So far, we went through every planet of our Solar System. If you can recall, after Mars, we went directly to Jupiter, skipping the Asteroid belt. Also, in the post about Neptune, Neptune was mentioned as the last planet of the Solar System. So below, we will go through the Asteroid belt and Asteroids in general. We will see why Pluto is not a planet, and which other objects are “nearby” (i.e., trans-Neptunian objects). Finally, we will see what comets are. 


Trans-Neptunian objects definition

All objects that classify either as dwarf planets or minor planets, and their orbits lie beyond Neptune, are classified as trans-Neptunian objects (TNO). This means, of course, that Pluto is not a planet. So let’s see why that is. 

The largest trans-Neptunian objects, with the Earth below for comparison. Note that Haumea is most likely the largest egg in the Solar System, and Sedna the only large enough object there that has no satellite.

#What is the definition of a planet?

First of all, we need to see what a planet is. As planet, is defined an astronomical body that orbits a star, or a stellar remnant. The object is massive to be rounded by its own gravity, and is not a satellite of another object, and has removed debris and small objects from the area around its orbit.

Pluto does not meet the third condition, since its mass is significantly lower than the combined mass of the other objects in its orbit. Also, in 2005, astronomers announced the discovery of another object beyond Pluto that has a higher mass. The object got the name Eris. So in 2006, it was decided that objects that meet the first two criteria of a planet, but not the third, will be called dwarf planets. Note that all dwarf planets and asteroids go under the category of minor planets. This leaves comets outside, which are a category of their own.

So let’s see what is included in the TNO zone and how it can be divided. This region of the outer Solar system is divided into three categories. These are:

#Kuiper belt – definition

The Kuiper belt is a circumstellar disk, located beyond the orbit of Neptune in the outer Solar System. It is made mainly of small bodies, and it also hosts three dwarf planets (Pluto, Haumea, and Makemake). The belt extends from a distance as low as 30 AU (i.e., one Astronomical Unit is the mean distance between the Earth and the Sun. This is equal to 149.6 million km), up to 50 AU from the Sun. It is similar to the Asteroid belt, but it’s both broader (20 times) and more massive (20-200 times).

#What is the scattered disc?

The TNOs that have perihelia (i.e., the minimum distance from the Sun) distances greater than 30 AU, belong to the scattered disk objects. These objects are also affected gravitationally by Neptune. Eris is the most massive one here.

#The detached objects, and the sednoids

The detached objects are a category of trans-Neptunian objects, which have orbits with a perihelion sufficiently distant from the gravitational effect of Neptune, or any other planet. Thus the only effect on them comes from the Sun. Finally, Sednoids (the name comes from the dwarf planet Sedna) are objects with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU. 

Unlike Asteroids, which are composed of rock and metal, these objects are mostly composed of “ices” such as methane, ammonia, and water. The origin of the belt is still not clear, but it is believed that these are fragments from the original proto-planetary disk around the Sun that failed to form into planets.

#Pluto facts and information

True color image of Pluto from the New Horizons probe. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

Pluto has a radius of 1,188 km and a mass of 1.303×1022 kg. The mean density of Pluto is 1.854 g/cm3, and all these correspond to a surface gravity of 0.063g (0.62 m/s2). Pluto orbits the Sun at a mean distance of 5.9 billion km (this 39.5 times the Earth-Sun distance). A Plutonian day lasts ~6.4 days, while one year there lasts 248 years. The axial of Pluto is 122.53° to its orbital plane. Pluto has an ultra-thin atmosphere that consists of nitrogen, methane, and carbon monoxide, while the mean temperature on this frozen world is -229°C (-380°F).

Pluto was discovered in 1930 by Clyde Tombaugh, and it is the second most massive object in the Kuiper Belt. Pluto has five known moons Charon, Styx, Nix, Kerberos, and Hydra. Due to the size of Charon, they are considered a binary system. This means that the barycenter of the orbits does not lie within any of them.


Eris is the most massive TNO (~ 1.66×1022 kg) and second in size (~ 1163 km). The mean density of Eris is 2.52 g/cm3. All these give a surface gravity is 0.083g (0.81 m/s2). The dwarf planet orbits the Sun at a mean distance of ~ 10.1 billion km (this is 67.8 times the Earth-Sun distance), and one year on Eris is equal to 559 years. The planet orbits slowly around its axis, with a day there lasting ~6.4 days, having an axial tilt of 122.53° to its orbital plane. The mean temperature is -231°C (-383°F), but so far we don’t have any information for the existence of an atmosphere. Eris has a natural satellite called Dysnomia.

