Answer to Question #126607 in Astronomy | Astrophysics for Celiwe

Question

Gerard Kuiper belt proposed that certain comets come from vast, extremely distant, spherical shell of icy bodies surrounding the solar?
Is it True or false

Answer 1

False.

The Kuiper Belt was proposed by Kuiper in 1951 and consists of a disc-shaped region of minor planets outside the orbit of Neptune as a source of short-period comets. Short-period comets make complete orbits around the Sun in less than 200 years.

The region of minor planets beyond Neptune is referred to as 'Kuiper belt' since Kuiper had suggested that such small planets or comets may have formed there. However, he also believed that such objects would have been swept clear by planetary gravitational perturbations so that none or few would exist there today.

Beyond the gas giant Neptune lies a region of space filled with icy bodies. Known as the Kuiper Belt, this chilly expanse holds trillions of objects which are remnants of early solar system.

 Kuiper predicted existence of a belt of icy objects just beyond the orbit of Neptune. In 1950, astronomer Jan Oort theorized that a vast population of comets may exist in a huge cloud on distant edges of our solar system.

Most of the discovered comets arrived near the Sun from distant regions far beyond the major planets. They have very elongated trajectories which take them back to distant regions they came from. These comets are the long-period comets, with orbital periods larger than 30 Earth years. Long-period comets are observed in the inner part of solar system only once, arriving unpredicted. They come from far away, at the outer fringes of the solar system.

Most discovered comets have arrived near the Sun from distant regions far beyond the major planets. They have very elongated trajectories that take them back to the distant regions they came from. These comets are known as the long-period comets, with orbital periods larger than 30 Earth years. The long-period comets are observed in the inner part of the solar system just once, arriving unannounced and unpredicted. As you might expect, they come from very far away, at the outer fringes of the solar system.

Some 150 comets have appeared more than once during past two centuries. These are the short-period comets, which revolve around the Sun with orbital periods of less than 30 Earth years. The short-period comets are seen time and again, trapped in tight orbits within planetary realm which have low orbital inclinations near plane of Earth's orbit, with mean distances of a few times (in AU) that of the Earth from the Sun.

The Oort Cloud:

Size and orientation of the trajectories of long-period comets can be explained if they come from a remote, spherical shell belonging to outer parts of the solar system. This vast comet repository is known as Oort cloud, named after Dutch astronomer Jan Oort, who first postulated its existence. Since the long-period comets approach the Sun from enormous distances of 100,000 AU or more, the Oort cloud has a diameter of nearly twice this size. Average distance between the Earth and Sun is just 1 AU, about 150 billion meters and because long-period comets enter planetary realm at all possible angles, with every inclination to Earth's orbital plane, they must have come from a spherical shell. This also explains the fact that long-period comets move in all directions.

But, how do comets fall from Oort comet cloud to the heart of the solar system? The distant comets are only weakly bound to solar system and are easily perturbed by gravitational force of nearby moving objects, which throw some of the comets back into planetary system. The random gravitational jostling of individual stars passing nearby knocks the comets in the Oort cloud from their stable orbits, either injecting them into interstellar space or gradually deflecting their paths toward the Sun. Every one million years, a dozen stars pass close enough to stir up cometary objects, sending a steady trickle of comets into the inner solar system on very long elliptical orbits.

A giant interstellar molecular cloud can also impart a gravitational tug when it moves past the comet cloud, helping to jostle some of them out of their remote resting-place. Tidal forces generated in the cloud by the disk of our galaxy, the Milky Way, also aid in feeding new long-period comets into the planetary region. As time progresses, accumulated effects of these tugs send a few comets toward the Sun or outward to interstellar space. If the many hundred new comets observed during recorded history have been put forward into view by perturbing action of nearby molecular clouds, then there are at least one hundred billion (10¹¹) comets in the Oort cloud. There may be a trillion (10¹²) or even ten trillion (10¹³) of them. This huge population of unseen comets can sustain the visible long-period comets and persist without large-scale depletion for billions of years, until long after the Sun expands to consume Mercury and boil Earth's oceans away.

