Although relatively few white dwarfs have disks, Manser explained that between 25 and 50 percent of known white dwarfs show signs of having consumed planetary material in their past. And before that material sinks below the white dwarf's surface, it would circle the dead star in the form of a dusty disk. So, although this might explain why dozens of white dwarfs have dusty disks, it doesn't explain why some have gas too."We are still uncertain why these disks that host gas are so rare," Manser said, "but we hypothesize that it is the presence of a planetesimal in the disc which is producing the gas that we see." It could be that gases are evaporating off the planetesimal's exposed surface, he says, or it could be that the planetesimal is acting like a wrecking ball as it barrels through material in SDSS1228's debris disk, releasing gases as it collides with dust particles. In order to learn more about the origins and evolution of planetesimals around gas-disk white dwarfs, Manser and his team are already planning to hunt for more examples.



"There are six other known gas disks around white dwarfs," he says, "and there are a few that are bright enough to apply the method we developed here to search for additional planetesimals." So by bolstering our catalog of known planetesimals in aging planetary systems, we can begin to understand how our own planets will react when the Sun shuts down.



"Discovering more planetesimals would teach us about the extreme conditions that planetary bodies — or their fragments — can survive in," Manser says, "as well as help our overall understanding of remnant planetary systems around white dwarfs. These systems are great tools to learn about the composition of exoplanetary material."

Get psyched for Psyche

Although studying planetary remains around long-dead stars is informative, presently, there are not many locations where the methods presented here are effective. But fortunately, the solar system is likely harboring its own piece of a planetary core — 16 Psyche — which is ripe for exploration.This iron-nickel asteroid orbits between Mars and Jupiter (about where Manser and his team think their planetesimal started out) and was discovered by Italian astronomer Annibale de Gasparis in 1852, making it the 16th asteroid ever found. At some 125 miles (200 km) wide and composed of up to 95 percent metal, Psyche offers a unique natural laboratory where researchers can study an ancient planetary core that likely underwent a series of violent collisions in its past.</s>Astronomers from University of Warwick in Coventry, England, said on April 4, 2019, that they’ve detected

 a relatively large fragment from a former planet, orbiting in a disk of debris encircling a dead star. The star is a white dwarf, and it’s located 410 light-years away. The white dwarf should have destroyed its solar system in a system-wide cataclysm that followed its death. But the newly discovered planet fragment is thought to be rich in heavy metals – iron and nickel – which helped it survive destruction. The astronomers said the fragment is orbiting the white dwarf:

They also said the planet fragment has a “comet-like tail” of gas, creating a ring within the debris disk. And they said this system offers us a hint as to the future of our own solar system, 6 billion years from now. The discovery is being reported in the peer-reviewed journal Science today (April 4). These astronomers’ statement explained:

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.

This is the second time astronomers have found a solid planetesimal in a tight orbit around a white dwarf. It’s the first time that scientists have used spectroscopy for this sort of discovery. These astronomers used the Gran Telescopio Canarias in La Palma in Spain’s Canary Island. They examined:

… the debris disk orbiting the white dwarf, formed by the disruption of rocky bodies composed of elements such as iron, magnesium, silicon, and oxygen — the four key building blocks of the Earth and most rocky bodies. Within that disk they discovered a ring of gas streaming from a solid body, like a comet’s tail. This gas could either be generated by the body itself or by evaporating dust as it collides with small debris within the disk. The astronomers estimate that this body has to be at least a kilometer (.6 miles) in size, but could be as large as a few hundred kilometers in diameter, comparable to the largest asteroids known in our solar system.

According to astronomers’ theories, our sun will become a white dwarf when it has burnt all the thermonuclear fuel (especially the light elements hydrogen and helium) that now enables it to shine. When this happens, our sun is expected to shed its outer layers, these astronomers said, leaving behind a white dwarf:

… a dense core which slowly cools over time. This particular star has shrunk so dramatically that the planetesimal 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 torn apart as the star began its cooling process.

Lead author Christopher Manser, a Research Fellow in the Department of Physics, said in the statement:

The star would have originally been about two solar masses, but now the white dwarf is only 70 percent of the mass of our sun. It is also very small – roughly the size of the Earth – and this makes the star, and in general all white dwarfs, extremely dense. 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.

Co-author Boris Gaensicke, also of University of Warwick, added:

The planetesimal 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. That is only possible because it must be very dense and/or very likely to have internal strength that holds it together, so we propose that it is composed 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 the planetesimal being a fairly massive fragment of a planet core. If correct, the original body was at least hundreds of kilometers in diameter because it is only at that point planets begin to differentiate – like oil on water – and have heavier elements sink to form a metallic core.

