A discovery of a planetary body orbiting a white dwarf — a dense dead star as massive as the sun, with a volume comparable to that of the Earth — offers a hint at what our solar system might encounter 6 billion years from now.
California State University, Northridge assistant professor of astronomy Wladimir Lyra was part of a 32-person international research team of astronomers that discovered the planetary fragment more than 400 light-years away.
The relatively large fragment is believed to be a remnant of a former planet that should have been destroyed by the star. According to astronomers, the iron-and-nickel-rich planetesimal — a body formed from rock, dust and other material — survived a system-wide- cataclysmic event that followed the death of its host star. Its survival is more astonishing to researchers, as the fragment orbits closer to the dead star than previously thought possible, going around it once every two hours.
“When any massive object gets very close to a white dwarf, the tides — gravitational force — of the dead star will rip them apart,” Lyra said. “What we found now is a very cohesive body. If the fragment was primarily composed of rock, then the tides would have torn it apart, so the only conclusion we can draw is that it must be composed of iron, which is dense enough to withstand that those strong tides.”
An article on the discovery, “A planetesimal orbiting within the debris disc around a white dwarf star,” appeared last month the journal Science
In order to find the planetesimal, researchers used a method called spectroscopy, which involves analyzing the different wavelengths of light emitted by an object. With the assistance of the Gran Telescopio Canarias, a technologically modern telescope in La Palma, Canary Islands, astronomers identified changes in the color of light emitted by a disk of debris around the white dwarf. Within the disk, a ring of gas streaming from a solid body, like a comet’s tail, was discovered and identified as the fragment.
“A big part of my own participation was explaining the variations in the pattern of star light emitted by the white dwarf on the timescale of days or hours,” Lyra said. “They found a very clear variation in the amount of light that they were getting in a period of two hours. This was clear indication that something was orbiting the white dwarf. My job, as a theoretical astrophysicist, was to rule out other explanations for the observed variation.”
This discovery is only the second solid planetesimal found in a tight orbit around a white dwarf.
Astronomers estimate that this body is at least a kilometer in size but could be as large as a few hundred kilometers in diameter — comparable to the largest asteroids known in the solar system.
Christopher Manser, the lead astronomer in the research, said that as stars age, they grow into red giants, which wipe out much of their inner planetary system. He added that in our solar system, the sun will eventually expand exponentially and wipe out Mercury, Venus, and the Earth.
“The general consensus is that 5 to 6 billion years from now, our solar system will be a white dwarf in place of the sun, orbited by Mars, Jupiter, Saturn, the outer planets, as well as asteroids and comets,” Manser said.
Lyra added: “The importance of this, is that we can understand what is going to happen to planetary systems after the death of their stars, to solve the puzzle of the future of our solar system.”