spotted one—or what’s left of it—that looks like the ghost of future yet to come.

Catherine Zuckerman at National Geographic reports that a new study identified the metal remnant of planet that is currently zipping around a slowly cooling white dwarf at a high rate of speed, giving astronomers a preview of our own solar system’s demise.

The planetary nugget was discovered by astronomers using the Gran Telescopio Canarias in the Canary Islands, according to a new study published in the journal Science. Researchers looked at a disc of debris orbiting a white dwarf about 410 light years away using spectroscopy, which can determine the elements in a celestial body depending on the variations in light they give off. They found lots of iron, magnesium, silicon, and oxygen in the debris disc, likely the leftovers from rocky bodies ripped apart by the white dwarf, as well as gas similar to a comet's tail streaming from a solid chunk.

The researchers hypothesize that the iron and nickel-rich object is part of the core of a minor planet that got caught in the dwarf's gravity. The fragment, known as a planetesimal, could be anywhere from one kilometer to several hundred kilometers in diameter, rivaling the size of the largest known asteroids in our own solar system. It is also very close to the star, about 320,000 miles away and orbits it once every two hours. It is only the second planetesimal found orbiting a white dwarf, and the first found using spectroscopy.

The fact that the fragment survives is surprising. In general, most of the planets catalogued by astronomers thus far orbit main sequence stars, like our own sun, which happily fuse hydrogen into helium for billions of years and are relatively stable in energy output and size. Eventually, as that fuel runs out, those stars fall off the main sequence and swell into red giants that fry the planets orbiting closest to them. Depending on its mass, the star will either explode in a supernova or collapse into a white dwarf, which, after billions of years, will cool into a dead black dwarf.

Lead author Christopher Manser of the University of Warwick says in a press release that the gravity of the white dwarf—about 100,000 times as strong as gravity on Earth—would rip apart any normal asteroid that came so close. To survive that type of stress, the researchers believe it must be made of the type of iron or iron and nickel found in most planetary cores. “The fact that we have discovered a body orbiting on a two-hour period is clear evidence that a planetary body can survive this destructive process,” Manser tells Dennis Overbye at The New York Times.

Writing for The Conversation, Manser says it’s unlikely the bit of core came from a planet originally orbiting the dwarf at such a tight proximity. Instead, he hypothesizes that larger planets in its solar system pushed the minor planet-sized object close to the white dwarf. Then, the white dwarf’s intense gravity ripped apart its crust and mantle, leaving only the chunk of metal core.

Manser says a similar process may happen when the end of our own solar system comes in about 5 billion years. The sun is too small to go out in a supernova explosion. Instead, it will swell into a red giant that will gobble up Mercury, Venus and likely vaporize Earth as well—though Mars will probably survive. During this disruptive process, the gravity of large planets like Jupiter could fling asteroids and minor planets toward the white dwarf, creating the type of debris field the team observed.

The astronomers have identified six other white dwarfs with debris fields they hope to study to see if the minor planet theory is correct. Lisa Kaltnegger, director at Cornell University’s Carl Sagan Institute who was not involved in the study, tells Zuckerman at National Geographic that these planetesimals could actually collide with one another and create habitable planets. Comets hitting the planet could bring in supplies of water and jumpstart life.

“After the white dwarf cools down further, we have shown that such a planet could maintain balmy conditions for billions of years,” she says. “[I]nstead of a hot dry zombie planet, you could get a planet where life could potentially start all over again.”