Cool Stars Have Different
Mix of Life-Forming Chemicals
Thursday, April 9,
2009
Life on Earth is thought to have arisen from a hot soup of
chemicals. Does this same soup exist on planets around other stars?
A new study from NASA's Spitzer Space Telescope hints that planets
around stars cooler than our sun might possess a different mix of
potentially life-forming, or "prebiotic," chemicals.
Astronomers used Spitzer to look for a prebiotic chemical, called
hydrogen cyanide, in the planet-forming material swirling around
different types of stars. Hydrogen cyanide is a component of
adenine, which is a basic element of DNA. DNA can be found in every
living organism on Earth.
The researchers detected hydrogen cyanide molecules in disks
circling yellow stars like our sun -- but found none around cooler
and smaller stars, such as the reddish-colored "M-dwarfs" and
"brown dwarfs" common throughout the universe.
"Prebiotic chemistry may unfold differently on planets around cool
stars," said Ilaria Pascucci, lead author of the new study from
Johns Hopkins University, Baltimore, Md. The study will appear in
the April 10 issue of the Astrophysical Journal.
Young stars are born inside cocoons of dust and gas, which
eventually flatten to disks. Dust and gas in the disks provide the
raw material from which planets form. Scientists think the
molecules making up the primordial ooze of life on Earth might have
formed in such a disk. Prebiotic molecules, such as adenine, are
thought to have rained down to our young planet via meteorites that
crashed on the surface.
"It is plausible that life on Earth was kick-started by a rich
supply of molecules delivered from space," said Pascucci.
Could the same life-generating steps take place around other stars?
Pascucci and her colleagues addressed this question by examining
the planet-forming disks around 17 cool and 44 sun-like stars using
Spitzer's infrared spectrograph, an instrument that breaks light
apart, revealing signatures of chemicals. The stars are all about
one to three million years old, an age when planets are thought to
be growing. The astronomers specifically looked for ratios of
hydrogen cyanide to a baseline molecule, acetylene.
They found that the cool stars, both the M-dwarf stars and brown
dwarfs, showed no hydrogen cyanide at all, while 30 percent of the
sun-like stars did. "Perhaps ultraviolet light, which is much
stronger around the sun-like stars, may drive a higher production
of the hydrogen cyanide," said Pascucci.
The team did detect their baseline molecule, acetylene, around the
cool stars, demonstrating that the experiment worked. This is the
first time that any kind of molecule has been spotted in the disks
around cool stars.
The findings have implications for planets that have recently been
discovered around M-dwarf stars. Some of these planets are thought
to be large versions of Earth, the so-called super Earths, but so
far none of them are believed to orbit in the habitable zone, where
water would be liquid. If such a planet is discovered, could it
sustain life?
Astronomers aren't sure. M-dwarfs have extreme magnetic outbursts
that could be disruptive to developing life. But, with the new
Spitzer results, they have another piece of data to consider: these
planets might be deficient in hydrogen cyanide, a molecule thought
to have eventually become a part of us.
Said Douglas Hudgins, the Spitzer program scientist at NASA
Headquarters, Washington, "Although scientists have long been aware
that the tumultuous nature of many cool stars might present a
significant challenge for the development of life, this result begs
an even more fundamental question: Do cool star systems even
contain the necessary ingredients for the formation of life? If the
answer is no then questions about life around cool stars become
moot."
Other authors include Daniel Apai of the Space Telescope Science
Institute, Baltimore, Md.; Kevin Luhman of Pennsylvania State
University, University Park; Thomas Henning and Jeroen Bouwman of
the Max Planck Institute for Astronomy, Germany; Michael Meyer of
the University of Arizona, Tucson; Fred Lahuis of the SRON
Netherlands Institute for Space Research, the Netherlands; and
Antonella Natta of the Arcetri Astrophysical Observatory,
Italy.
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NASA
Thanks to NASA for this
article.