There’s something strange orbiting a dead star 450 light years from Earth. Spotted in a cloud of debris not acting as it should, the likely culprit is improbable—but not impossible.
In a paper reported today in Science, a University of Warwick-led team reported the tentative identification of a massive chunk of debris orbiting the white dwarf SDSS J122859.93+104032.9. If their suspicions are correct, it could be a 125-mile round fragment of an Earth-like planetary core, leftover from the death of its original star.
Every star reaches an endpoint in its life and goes out in a violent cataclysm. For massive stars, that’s usually a supernova that then creates a black hole or a pulsar. But for stars closer in size to the sun, there’s an end stage where—when all the hydrogen reserves are spent—the star expands outward, shredding planets in the path. What’s left at the core is a white dwarf.
This is the eventual fate of the sun, and Mercury, Venus, and Earth (and maybe even Mars) will be shredded as the sun expands into this red giant phase. This could lead to an event similar to what created the mysterious object orbiting this white dwarf. Christopher J. Manser of the University of Warwick says its also possible that the chunk of core became exposed from a collision between two planets in this phase.
If we were to look for something similar in our own solar system, we’d have to turn to the asteroid belt and a bizarre world named Psyche. Psyche, originally discovered in 1852, is roughly the same size as the exposed core at SDSS J1228, and is similarly made of dense iron-nickel metals. In other words, Psyche, too, is the exposed core of a planet-like body, if current theories hold.
“Iron-rich fragments like these have also been seen at other white dwarfs, although only after they have been destroyed and consumed by the white dwarf,” Manser says. The core itself is 1 quadrillion to 1 sextillion kilograms, which is a little lighter than Mercury.
The planetary fragment at SDSS J1228 was found by examining the spectra of the star, or the breaking down of its light into constituent elements with a device called a spectroscope. Periodic changes in the spectra—repeating every 123 minutes—seemed to bear out a fairly substantially sized object, rather than dust.
Though only a few dozen to a few hundred miles in diameter now, the object was likely an Earth-like planet gone drastically wrong. The exposed core may be protected by a dust ring around it—leftover debris from whatever violent event formed it. Manser says it’s unlikely to have formed close to the star, but probably had a more conventional orbit and migrated in, possibly through some not-so-gentle nudging from other planets in the system.
We seem to be catching it at the right moment, too. Eventually, the object will fall into the star and be vaporised, polluting its light output with heavy metals. We’ve never witnessed an existing planetesimal at a white dwarf before, but have instead had to infer them after they’ve been devoured by the strange matter in the heart of a white dwarf. So far, the planetesimal hypothesis seems to be the only explanation that fits the spectra seen.
“We have not been able to come up with an alternative hypothesis to a small body orbiting in the disc that explains the data that we have obtained,” Manser says. “We have future observations that we are hoping to obtain that, if successful, will reinforce the presence of a planetesimal even more strongly.”