Oort Cloud & Sol b?

In 1950, Jan Hendrik Oort (1900-1992) inferred the existence of the Oort Cloud from the physical evidence of long-period comets entering the planetary system. These comets are observed to come into the Solar System from all directions, which implies an immense spherical cloud of trillions of small icy, planetary objects -- all potentially active but currently dormant comets -- that extend as much as two light-years outward from Sol. In contrast, the Edgeworth-Kuiper Belt is roughly 100 times closer to Earth than this hypothesized Oort Cloud. Estimates of the total mass of this Oort Cloud range from about 40 times that of Earth to greater than that of Jupiter.

Southwest Research Institute


Larger illustration.


Sandblasted by interstellar dust gains and
irradiated over eons, long-period comets from
the Oort Cloud are not exactly pristine relics
from the birth of the Solar System (more).

Orbit diagrams of many comets in relation
to planetary orbits are available at the
Comet Lists of the Minor Planet Center.

Oddly enough, Oort Cloud objects were probably formed in a region of the protoplanetary disk that was located closer to the Sun than the Edgeworth-Kuiper Belt objects that persist in the orbital plane of the planets (ecliptic) to this day. During the first 100 million years of the System's birth, small planetary bodies that formed near the developing giant planets would have been ejected from their neighorhood through gravitational encounters (Levison et al, 1999; and Dones et al, 1998). Those that did not escape completely from our Sun's gravitational pull eventually became part of the distant Oort Cloud, which became a vast repository of icy bodies tossed out of the early Solar System.

Dormant Comets

NASA, JPL, Caltech, Deep Space 1 -- larger image


Comet Borrelly's 8-km (5-mile) long nucleus (more).


Although Borrelly is a short-period comet, it may
be composed mostly of frozen gases and dust
grains like Oort Cloud comets.

The members of the Oort Cloud are fragile and irregularly-shaped bodies of primordial material made of a mixture of non-volatile dust grains and frozen gases maintained at a typical temperature of about four degrees Celsius above absolute zero. Most lie beyond our Solar System's plane of planetary orbits, between 10,000 to 20,000 AUs outward from the Sun. However, many travel in highly elliptical orbits that bring them very close to the Sun as well as deep into space beyond the orbit of Pluto.

Within the Cloud, these dormant 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 planetary plane (ecliptic) and gradually replenishes the outer boundaries, creating a steady state. One sixth of an estimated six trillion icy objects or comets are believed to lie in the outer region, with the remainder in the relatively dense core.

The Oort Cloud is affected by stellar perturbations where another star's Oort Cloud passes through or close by. In addition, there are the influences of giant molecular clouds of cold hydrogen and dust massing many suns and galactic tidal forces. Our Sun, Sol, encounters molecular clouds about every 300 to 500 million years, which may violently redistribute comets within the Oort cloud. In addition, tidal forces affecting the Oort Cloud come from the differential gravitational forces exerted by stars in the Milky Way's galactic disk and by the galactic core on the Sun and comets as a result of their relative location in the Solar System. Tidal forces have a greater impact on comets than the perturbations of passing stars, and so comets located beyond 200,000 AUs are easily lost to interstellar space. However, tidal forces also replenishes the outer Oort Cloud by pulling out inner comets.

The Oort Cloud is the source of long-period comets and possibly higher-inclination intermediate comets, such as Halley and Swift-Tuttle, that were pulled into shorter period orbits by the planets. 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 region of the planets for the first time, come from an average distance of about 44,000 times the Earth-Sun distance ("astronomical unit" or AU).

Active Comets

NASA





Another false-color image
of Halley's Comet





Long-period, Oort-Cloud
comets may have formed
even closer to the Sun
than Edgeworth-Kuiper Belt
comets like Halley's.

Comets are diverse and very dynamic, but all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma, but the coma and the nucleus together constitute the "head" of the comet. As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun.

Science@NASA


More on Halley's orbit.



More on the source of the October
Orionid and eta Aquarid meteors.

When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as "dirty" icebergs or snowballs, since over half of their material is ice. When a comet approaches within a few AUs of the Sun, the surface of the nucleus begins to warm, and volatile gas ices evaporate. The evaporated gases boil off and carry dust particles with them, forming the comet's coma. When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about five AUs from the Sun, fluorescence usually becomes more intense than reflected light.

Giotto, ESA





Larger image.





Comet Halley's nucleus
at a distance of 0.89 AU
from Sol (more on the
ESA's Giotto Mission
and Comet Halley).

As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a gaseous envelope. This envelope cannot be seen from Earth because its light is absorbed by Earth's atmosphere, but it has been detected by spacecraft.

Lowell Observatory,
NOAO, AURA, NSF



Larger false-color image.



Comet Halley's tail, with colors indicating
varying levels of brightness and a type-I,
ion tail visible below a type-II, dust tail
(more on 1910 photo from NOAO).

The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun. Thus, comets should have two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles.

More images and information on comets are available at the Comet Page. Try the Orbit Viewer, originally written by Osamu Ajiki of AstroArts and modified by Ron Baalke of NASA's Jet Propulsion Laboratory, to see real-time orbit animations of the known comets, Edgeworth-Kuiper ice bodies, asteroids, and planets.

In 1999, U.K. and U.S. astronomers independently reported finding evidence that one or more large planets or brown dwarfs gravitationally bound to our Sun, Sol may be perturbing the orbits of two different groups of long-period comets at the outer reaches of the Oort Cloud into the inner Solar System with the assistance of galactic tidal forces. The U.S. team (led by John J. Matese) estimated that the substellar object may have a mass around three to five Jupiter-masses and was recently orbiting Sol at around 25,000 AUs in a wide band running through Constellation Cassiopeia and the the North Star, Polaris, while calculations by John B. Murray of the U.K. focus on a smaller region centered around Constellation Delphinus at an estimated distance of 32,000 AUs. Some astronomers believe that Matese and Murray are being misled by random statistical fluctuations or the past gravitational effects of passing stars. However, confirmation through direct observation by the upcoming, new generation of infrared telescopes such as the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Space Infrared Telescope Facility (SIRTF) may be feasible if the positions of the hypothesized objects can be adequately constrained.