“We can confidently say that it is not going to be the core of a gas giant, as these should have a different composition,” Manser says, “and the material we see that has been accreted by the white dwarf doesn’t match the material you would expect to see from a gas giant’s outer layers or core.”
Instead, the planetesimal likely originated deep within a rocky planet that was torn apart as its host star went from a normal star, to an inflated red giant, to a white dwarf.
“In our own Solar System, when the Sun becomes a red giant, it will engulf Mercury, Venus, and most definitely the Earth,” Manser says. But the planetesimal revealed in this research probably is “from farther out in the system, like an asteroid-belt analog or a planet orbiting farther away, that is then scattered toward the white dwarf.”
Based on the planetesimal’s iron-loaded composition, Manser says it was either pushed toward the white dwarf through interactions with giant planets or a super-Earth and partially destroyed, or it was “broken apart in a planet-planet collision, producing a core-like asteroid similar to 16 Psyche in our own solar system,” before migrating inward.
The planetesimal spins about once every 40 minutes, and it orbits the white dwarf about once every two hours. With such a short orbital period, the researchers expect the planetesimal to sit very near its host star. In fact, according to the study, if the object was instead orbiting the Sun (which is roughly 100 times the diameter of the white dwarf), it would literally sit inside our star.
Furthermore, the shape of the planetesimal’s orbit seems to be more elongated (or eccentric) than the near-circular orbit of your average planet, though Manser points out further data is needed to confirm this eccentricity. Often, when an object has a highly eccentric orbit, it indicates a powerful past interaction kicked it from the round orbit it received when the object was first forming within a circular protoplanetary disk. So, if the eccentric orbit is confirmed, it could serve as further evidence for the planetesimal’s violent past.
Gassy White Dwarf
The white dwarf, named SDSS J122859.93+104032.9 (SDSS1228 for short), is currently about 70 percent the mass of the Sun and located some 410 light-years away in the constellation Virgo. SDSS1228 is especially notable because it’s a member of an ultra-rare class of white dwarfs that host planetary disks made of both gas and dust. Though Manser says there are about 40 to 45 white dwarfs known to host disks solely made of dust, only seven (or 0.07 percent of all white dwarfs) have disks that also include gas.