Retired Star Found with Planets and Debris Disk


Feature • April 9, 2013

The Herschel Space Observatory, a European Space Agency mission with important NASA contributions, has provided the first images of a dust belt - produced by colliding comets or asteroids - orbiting a subgiant star known to host a planetary system.


At least during the subgiant phase, planets, asteroids and comet belts around these 'retired' stars are expected to survive, but observations are needed to measure their properties. One approach is to search for disks of dust around the stars, generated by collisions between populations of asteroids or comets.

Thanks to the sensitive far-infrared detection capabilities of the Herschel space observatory, astronomers have been able to resolve bright emission around Kappa Coronae Borealis (x CrB, or Kappa Cor Bor), indicating the presence of a dusty debris disk.



Herschel's detection provides rare insight into the life of planetary systems orbiting subgiant stars, and enables a detailed study of the architecture of its planet and disk system.

"This is the first 'retired' star that we have found with a debris disk and one or more planets," says Amy Bonsor of the Institute de Planétologie et d'Astrophysique de Grenoble, and lead author of the study. "The disk has survived the star's entire lifetime without being destroyed. That's very different to our own Solar System, where most of the debris was cleared away in a phase called the Late Heavy Bombardment era, around 600 million years after the Sun formed."

Dr Bonsor's team used models to propose three possible configurations for the disk and planets that fit Herschel's observations of Kappa Cor Bor.


By comparison, the icy debris disk in our Solar System - known as the Kuiper Belt - spans a narrower range of distances, 30-50 AU from the Sun.


A variation on this model has the disk being stirred by the gravitational influence of both companions, mixing it up such that the rate of dust production in the disk peaks at around 70-80 AU from the star.


"It is a mysterious and intriguing system: is there a planet or even two planets sculpting one wide disk, or does the star have a brown dwarf companion that has split the disk in two?" says Dr Bonsor.

As this is the first known example of a subgiant star with planets and a debris disk orbiting it, more examples are needed to determine whether Kappa Cor Bor is unusual or not.

"Thanks to Herschel's sensitive far-infrared capabilities and its rich dataset, we already have hints of other subgiant stars that may also have dusty disks. More work will be needed to see if they also have planets," says Göran Pilbratt, ESA's Herschel project scientist.
