Planets and Debris Disk Found Around Retired Star

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ESA’s Herschel space observatory has provided the first images of a dust belt – produced by colliding comets or asteroids – orbiting a sub-giant star known to host a planetary system.

Kappa Coronae Borealis, based on Herschel PACS observations at 100 μm. North is up and east is left. The star is in the center of the frame (not visible in this graphic) with an excess of infrared emission detected around it, interpreted as a dusty debris disk containing asteroids and/or comets. The inclination of the planetary system is constrained at an angle of 60º from face-on. Image Credit: ESA/Bonsor et al (2013)

After billions of years steadily burning hydrogen in their cores, stars like our Sun exhaust this central fuel reserve and start burning it in shells around the core. They swell to become sub-giant stars, before later becoming red giants.

At least during the sub-giant 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 (κ CrB, or Kappa Cor Bor), indicating the presence of a dusty debris disk.

The star is a little heavier than our own Sun at 1.5 solar masses, is around 2.5 billion years old and lies at a distance of roughly 100 light-years. From ground-based observations, it is known to host one giant planet roughly twice the mass of Jupiter orbiting at a distance equivalent to the Asteroid Belt in our own Solar System. A second planet is suspected, but its mass is not well constrained.

Herschel’s detection provides rare insight into the life of planetary systems orbiting sub-giant 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. The first model has just one continuous dust belt extending from 20 AU to 220 AU (where 1 AU, or Astronomical Unit, is the distance between Earth and Sun).

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.

In this model, one of the planets orbits at a distance of greater than 7 AU from the star, and its gravitational influence may sculpt the inner edge of the disk. 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.

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