The Scientific Importance of Comets
Solar System Formation
Comets are important scientifically because
they represent the remnants from early in the era of the formation of
our Solar System, and may provide clues to the physical and chemical
conditions within the nebula out of which our Solar System formed.
Comets are potentially our only direct samples of this early epoch in
our history, and like the archaeologist who uses relics to piece
together the chronology and structure of an ancient civilization,
astronomers hope that by understanding the composition and physical
structure of comets that we can constrain the models which describe the
process of planet formation.
Our Solar System was formed approximately
4.55 billion years ago out of a molecular cloud - a large concentration
of gas (roughly 75% H and 21-24% He with trace amounts of other
molecules) and dust grains. In addition to hydrogen and helium, the
interstellar medium (ISM) gas consists of a rich array of organic and
inorganic molecules, some of which are include: H2O, CO, NH3, CN, CO2, OH, HCN, CH3OH, H2CO, CH3C2H,
HNCO, CH3CN etc.) The dust, which comprises
roughly 1% of the ISM is made of refractory cores (silicates and
carbon) coated with organics and ice. Molecular clouds are large, with
diameters between 50-200 light years, and extremely cold
temperatures (10-50K). Although dense compared to the surrounding ISM,
their densities (10^4 to 10^5 particles per cubic cm) are about 100
million times lower than the best vacuums we can create in the
laboratory. The nebula in the image below left, of Barnard 86, is what
one of these dark molecular clouds might look like as the star
formation process is just beginning.
The process of star and planet formation
begins with the collapse of the molecular cloud, which can be triggered
by factors such as a nearby supernova explosion, or collision with
another cloud. As the cloud begins to collapse, material gets drawn at
an ever-increasing rate toward the center of the cloud, where it begins
to heat up. At the same time, the decreasing size of the cloud cause
it to spin faster (due to the conservation of angular momentum). Near
the center of the cloud, where the density is the greatest, collisions
between dust and gas remove energy until the particles no longer
collide. This causes a general flattening of the system and causes the
particles to orbit the center in the same direction. This flattening
occurs first near the center where the densities are the highest.
During the collapse, the gravitational energy is converted to heat. At
first the heat can easily escape, but as the density increases in the
center, the heat cannot easily radiate away, and the temperature starts
to rise. Temperatures will eventually rise high enough to break apart
the molecules and vaporize of the dust in the central regions. As more
and more material settles to the core and midplane of the young nebula,
enough material will eventually accumulate to shield the outer regions
from the growing temperatures in the central region. At this point, as
the gases cool, some of the vaporized materials will re-condense into
micron-sized grains. Closest to the central regions, to the young
protostar, only high temperature refractory materials can
condense, but farther from the star volatiles can also
condense. Since the volatiles are the most abundant species in the
original ISM, they will overwhelm the refractory material farther out
in the nebula. This provides a natural explanation for the difference
in composition between the terrestrial planets (rocky) close to the
sun, and the Giant planets (volatile-rich) farther out.
The condensing material will slowly start
to clump together. Eventually, some clumps grow larger than others,
becoming planetesimals, and these begin to sweep up all the other
debris along their orbits about the central protostar. As the
planetesimals get larger, the velocities get higher and the collisions
get more violent. In the vicinity of the giant planets, some of the
volatile-rich planetesimals get thrown into the inner solar system,
bringing volatiles to the inner planets, and some got thrown out into a
vast spherical swarm of small bodies surrounding the planets. Once the
center of the nebula becomes hot enough that thermo-nuclear reactions
can begin, the collapse of the nebula stops, and any remaining material
(gas and dust) is blown out of the system, and the era of planet
formation has ended. The entire process occurs relatively quickly,
taking only a few million years. The center picture above, of the
Eagle Nebula (M16) shows a star forming region 2 million years old.
The hydrogen gas is emitting light from atoms excited by newly formed
hot stars. In a Hubble Space Telescope close up of the same region (at
right), we get a high resolution view of the small dense regions which
are embryonic stars in the process of contraction. The icy
planetesimals which are thrown to vast distances (near 50,000 AU - the
Oort Cloud) were then stored unaltered from this early epoch in the
history of the Solar System, and are what we call comets today.
Subtleties - Comets as Cosmic Thermometers
The early models of the Solar System
formation assumed that the comets (e.g. the icy planetesimals)
formed in the vicinity of the giant planets, and that they may have
contained some pristine unaltered material from the ISM. After being
thrown into the Oort cloud, perturbations by nearby passing stars could
alter their orbits and send them toward the inner Solar System where
they would be observed as a long-period comet or captured into a
low-inclination short-period cometary orbit after a close
passage to Jupiter. It was believed that comets could not form too far
out in the Solar System because the density of material would be
insufficient for planetesimal growth. However, this scenario has
changed significantly in the past few years:
- New technology, particularly in the infrared wavelengths, has
permitted a detailed study of the processes of star formation.
Observations of young stellar objects show that 20-50% of these objects
have extended disks out to a few hundred AU associated with them. This
suggests that the disks associated with the star / planet formation
process are common and that they are much more massive that previously
believed.
