Dirty Stars Make Great Hosts for New Worlds

Dirty stars ? those rich in heavier elements ? make the best hosts of planetary systems, new research suggests.

"When you observe stars, the ones with more heavy elements have more planets," said Mordecai-Mark Mac Low, Curator of Astrophysics at the American Museum of Natural History and co-author of the study detailing the research in the journal Astrophysical Journal Letters.

Some stars observed in our galaxy appear to be lonely gas furnaces, radiating their heat out into the surrounding cold emptiness of space, while others shine their light on a skirt of orbiting planets.

Whether a star has planets or not often depends on its composition: Observations of distant solar systems show that exoplanets, or planets that orbit stars other than the sun, are much more abundant around stars that have a greater abundance of elements heavier than helium, including iron and oxygen. These elements are building blocks for rocks and ice.

"In other words, what's in the disk reflects what's in the star. This is a common sense result," Mac Low said.

The new research explains why planets tend to form around these "dirtier" stars, the scientists say.

New simulations by Mac Low and his colleagues Anders Johansen of Leiden Observatory in the Netherlands and Andrew Youdin of the University of Toronto compute just how planets and other bodies form when pebbles clump together and build up to form mini-planets, referred to as planetesimals.

Their results show that when pebbles, made of heavy elements, constitute less than one percent of the gas mass of the protoplanetary disk surrounding the star, clumping is weak. But if the fraction of pebbles is increased slightly, the clumping increases dramatically and quickly results in the accretion of sufficient material to make larger-scale planetesimals.

These mini-planets work as planetary building blocks, merging over millions of years to form planets, theory holds. In short, clumping of pebbles, when the fraction of solids in the gas is high enough, is the recipe for mini-planet formation, a crucial intermediate step in forming planets.

"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," Johansen, lead author of the study, said. "The probability of having planets almost explodes."

Youdin adds that "There's an inherent advantage in being born rich, in terms of solid rocks. But less advantaged systems, like our own solar system, can still make planets if they work to marshal their resources and hang onto their solids as the gas evaporates away. So the sun is middle-class, rather than rich."

The results of the study were presented this week at a meeting of the Division of Planetary Sciences of the American Astronomical Society in Puerto Rico.

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