The Oort cloud is an immense spherical cloud surrounding the planetary system and extending approximately 3 light years, about 30 trillion kilometers
from the Sun. This vast distance is considered the edge of the Sun's orb of physical, gravitational, or dynamical influence.
Within the cloud, comets are typically tens of millions of kilometers apart. They are weakly bound to the sun, and passing stars and other forces can
readily change their orbits, sending them into the inner solar system or out to interstellar space. This is especially true of comets on the outer
edges of the Oort cloud. The structure of the cloud is believed to consist of a relatively dense core that lies near the ecliptic plane and gradually
replenishes the outer boundaries, creating a steady state. One sixth of an estimated six trillion icy objects or comets are in the outer region with
the remainder in the relatively dense core.
In addition to stellar perturbations where another star's Oort cloud passes through or close to the Sun's Oort cloud, are the influences of giant
molecular clouds and tidal forces. A giant molecular-cloud is by far more massive than the Sun. It is an accumulation of cold hydrogen that is the
birthplace of stars and solar systems. These are infrequently encountered, about every 300-500 million years, but when they are encountered, they can
violently redistribute comets within the Oort cloud.
Tidal forces affecting the Oort cloud come from stars in the Milky Way's galactic disk with some pull from the galactic core. The tide results from
the sun and comets being different distances from these massive amounts of matter. The force on the comets from these tides is greater than the
perturbations of passing stars, and comets beyond 200,000 AU are easily lost to interstellar space. This pull contributes to the steady state which
replenishes the outer comets that are randomly distributed away from the ecliptic plane.
The total mass of comets in the Oort cloud is estimated to be 40 times that of Earth. This matter is believed to have originated at different
distances and therefore temperatures from the sun, which explains the compositional diversity observed in comets.
Typical noontime temperatures are four degrees Celsius above absolute zero. As temperatures move toward absolute zero, the kinetic energy of the
molecules approach a finite value. Absolute zero should not be considered a state of zero energy without motion. There still remains some molecular
energy, although it is at a minimum, at absolute zero.
The Oort cloud is the source of long-period comets and possibly higher-inclination intermediate comets that were pulled into shorter period orbits by
the planets, such as Halley and Swift-Tuttle. Comets can also shift their orbits due to jets of gas and dust that rocket from their icy surface as
they approach the sun. Although they get off course, comets do have initial orbits with widely different ranges, from 200 years to once every million
years or more. Comets entering the planetary region for the first time, come from an average distance of 44,000 astronomical units.
Long period comets can appear at any time and come from any direction. Bright comets can usually be seen every 5-10 years. Two recent Oort cloud
comets were Hyakutake and Hale-Bopp. Hyakutake was average in size, but came to 0.10 AU (15,000,000 km) from Earth, which made it appear especially
spectacular. Hale-Bopp, on the other hand, was an unusually large and dynamic comet, ten times that of Halley at comparable distances from the sun,
making it appear quite bright, even though it did not approach closer than 1.32 AU (197,000,000 km) to the Earth.
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In 1950, Dutch astronomer Jan Oort hypothesized that comets came from a vast shell of icy bodies about 50,000 times farther from the Sun than Earth
is. A year later astronomer Gerard Kuiper suggested that some comet-like debris from the formation of the solar system should also be just beyond
Neptune. In fact, he argued, it would be unusual not to find such a continuum of particles since this would imply the primordial solar system has a
This notion was reinforced by the realization that there is a separate population of comets, called the Jupiter family, that behave strikingly
different than those coming from the far reaches of the Oort cloud. Besides orbiting the Sun in less than 20 years (as opposed to 200 million years
for an Oort member), the comets are unique because their orbits lie near the plane of the Earth's orbit around the Sun. In addition, all these comets
go around the Sun in the same direction as the planets.
Kuiper's hypothesis was reinforced in the early 1980s when computer simulations of the solar system's formation predicted that a disk of debris
should naturally form around the edge of the solar system. According to this scenario, planets would have agglomerated quickly in the inner region of
the Sun's primordial circumstellar disk, and gravitationally swept up residual debris. However, beyond Neptune, the last of the gas giants, there
should be a debris-field of icy objects that never coalesced to form planets.