The discovery offers a glimpse into the future of our own solar system. Manser said:

As stars age 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 up to where the Earth currently orbits, and will wipe out Earth, Mercury, and Venus. Mars and beyond will survive and will move further out. 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. Gravitational interactions are likely to happen in such remnants of planetary systems, meaning the bigger planets can easily nudge the smaller bodies onto an orbit that takes them close to the white dwarf, where they get shredded by its enormous gravity.

Bottom line: Astronomers have identified a heavy metal planet fragment orbiting the white dwarf star SDSS J122859.93+104032.9. The system may give us a glimpse of what our solar system will become 6 billion years from now.

Source: A Planetesimal Orbiting Within the Debris Disc Around a White Dwarf Star</s>For the first time, a small planet has been confirmed orbiting a white dwarf star.

White dwarfs represent the end of the line for main-sequence stars such as the sun. At the conclusion of their nuclear-burning stage, healthy stars shed most of their outer material, leaving only a dense hot core. A typical white dwarf has a mass equivalent to that of the sun, but is only about the size of the Earth.

Astronomers have long theorised that it was possible for planets to survive the explosive degeneration of a main-sequence star into a dwarf, but evidence has been thin.

There have been several sets of observations from which researchers have deduced the existence of rubble and gas clouds – both assumed to be the remnants of destroyed planets – orbiting white dwarfs.

Until now, however, there has only been one tentative identification of intact small planet, or planetesimal, orbiting such a star. That involves regular light-curve variations observed at a white dwarf called WD 1145+017, which astronomers have inferred are the product of dust clouds being thrown off a planetesimal orbiting every four-or-so hours

Now, however, Christopher Manser from the UK’s University of Warwick, and colleagues, present what they believe is much firmer evidence, in the matter of a white dwarf known as SDSS J122859.93+104032.9.

In a study published in the journal Science the researchers detail variation in gas emission lines that occur every 123.4 minutes. Their results were gained using the Gran Telescopio Canarias (GTC) on La Palma, Canary Islands, in two observation sessions roughly a year apart.

The variations were unchanged between the first and second sessions, they report, meaning that they must also have been happening for the approximately 4400 orbital cycles that took place in between.

Manser and colleagues estimate the planetesimal is around 600 kilometres in diameter, and may lie just 200 kilometres above the star’s surface. Its orbit, they suggest, appears to be eccentric – a not unexpected situation, given that at some stage it weathered some enormous explosions as its host entered its final stages of stellar existence.</s>When the majority of stars reach the end of their life, they make their own funeral pyre, throwing off most of their remaining gas in a glorious last display of their immense power and leaving behind a dense, cinderlike core known as a white dwarf. But what becomes of any planets orbiting such a star? This question has been difficult to answer, but now scientists say they have detected the suspected remnant of a white dwarf world, a first-of-its-kind discovery that could offer fascinating insights into planetary systems’ dying days across the universe.

In a paper published Friday in Science, a team led by Christopher Manser of the University of Warwick in England reports unprecedented observations of a white dwarf slightly larger than Earth but containing almost the mass of the sun. Called SDSS J1228+1040, the star is located about 410 light-years away, and it is more specifically known as a “polluted” white dwarf, owing to the heavy elements astronomers can detect raining down onto its surface. That material is believed to originate from a disk of gas that surrounds the object. And within that disk, the paper’s authors say, they have found a planet’s ghostly remains.

“It’s like the iron core left behind by a larger body that’s been ripped apart,” Manser says. “The crust and mantle have come off this body. We’re left with this iron core orbiting around that has a high internal strength holding it together, not just gravity.”

The white dwarf’s three-million-kilometer-wide debris disk was first discovered more than 15 years ago, making the star one of fewer than a dozen polluted white dwarfs known to be surrounded by disks. When a star of less than about eight solar masses exhausts its hydrogen fuel, it expels its outer layers, engulfing any close-in planets in the process. Some of these worlds, it is thought, might then be ripped apart, after which their shredded remains gather into such a disk. And although more than 4,000 exoplanets have been discovered to date, scientists have only once before seen smoking-gun evidence of this process: the remains of a planet in a disk around the white dwarf WD 1145+017, which was closely studied in 2015.

Whereas those planetary remnants orbit around WD 1145+017 in 4.5 hours, the heavy-metal core circling SDSS J1228+1040 completes a revolution in less than half of that time. The new study’s authors detected the star’s dense companion using the 10.4-meter Gran Telescopio Canarias (GTC) on the Canary Islands in two separating observing periods in April 2017 and in April and May 2018, respectively. By splitting up the white dwarf’s emitted light into its constituent wavelengths — much as a prism splits white light into a rainbow — the team detected the disk and recorded shifts in its color that corresponded to its orbital motion.

