Theories Attempting to Explain the Origin of Comets
Seven modern theories have been proposed to explain the origin of comets. Each theory will be described as an advocate would. Later, we will test each theory with the characteristics of comets, listed above, that require an explanation.
Hydroplate Theory. Comets are literally out of this world. As the flood began, the extreme pressure in the interconnected subterranean chambers and the power of supercritical water exploding into the vacuum of space launched material that later merged to become about 50,000 comets, containing less than 1% of the water in the chambers. (These estimates will be derived later.) The chapter on “The Origin of Earth’s Radioactivity,” beginning on page 391, will explain the source of most of this launch energy and the great abundance of heavy hydrogen in comets.
As subterranean water escaped, the chambers’ pillars were crushed. Also, the 60-mile-high walls along the rupture were unstable, because granitic rock is not strong enough to support a cliff greater than 5 miles high.82 The bottom portions of the walls were crushed into large blocks which were swept up and launched by the fountains of the great deep. Carried up with the water were eroded dirt particles, minerals that form only in scalding-hot, high-pressure, liquid water, pulverized organic matter (especially cellulose from preflood forests), and even bacteria.
As explained in “Rocket Science” on pages 598– 599, droplets in this muddy mixture froze quickly in outer space. The expanding spheres of influence of the larger rocks captured more and more smaller rocks and ice, which later merged gravitationally to form comets. Five days later, comets and rocks began hitting the Moon and formed large basins. [See pages 603– 609.] Those impacts produced lava flows and debris, which then caused secondary impacts. Water vapor condensed in the permanently shadowed, very cold craters near the poles of Mercury and the Moon.
Hyperbolic comets never returned to the solar system. Near-parabolic comets now being detected are returning to the inner solar system for the first time. Comets with slower velocities received most of their orbital velocity from Earth’s orbital motion around the Sun. They are short-period comets with elliptical, prograde orbits lying near the Earth’s orbital plane. Since the flood, many short-period comets have been pulled gravitationally into Jupiter’s family. Small comets are composed of material that escaped Earth with the least velocity. [Pages 118– 157 give a more detailed description of the hydroplate theory.]
Exploded Planet Theory.83 Consistent with Bode’s “law,” 84 a tenth planet once existed 2.8 AU from the Sun, between the orbits of Mars and Jupiter. It exploded about 3,200,000 years ago, spewing out comets and asteroids. Many fragments collided with other planets and moons, explaining why some planets and moons are cratered primarily on one side. The fragments visible today are those that avoided the disturbing influence of planets: those launched on nearly circular orbits (asteroids) and those launched on elongated ellipses (comets). This theory also explains the origin of asteroids and some similarities between comets and asteroids.
Volcanic Eruption Theory.85 The large number of short-period comets, as compared with intermediate-period comets, requires their recent formation near the center of the solar system. Volcanic eruptions, probably from the giant planets (Jupiter, Saturn, Uranus, and Neptune) or their moons, periodically launch comets. Jupiter’s large, recently-acquired family suggests that Jupiter was the most recent planet to erupt. The giant planets are huge reservoirs of hydrogen, a major constituent of comets. New eruptions replenish comets that are rapidly lost through collisions with planets or moons, evaporation when passing near the Sun, and ejection from the solar system.
Oort Cloud Theory.86 As the solar system formed 4.5-billion years ago, a cloud of about 1012 comets also formed approximately 50,000 AU from the Sun87—more than a thousand times farther away than Pluto and about one-fifth the distance to the nearest star. Stars passing near the solar system perturbed parts of this Oort cloud, sending randomly oriented comets on trajectories that pass near the Sun. This is why calculations show so many long-period comets falling into the inner solar system from about 50,000 AU away. As a comet enters the planetary region (0 – 30 AU from the Sun), the gravity of planets, especially Jupiter, either adds energy to or removes energy from the comet. If energy is added, the comet is usually thrown from the solar system on a hyperbolic orbit. If energy is removed, the comet’s orbital period is shortened. With so many comets in the initial cloud (1012), some survived many passes through the inner solar system and are now short-period comets.
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Explained by theory. |
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Theory has moderate problems with this item. |
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Theory has serious problems with this item. |
| Numbers in this table refer to amplifying explanations on pages 326–337. |
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Revised Oort Cloud Theory.88 As the solar system began 4.5-billion years ago, all comets formed in a comet nursery near or just beyond the outer giant planets. Because these comets were relatively near the Sun, passing stars and the massive galactic clouds (molecular clouds) could not eject them from the solar system. As with planets, these early comets all had prograde orbits near the plane of the ecliptic. Perturbations by the giant planets gave some comets short periods with prograde orbits near the ecliptic plane. Other perturbations ejected other comets out to form and resupply an Oort cloud, 50,000 AU from the Sun. Over millions of years, passing stars have circularized these latter orbits. Then, other passing stars perturbed some Oort cloud comets back into the planetary region, as described by the original Oort cloud theory. Therefore, large numbers of near-parabolic comets are still available to fall into the inner solar system from about 50,000 AU away. An unreasonably large number of comets did not have to begin in the Oort cloud 4.5-billion years ago (where, after a few billion years, passing stars, galactic clouds, and the galaxy itself would easily strip them from the cloud). Short-period comets cannot come from the Oort cloud.
Meteor Stream Theory.89 When particles orbiting the Sun collide, they exchange some energy and momentum. If the particles are sufficiently absorbent (squishy), their orbits become more similar.90 After millions of years, these particles form meteor streams. Water vapor condenses on the particles in the meteor streams as they pass through the cold, outer solar system. Thus, icy comets form continually. This is why so many meteor streams have cometlike orbits, and why more short-period comets exist than an Oort cloud could provide.
Interstellar Capture Theory.91 Comets form when the Sun occasionally passes through interstellar gas and dust clouds. As seen from the Sun, gas and dust stream past the Sun. The Sun’s gravity deflects and focuses these particles around and behind the Sun. There, they collide with each other, lose velocity, enter orbits around the Sun, and merge into distinct swarms of particles held together by their mutual gravity. These swarms become comets with long and short periods, depending on how far the collisions were from the Sun.