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07/13/2005
Expanding and Contracting Dust Clouds
In contrast to humanity’s evil side shown in the explosions in
London last week, the July 4th Deep Impact explosion on the
comet Tempel 1 demonstrates one of the finer human endeavors,
the exploration of our surroundings and the search for our roots.
Clearly, the formation of our solar system was crucial to
providing a home for our ancestors and us. How do solar
systems form and what is the stuff from which they are formed?
Tempel 1 is thought to be composed of the same stuff that
formed our solar system.
A July 8 joint JPL/NASA/University of Maryland press release
gives some data on the impact. (Michael A’Hearn of the U. of
Maryland is the Deep Impact Principal Investigator.) The 820-
pound Deep Impact probe slammed into the roughly 3- by 7-mile
potato-shaped comet at a 25 degree angle to its surface at a speed
of about 23,000 miles an hour. The result was a huge cloud of
powdery dust expanding into space at 3 miles a second. The
material in the cloud was like talcum powder, not like grains of
sand or pebbles. If the comet is composed of such powdery
material, how does it hang together in its travels through space?
One of the Deep Impact scientists, Pete Schultz of Brown
University, says you have to realize that the comet spends its
time undisturbed in the vacuum of deep space. When it gets near
the Sun, the Sun cooks off some of the material to form the fan-
shaped coma we typically see around a comet.
The fan is, however, mostly show with lots of light but little
substance, more like a fog. More substantial particles shed from
a comet don’t get fanned out but form a debris trail that follows
the comet in its orbit. This debris consists of small chunks of
material millimeters or centimeters in size, more like pebbles.
The debris trail particles are too small to be detected by optical
telescopes; however, there’s a telescope out in space that can see
these debris trails. It’s the Spitzer Space Telescope, which is
equipped to detect infrared radiation. The “pebbles” in the debris
trail pick up heat from the Sun and reradiate the heat in the form
of infrared radiation.
The infrared radiation picked up by Spitzer complements the
visible light sent back from the Hubble Space Telescope, X-ray
data from the Chandra X-ray Observatory and gamma-ray data
from the Compton Gamma-Ray Observatory. (These four space
telescopes form NASA’s Great Observatories Program, while
Spitzer is also part of NASA’s Astronomical Search for Origins
Program aimed at searching for our cosmic roots.) The long
wavelength infrared radiation can travel through dust and gas
clouds in space unhindered. Thus we can see things out in deep
space that can’t be seen in visible light because of intervening
dust clouds. Similarly, a report from the National Gallery in
London last week told how an infrared technique was used to
peer through one of Leonardo Da Vinci’s paintings to reveal a
heretofore-unknown sketch on the canvas under his painting.
Spitzer was launched in August of 2003 and has performed
brilliantly, finding much more important things than the debris
trails of comets. Spitzer really shines when it looks out in space
and sees stars with clouds of dust and gas spinning around them.
If you see a star surrounded by a disk of dust and gas, that’s the
recipe for forming a solar system. With luck, there might even
be planets already formed or in the process of forming. Here’s
where Spitzer’s infrared telescope has an advantage over the
Hubble’s visible light telescope. (NASA’s Spitzer Web site has
lots of information on the technical aspects of the mission.)
Looking at an incipient solar system in visible light, the star is
thousands of times brighter than the light reflected from the dust
or any planets. This makes it very hard to see a dimly lit planet,
so outshined by its star. Temperature-wise, however, the star is
likely to be only a few hundred times hotter than its planet(s).
Remember, it’s heat that is associated with infrared radiation. In
an article titled “How Nature Builds a Planet” in the July issue of
Discover magazine, Adam Frank of the University of Rochester
describes the excitement last summer when he and his colleagues
first analyzed Spitzer infrared data on a “baby” star named
Cohen-Kui Tau/4 (CKT/4). CKT/4, some 420 light-years from
Earth, has a dusty disk around it.
Let’s get a bit technical and see what they found. If CKT/4 did
not have a dusty disk but was a “naked” star, what would the
infrared data look like? Just as we can split our Sun’s visible
light into its spectrum of different wavelengths, the familiar
rainbow of colors, Spitzer obtains the spectrum of the infrared
radiation at different wavelengths. Each wavelength corresponds
to a temperature; shorter wavelengths come from hotter regions,
longer wavelengths from cooler regions. When you plot the
energy of the infrared radiation for the different wavelengths you
get smooth curve that has a hump or a smooth peak.
That’s for a naked star. What about a star with a dusty disk?
Well, the dust/gas in the disk will obviously be cooler than the
star. It will also be hottest near the center, closest to the star. So,
the disk should emit longer wavelength infrared radiation, the
shortest wavelengths coming from the regions of the disk closest
to the star. Let’s go back to last summer at the University of
Rochester, where researchers Dan Watson and Bill Forrest in
Adam Frank’s group had worked on the spectrograph and camera
for Spitzer.
Watson was peering over the shoulder of student Joel Green,
who was plotting CKT/4 data from Spitzer. There was the hump
from the star and the longer wavelength radiation from the disk.
But wait, something was missing – the expected radiation from
the hotter part of the disk near the center was not there. There
was joy in Rochester – Watson realized immediately they had
found a planet! When a planet forms, it “cleans out” the dust and
gas around it by either incorporating the dust as it grows or it
pushes the dust away and the dust eventually ends up falling into
the star. The planet has created an empty gap in the disk; hence,
no radiation.
You might greet the finding of another planet with a yawn. After
all, we’ve talked before about finding planets and there are now
at least 150 or so known planets outside our solar system. What
really excited the researchers was that CKT/4 was a young star,
only a million years old. They figure the planet circling CKT/4
formed a mere few hundred thousand years ago. The finding
made the theorists rethink their models for how solar systems
form. Some models predict solar systems can form rapidly; other
models predict very slowly. Which is correct?
Spitzer has now looked at many stars with dusty disks and the
conclusion is that things are really messy out there. They’ve
found young stars with no disks and really old stars with massive
disks. Some old stars known to have planets have disks with a
hundred times more dust than in our solar system. Asteroid belts
have been found, one 25 times denser than our own asteroid belt.
We still have dust in our solar system from comets breaking up
and asteroids banging into each other. This dust is making its
way into the Sun and you can see it as a cone of so-called
zodiacal light hanging in the west after sunset.
Frankly, I don’t see why anyone should be surprised that forming
solar systems is so messy. If you look at pictures from Hubble,
Spitzer and other telescopes of those dusty/gassy clouds where
all the new stars are being born or have been born, it’s clear that
there are dense clouds and sparse clouds. It seems to me that if
you have lots of dust and gas, you could form planets fast and
vice versa. In addition, with stars blowing up and shooting jets
of stuff into space and some stars and planets banging into each
other, you never know what sort of stuff might barge into the act.
With all that uncertainty, we’re certainly lucky our solar system
formed the way it did!
Allen F. Bortrum
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