- New models of the collapse of the Solar Nebula are suggesting that
because of these larger disks, comets may form much farther out in the
nebula. Likewise, it may not be likely that totally pristine,
unaltered interstellar grains survive the infall process. As they
settle through the nebula to the mid-plane, frictional heating may
vaporize the icy coating, which can later recondense on the cold grain
cores at the midplane.
- Laboratory experimentation has shown that when ice condenses below
temperatures of 100K, it does not develop the regular crystal
structure, rather adopts an amorphous irregular structure (because
there is not enough energy for the ordered crystal structure). In the
amorphous phase, the water-ice can trap a large number of other molecules
in the voids. The lower the temperature of condensation, the more gas
can be trapped. This is shown in the figure above (Owen et al., 1995),
where it can be see that teh difference in the amount of trapped gases
varies by 6 orders of magnitude between 20 and 100K. As the ice is
heated, the trapped gases are released as the voids close up, and the
release of gas may be observable as activity on the comet.
- The recent discovery of objects in the Kuiper Belt, a region
between 35-50 AU (beyond the orbit of Pluto) with inclinations ranging
between 0-30 degrees, has prompted a detailed investigation of the
dynamics of small bodies in the outer Solar System. Dynamical models
show that the short period comets, which unlike the long period comets
have a flattened inclination distribution, cannot have a source in the
Oort cloud, but most likely come from the vicinity of the Kuiper Belt.
Since the amount of gas trapping and release is
a very sensitive function of temperature, and since we now believe that
comets can form at a wide range of distances from perhaps the Uranus-Neptune
zone (20-30AU) out to perhaps 50-100 AU (the Kuiper belt), observations of
the activity of comets at various distances from the sun will serve as a
very sensitive thermometer to probe the conditions and distances at which
the planetesimals formed.
Known Extra-Solar Planets
Within the past few years, we are not only
developing a new understanding of the process of planet formation from
both new observational techniques and theories, but we have recently
detected direct evidence of extra solar planetary systems. The first
extra-solar planets detected were objects around pulsars the
ultra-dense remnants of a supernova explosion (see the first 3 listings
in the table below; ref. xxx). However, during the past year,
astronomers have finally discovered planets orbiting around solar-type
stars (i.e. star systems which would be capable of supporting
life). Many of the characteristics of the planets (shown in the table
and figure below, from Beichman, 1996; Marcy & Butler, 1996) are
challening our ideas of planet formation. The combination of new
technology and high- quality observations with new theories is
converging on a new more sophisticated understanding of the process of
planet formation and evolution, and it is believed that the detailed
study of the most primitive samples we have from the era of our Solar
System's formation, the comets, will contribute greatly towards our
understanding of these processes.
Star System Name | Temp [K] | Mass [MJup] | a [AU] | Period [yr] | Dist [pc] | Spectrum |
PSR1257+12 | 20,000 | 0.015 | 0.19 | 25.34dy | 491 | Pulsar |
- | - | 3.4 | 0.36 | 66.54dy | - | - |
- | - | 2.8 | 0.47 | 98.22dy | - | - |
PSR1828-11 | 20,000 | 3 | 0.93 | 0.68 | - | - |
- | - | 12 | 1.32 | 1.35 | - | - |
- | - | 8 | 2.1 | 2.71 | - | - |
PSR1620-20 | 20,000 | 10? | - | 100? | - | - |
Beta Pic | 7,000 | 1-10 | 2.5-8 | 2-19 | 16.4 | - |
51 Peg | 5,770 | 0.46 | 0.005 | 4.2 dy | 15.3 | G2-3V |
55 Cancri | 5,570 | 0.8 | 0.11 | 14.8dy | 13.0 | - |
- | - | 5 | >5 | 1-20 | - | - |
Lalande 21185 | 3,580 | 0.9 | 2.3 | 5.8 | 2.5 | M2 |
- | - | >1 | 7 | 30.1 | - | - |
Ups And | 6,200 | 0.6 | 0.054 | 0.126 | 16.6 | F8V |
47 UMa | 5,880 | 2.39 | 2.1 | 2.98 | 14.1 | G0V |
Tau Boo | 6,300 | 3.87 | 0.046 | 3.3dy | 18.4 | F7V |
70 Vir | 5,490 | 6.6 | 0.43 | 116.7dy | 18.1 | G4V |
HD 114762 | 6,100 | 10 | 0.41 | 84dy | - | F9V |
16 Cyg B | 5,800 | 1.5 | 1.68 | 2.2 | 26.1 | G2.5V |
Gleiss 229 | 3,720 | >20 | >44 | - | - | >td>-- |
Figure Credit: NASA ExNPS report, updated by the San Francisco
State University Astronomy Department.
References
Beichman, C. A. (1996). A Road Map for the Exploration of
Neighboring Planetary Systems, JPL Publication 96-22.
Marcy, G. W., and R. P. Butler (1996). "Planets Around Solar Type
Stars", Bull. Amer. Ast. Soc., 28, 1056.
Owen, T., D. Cruikshank, C. de Bergh and T. Begalle (1995). "Dark
Matter in the Outer Solar System", Adv. in Space Res. 16,
41.
Willman, A. (1997). http://www.princeton.edu/~willman/planetary_systems/