Cool Stars Have Different Mix of Life-Forming Chemicals

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Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

NEWS RELEASE: 2009-064 April 7, 2009

Cool Stars Have Different Mix of Life-Forming Chemicals

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.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope
mission for NASA's Science Mission Directorate, Washington. Science operations are
conducted at the Spitzer Science Center at the California Institute of Technology, also in
Pasadena. Caltech manages JPL for NASA. Spitzer's infrared spectrograph, which made the
new observations, was built by Cornell University, Ithaca, N.Y. Its development was led by
Jim Houck of Cornell. For more information about Spitzer, visit
http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .

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