The Kuiper belt remained theory until the 1992 detection of a 150-mile wide body, called 1992QB1 at the distance of the suspected belt. Several
similar-sized objects were discovered quickly confirming the Kuiper belt was real. The planet Pluto, discovered in 1930, is considered the largest
member of this Kuiper belt region. Also, Neptune's satellites, Triton and Nereid, and Saturn's satellite, Phoebe are in unusual orbits and may be
captured Kuiper belt objects.
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Far beyond the orbit of Neptune, nearly halfway to the nearest stars, our solar system is surrounded by a vast spherical reservoir of comets known as
the Oort cloud. In the classical view, first proposed by the Dutch astronomer Jan Oort in 1950, these comets remain in the distant reservoir until a
passing star perturbs the cloud, diverting some of the comets toward the inner solar system. Once close to the Sun, the comets may careen through the
solar system for thousands of years until they are ejected into interstellar space or until they collide with another body such as a planet.
This scenario has largely been accepted for several decades now as the most likely explanation for the orbital habits of certain comets--except that a
passing nearby star is no longer seen as the primary perturber of the Oort cloud. Some recent developments suggest other explanations. The resulting
debate has implications not just for the celestial mechanics of comets, but for theories about the mass extinctions of species that shape life on
For those unfamiliar with the theories, a brief history lesson should provide some background for the present debate. As the story goes, every 26
million years or so the fossil record seems to record the extinction of an abnormally great number of species. The cycle can be traced back through
the past 250 million years and just happens to include the extinction event at the end of the Cretaceous period, 65 million years ago, that marked the
end of the dinosaurs. This finding alone is remarkable, but it became even more so when several groups of scientists independently proposed that the
cause of the extinctions was nothing less than a periodic bombardment of projectiles from space.
The intellectual climate was ripe for the idea: Only a few years earlier the discovery of a deposit of iridium in the rocks of the Cretaceous-Tertiary
boundary suggested to some scientists that an extraterrestrial impactor had wiped out the dinosaurs. Especially intriguing was the finding that the
record of terrestrial impacts seemed to have a 28- to 32-million-year cycle. The apparent convergence of the extinctions and the impacts on a near
30-million-year cycle kindled a cosmic question: What mechanism could drive a cycle of extinctions and impacts with such an enormously long period?
Among the more intriguing responses was a theory that linked the extinction cycle to another well-known cycle: the periodic oscillation of the solar
system, back and forth across the plane of the Milky Way galaxy, about once every 30 million to 35 million years. The extinctions appeared to be
occurring just when the solar system was crossing the densest part of the galactic disk. According to the proponents of the theory, the Oort cloud was
being greatly perturbed by something in the galactic midplane, and this caused a catastrophic rain of comets on the inner solar system (including the
earth) every 30 million years.
Some thought that the perturbing something was the gravitational effect of giant molecular clouds situated in the midplane of the galactic disk.
Others disagreed with this notion, arguing that the effects of the giant molecular clouds should be about as strong in the midplane as they are above
and below the plane at the galactic latitudes traversed by the solar system.
At just about the time this issue was being debated, several Oort-cloud experts proposed that the cumulative effects of the local matter in the plane
perpendicular to the galactic disk--the so-called disk tides--were far more significant than the intermittent gravitational effects of passing stars
or giant molecular clouds. This threw a small wrench into the oscillating-solar-system theory because it wasn't clear just how the disk tides would
modulate the flux of comets at different heights above or below the galactic midplane. Some scientists were unperturbed by the absence of a precise
understanding, however, taking it on faith that the strength of the disk tides would be sufficient to give the Oort cloud a good kick every 30 million
And so the matter stood until 1995, when John Matese and Patrick Whitman of the University of Southwestern Louisiana and their colleagues Mauri
Valtonen of Finland and Kimmo Innanen of Canada attempted to assess the quantitative effects of the disk tides. Their numerical models of Oort-cloud
dynamics suggested that as the solar system oscillates through the galactic plane, the disk tides modulate the comet flux from the Oort cloud by a
ratio of about 4 to 1, with the greatest effect in the midplane of the galaxy (Icarus 1995, 116:255). The results brought new life to the theory by
providing a mechanism for the 30-million-year galactic clock.
