Sitting 410 light years from Earth, an exoplanet orbiting the dead star SDSS J122859.93+104032.9 survived the violent death of its stellar companion. The demise of this star left behind a fragmentary core of a once-larger planet, orbiting in its own debris around the stellar corpse, where it developed a comet-like tail. This alone could make it the most metal planet in the galaxy — but this odd world is even made of heavy metal.
The rich iron and nickel content of this planet may have helped this world survive the cataclysm which destroyed its parent star. Researchers found the planetesimal still orbiting in a disk of debris left over from the planets with which it once shared a solar system. The relatively small size of the world prior to the cataclysm may have played a role in its survival, but in its place, our own world would not fare as well.
“While the high density and internal strength of the planetesimal has helped it survive, the Earth’s core is above our estimate on the largest size an iron-rich body could be — around 720km [450 miles] in diameter (the Earth’s inner core is about 2400km [1500 miles] in diameter). Maybe a smaller fragment of the core could survive though,” explained Dr. Christopher Manser of the University of Warwick, in an exclusive interview with The Cosmic Companion.
The Sun will, one day, expand as a red giant, engulfing the planets Mercury, Venus, and (likely) the Earth. Gravitational interactions will occur, and Jupiter and Saturn could nudge smaller bodies closer to the remains of our Sun. However, this chain of events will not take place for another five to six billion years.
Astronomers utilized the Gran Telescopio Canarias in La Palma, Spain to analyze a debris ring circling the white dwarf. This disc is composed of many of the same elements which make up the inner rocky planets of our own solar system — iron, magnesium, silicon, and oxygen.
That’s A Long Time for Such a Short Journey
When this world was in it’s heyday, the planet was small — closer to the size of a large asteroid than a rocky world like the one we call home. The planet may have been composed nearly entirely of metal, but evidence suggests it was once a larger body, where differentiation would have brought heavier elements, like metals, into a solid core, surrounded by lighter compounds.
“We currently have a large range of possible sizes for the planetesimal, ranging from a several kilometers to 720km [450 miles] in diameter. The maximum mass we would estimate would be somewhere on the order of 10²¹ kg, which is a few times more massive than Ceres [the largest member of the asteroid belt],” Dr. Manser explains to The Cosmic Companion.
Orbiting once every two hours, the remains of this world orbits so close to the remains of the star, that it is actually within a distance which would have been filled by the star prior to its collapse as a white dwarf. Logically, this means the planet must have formed at a greater distance from the center of its solar system than it currently resides, and been drawn inward, following the collapse of the star to a white dwarf. However, the original distance between the once-extant planet and its star is unknown.
“It is possible that the body came from an asteroid-belt analogue, or even that it is the result of a planet-planet collision. If the later is true, then it is even harder to pin down a location in its planetary system. However, one thing we do know is that it is going to have to come from somewhere beyond the maximum radius of the star when it becomes a red-giant. The star that this white dwarf was born from was about two times the mass of the Sun, so this radius would extend beyond Earth’s orbit for sure,” Dr. Manser describes.
Thinking About the Gravity of the Situation
Following the collapse of the star, this stellar body lost much of its material, shrinking from two solar masses to just 70 percent as massive as the Sun.
“The white dwarf’s gravity is so strong — about 100,000 times that of the Earth’s — that a typical asteroid will be ripped apart by gravitational forces if it passes too close to the white dwarf,” said Dr. Manser.
This system could provide a look at the future of our own solar system, in five to six billion years, following the death of the Sun.
There’s a First Time for Everything
This discovery marks the first time a solid body orbiting a white dwarf has been found using spectroscopy (examining light from a distant object broken up into a spectrum like a rainbow). Only once before has a planetesimal been found in a close orbit around a white dwarf, as that world passed in front of its parent star as seen from Earth. For systems without this fortuitous alignment, spectroscopy offers a way to find these otherwise-hidden worlds.
Analysis of the region indicates the planetesimal is releasing gas in the shape of a tail, much like a comet near the Sun. It is uncertain whether this gas is being produced by the planetesimal alone, or is the result of dust impacting the body as it orbits the white dwarf.
Astronomers now of several similar disks surrounding white dwarfs, which may hold other, similar, planetesimals. For the meantime, though, SDSSJ1228+1040 rocks on as the most metal planet we’ve ever seen.