Cannibal star ate planet
May 9, 2001
A distant sun-like star with at least two orbiting planets probably consumed a giant planet when it was about 20 million years old, according to a team led by Garik Israelian of the Instituto de Astrofisica de Canarias in Tenerife. The astronomers believe that tell-tale traces of lithium in the star's atmosphere - previously unknown in this kind of star - are the remains of a giant gas planet that the star engulfed (G Israelian et al 2001 Nature 411 163).
The star - known as HD82943 - is a 'metal-rich' star. These mature stars have an abundance of heavy elements, and current theories of stellar evolution propose that they burn off the rare isotope lithium-6 early in their lives. Lithium-6 is, however, plentiful in the atmospheres of giant gas planets like Jupiter.
Astronomers studying extra-solar planets know that some of these systems contain giant gas planets in very tight orbits around their parent stars. Israelian and colleagues believe that the gravitational pull of a central star on such a nearby planet is strong enough to draw matter from the planet - or even engulf it entirely. They propose that this kind of gravitational interaction caused HD82943 to swallow an orbiting planet.
Israelian's group used models of the relative abundance of elements in the primordial universe to estimate the amount of lithium-6 in massive gas planets. To explain the quantity of lithium-6 detected in its spectrum, the researchers calculated that HD82943 absorbed one giant planet - or several smaller ones - equivalent in mass to two Jupiters. The whole process took between 10 and 20 million years.
Small quantities of lithium-6 are produced by flares erupting from the surface of the Sun, but the team calculated that such explosions on HD82943 could only account for a thousandth of the lithium-6 detected in its spectrum. Israelian and colleagues also mention that theories of nucleosynthesis are uncertain and the primordial universe - and therefore the gas planets - could have contained more lithium. This would suggest that HD82943 ingested a smaller planet or that processes within the star have depleted the levels of lithium-6.