An iron and nickel-rich planetesimal is apparently all that survives of a planet following the death of its star, SDSS J122859.93+104032.9. We are talking about an object in an orbit around a white dwarf so tight that it completes a revolution every two hours. Significantly, spectroscopic methods were used to make the identification, the first time a solid body has been found around a white dwarf with spectroscopy. Variations in emitted light were used to identify the gases generated by the planetesimal, with data from the Gran Telescopio Canarias in La Palma.

Lead author Christopher Manser (University of Warwick) notes the advantages of the method the team developed to study a white dwarf 400 light years away:

“Our discovery is only the second solid planetesimal found in a tight orbit around a white dwarf, with the previous one found because debris passing in front of the star blocked some of its light — that is the “transit method’’ widely used to discover exoplanets around Sun-like stars. To find such transits, the geometry under which we view them has to be very finely tuned, which means that each system observed for several hours mostly leads to nothing. The spectroscopic method we developed in this research can detect close-in planetesimals without the need for a specific alignment.”

Image: A planetary fragment orbits the star SDSS J122859.93+104032.9, leaving a tail of gas in its wake. Credit & copyright: University of Warwick/Mark Garlick.

This is an extreme environment, the white dwarf in question being surrounded by a debris disk through which the object passes in its orbit. The star itself is about 70 percent of the mass of the Sun and, like all white dwarfs, this one — roughly the size of Earth — is quite dense, a survivor of the star’s red giant phase. An object moving this close to the white dwarf will be under extreme gravitational stress; the gravity of SDSS J122859.93+104032.9 is fully 100,000 times that of the Earth. The fact that the team could identify a planetesimal deep within the gravitational well indicates it must be an object of great density, probably made up of iron and nickel.

On where the object came from, the paper offers this intriguing possibility:

This object may be the differentiated iron core of a larger body that has been stripped of its crust and mantle by the tidal forces of the white dwarf. The outer layers of such a body would be less dense and would disrupt at greater semimajor axes and longer periods than those required for core disruption. This disrupted material would then form a disc of dusty debris around SDSS J1228+1040, leaving a stripped corelike planetesimal orbiting within it.

Manser’s colleague and co-author Boris Gaensicke adds if the assumption that we are dealing with a planetary core is correct, then the original body would have been at least hundreds of kilometers in diameter, because it is only at this size that planets begin to differentiate, with heavier elements sinking to form a metal core. It could, of course, have been much larger.

Thus the survival of a planetesimal here, actually orbiting within the original radius of its star, suggests a large object ultimately shredded by gravitational forces. We are glimpsing what our own Solar System may resemble in 5 to 6 billion years, when it will be a white dwarf orbited by the outer planets along with asteroids and comets. Our star’s expansion into a red giant will savage the inner system, perhaps leaving debris like what we see around SDSS J122859.93+104032.9. Bear in mind, too, that the vast majority of the stars known to host planets will end their lives as white dwarfs, so we are looking at a common destiny.

The debris disk of the white dwarf is rich in magnesium, iron, silicon and oxygen, and it is within that disk that the scientists found gas streaming from the evidently solid body. The object appears to be about a kilometer in size but could be as large as a few hundred kilometers in diameter. Whether it is the source of the gas or simply the cause of the gaseous ‘tail’ as it collides with debris in the disk is not yet known. Learning more will involve studying other debris disks similar to SDSS J122859.93+104032.9 (eight gaseous white dwarf debris discs are currently known), where the spectroscopic method will perhaps find other instances of planetesimals orbiting near or within the parent star’s debris disk.

The paper is Manser et al., “A Planetesimal Orbiting Within the Debris Disc Around a White Dwarf Star,” Science April 4 2019 (abstract).