A fragment of the planet that survived the death of its star has been discovered by the University of Warwick astronomers in a disk of debris formed from destroyed planets, which the star ultimately consumes.  The iron and nickel rich planetesimal survived a system-wide cataclysm that followed the death of its host star, SDSS J122859.93 + 104032.9. Believed to have once been part of a larger planet, its survival is all the more astonishing as it orbits closer to its star than previously thought possible, going around it once every two hours.
The discovery, reported in journal Science, white dwarf using subtle variations of the emitted light to identify the additional gas that the planetesimal generates.
Using the Gran Telescopio Canarias in La Palma, the researchers studied the disc orbiting a white dwarf 410 light years away, formed by the rocky bodies, consisting of elements such as iron, magnesium, silicon and oxygen – the four key building blocks of the Earth and most rocky bodies. Within that disc they discovered a circle of gas streaming from a solid body, like a comet's tail.
The astronomers estimate that this body should be at least a kilometer in size but may be as large as a A few hundred kilometers in diameter, comparable to the largest asteroids known in our Solar System.
White dwarfs are the remains of stars like our sun that have burnt all their fuel and shed their outer layers, leaving behind a lush cool core over time This particular star has shrunk so dramatically that the planestinal orbits within its sun's original radius. Evidence suggests that it was once part of a larger body further out in its solar system and is likely to have been a planet apart as the star began its cooling process.
Lead author Dr. Christopher Manser, Department of Research of Physics, said: "The star would have originally been about two solar masses, but now the white dwarf is only 70% of the mass of our Sun. "The white dwarf's gravity is so strong – about 100,000 times that of the Earth – that A typical asteroid will be ripped apart by gravitational forces if it passes too close to the white dwarf. "
Professor Boris Gaensicke, co-author of the Department of Physics, adds:" The planestinal we have discovered is deep into the gravitational well of the white dwarf, much closer to it than we would expect to find anything still alive. This is only possible because it should be very dense and / or very likely to have a internal strength that holds it together, so we propose that it consists largely of iron and nickel.
"If it was pure iron it could survive where It lives now, but equally it could be a body that is rich in iron but with internal strength to hold it together, which is consistent with planetesimal being a fairly massive fragment of planet core. If it was correct, the original body was at least a few hundred kilometers in diameter because it is only at that point that planets begin to differentiate – like oil on water – and have heavier elements to sink to form a metallic core. "
The discovery offers a
Dr. Christopher Manser said: "As stars, they grow into red giants, which" clean out "much of the inner part. of their planetary system. In our Solar System, the Sun will expand to where the Earth is currently orbiting, and will wipe out Earth, Mercury, and Venus. "The general consensus is that between 5-6 billion years from now, our Solar System will be a white dwarf in the place of the Sun, orbited by Mars, Jupiter, Saturn, the outer planets, as well as asteroids and comets. Gravitational interactions are likely to happen in such remnants of planetary systems, meaning larger planets can easily push the smaller bodies to the orbital, which takes them close to the white dwarf, where they get shredded by its tremendous gravity.
"Learning about the masses of asteroids, or planetary fragments that can reach a white dwarf can tell us something about the planets that we know to be further out in this system, but we currently have no way to detect.
"Our discovery is only the The second solid planetesimal was found in a tight orbit around a white dwarf, with the former one being found, because the debris passing in front of the star is blocked by some of its light, which is the "transit method" widely used to discover exoplanets around Sun-like stars . To find such transits, the geometry under which we view them should be very fine tuned, which means that each system observed for several hours mostly leads to nothing. The spectroscopic method we have developed in this research can detect close-in planetesimals without the need for a particular alignment. We already know of several other systems with bulk discs very similar to SDSS J122859.93 + 104032.9, which we will study next. We are confident that we will discover additional planetesimals for orbiting white dwarfs, which will then allow us to learn more about their general properties. "
Publication: Christopher J. Manser, et al.," A planetary orbiting within the rubbish disk around a white dwarf star, "Science, April 5, 2019: Vol. 364, Issue 6435, pp. 66-69; DOI: 10.1126 / science.aat5330