Herschel Discovers First Ever 'Retired' Star with Planets and Debris Disc

About 100 light-years away, a subgiant star named Kappa Coronae Borealis hosts a planetary system. Now, a team of scientists have produced the very first images of a dust belt orbiting this massive star.

The findings, which are published in the journal Monthly Notices of the Royal Astronomic Society, were made with the European Space Agency's Herschel space observatory. Using its sensitive far-infrared detection capabilities, astronomers resolved bright emissions around the star, which indicated the presence of a dusty debris disc.

The 2.5-billion-year-old star is slightly heavier than our own Sun at about 1.5 solar masses. Like other subgiant stars, it was created after billions of years of steadily burning hydrogen at its core. Eventually, it exhausted this central fuel reserve and started burning it in shells around the core. The star then swelled, becoming the subgiant that it is today. It currently plays host to only one planet--a behemoth that's roughly twice the mass of Jupiter. There may be another planet, though evidence for it has yet to be confirmed. Yet it's the debris disc that is unusual for a subgiant star.

"This is the first 'retired' star that we have found with a debris disc and one or more planets," said Amy Bonsor, lead author of the study, in a press release. "The disc has survived the star's entire lifetime without being destroyed. That's very different to our 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."

Exactly how the disc and the planets are aligned with the star is another matter entirely. The researchers have used models to propose three possible configurations for the disc and planets that fit Herschel's observations. The first one possesses just one continuous belt with the planet orbiting far from the star, its gravitational force sculpting the inner edge of the disc. The second has the disc being stirred by the gravitational influenced by both companions, while the third has the dust disc divided into two narrow belts.

This subgiant star is the first known example of one with both planets and a debris disc orbiting it. Currently, researchers are searching for other examples to know whether this star is unusual or not.