In a shard of galactic archaeology that offers a less-than-inviting hint at our own future, astronomers have discovered a chunk of a former planet orbiting the remains of its former star, now a smoldering cinder known as a white dwarf.
The fragment, made mostly of iron, nickel and other metals, lies 410 light-years from Earth in the constellation Virgo. It could be a mile, or hundreds of miles, in diameter, but to be dense enough to have survived the explosive demise and subsequent evolution of its host star, it likely was part of a large planet with a wider orbital radius. Now it circles the white dwarf so closely that it completes an orbit every 123 minutes.
“The fact that we have discovered a body orbiting on a two-hour period is clear evidence that a planetary body can survive this destructive process,” said Christopher Manser, a physicist at the University of Warwick in England, and the leader of an international team that reported their results in Science on Thursday.
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A white dwarf is the end product left behind when a star as large as the sun or slightly bigger runs out of fuel, expires and eventually shrinks into a dense ember about the size of Earth. The universe is littered with these dense, cooling tombstones.
In the process of dying, however, these stars first puff up into so-called red giants, enveloping and destroying their inner planets. When our sun goes through this process in 5 or 6 billion years, it will incinerate everything inside the orbit of Mars and probably disrupt the orbits of planets further out. There is no chance of life on this planet surviving the event, and it’s a toss up whether the physical object now known as Earth will persevere or be dragged to its doom in the sun.
The newly discovered planetary fragment is only about 320,000 miles from the white dwarf. That is well inside the presumed radius of the original star, and far closer than astronomers would expect to find anything solid. “So if the sun was positioned where the white dwarf is, the planetesimal would be orbiting within the sun,” said Dr. Manser.
The original star is estimated to have been about twice as massive as the sun, and it exploded about 100 million years ago, Dr. Manser said.
Dr. Manser, along with Boris Gänsicke, also at the University of Warwick, has been studying the disc around a white dwarf known as SDSS J122859.93+10432.9; they have used a variety of telescopes, most recently the Gran Telescopio Canarias in La Palma, in Spain’s Canary Islands. It is one of a handful of white dwarfs known to be encircled by disks of debris.
Within the orbiting debris, the researchers noticed a solid object spraying a cometlike trail of gas. The object, a dense chunk of metal, was probably a remnant of the core of a planet, Dr. Gänsicke said. “If correct, the original body was at least hundreds of kilometers in diameter.”
It is only the second solid remnant of a planet to have been discovered orbiting a white dwarf. And it marks the first time such an object has been identified spectroscopically, by analysis of the light emitted by the gas in its “tail.”
It came as a surprise, Dr. Manser said. Such debris disks were typically understood to be the remains of a small body, such as an asteroid or comet, that had come near the white dwarf and been torn apart by tidal forces. The newly found fragment would have to be solid metal, perhaps iron, to have withstood the tidal stretching, Dr. Manser said. But it would have orbited too close to the white dwarf for life as we know it to have existed on it.
Nor is there better news to be found elsewhere around the white dwarf. Other planets may exist, orbiting farther out, but the star is now too faint to deliver sustenance at such a distance. “It is unlikely that the system is habitable,” Dr. Manser said.
The discovery could yield additional insights into what kind of planetary body this once was, such that it could endure the wall of fire embrace of an expanding star. If a fragment can survive a solar blast furnace, maybe some piece of our own could, too, if only a few inscriptions on some very hard surface.
“We are confident that we will discover additional planetesimals orbiting white dwarfs, which will allow us to learn more about their general properties,” Dr. Manser said.
