Dirty Stars Make Great Hosts for New Worlds

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

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

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

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

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

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

New simulations by Mac Low and his colleagues AndersJohansen of Leiden Observatory in the Netherlands and Andrew Youdin of theUniversity of Toronto compute just how planets and other bodies form whenpebbles clump together and build up to form mini-planets, referred to asplanetesimals.

Their results show that when pebbles, made of heavyelements, constitute less than one percent of the gas mass of the protoplanetarydisk surrounding the star, clumping is weak. But if the fraction of pebblesis increased slightly, the clumping increases dramatically and quickly resultsin 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, isthe recipe for mini-planet formation, a crucial intermediate step in formingplanets.

"There is an extremely steep transition from not beingable to make planets at all to easily making planets, by increasing theabundance of heavy elements just a little," Johansen, lead author of thestudy, said. "The probability of having planets almost explodes."

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

The results of the study were presented this week at ameeting of the Division of Planetary Sciences of the American AstronomicalSociety in Puerto Rico.

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