Against all odds, a small planetary body called a planetesimal has survived the infernal death of its sunlike star and now orbits the white dwarf that remains.
When most planet-hosting stars run out of hydrogen fuel, they blow out their outer shells of gas, obliterating anything within their inner solar systems and leaving behind a dead star called a white dwarf. Planets orbiting farther out can survive this initial cataclysm, but if those planets move in closer, they also get ripped apart (SN Online: 10/21/15) and gobbled up by the dead star’s intense gravity (SN: 9/24/11, p. 10).
The rare discovery of the intact planetesimal still orbiting close to the white dwarf SDSS J1228+1040, reported in the April 5 Science, could offer insight into the fate of solar systems like our own and the chemical makeup of planets.
Astronomers led by Christopher Manser, at the University of Warwick in England, used the Gran Telescopio Canarias in Spain’s Canary Islands to peer at the debris disk surrounding the white dwarf over two nights in spring 2017 and three nights in spring 2018. The observations of the dead star about 400 light-years away revealed a slight brightening and dimming of certain wavelengths of light in the disk every two hours. That indicated the presence of a cometlike tail of calcium gas trailing a planetesimal as it whips around its star.
The gaseous jet stream may be generated by the white dwarf’s radiation blasting calcium off the planetesimal, or by the planetesimal vaporizing calcium dust as it plows through surrounding rubble, Manser says. This hardy hunk of rock hugs its star extremely closely, orbiting almost 300 times closer than Earth orbits the sun. Since it is able to survive the intense gravity without being shredded, the planetesimal is probably a dense, iron-rich object just a few hundred kilometers across — possibly the core of a planet whose outer layers have been stripped away, the researchers suggest.
Studying this relic and other bits of planetary matter around white dwarfs could help astronomers better understand the composition of planets, something that’s hard to do even here on Earth. “We can’t drill into the core of the Earth to figure out what it’s made of,” says Andrew Vanderburg, an astronomer at the University of Texas at Austin not involved in the study. “We have to rely on indirect methods.”