The Kuiper Belt:

The Oort cloud cannot easily explain the comets with shortest periods, the Jupiter-family comets with periods less than 20 Earth years. These comets have smaller orbits tilted slightly from orbital plane of Earth and they often move in same prograde direction as the planets. Unlike their longer-period cousins, motions of the Jupiter-family comets resemble those of the planets. Main source of these comets was thought to be a ring of small icy objects at outer edge of planetary realm, just beyond orbit of Neptune and a thousand times closer than the Oort cloud. It is called the Kuiper belt.

Density in this outer region of primeval planetary disk was so low that small objects did not coalesce into a single larger planet. Instead, they formed the flattened Kuiper belt of 100 million to 10 billion small frozen worlds which have remained there for billions of years.

Once a comet is launched into planetary realm, from either Kuiper belt or Oort belt, it may not stay on the same trajectory. Its orbit can be transformed if it passes near Jupiter, the most massive of all the planets. Jupiter's gravity can perturb the comet into a new elliptical orbit around the Sun.

In the 1950's, Jan Oort hypothesized a zone even further out in the solar system. This zone explained the long-period comets. These comets take longer than 200 years to orbit the Sun. This theoretical zone is called Oort Cloud. It is theoretical because even though there is evidence for its existence, no one has seen an Oort cloud object other than a few comets thought to have come from this zone. Oort cloud is a cloudy sphere of icy bodies around entire solar system. It extends between 5,000 and 100,000 AU. 1,00,000 AU is approximately 2 light years away from Earth. This means that it takes almost two full Earth years for light to reach this zone from the Sun.

In 1950, Dutch astronomer Jan Oort proposed that certain comets come from a vast, extremely distant, spherical shell of icy bodies surrounding the solar system. This giant swarm of objects is named the Oort Cloud, occupying space at a distance between 5,000 and 100,000 astronomical units. (One AU is mean distance of Earth from the Sun which is about 93 million miles). Outer extent of Oort Cloud is the 'edge' of our solar system, where Sun's physical and gravitational influence ends. Oort Cloud contains 0.1 to 2 trillion icy bodies in solar orbit. Giant molecular clouds, stars passing nearby or tidal interactions with Milky Way's disc disturb the orbit of one of these bodies in outer region of Oort Cloud, causing the object to streak into inner solar system as a long-period comet.

J. H. Oort proposed that all comets come from an enormous spherical cloud of objects extending halfway to nearest stars and only weakly bound by Sun's gravity. This Oort cloud is often perturbed gravitationally as it moves through galaxy. Such disturbances hurl comets from the cloud into inner solar system, where people can see them.

In 1951, Dutch-American astronomer Gerard Kuiper wrote a paper on the origin of solar system. He assumed that planets had condensed from solar nebula, a disc-shaped cloud of gas and dust in orbit around the Sun. When Kuiper estimated the original amounts of material in solar nebula at various distances from Sun, he found that distribution increased to a peak at the distance of giant planets Jupiter and Saturn & which declined beyond the orbit of Neptune. His model of solar nebula beyond Neptune had insufficient mass to form another large planet but a smooth distribution of leftover dust and gas extended to greater distances. Kuiper calculated that this material would have condensed to form billions of small icy bodies with the composition of comets. He also inferred that gravitational perturbations by planets would have thrown most of these bodies out to the distance of Oort cloud, where stellar perturbations would have altered their orbits once more, making them rounder and of random inclination. But not all the comets would have been ejected to Oort cloud. Kuiper assumed that beyond orbit of Pluto, 'remnants of circular comet ring are still present'. This donut-shaped region of comets beyond Neptune became known as Kuiper Belt.

The Kuiper Belt is oldest surviving remnant of the original solar nebula. It represents original source of far more distant Oort cloud comets. Kuiper Belt may also be the source of some of the short-period comets. It was found that gravity of Neptune can hurl enough KBOs into the inner solar system to account for the population of short-period comets moving around the Sun. Since most of the known KBOs have periods of only a few hundred years (unlike long-period comets), perturbation by Neptune can convert enough of them to account for observed short-period comets.