Life on Earth is thought to have arisen from a hot
soup of chemicals. Does this same soup exist
on planets around other stars? Led by a Johns Hopkins
University researcher, 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 carbon- and
nitrogen-containing component of adenine, one of the four
basic elements of the molecule DNA, which
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 lead author
Ilaria Pascucci, a research scientist in the Henry A. Rowland
Department of Physics and Astronomy in
Johns Hopkins' Krieger School of Arts and Sciences. The
study appears 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," Pascucci said.
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 sunlike stars
using Spitzer's infrared spectrograph, an instrument that
breaks light apart, revealing signatures of
chemicals. The stars are all about 1 million to 3 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-dwarfs and
brown dwarfs, showed no hydrogen
cyanide at all, while 30 percent of the sunlike stars
did.
"Perhaps ultraviolet light, which is much stronger
around the sunlike stars, may drive a higher
production of the hydrogen cyanide," Pascucci said.
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 rocky
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.
Douglas Hudgins, the Spitzer program scientist at NASA
headquarters in Washington, D.C.,
said, "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."
NASA's Jet Propulsion Laboratory manages the Spitzer
Space Telescope mission for NASA's
Science Mission Directorate. Science operations are
conducted at the Spitzer Science Center at the
California Institute of Technology. Caltech manages JPL for
NASA. Spitzer's infrared spectrograph,
which made the new observations, was built by Cornell
University.
For more about Spitzer, go to:
www.spitzer.caltech.edu/spitzer and
www.nasa.gov/spitzer.