Eris was discovered in January 2005 by Mike Brown and his team at Palomar observatory. In September 2006, it was named after Eris, the goddess of strife and discord. The discovery of Eris and its satellite was one of the reasons that Pluto fell from grace.

Asteroids facts and information

Asteroids are divided into three main categories. Those that are located in the Asteroid-belt, the Trojans, and the Near Earth Objects.


The Asteroid belt is located between the orbits of Mars and Jupiter. Note that the Trojans are on the same orbit with Jupiter. Those that that 60 behind Jupiter and called Trojans while those that are 60 degrees ahead are called Greeks.

#Asteroid belt facts

The asteroid belt is a torus-shaped region in the Solar System located between Jupiter and Mars. This region is occupied by many great irregularly shaped bodies that are called asteroids or minor planets. The region between the orbits of Mars and Jupiter is known as the main belt, thus distinguishing it from the Trojan asteroids and the Near-Earth Asteroids.

What is the composition of an asteroid?

In terms of composition, Asteroids classify into three main categories. These are:

How were asteroids formed?

The initial model for the Asteroids formation was that they are fragments from a planet that was present between Mars and Jupiter. The low mass of the whole belt though doesn’t favor the case. The most favorable formation model suggests that: Carbonaceous asteroids are most likely the remains from primordial material. Metal and silicate-rich asteroids are fragments of formerly larger differentiated bodies that through collisions were shattered into the smaller pieces observed today.

Names of Asteroids come from Ancient Greek mythology, but also from famous scientists and musicians.


#What are the trojan asteroids

A trojan is a small celestial body that shares its orbit with a larger one, that is on a stable orbit approximately 60° ahead or behind the main body. Thus trojans share orbits with planets or large satellites. Most of these celestial bodies share their orbit with Jupiter (~ one million). Trojans have also been found around other planets (Mars, Neptune, Uranus, even Earth). Also, trojans have been found around Tethys and Dione, the satellites of Saturn.

#What is a Near-Earth Object

As a near-Earth object (NEO) is classified, a celestial body that its orbit brings it in proximity with Earth. A celestial body in the Solar system, classifies as a NEO if its closest approach to the Sun is less than 1.3 AU. Objects with a diameter greater than 140 meters, they are considered as potentially hazardous objects since an impart event will cause devastating damage.


Interesting facts – Comets

A comet is a small (its radius can be from a few hundred meters up to tens of km) Solar System body. When it will approach the Sun, due to temperature increase, it will release gases. This phenomenon is known as out-gassing. This process creates a visible atmosphere known as coma, and occasionally a tail is formed. The Solar radiation and the Solar wind play their part in the formation of these features. So let see some comets fun facts:

Halley’s comet during its 1986 visit. The next one will be in 2061. Image Credit: NASA/W. Liller – NSSDC’s Photo Gallery


Comets are classified according to their orbital periods. It is believed that comets with short periods come from the Kuiper belt or the scattered disk, meaning that they are also Trans-Neptunian objects.

On the extreme of short-period, comets like Encke’s comet have an orbit that doesn’t take them further away than Jupiter. These comets have periods of less than 20 years, and the family is known as Encke-type comets or Jupiter family comets (JFC). As of 2020, 91 HTCs have been observed, compared with the 691 identified as JFCs.

Periodic comets or short-period comets are those with orbital periods less than 200 years. Their orbits lie beyond Jupiter’s orbit. A typical example of such a comet is the Halley’s Comet, which reaches its maximum distance from the Sun beyond Neptune. Thus, comets who are near this region are called Halley Type Comets (HTC).

For the long-period comets, it is believed that they originate in the Oort Cloud. The Oort Cloud is a spherical cloud of icy bodies that extends beyond the Kuiper belt and its associated structures, up to halfway to the nearest star. Passing by stars and the galactic tide may trigger the motion towards the Sun of these objects. These comets may have a period that can be as high as 70,000 years or more. Finally, some objects will only pass once through the Solar System before escaping to interstellar space. 

Comet West was discovered by Richard West on August 10, 1975 by using photographs. The comet reached its peak brightness in March 1976. During that time it was visible even at daytime. Despite the fact that it was referred as the comet of century, it did not attract enough attention in the media. The orbital period of the comet is around 558,000 years. Image Credit: J. Linder/ESO
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