In another star system, a Pluto-like world survived until the bitter end
For the first time, astronomers have observed something like a Pluto colliding with a white dwarf, the shriveled corpse of a medium-size star.
The white dwarf, dubbed WD 1647+375, is about 260 light-years away from Earth in space. Despite its distance, this dead star's realm might have had striking similarities to our own solar system, including something akin to a Kuiper Belt, a disk past Neptune of comets and icy dwarf planets, according to new research.
NASA's Hubble Space Telescope made the discovery by playing the part of a crime scene detective. By studying the material falling onto the white dwarf, scientists could reconstruct what kinds of planets and other cosmic objects — their ingredients, sizes, and compositions — probably orbited the star.
Put another way: The trash around a white dwarf tells researchers a lot about the exoplanets a star once had.
"I’ve been absolutely thrilled that we now identified a system that resembles the objects in the frigid outer edges of our solar system," said Boris Gänsicke, the Hubble program's principal investigator and a University of Warwick researcher, in a statement. "Measuring the composition of an exo-Pluto is an important contribution toward our understanding of the formation and evolution of these bodies."
The new findings suggest icy bodies in the outskirts of a planetary system may be able to survive for a long time after its star has run out of fuel. This may even provide insight into what will come of our own solar system after the sun dies. The results were published in the Monthly Notices of the Royal Astronomical Society.
Normally when a medium-size star dies, it blasts away the bulk of material in its system. The Hubble discovery was surprising to astronomers because most would predict the distant icy worlds to be destroyed or cast out long before the star reached the white-dwarf stage.
But in this case, Hubble observed the debris around the white dwarf in ultraviolet light and saw the chemical fingerprints of carbon, nitrogen, sulfur, and water ice — similar to the mix of elements found on Pluto. Because white dwarfs have simple atmospheres — mostly hydrogen and helium — the team knew any heavier elements detected there had to have come from something else crashing into the dead star, according to the paper.
"We know that Pluto's surface is covered with nitrogen ices," said Snehalata Sahu, a University of Warwick astronomer and the paper's first author, in a statement. "We think that the white dwarf [accumulated] fragments of the crust and mantle of a dwarf planet."
Still, the researchers can't rule out some other origin. Based on the chemical composition alone, they can't determine if the Pluto-like object was native to the star's system or an interstellar visitor, not unlike the comet 3I/ATLAS hurtling through our solar system now.
But if this object were a far-flung dwarf planet from the outskirts of its planetary system, how then did it end up so close to the star?
As a sun-like star ages, it swells into a red giant before becoming a white dwarf, bloating so much that it either scorches or engulfs the inner planets. Around the sun, the first victims are expected to be Mercury and Venus, followed by Earth.
By the time the star withers into a white dwarf, the survivors are the gas and ice giant worlds in the outer reaches. That's when things get chaotic. In the dead star's shrunken state, it has much less mass — and less gravity — to hold everything in place. Formerly stable orbits for planets and space rocks could weaken, and any remaining giant planets could shove little icy worlds into extreme, oval-shaped orbits, eventually bringing them near the white dwarf.
This Pluto-like world likely spent billions of years far from the star, remaining frozen solid. That may be how it stayed intact. But when it approached the white dwarf, it was shredded in one brief-but-violent episode.
Not only does the discovery shed light on what happens to planetary systems when their stars die, it also helps scientists piece together how water and other ingredients move between worlds.
Many astronomers believe Earth's oceans formed from comets and asteroids smashing into the planet. While some scientists believe primitive Earth vented out gases 4.5 billion years ago, creating an atmosphere that allowed rain to fall and pool, others think the larger bodies of water formed because space rocks from the outer solar system brought the water — or at least some of it.
Because the research team found a lot of water ice — making up about 64 percent of the Pluto-like fragments — the study ties into bigger mysteries about how icy objects survive and perhaps deliver water in space, the researchers said.
"If an alien observer looks into our solar system in the far future," Sahu said, "they might see the same kind of remains we see today around this white dwarf."