Generally, this method is used to observe systems such as SDSS J1228 over long periods of years, but by studying this white dwarf over the course of several hours, the team discerned a clump of material within the disk that exhibited a shift that was entirely its own. The shift’s strength suggested its source was a solid object some 600 kilometers across — a suspected planetary core, with a density between 7.7 and 39 grams per cubic meter, comparable to the pure iron found within Earth’s core. “The density of the piece of rock is consistent with what we think the cores of planets [are],” says Luca Fossati of the Austrian Academy of Sciences, who was not involved in the paper. “That’s a big thing. It essentially confirms the idea we have that these disks of dust and debris that orbit around some of the white dwarfs are [the remains of] ancient planets.”

The shift also indicated that the clump orbits at just a half-million kilometers from the star — an extremely close distance at which larger, less dense objects would be stretched and torn apart by the white dwarf’s intense gravitational field. This messy disassembly, which, at its most extreme, is dubbed “spaghettification” by astrophysicists, is the typical fate of anything falling into a black hole. But around the white dwarf, where gravity is not as extreme, a small, dense heavy-metal planetary core could survive. “If you have a small enough object, it’s easier to resist spaghettification,” says Andrew Vanderburg of the Harvard-Smithsonian Center for Astrophysics, who was also not involved in the paper. “What [the researchers] found here is that this planet is orbiting so close to its star that it must be held together by material strength.”

With a specific orbital period of 123 minutes, this core would set a new record for the fastest planetary object orbiting a star; the previous record was the exoplanet KOI 1843.03, which completes an orbit in 4.25 hours. The bizarre system also provides a one-of-a-kind scientific opportunity to scrutinize the exposed core of an exoplanet, even though astronomers presently lack any sufficiently large telescope to directly see the core.

Perhaps, however, similar objects await discovery around other white dwarfs. Whether studying them singly or in multitudes, such heavy-metal worlds may offer grim previews of our own solar system’s future, when our sun’s fuel runs out some five billion years hence. “It’s really exciting, because this is sort of the missing piece, if you like, [of white dwarf planetary systems],” says Sarah Casewell of the University of Leicester in England, who was not involved in the research. “We know big things can survive, and we know tiny things get destroyed. This is quite a sizable rocky object that has survived.”

This article is reproduced with permission from Scientific American. You can view the original story here.</s>There's something strange orbiting a dead star 450 light years from Earth. Spotted in a cloud of debris not acting like it should, the likely culprit is improbable—but not impossible.

In a paper reported today in Science, a University of Warwick-led team reported the tentative identification of a massive chunk of debris orbiting the white dwarf SDSS J122859.93+104032.9. If their suspicions are correct, it could be a 125-mile round fragment of an Earth-like planetary core, leftover from the death of its original star.

Every star reaches an end point in its life and goes out in a violent cataclysm. For massive stars, that's usually a supernova that then creates a black hole or a pulsar. But for stars closer in size to the sun, there's an end stage where—when all the hydrogen reserves are spent—the star expands outward, shredding planets in the path. What's left at the core is a white dwarf.

This is the eventual fate of the sun, and Mercury, Venus, and Earth (and maybe even Mars) will be shredded as the sun expands into this red giant phase. This could lead to an event similar to what created the mysterious object orbiting this white dwarf. Christopher J. Manser of the University of Warwick says its also possible that the chunk of core became exposed from a collision between two planets in this phase.

If we were to look for something similar in our own solar system, we'd have to turn to the asteroid belt and a bizarre world named Psyche. Psyche, originally discovered in 1852, is roughly the same size as the exposed core at SDSS J1228, and is similarly made of dense iron-nickel metals. In other words, Psyche, too, is the exposed core of a planet-like body, if current theories hold.

"Iron-rich fragments like these have also been seen at other white dwarfs, although only after they have been destroyed and consumed by the white dwarf," Manser says. The core itself is 1 quadrillion to 1 sextillion kilograms, which is a little lighter than Mercury.

The planetary fragment at SDSS J1228 was found by examining the spectra of the star, or the breaking down of its light into constituent elements with a device called a spectroscope. Periodic changes in the spectra—repeating every 123 minutes—seemed to bear out a fairly substantially sized object, rather than dust.

Though only a few dozen to a few hundred miles in diameter now, the object was likely an Earth-like planet gone drastically wrong. The exposed core may be protected by a dust ring around it—leftover debris from whatever violent event formed it. Manser says it's unlikely to have formed close to the star, but probably had a more conventional orbit and migrated in, possibly through some not-so-gentle nudging from other planets in the system.

We seem to be catching it at the right moment, too. Eventually, the object will fall into the star and be vaporized, polluting its light output with heavy metals. We've never witnessed an existing planetesimal at a white dwarf before, but have instead had to infer them after they've been devoured by the strange matter in the heart of a white dwarf. So far, the planetesimal hypothesis seems to be the only explanation that fits the spectra seen.