It was enough to convince some scientists that there might be something to the theory after all. Notable among these is Gene Shoemaker of the U.S.
Geological Survey, who at one time believed that the periodicity was a "statistical fluke." The work of Matese and his colleagues convinced him that
the "impact surges are real... and that [the comet flux is] controlled by the fluctuating galactic tidal forces." The Matese study, he said, "is a
landmark contribution in understanding the history of bombardment of the earth."
Recently, Matese and his colleague Daniel Whitmire have taken their studies of the Oort-perturbing effects of the galaxy a step further (The
Astrophysical Journal Letters, November 20, 1996). Their analysis of a selected group of comet orbits indicates that the entire galaxy, including the
distant matter at its central core, plays a role in jostling some comets free of the cloud. Unlike the disk tides, these distant-matter tides exert
their effects within the plane of the galactic disk. Whereas the disk tides might account for about two-thirds of all Oort-cloud comets that we
observe, the distant-matter tides may be responsible for nearly another one-third. (Perturbing effects of nearby stars and giant molecular clouds
account for a small remainder.) The distant-matter tides turn out to be significantly more important than anyone would have imagined, including the
authors. "We didn't expect to detect an effect from distant galactic matter at all," said Matese. "It was a completely serendipitous discovery."
Not everyone agrees that Matese and Whitmire have made an adequate case for the existence of distant-matter tides. Paul Weissman of the Jet Propulsion
Laboratory in Pasadena, California points out that "the effects of the [distant-matter] tides only show up when a small subset of the comet data are
used." Matese responds that the subset was selected so that only "high-quality classes of comets with well-determined orbits" would be included in
As for the role of the distant-matter tides in the 30-million-year cycle of Oort-cloud perturbations, Matese admits that it is too soon to say. "We
don't know whether it will increase or decrease the 4-to-1 modulation effect of the disk tides, but it probably won't be a large change."
Either way, Weissman doesn't think it will make a difference for the oscillating-solar-system theory of cosmic impacts and periodic extinctions. "If
you consider that [Oort-cloud] comets only account for 25 percent of the impacts on the earth at present, and the fact that the comet flux is
currently at the maximum point of the 4-to-1 modulation, then the [disk tide] is only modulating the frequency of one-quarter of all impacts." He
adds, "Going through the galactic plane is not like going over a speed bump. There is a broad distribution of matter, and the solar system oscillates
slowly through it. The modulation effect is gradual; it shouldn't produce a dramatic spike in the comet flux." Weissman believes that asteroids,
which account for most of the other 75 percent of the impact craters on the earth, play a greater role in the impact extinctions.
Matese counters that although Oort-cloud comets may only account for 25 percent of the terrestrial impacts, they are disproportionately the larger
impact craters: "Impactors that make craters greater than 100 kilometers in diameter are the ones that play the key role in the extinction events."
He notes that comet impacts appear to be potentially responsible for the four largest terrestrial-impact craters known: Chesapeake Bay on the East
Coast of the United States and Popagai in Siberia (both dated at 35 million years), Chicxulub on the Yucatán peninsula (65 million years old and
suspected of being produced by the impactor that may have killed the dinosaurs) and Manicouagan in Quebec (dated at 210 million years). In turn,
Weissman points out that "it is very difficult to determine what type of object caused a particular crater because the impactor is vaporized in the
impact. Others have claimed that Chicxulub was caused by an asteroid, not a comet."
The supporting or confuting evidence for these ideas should be in the rocks and perhaps in the orbital dynamics of distant comets. For the time being
it appears that the Oort comet cloud and the galaxy will continue to be entangled with the dinosaur extinctions and extraterrestrial impactors.
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