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February 20, 2004 -- UNIVERSITY OF UTAH NEWS ADVISORY
The news release below was issued by the Harvard-Smithsonian Center
for Astrophysics in Cambridge, Mass., and involves research by
one of their scientists and by University of Utah physicist Ben
Bromley. The study described below continues the research outlined
in a 2001 University of Utah news release that can be found at:
http://www.utah.edu/news/releases/02/sep/planets.html
SEEKING NEW EARTHS? LOOK FOR DUST
Cambridge, MA -- If alien astronomers around a distant star
had studied the young Sun four-and-a-half billion years ago, could
they have seen signs of a newly-formed Earth orbiting this innocuous
yellow star? The answer is yes, according to Scott Kenyon (Smithsonian
Astrophysical Observatory) and Benjamin Bromley (University of
Utah). Moreover, their computer model says that we can use the
same signs to locate places where Earth-size planets currently
are forming -- young worlds that, one day, may host life of their
own.
The key to locating newborn Earths, say Kenyon and Bromley,
is to look not for the planet itself, but for a ring of dust orbiting
the star that is a fingerprint of terrestrial (rocky) planet formation.
"Chances are, if there's a ring of dust, there's a planet,"
says Kenyon.
** Good Planets Are Hard To Find **
Our solar system formed from a swirling disk of gas and dust,
called a protoplanetary disk, orbiting the young Sun. The same
materials are found throughout our galaxy, so the laws of physics
predict that other star systems will form planets in a similar
manner.
Although planets may be common, they are difficult to detect
because they are too faint and located too close to a much brighter
star. Therefore, astronomers seek planets by looking for indirect
evidence of their existence. In young planetary systems, that
evidence may be present in the disk itself, and in how the planet
affects the dusty disk from which it forms.
Large, Jupiter-sized planets possess strong gravity. That gravity
strongly affects the dusty disk. A single Jupiter can clear a
ring-shaped gap in the disk, warp the disk, or create concentrated
swaths of dust that leave a pattern in the disk like a wake from
a boat. The presence of a giant planet may explain the wake-like
pattern seen in the disk around the 350 million-year-old star
Vega.
Small, Earth-sized worlds, on the other hand, possess weaker
gravity. They affect the disk more weakly, leaving more subtle
signs of their presence. Rather than looking for warps or wakes,
Kenyon and Bromley recommend looking to see how bright the star
system is at infrared (IR) wavelengths of light. (Infrared light,
which we perceive as heat, is light with longer wavelengths and
less energy than visible light.)
Stars with dusty disks are brighter in the IR than stars without
disks. The more dust a star system holds, the brighter it is in
the IR. Kenyon and Bromley have shown that astronomers can use
IR brightnesses not only to detect a disk, but also to tell when
an Earth-sized planet is forming within that disk.
"We were the first to calculate the expected levels of
dust production and associated infrared excesses, and the first
to demonstrate that terrestrial planet formation produces observable
amounts of dust," says Bromley.
** Building Planets From The Ground Up **
The most prevalent theory of planet formation calls for building
planets "from the ground up." According to the coagulation
theory, small bits of rocky material in a protoplanetary disk
collide and stick together. Over thousands of years, small clumps
grow into larger and larger clumps, like building a snowman one
handful of snow at a time. Eventually, the rocky clumps grow so
large that they become full-fledged planets.
Kenyon and Bromley model the planet formation process using
a complex computer program. They "seed" a protoplanetary
disk with a billion planetesimals 0.6 miles (1 kilometer) in size,
all orbiting a central star, and step the system forward in time
to see how planets evolve from those basic ingredients.
"We made the simulation as realistic as we could and still
complete the calculations in a reasonable amount of time,"
says Bromley.
They found the planet formation process to be remarkably efficient.
Initially, collisions between planetesimals occur at low velocities,
so colliding objects tend to merge and grow. At a typical Earth-Sun
distance, it takes only about 1000 years for 1-kilometer objects
to grow into 100-kilometer (60-mile) objects. Another 10,000 years
produces 600-mile-diameter protoplanets, which grow over an additional
10,000 years to become 1200-mile-diameter protoplanets. Hence,
Moon-sized objects can form in as little as 20,000 years.
As planetesimals within the disk grow larger and more massive,
their gravity grows stronger. Once a few of the objects reach
a size of 600 miles, they begin "stirring up" the remaining
smaller objects. Gravity slingshots the smaller, asteroid-sized
chunks of rock to higher and higher speeds. They travel so fast
that when they collide, they don't merge -- they pulverize, smashing
each other apart violently. While the largest protoplanets continue
to grow, the rest of the rocky planetesimals grind each other
into dust.
"The dust forms right where the planet is forming, at the
same distance from its star," says Kenyon. As a result, the
temperature of the dust indicates where the planet is forming.
Dust in a Venus-like orbit will be hotter than dust in an Earth-like
orbit, giving a clue to the infant planet's distance from its
star.
The size of the largest objects in the disk determines the dust
production rate. The amount of dust peaks when 600-mile protoplanets
have formed.
"The Spitzer Space Telescope should be able to detect such
dust peaks," says Bromley.
Currently, Kenyon and Bromley's terrestrial planet formation
model covers only a fraction of the solar system, from the orbit
of Venus to a distance about halfway between Earth and Mars. In
the future, they plan to extend the model to encompass orbits
as close to the Sun as Mercury and as distant as Mars.
They also have modeled the formation of the Kuiper Belt -- a
region of small, icy and rocky objects beyond the orbit of Neptune.
The next logical step is to model the formation of gas giants
like Jupiter and Saturn.
"We're starting at the edges of the solar system and working
inward," Kenyon says with a grin. "We're also working
out way up in mass. The Earth is 1000 times more massive than
a Kuiper Belt object, and Jupiter is 1000 times more massive than
the Earth."
"Our ultimate goal is to model and understand the formation
of our entire solar system." Kenyon estimates that their
goal is attainable within a decade, as computer speed continues
to increase, enabling the simulation of an entire solar system.
This research was published in the February 20, 2004, issue
of The Astrophysical Journal Letters. Additional information and
animations are available online at: http://cfa-www.harvard.edu/~kenyon/
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center
for Astrophysics is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory.
CfA scientists, organized into six research divisions, study the
origin, evolution and ultimate fate of the universe.
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