"We have not been able to come up with an alternative hypothesis to a small body orbiting in the disc that explains the data that we have obtained," Manser says. "We have future observations that we are hoping to obtain that, if successful, will reinforce the presence of a planetesimal even more strongly."</s>If it weren’t for the sun constantly showering us with energy, there would be no life on Earth. But eventually stars like it run out of fuel, expand into red giants and finally collapse into small, faint objects called white dwarfs. So what will happen to us and the other planets in our solar system when the sun dies? It’s not been entirely clear.

Now my colleagues and I have spotted the possible core remnant of a planet orbiting the white dwarf SDSSJ122859.93+104032.9, residing some 410 light years away. Our results, published in Science, offer important clues about the fate of the planets in our own solar system.

Scientists have identified thousands of “exoplanets” orbiting stars other than the sun, many of them very similar to our own. They can do this by measuring the small movement a star makes as it responds to the gravitational tug of an orbiting planet. They have also discovered discs of gas and debris surrounding white dwarfs, but only one case of a planetary fragment. The reason such remnants are so hard to spot is that they are so tiny that they have a very small gravitational pull on their parent star.

We came up with a completely new approach to probe the gas around white dwarfs. The technique, called spectroscopy, splits the light we see from an object into its separate colours, producing a spectrum in a similar way to a prism. The observations were made using the Gran Telescopio Canarias – the largest telescope in the world with a 10.4-metre diameter mirror.

The planetary fragment we found produced a stream of gas that could be detected by our spectrometer. We could spot it orbiting the star by looking at how its spectrum shifted in colour as the body moved towards and away from us. This change in colour is called a doppler shift, which is essentially a stretching or squashing of waves because of motion. It is similar to the pitch of the sound of an ambulance being higher when it is coming towards you, and lower when it is moving away.

The body completed one passage around its host star in just over two hours, orbiting at a distance that is smaller than the radius of the sun in a disc of gas and dust.

The discovery is surprising, as we didn’t think anything could survive so close to a white dwarf. A white dwarf is only about the size of the Earth but it contains around 60-70% of the sun’s mass – making it extremely dense. If a body orbits too close to a white dwarf, its immense gravity will tear it apart. This was the fate of the material that formed the disc around it.

So how did it survive without getting ripped apart? It would have to be very dense or have some amount of internal strength holding it together. We calculated that it would have a maximum diameter of 720km, which is the size of a small minor planet. To compare, the dwarf planet Ceres in our own solar system has a diameter of 946km.

The origin of this body remains a mystery. One possibility is that this is the core of a minor planet that was pushed close to the white dwarf by a larger planet further out in the remnant planetary system – like a Jupiter. As the minor planet passed close to the white dwarf, its crust and mantle layers would have been ripped apart.

All that would be left of the body would be its dense, iron-dominated core. This kind of object is quite common, with one famous resident in our own solar system: the asteroid 16-Psyche.

Systems such as the one we’ve just discovered can help us understand the future of our own planetary system. In about five billion years, the sun will eventually start to expand into a red giant. At this point, it will engulf Mercury, Venus and most likely Earth – unless we manage to move our planet into a wider orbit, which should be possible in theory. However, Mars, the asteroid belt and the rest of the solar system will survive engulfment and continue orbiting it as the sun collapses into a white dwarf.

During this process, planets like Jupiter could also scatter asteroids, comets or even minor planets towards the white dwarf. There they would undergo partial or complete disruption, forming a disc like the one we’ve just investigated. It is unlikely that any living organisms on planetary or moon fragments could survive this process. And even if they did, they would struggle to live on in the faint light of a white dwarf.

This is not only the solar system’s fate, but that of practically all known exoplanet systems. In the much much closer future, we hope to use the method we have developed here to find more planetary bodies around other white dwarfs. We know of six candidate white dwarfs that are orbited by discs made of dust and gas, and we want to test whether these discs are the “smoking gun” for the presence of minor planets. The more such planets we find, the more we can learn about what happens to a planetary system as its star dies.

But our technique could also help us discover more about the composition of exoplanets – something that is extremely hard to do with exoplanets orbiting stars like our sun. The atmosphere of a standard white dwarf is very pure, made of either hydrogen or helium. But while it is consuming planetary material, its atmosphere gets polluted – enabling us to calculate how much of each element there is. This could tell us what the disintegrating planet was made of and even if it had water, helping us to build a better picture of what kind of exoplanets are the most likely to host life.
A small 5-200 kilometer planetesimal, with a similar mineral composition to Earth, is found orbiting in the debris disk of a white dwarf star. This discovery is among the first of its kind and expected to provide insight into the future of our own planetary system.