Planets form out of 'dirty' stars

On Earth cleanliness is next to godliness, but in deep space it seems it is a recipe for a lonely existence.

A recent study published in the Astrophysical Journal Letters explains why 'dirty' stars, those rich in elements heavier than helium, are more likely to be surrounded by a system of planets and asteroids.

The findings suggest that the make-up of the original molecular cloud from which a star is formed will determine whether it can host a solar system.

When stars form from the collapse of a molecular cloud, they are surrounded by what is known as a protoplanetary disk - a gaseous dust cloud that orbits the star.

The team led by Dr Anders Johansen of Leiden Observatory in the Netherlands, has shown that if the gas disk has an abundance of heavy elements it is more likely to form extrasolar planets, those planets that orbit stars other than the Sun.

Dr Sarah Maddison, of Swinburne University of Technology's Centre for Astrophysics and Supercomputing in Melbourne, says observations of the known 350 extrasolar planets show the majority are found around stars with an abundance of heavy elements.

She says planets form by the accumulation of tiny dust grains that grow in size to form pebbles, then larger planetesimals and finally protoplanetary bodies.

Clumping together

Maddison says the latest paper builds on the previous "good work" by the researchers.

In a Nature paper in 2007 they reported that in these dusty-gas disks, the "dust grains feel a headwind" from the more slowly moving gas, which stops the dust from rapidly falling into the central star.

Maddison says the astronomers have now shown the more dust in the cloud, the easier it is for pebble-sized grains to clump.

This is important, she says, because it is this clumping that allows the pebbles to grow in size and form larger preplanetary material.

Using a 3D simulation of the "drag resulting in clumping" effect, the researchers were able to demonstrate how a cloud of one centimetre pebbles is capable of "creating" a planetesimal up to 100 kilometres in radius.

These planetary building blocks merge over millions of years to form planets.

The researchers also found that when pebbles constitute less than 1% of the gas disk mass, clumping is weak.

"There is an extremely steep transition from not being able to make planets at all to easily making planets, by increasing the abundance of heavy elements just a little," says lead author Johansen. "The probability of having planets almost explodes."