Epsilon Aurigae

Epsilon Aurigae

Theory Proposes Planetary System Forming in Binary

Epsilon Aurigae, a long-standing puzzle star for astronomers, is the remarkable instance of a binary system in which a planetary system is currently forming, according to a theory proposed by two British mathematicians.

A third magnitude yellow supergiant star. Epsilon Aurigae is eclipsed every 27 years by a mysterious dark companion. The fact that this companion star is invisible has led to speculation that is a black hole, although this has not been borne out by subsequent x-ray observations.

Michael Handbury and Iwan Williams of the University of London suggest that Epsilon Aurigae is actually a very young star about nine times as massive as our sun, and still contracting onto the main sequence. Its dark companion is so young that it has not yet “switched on” to become a star. The companion is surrounded by a disk shaped nebula in which planets may be forming.

Epsilon Aurigae being eclipsed by a dark dust cloud. Source of the photo: astronomycentral.co.uk

The true shape and nature of the secondary companion to Epsilon Aurigae is revealed by the peculiar light curve of the eclipses it produces. The whole eclipse lasts a remarkable two years. Minimum light output, during which Epsilon Aurigae is dimmed by 50 percent, lasts 330 days.

To account for this behavior, Handbury and Williams believe the obscuring body cannot be a simple spherical star (or a black hole), but instead must be shaped like a flattened disk, growing transparent toward its edges. The obscuring disk is about one billion miles in diameter and contains approximately 10 solar masses. Most of the mass is concentrated in a forming central star, with the remainder collecting into a planetary system around it.

Because the two stars of the binary are so far apart – about two billion miles – planets can form in stable orbits, similar to the stable orbits of Jupiter’s satellites.

The Handbury-Williams model is particularly important because it means that the binary becomes one of the best candidates for observing solar system formation in action. So far, only RU Lupi, a star of the T Tauri variety, has looked as promising. In 1974, four Swedish astronomers suggested that RU Lupi was surrounded by a swarm of protoplanets.

Confirmation of planets in the Epsilon Aurigae system will settle the longstanding argument among astronomers of whether planets can ever form around double stars, with consequent implications for the existence of extraterrestrial life.

If the dark companion of Epsilon Aurigae is a star in the process of formation, it might become visible in the near future. Certainly, the next eclipse of the system in 1982 will be the object of a great deal of astronomical attention.

(History of astronomy. Astronomy Magazine, July 1976).