June 14, 2012
Small Planets Less Pickier For Host Stars Than Previously Thought
Lee Rannals for redOrbit.com
New observations show that small planets may be more widespread in our galaxy than previously thought.
Lars A. Buchhave, an astrophysicist at the Niels Bohr Institute and the Centre for Star and Planet Formation at the University of Copenhagen, and colleagues studied the composition of over 150 stars harboring 226 planet candidates smaller than Neptune for the research.
Scientists previously thought that the formation of small planets like Earth occurred mostly around stars rich in heavy elements like iron and silicon. However, the latest findings suggest these small planets form around stars with a wide range of heavy element content.
"I wanted to investigate whether small planets needed a special environment in order to form, like the giant gas planets, which we know preferentially develop in environments with a high content of heavy elements," Buchhave said in recent statement. "This study shows that small planets do not discriminate and form around stars with a wide range of heavy metal content, including stars with only 25 percent of the sun's metallicity."
Astronomers refer to all chemical elements heavier than hydrogen and helium as metals, and say metallicity is the metal content of heavier elements in a star.
Stars that have a higher fraction of heavy elements than the sun are considered metal-rich, while stars with a lower faction of heavy elements are metal-poor.
According to NASA, planets are created from disks of gas and dust around new stars, and those like Earth are made-up mostly of elements like iron, oxygen, silicon and magnesium.
Astronomers believe that large quantities of heavy elements in the disk would lead to more efficient planet formation, and it has long been thought that giant plants with short orbital periods are associated with metal-rich stars.
Planets that are four times the size of Earth can form around stars with a wide range of heavy element content, including stars with a lower metallicity than the sun.
According to the findings published in the journal Nature, small planets may form around stars with a wide variety of metallicities, including systems with only one-quarter of the Sun's metal content.
The latest discovery by Buchhave and the team supports the "core accretion" model of planet formation. Under this model, primordial dust accumulates into mile-sized planetesimals that then coalesce into full-fledged planets. The largest of these planets developed under this model can weigh 10 times Earth, gather surrounding hydrogen and become a gas giant.
"This work suggests that terrestrial worlds could form at almost any time in our galaxy's history," Smithsonian astronomer David Latham, Harvard-Smithsonian Center for Astrophysics, said in a statement. "You don't need many earlier generations of stars."