[Spitzer News] Many, Perhaps Most,
Nearby Sun-Like Stars May Form Rocky Planets
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spitzer-news at lists.ipac.caltech.edu
Sun Feb 17 07:45:41 PST 2008
In this issue:
1) Many, Perhaps Most, Nearby Sun-Like Stars May Form Rocky Planets
MANY, PERHAPS MOST, NEARBY SUN-LIKE STARS MAY FORM ROCKY PLANETS
Astronomers have discovered that terrestrial planets might form around
many, if not most, of the nearby sun-like stars in our galaxy. These
new results suggest that worlds with potential for life might be more
common than we thought.
University of Arizona, Tucson, astronomer Michael Meyer and his
colleagues used NASA's Spitzer Space Telescope to determine whether
planetary systems like ours are common or rare in our Milky Way
galaxy. They found that at least 20 percent, and possibly as many as
60 percent, of stars similar to the sun are candidates for forming
Meyer is presenting the findings at the annual meeting of the American
Association for the Advancement of Science in Boston. The results
appear in the Feb. 1 issue of Astrophysical Journal Letters.
The astronomers used Spitzer to survey six sets of stars, grouped
depending on their age, with masses comparable to our sun. The sun is
about 4.6 billion years old. "We wanted to study the evolution of the
gas and dust around stars similar to the sun and compare the results
with what we think the solar system looked like at earlier stages
during its evolution," Meyer said.
The Spitzer telescope does not detect planets directly. Instead it
detects dust -- the rubble left over from collisions as planets form
-- at a range of infrared wavelengths. The hottest dust is detected at
the shortest wavelengths, between 3.6 microns and 8 microns. Cool dust
is detected at the longest wavelengths, between 70 microns and 160
microns. Warm dust can be traced at 24-micron wavelengths. Because
dust closer to the star is hotter than dust farther from the star, the
"warm" dust likely traces material orbiting the star at distances
comparable to the distance between Earth and Jupiter.
"We found that about 10 to 20 percent of the stars in each of the four
youngest age groups shows 24-micron emission due to dust," Meyer said.
"But we don't often see warm dust around stars older than 300 million
years. The frequency just drops off.
"That's comparable to the time scales thought to span the formation
and dynamical evolution of our own solar system," he added.
"Theoretical models and meteoritic data suggest that Earth formed over
10 to 50 million years from collisions between smaller bodies."
In a separate study, Thayne Currie and Scott Kenyon of the Smithsonian
Astrophysical Observatory, Cambridge, Mass., and their colleagues also
found evidence of dust from terrestrial planet formation around stars
from 10 to 30 million years old. "These observations suggest that
whatever led to the formation of Earth could be occurring around many
stars between three million and 300 million years old," Meyer said.
Kenyon and Ben Bromley of the University of Utah, Salt Lake City, have
developed planet formation models that provide a plausible scenario.
Their models predict warm dust would be detected at 24-micron
wavelengths as small rocky bodies collide and merge. "Our work
suggests that the warm dust Meyer and colleagues detect is a natural
outcome of rocky planet formation. We predict a higher frequency of
dust emission for the younger stars, just as Spitzer observes," said
The numbers on how many stars form planets are ambiguous because
there's more than one way to interpret the Spitzer data, Meyer said.
The warm-dust emission that Spitzer observed around 20 percent of the
youngest cohort of stars could persist as the stars age. That is, the
warm dust generated by collisions around stars three to 10 million
years old could carry over and show up as warm dust emission seen
around stars in the 10- to 30- million-year-old range and so on.
Interpreting the data this way, about one out of five sun-like stars
is potentially planet-forming, Meyer said.
There's another way to interpret the data. "An optimistic scenario
would suggest that the biggest, most massive disks would undergo the
runaway collision process first and assemble their planets quickly.
That's what we could be seeing in the youngest stars. Their disks live
hard and die young, shining brightly early on, then fading," Meyer
said. "However, smaller, less massive disks will light up later.
Planet formation in this case is delayed because there are fewer
particles to collide with each other."
If this is correct and the most massive disks form their planets first
and the wimpiest disks take 10 to 100 times longer, then up to 62
percent of the surveyed stars have formed, or may be forming, planets.
"The correct answer probably lies somewhere between the pessimistic
case of less than 20 percent and optimistic case of more than 60
percent," Meyer said.
The next critical test of the assertion that terrestrial planets like
Earth could be common around stars like the sun will come next year
with the launch of NASA's Kepler mission.
Meyer's 13 co-authors include John Carpenter of the California
Institute of Technology in Pasadena. NASA's Jet Propulsion Laboratory
in Pasadena manages the Spitzer Space Telescope mission for NASA's
Science Mission Directorate, Washington. Science operations are
conducted at the Spitzer Science Center at Caltech. Caltech manages
JPL for NASA.
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