No Heavy Metals? No Problem!

The above image is an artist's concept of a planet transiting (passing across) the disc of a distant star. The Kepler mission discovers and studies extra-solar planets (exoplanets) by watching distant stars for the dimming starlight caused by planet transits. Image Credit: Ames Wendy Stenzel/NASA

Until recently, astronomers thought the formation of small worlds like Earth occurred mostly around stars rich in heavy elements such as iron and silicon. But new ground-based observations, combined with data collected by NASA's Kepler space telescope, show small planets form around stars with a wide range of heavy element content and suggest they may be widespread in our galaxy.

A research team led by Lars A. Buchhave, an astrophysicist at the Niels Bohr Institute and the Centre for Star and Planet Formation at the University of Copenhagen, studied the elemental composition of more than 150 stars harboring 226 planet candidates smaller than Neptune.

Buchhave wanted to investigate whether small planets needed a special environment in order to form, like the giant gas planets, which prefer to develop in environments with a high content of heavy elements. The current study shows that small planets do not discriminate, forming around stars with a wide range of heavy metal content, including stars with only 25 percent of the metallicity of our sun.

I'm sorry. Metal-what-ity?

Astronomers refer to all chemical elements heavier than hydrogen and helium as metals. They define metallicity as the metal content of heavier elements in a star and our own star is used as a baseline. Stars with a higher fraction of heavy elements than the sun are considered metal-rich. Stars with a lower fraction of heavy elements are considered metal-poor.

Planets are created in disks of gas and dust around new stars. Planets like Earth are made almost entirely of elements like iron, oxygen, silicon and magnesium. The metallicity of a star mirrors the metal content of the planet-forming disk. Astronomers have thought that large quantities of heavy elements in the disk would lead to more efficient planet formation. It has long been noted that giant planets with short orbital periods tend to be associated with metal-rich stars.

Unlike gas giants, the occurrence of smaller planets is not strongly dependent on the heavy element content of their parent stars. Planets up to 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. The findings are described in a new study published in the journal Nature. The journal may be visited online at this URL: www..nature.com/nature .

The mission team notes that Kepler has identified thousands of planet candidates, which makes it possible to study big-picture questions like whether small planets need a special environment in which to form. And the new data suggest that small planets may form around stars with a wide range of metallicities.

The ground-based spectroscopic observations for this study were made at the Nordic Optical Telescope on La Palma in the Canary Islands; Fred Lawrence Whipple Observatory on Mt. Hopkins in Arizona; McDonald Observatory at the University of Texas at Austin; and W.M. Keck Observatory atop Mauna Kea in Hawaii.

Launched in March 2009, Kepler searches for planets by continuously monitoring more than 150,000 stars, looking for telltale dips in their brightness caused by passing, or transiting, planets. At least three transits are required to verify a signal as a planet. Follow-up observations from ground-based telescopes are also needed to confirm a candidate as a planet.

And now, the mission particulars...

Ames manages Kepler's ground system development, mission operations and science data analysis. NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, managed the Kepler mission development. JPL is managed by the California Institute of Technology, also in Pasadena, for NASA.

Ball Aerospace & Technologies Corp. in Boulder, Colorado, developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

The Space Telescope Science Institute in Baltimore archives, hosts and distributes Kepler science data. Kepler is NASA's 10th Discovery Mission and is funded by NASA's Science Mission Directorate at the agency's headquarters in Washington.

For more information about the Kepler mission, visit: www.nasa.gov/kepler .

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