Scientists have discovered what they believe to be the super-dense, metallic remains of a decimated planet surviving closer to the core of a dying star than had previously been thought possible. The discovery offers a tantalizing glimpse into what may become of our home solar system in the distant future, when our Sun transforms into a white dwarf in five to six billion years.

The new study focussed on the white dwarf SDSS J122859.93+104032.9, which is orbiting within the Milky Way some 410 light-years from Earth. White dwarfs are the relatively cold, dim remains of once magnificent stellar bodies like our Sun.

When our star has used up the last of its nuclear fuel, it will swell to become an enormous red giant, and subsequently throw off its outer layers. When this dramatic process has run its course, all that will remain of the once-great star will be a small, super-dense core, which astronomers refer to as a white dwarf.

This transition will be catastrophic for the planets of our solar system. When the Sun transforms into a red giant, it will swell out to the current orbit of Earth. This will likely be the end of the road for our home planet, and will certainly be the death of Mercury and Venus.

Upon becoming a white dwarf, it is possible that the remaining large planets, including Saturn and Jupiter, will push smaller bodies into close proximity with the star, where they will be torn apart and form a disk.

By observing white dwarfs that have already transformed, along with what remains of the planets that once orbited them, astronomers can gain a greater understanding as to the possible nature of our home solar system as it progresses through its twilight years.

SDSS J122859.93+104032.9 was observed using the spectroscopic capabilities of the Gran Telescopio Canarias, located at the Observatorio del Roque de Los Muchachos on the island of La Palma, within the Canary Islands. The white dwarf was observed for a total of five nights spread out between April 20, 2017 to May 2, 2018.

The study authors estimate that SDSS J122859.93+104032.9, which is in desperate need of a nickname, was once roughly twice the size of the Sun. However, after casting off its outer layers and condensing during its transformation from a main sequence star to a white dwarf, it is now likely the size of Earth.

Despite being a shadow of its former glory, SDSS J122859.93+104032.9 still boasts a mass the equivalent of about 70 percent that of our Sun, which gives it a gravitational influence of approximately 100,000 times that of our planet.

SDSS J122859.93+104032.9 is surrounded by a disk of dust and gas, the remains of solar system bodies destroyed by the gravity of the dying star. However, the researchers were surprised to discover the signature of what they believe to be a planetary remnant orbiting within the debris surrounding the white dwarf.

"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," said Prof. Boris Gaensicke, co-author from the University of Warwick's Department of Physics. "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."

The planetary remnant was found to orbit within its star's original radius, and is most likely the core of a once much larger planet that has had its crust and mantle stripped away by the tidally disruptive gravity of the white dwarf.

"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," said Gaensicke. "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 tortured object completes an orbit of its star once every two hours, and is estimated to be anywhere between 4 and 600 km (2.5 to 373 miles) in diameter. The planetary remnant also appears to have a comet-like debris tail trailing in its wake. It is possible that the tail is comprised of gas that has sublimated out from within the planetesimal. Alternatively, it may be debris created by the large body as it smashes through the matter in its path.

Alongside providing a glimpse into the final evolutionary stages of our home solar system, the new discovery could help astronomers understand the types of planets lurking in the space outside of SDSS J122859.93+104032.9's debris disk, that cannot be directly detected.

The paper has been published in the journal Science.