Half a century ago, we were wondering if other stars had planets, and although we assumed so, there was always the possibility that planets were rare. Now we know that they’re all over the place. In fact, recent research out of Katholieke Universiteit Leuven in Belgium suggests that under certain circumstances, planets can form around stars that are going through their death throes, beginning the transition from red giant to white dwarf. The new work homes in on certain binary stars, and therein hangs a tale.

After a red giant star has gone through the stage of helium burning at its core, it is referred to as an asymptotic giant branch star (AGB), on a path that takes it through a period of expansion and cooling prior to its becoming a white dwarf. These expanding stars lose mass as the result of stellar wind, up to 50 to 70 percent of the total mass of the star. The result: An extended envelope of material collecting around the object that will become a planetary nebula, a glowing shell of ionized gas.

In binary systems, that stellar material coming off the star can evolve in interesting ways. While our standard view of planet formation involves circumstellar disks and planets emerging not all that long after the birth of their star, the KU Leuven work, led by Jacques Kluska, notes that in binary stars, a second star can gravitationally shape the gas and dust being ejected by a late stage red giant. From an observational perspective, this disk looks much like the disks that form around young stars.

To analyze the matter, the KU Leuven team assembled a catalog of all known post-AGB binaries that showed disks, compiling spectral energy distributions of 85 systems and examining the infrared characteristics of the various disk formations. Out of this emerged a catalog of different disk types. This is where things get intriguing. Between 8 and 12 percent of the cataloged systems are surrounded by what the paper calls ‘transition disks,’ meaning disks that show little or no low-infrared excess.

At the same time, these disks demonstrate a link with the depletion of refractory elements (metals highly resistant to heat) on the surface of the red giant star.

Jacques Kluska explains the significance of this result:

“In ten per cent of the evolved binary stars with discs we studied, we see a large cavity in the disc. This is an indication that something is floating around there that has collected all matter in the area of the cavity… In the evolved binary stars with a large cavity in the disc, we saw that heavy elements such as iron were very scarce on the surface of the dying star. This observation leads one to suspect that dust particles rich in these elements were trapped by a planet.”

Image: Discs surrounding so-called evolved binary stars not uncommonly show signs that could point to planet formation. Credit: N. Stecki.

In this scenario, it is possible that not just single planets but an entire system’s worth could eventually grow. We’ll have to see whether planets can be confirmed in some of these systems, and to do that the team intends to home in on the ten pairs of binary stars whose disks demonstrate a large cavity. Such confirmation would denote yet another method of planet formation, to add to what we’re learning about planets in white dwarf systems. The tendency of stars to grow planets, it seems, can manifest itself even in stellar death, a useful fact given that perhaps a third of all stars in the Milky Way are found in multiple star systems. And there is precedent:

…if the planetary explanation is correct for transition disks, it would mean that there is a pressure maximum outside the planet’s orbit, trapping the dust and creating a favorable environment for second-generation planet formation… The possibility of second-generation planet formation is further supported by the detection of two giant planets around the white-dwarf binary system NN Ser. These planets are candidates for having been formed in such second-generation disks… If the planetary scenario is confirmed, these disks would become a promising site for studying second-generation planet formation and, hence, planet formation scenarios in an unprecedented parameter space.

About 1700 light years away, NN Serpentis is a system containing an eclipsing white dwarf and an M-class dwarf. The existence of what are evidently two gas giants here has been inferred from transit timing variations that can be attributed to one planet of about 6 times Jupiter’s mass, the other of about 1.6 times Jupiter’s mass, both in circumbinary orbits around this extremely tight binary. This is an interesting system for models of planet formation that go well beyond the infancy of the host stars.

The paper is Kluska et al., “A population of transition disks around evolved stars: fingerprints of planets. Catalog of disks surrounding Galactic post-AGB binaries,” Astronomy & Astrophysics Vol. 658, A36 (01 February 2021). Full text. On NN Serpentis, see Völschow et al. ”Second generation planet formation in NN?Serpentis?” Astronomy & Astrophysics Vol. 562 (February 2014) A19 (abstract). For more on late binary systems, see Winckel, “Binary post-AGB stars as tracers of stellar evolution,” in Beccari, ed., The Impact of Binary Stars on Stellar Evolution, Cambridge University Press, 2019 (preprint).