Citing the first direct evidence that the fiery debris of a dying star may swirl around long after the star is obliterated, MIT astrophysicists report in the April 6 issue of Nature that this orbiting disk of debris could also lead to the birth of strange new planets.
This first-of-its-kind observation of a disk of debris around a long-dead star, made with NASA's infrared Spitzer Space Telescope, could be the long-sought missing link behind the existence of the first planets discovered outside our solar system. In 1992, three Earth-sized planets were observed circling an exploded star called a pulsar. The MIT finding confirms what researchers had surmised from indirect evidence: These exotic planets were probably formed out of a dusty debris disk.
"When the planetary system around the pulsar was discovered, people generally agreed that the planets were probably formed from a disk," said lead author Zhongxiang Wang, a postdoctoral fellow with the MIT Kavli Institute for Astrophysics and Space Research.
Yet searches for disks around old pulsars proved fruitless -- until now. "Our work, the discovery of such a disk, strongly supports the suggestion that planets form around pulsars from residual disks," Wang said.
"Pulsars emit a tremendous amount of high-energy radiation, yet within this harsh environment, we have a disk that looks a lot like those around young stars where planets are formed," said principal investigator Deepto Chakrabarty, associate professor of physics with the MIT Kavli Institute.
Evidence for fallback
Massive stars more than 10 times the mass of the sun end their lives in supernova explosions. These dying stars collapse under their own weight, flinging material far into space. The incredibly dense remaining core can become a rapidly spinning pulsar, a type of neutron star. A neutron star is the size of a city but with about the same mass as the sun.
Researchers believe that the supernova explosion blows away most of the star and any existing local planets, but that some small amount of debris ends up falling back toward what had been the core of the star, now the neutron star. "Nobody really knows how much of that material will fall back, but there should be some," said Pappalardo Postdoctoral Fellow David Kaplan of physics. "Up until now, people had searched for some evidence of this in the universe, but had not found anything direct."
When Wang, Chakrabarty and Kaplan used the Spitzer Space Telescope to look at a young X-ray pulsar 13,000 light years away in the constellation Cassiopeia, they saw a cold disk of material glowing around it in infrared light. This, they said, is the so-called "fallback disk," material that has not escaped from the supernova explosion that occurred about 100,000 years ago.
"Our work provides the first direct evidence for such a scenario, because the debris disk we detected was likely formed from the fallback material," Wang said.
Birth of strange planets
The research provides a missing link that would help explain a new, exotic type of planetary system that forms around pulsars.
Like most of the objects in space, the material that falls back toward the pulsar will keep rotating. If it rotates fast enough, it will end up forming a disk in orbit around the pulsar.
The disk is unlike anything in our solar system. Our sun, the planets in our solar system and most of the other stars and planets we know formed out of the same mix of materials: mostly hydrogen, some helium and tiny amounts of other elements such as oxygen and carbon.
However, a massive star is a nuclear furnace that converts hydrogen into all of the other elements. And, the supernova explosion itself can provide more nuclear tricks. So the material that makes up the disk, instead of being mostly hydrogen, could be mostly iron, nickel or cobalt, and will likely contain all sorts of other interesting elements. What's more, the disk is constantly bathed in the X-ray light of the pulsar, creating an extremely harsh environment.
In most solar systems, the star at the center collapses out of a cloud of gas. The remains of that cloud form a big disk around the star that eventually makes planets. "So, by analogy, the disk of debris that we see could eventually form some planets," Kaplan said. "However, those planets would be constantly hit with X-rays, so anything on them would have a very tough time."
This work also may help answer questions about how black holes form. Some theories suggest that black holes form due to fallback. For instance, if the debris from the exploded star isn't rotating fast enough, it could fall until it hits the pulsar. Then, what had been a neutron star will become too massive and collapse quickly into a black hole. The observations should help to decide whether this is possible.
NASA's Jet Propulsion Laboratory manages the Spitzer Space Telescope Mission. This work is supported by NASA.
A version of this article appeared in MIT Tech Talk on April 12, 2006 (download PDF).