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Alien Earths Could Form Earlier Than Expected

From: Ray Dickenson <r.dickenson.nul>
Date: Wed, 13 Jun 2012 23:55:21 +0100
Archived: Thu, 14 Jun 2012 07:58:43 -0400
Subject:  Alien Earths Could Form Earlier Than Expected

Hello List,

An obvious implication of this finding is that "Earth
equivalents" have already formed in our Galaxy, some maybe
millions or billions of years ahead of ours.




June 13, 2012

Alien Earths Could Form Earlier Than Expected

Building a terrestrial planet requires raw materials that
weren't available in the early history of the universe. The Big
Bang filled space with hydrogen and helium. Chemical elements
like silicon and oxygen - key components of rocks - had to be
cooked up over time by stars. But how long did that take? How
many of such heavy elements do you need to form planets?

Previous studies have shown that Jupiter-sized gas giants tend
to form around stars containing more heavy elements than the
Sun. However, new research by a team of astronomers found that
planets smaller than Neptune are located around a wide variety
of stars, including those with fewer heavy elements than the
Sun. As a result, rocky worlds like Earth could have formed
earlier than expected in the universe's history.

"This work suggests that terrestrial worlds could form at almost
any time in our galaxy's history," said Smithsonian astronomer
David Latham (Harvard-Smithsonian Center for Astrophysics). "You
don't need many earlier generations of stars."

Latham played a lead role in the study, which was led by Lars A.
Buchhave from the University of Copenhagen and will be published
in the journal Nature. The work is being presented June 13 at
the 220th meeting of the American Astronomical Society.

Astronomers call chemical elements heavier than hydrogen and
helium "metals". They measure the metal content, or
metallicities, of other stars using the Sun as a benchmark.
Stars with more heavy elements are considered metal-rich while
stars with fewer heavy elements are considered metal-poor.

Latham and his colleagues examined more than 150 stars known to
have planets, based on data from NASA's Kepler spacecraft. They
measured the stars' metallicities and correlated that with the
sizes of the associated planets. Large planets tended to orbit
stars with solar metallicities or higher. Smaller worlds,
though, were found around metal-rich and metal-poor stars alike.
"Giant planets prefer metal-rich stars. Little ones don't,"
explained Latham.

They found that terrestrial planets form at a wide range of
metallicities, including systems with only one-quarter of the
Sun's metal content.

Their discovery supports the "core accretion" model of planet
formation. In this model, primordial dust accumulates into mile-
sized planetesimals that then coalesce into full-fledged
planets. The largest, weighing 10 times Earth, can then gather
surrounding hydrogen and become a gas giant.

A gas giant's core must form quickly since hydrogen in the
protoplanetary disk dissipates rapidly, swept away by stellar
winds in just a few million years. Higher metallicities might
support the formation of large cores, explaining why we're more
likely to find a gas giant orbiting a metal-rich star.

"This result fits with the core accretion model of planet
formation in a natural way," said Latham.

Harvard-Smithsonian Center for Astrophysics (2012, June 13).
Alien Earths could form earlier than expected.



Ray D

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