Some clues to why Peter Pan discs exist

Peter Pan discs that seemingly “never grow up” have intrigued us since citizen scientists first made us aware of their existence.

NASA’s Disk Detective project discovered the oldest protoplanetary disc back in 2016 and six more Peter Pans have since been added to the list – four as recently as January this year.

These planet-forming discs are giant areas of gas and dust found circling young stars, but it isn’t clear how and why some live up to 10 times as long as others.

Now two scientists from Queen Mary University of London, writing in the Monthly Notices of the Royal Astronomical Society: Letters, say they have some clues.

Using computer simulations to look at a range of possible starting configurations and ways in which the disc evolves, they explored the combination of conditions needed to form Peter Pan discs – dubbing them Neverland’s parameters.

They found these discs only form in lonely environments, away from other stars, and that they need to start out much larger than normal discs.

“Most stars form in big groups containing around 100,000 stars; however, it seems that Peter Pan discs can’t form in these environments,” says first author Gavin Coleman.

“They need to be much more isolated from their stellar neighbours as the radiation from other stars would blow these discs away. They also need to start out massive, so they have more gas to lose and are therefore able to live for much longer.”

Coleman’s colleague Thomas Haworth says a particularly interesting point is that Peter Pan discs have so far only been found around low mass stars, and these low mass stars are generally being found to host lots of planets.

“The required external photoevaporation rates are so low that primordial Peter Pan discs will have formed in rare environments on the periphery of low-mass star-forming regions, or deeply embedded, and as such have never subsequently been exposed to higher amounts of UV radiation,” the researchers write in their paper.

“Given that such an external photoevaporation scenario is rare, the required disc parameters and accretion properties may reflect the initial conditions and accretion rates of a much larger fraction of the discs around low-mass stars.”

The large disc masses needed to end up with Peter Pan discs could be an important ingredient that allows these planets to exist, Coleman and Haworth suggest.