From: LARRY KLAES (ljk4_at_msn.com)
Date: Mon Oct 20 2003 - 17:11:50 PDT
----- Original Message -----
From: NASA Jet Propulsion Laboratory
Sent: Monday, October 20, 2003 3:14 PM
To: ljk4_at_msn.com
Subject: Spotlight: Missing Link Sought in Planetary Evolution
Spotlight Feature
October 20, 2003
Spotlight: Missing Link Sought in Planetary Evolution
Just as anthropologists sought "the missing link" between apes and
humans, astronomers are embarking on a quest for a missing link in
planetary evolution. Only instead of dusty fields and worn shovels,
their laboratory is the universe, and their tool of choice is NASA's
new Space Infrared Telescope Facility.
Launched on Aug.25, NASA's fourth and final Great Observatory will
soon set its high-tech infrared eyes on, among other celestial
objects, the dusty discs surrounding stars where planets are born.
While other ground- and space-based telescopes have spied these
swirling "circumstellar" discs, both young and old, they have missed
middle-aged discs for various reasons. The Space Infrared Telescope
Facility's unprecedented sensitivity and resolution will allow it to
fill in this gap and in the process answer fundamental questions
regarding how planets, including those resembling Earth, may form.
"With the Space Infrared Telescope Facility, we anticipate seeing many
planetary discs at all stages of development," says Dr. Karl
Stapelfeldt of JPL, a scientist with the mission. "By studying how
they change over time, we may be able to determine what conditions
favor planet formation."
Circumstellar discs are a natural step in the evolution of stars.
Stars begin life as dense cocoons of gas and dust, then as pressure
and gravity kick in, they begin to coalesce, and a flat ring of gas
and dust takes shape around them. As stars continue to age, they suck
material from this disc into their core. Eventually, a state of
equilibrium is reached, leaving a more mature star encircled by a
stable disc of debris.
It is around this time, about 10 million years into the lifetime of
the star, that astronomers believe planets arise. Dust particles in
the discs are thought to collide to form larger bodies, which
ultimately sweep out gaps in the discs, much like those lying between
the rings of Saturn.
"You can think of planets as wrecking balls that either clear away
debris or gather it up as if it were mud," says Dr. George Rieke,
principal investigator on one of the three science instruments onboard
the observatory.
Infrared telescopes can sense the glow of the cosmic dust that makes
up these discs; however, they cannot detect planets directly. Planets
have less surface area than their equivalent in dust grains and thus
give off less infrared light. This is the same reason coffee is ground
up before brewing: the larger combined surface area of the coffee
grains results in a more robust pot of coffee.
Past observations of circumstellar discs generally fall into two
categories: young, opaque discs (called protoplanetary discs) with
more than enough mass to match our own solar system's planetary
bodies; or older, transparent discs (called debris discs) with masses
equal to a few moons, and doughnut-like holes at their center.
Middle-aged discs linking these two developmental stages have gone
undetected.
One of the questions astronomers hope to address with the Space
Infrared Telescope Facility is: What happened to all the mass observed
in the younger discs? Somewhere in their evolution, mass is either
eaten up by the star, ejected by the star or transformed into planets
that lie in the doughnut holes of the discs. By analyzing the
composition and structure of the "missing link" discs, astronomers
hope to solve this riddle, and better understand how planetary systems
like our own evolved.
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