A comet is a small icy celestial body that orbits around the sun. It has a nucleus (solid, frozen ice), a gaseous coma (water vapor, CO2 and other gases) and a long tail.
The International Astronomical Union (IAU) defines a planet as a Sun-orbiting astronomical body that has undergone the rounding process to achieve hydrostatic equilibrium, an achievement that is known as sphericity. This allows for a wide range of size, from smaller rocky planets to gas giants like Jupiter.
Comets
Comets are icy celestial bodies that orbit around the sun in highly elliptical orbits. They can spend hundreds of years out in the depths of the solar system before they return to the Sun at their perihelion (closest approach to the Sun).
A comet is usually made up of ice, ultramicroscopic cosmic dust, and frozen gases that pass from solid to gaseous state (sublimation). When a comet approaches the Sun, these gases are heated up and expelled from the nucleus. The dust and gases form a cloud of material called a coma around the nucleus.
The coma can be as large as a planet, and it can contain water and other volatiles like carbon dioxide. When the coma is pushed away from the nucleus by the super-fast particles of the Sun’s solar wind, it forms a tail that extends far out beyond the nucleus.
This tail is the most spectacular feature of a comet, and it can be thousands of miles long. The tail is blown out from the coma by the solar wind, which can travel at speeds of over a million kilometers per hour.
Some comets have a second tail, too. This tail is generally bluish in color and points away from the sun because it’s blown back by the solar wind.
Comets are thought to form from collections of loosely bound, smaller bodies that have become gravitationally attracted to each other and coalesced into a single body. These are called “rubble piles.” The crowded halo of debris surrounding comet Hartley 2 (103P/Hartley) and 67P/Churyumov-Gerasimenko may indicate that such collections did form. However, some astronomers are skeptical of this theory.
Kuiper Belt
The Kuiper Belt is a vast ring of icy celestial bodies that orbit around the sun beyond Neptune’s orbit. It is named after the Dutch American astronomer Gerard Kuiper, who first postulated that such a region might exist.
The region is estimated to be containing hundreds of millions of small icy celestial bodies that are believed to have formed from the solar nebula about 4.6 billion years ago. The majority of this material merged to form the planets, while a small percentage was left in the form of these Kuiper belt objects.
Many astronomers believe that these objects are remnants from the earliest stages of the solar system. They may have formed from a nebula of dust and gas that was swept up by the planets during their formation.
Astronomers are attempting to learn more about how these bodies formed. They are also trying to understand how they have been changed by the planets over time.
KBOs are generally classified by their semimajor axis, perihelion distance and inclination to the orbital plane of the planets in the solar system. This classification can be used to determine the size and chemical makeup of these objects, which are difficult to observe from Earth because they are so distant and their light is dimmed by a factor of four (r-4).
These data are important for understanding how the KBOs have developed over time. They can help astronomers determine how the solar system formed and what kinds of objects it had in its early days.
Although there are several prevailing models for the evolution of the orbital distribution of KBOs, two common themes have emerged. One model is that Neptune gradually migrated into a larger mean orbital distance and this gradually trapped KBOs in orbital resonances with Neptune. This model does not produce the hot classical component seen in Figure 4, but it does explain the large range of orbital inclinations.
Oort Cloud
The Oort Cloud is a large collection of icy celestial bodies that orbit around the Sun. It is a hypothetical spherical distribution of comet-like objects extending from the Kuiper Belt to a distance of about 3000 to 100,000 AU (astronomical units, or Earth-Sun distances). The Oort Cloud probably contains 0.1 to 2 trillion icy bodies.
These bodies are scattered by the gravitational influence of other stars and giant molecular clouds. They are also affected by tidal interactions with the disc of the Milky Way. These factors cause some of these icy bodies to fall into the outer region of the Oort Cloud as so-called long-period comets.
Long-period comets are icy bodies that have spent thousands of years traveling around the Sun in random orbits. They are thought to have originated from a region of space beyond Neptune called the Kuiper Belt, which is a disc-shaped region with a density of about 100 times that of the solar system’s planets.
They are thought to have formed about 4.6 billion years ago when the solar system was still very young. The Oort Cloud is thought to have been created when some of the icy bodies in the scattered disk were booted out of the solar system through interactions with passing nearby stars, giant molecular clouds, or tidal forces.
The Oort Cloud is believed to have a half-life of about 10 Gyr, which means that it will gradually become smaller and less dense as time passes. However, close encounters with other stars are believed to be key in speeding up the erosion of the Oort Cloud.
Ganymede
Ganymede is the largest Jovian moon and the only known satellite to have its own magnetic field. It orbits around Jupiter at a distance of 665,000 miles (1.070 million kilometers), taking about seven Earth days to complete an orbit.
This icy celestial body has an almost negligible atmosphere and a surface pressure of about
Scientists believe that Ganymede has an ocean buried deep underground. The interior of the moon has a metallic iron core and a rocky crust. It also has a large shell of ice covering the rock.
Its surface is a mix of two types of terrain: highly cratered dark regions and younger, lighter regions marked by grooves. These features run for thousands of miles and are thought to have been formed more recently than the darker cratered regions.
The ridges and grooves on Ganymede may be the result of global tectonic processes that have affected the surface. They are a complex pattern with parallel ridges and troughs that have a vertical relief of a few hundred meters.
They were likely created by tidal heating and tectonic recycling. It is estimated that about 60% of the surface of Ganymede is made up of the lighter, grooved material.
A computer model of the inside of Ganymede created in 2014 suggests that this icy world might have a sea of liquid water in its rocky crust. This would mean that it has an environment very similar to our own oceans, where salt water interacts with the rocky mantle. The presence of these elements could help to foster life on Ganymede and allow it to evolve without needing sunlight to survive.
Callisto
Callisto is the outermost of the four Galilean satellites that orbit around Jupiter. It was discovered in 1610 by Italian astronomer Galileo and probably independently by German astronomer Simon Marius, and is named for the goddess Callisto from Greek mythology.
Its surface is heavily cratered, suggesting that it is older than most other planets in our solar system. Like Ganymede, it is composed of rock and water ice and scientists believe that it has a salty ocean beneath its crust.
The icy surface has been found to be heterogeneous at the small scale, with bright patches of pure water ice and patches of rock-ice mixtures, as well as extended dark areas made of a non-ice material. These areas are believed to have formed from the ejection of water ice and a thin layer of rock from Callisto’s core after it was impacted with a massive number of asteroids and comets.
There are also eight prominent crater chains on the surface that formed from the impact of comets that were tidally disrupted by Jupiter. One of these, called Gomul Catena, is
Another large impact feature on the surface is a multi-ring basin that extends from Valhalla to Asgard and consists of concentric rings surrounding a section of soft or liquid ice (possibly the ocean). Other notable impacts on the surface are a series of large troughs known as Tornasuk, which have scarps along their edges and craters centered on the palimpsest or central region.
The icy crust on Callisto is thought to be about 124 miles (200 km) thick and it has no magnetic field. However, it is likely that its magnetosphere is affected by Jupiter’s background magnetic field. This is because the varying flow of the magnetic field is related to how close Callisto is to Jupiter.