Sunday 12 February 2012

The impossible planets of HU Aquarii.

HU Aquarii is an eclipsing binary system in the constellation of Aquarius, 586.8 light years from Earth. It's principle component (HU Aquarii A) is a White Dwarf stellar remnant with 88% of the sun's mass, a radius of 1% of the sun's and 0.22% of the sun's luminosity, with a surface temperature of 12 500K (compared to 5578K for the sun). This is being orbited by a small Red Dwarf companion (HU Aquarii B) at a distance of 0.0032 AU (0.32% of the distance at which the Earth orbits the Sun, or 0.8% of the distance at which Mercury orbits the Sun), every 2.08 hours. The Red Dwarf companion has a mass 20% of that of the Sun, a radius 22% of the Sun's, 0.52% of the Sun's luminosity and a surface temperature of 3400K. As it orbits the larger star it's outer layer is being stripped of and forming an accretion disk about it. Systems like this are known as cataclysmic binaries.

An artists impression of a Cataclysmic Binary. Dana Berry/Space Telescope Science Institute

In 2011 the discovery of a pair of planets in the HU Aquarii system was announced, based upon perturbations in the orbit of HU Aquarii B; the small star speeds up and slows down as it orbits the larger, which is often indicative of the presence of a planet, or in this model, planets.

In a paper posted on the online arXiv database at Cornell University Library and accepted for publication in the peer-reviewed conference proceedings of the 11th annual Australian Space Science Conference, held in Canberra, September 2011, a team of scientists lead by Jonathan Horner of the Department of Astrophysics and Optics at the School of Physics at the University of New South Wales describe an attempt to build a working computer model of the HU Aquarii system with two planets, based upon the known data.

Horner et al. were unable to build a stable model of the HU Aquarii system based upon the proposed planetary alignments. They then tried a number of other potential planetary alignments, but came to the conclusion that any planetary alignment that could account for the variability in the orbit of HU Aquarii B would be fundamentally unstable. From this they conclude that the irregularity in the orbit of HU Aquarii B is not caused by the influence of a planetary system, but rather by the interactions of the magnetic fields of the two stars as HU Aquarii B is torn apart by its more massive companion. They caution against the assumption that orbital perturbations are always caused by the influence of planets, particularly in extreme or unstable systems such as HU Aquarii.