What image of the Solar System have people? Probably something like the Sun with the eight planets, their moons and the asteroid belt. But does this image correspond with reality? And what is in the asteroid belt? If Pluto is not a planet then what is it? What is beyond Pluto? What else is in the Solar System? These questions will be answered next.


Let’s start with Pluto. It was discovered in 1930 and it was considered a planet for some time. However, after 76 years the International Astronomical Union (IAU) moved Pluto to the rank of dwarf planets. But Pluto’s adventure is not over. In 1978 its main satellite Charon was discovered and it was considered the satellite of Pluto. And now it is believed that it is a double planet as the joint center of gravity is located outside of the main planet. I think it will be more understandable after the next image.


Pluto-Charon system in actual proportions





How such a double planet system could be formed? There are two most common hypotheses. Some scientists believe that the planets were formed in the Kuiper belt and were taken from there by the gravity of the outer planets. Another hypothesis assumes that the system was formed after the collision of the proto-Pluto with the proto-Charon. From the created fragments the current satellites were formed.


The formation of Charon. Perhaps, in the early Solar System the proto-Charon collided with proto-Pluto creating fragments. It is believed that these fragments formed four small satellites of Pluto

Pluto has several more satellites: Styx, Nix, Kerberos and Hydra. Theoretical calculations show that Pluto may have even more satellites.

Dwarf Planets

If Pluto is a dwarf planet, are there any other such planets in the Solar System? Yes, there are: the International Astronomical Union officially recognized five dwarf planets. They are: the largest known asteroid Ceres, trans-Neptunian objects of Makemake, Pluto, Haumea and Eris.


Dwarf planets

We have already discussed Pluto so let’s continue with Eris which comes next in size. Until 2006 Eris was a possible candidate for the tenth planet. However, on the 24 August 2006 the definition of a classic planet was approved which Eris did pass and so it gained the status of a dwarf planet. The planet has a satellite — Dysnomia.

The third-largest dwarf planet is Makemake and it is a trans-Neptunian object. Not a single satellite was detected on the orbit of the planet and that distinguishes from other large trans-Neptunian objects which have at least one.

Haumea is the fourth largest dwarf planet of the Solar System. Like Pluto and Makemake it is a trans-Neptunian object. It has a highly elongated form. The planet has two satellites — Namaka and Hiʻiaka. On the surface of the latter it is presumed to be a large amount of pure water ice what is rare for the objects of the Kuiper belt.

Ceres is the smallest of the dwarf planets but also the most interesting. It is the closest to the Earth dwarf planet and is not located in the Kuper belt but in the asteroid belt. For a time it was considered to be regarded as a planet of the Solar System and even in 1802, when it was classified as an asteroid, it was still considered as a planet for several decades. On that same meeting of August 24, 2006 IAU awarded it the dwarf planet status. Ceres has no satellites though there are asteroids with satellites.

Using the “Herschel” telescope, water vapor was found around Ceres in 2014. And it became the fourth object in the Solar System that has a proven water activity. Before, this activity was detected only on the Earth, Europe (Jupiter’s moon) and Enceladus (Saturn’s moon).

Sedna was another candidate for the status of a dwarf planet. Even though the IAU did not reward Sedna this status, a lot of scientists consider it a dwarf planet. It is larger than Ceres in size. Sedna is a trans-Neptunian object which is one of the most distant known objects of the Solar System with the exception for the long-period comets.


An artistic NASA image of Sedna. Its great distance from the Sun and the red color of the surface are correctly displayed


The orbit of Sedna (red) compared to the orbits of: Jupiter (orange), Saturn (yellow), Uranus (green), Neptune (blue) and Pluto (purple)

Why are these planets called “dwarf planets”? The main difference is that the dwarf planet cannot clear its orbit of other bodies using its gravity.

It is assumed that another forty of other known objects of the Solar System belong to this category. According to the scientists’ estimates, about two hundred dwarf planets can be found in the Kuiper belt and up to two thousand dwarf planets beyond it.

Trans-Neptunian Objects

“Trans-Neptunian objects” have already been mentioned, but what is it? These are cosmic bodies which orbit around the Sun and their average distance to the Sun is larger than that of Neptune.

Trans-Neptunian objects form the Kuiper belt, the scattered disc and the Oort cloud. We will talk about them a bit later.

Pluto is considered the first object of this kind which was discovered in 1930. 1992 QB1 was the second trans-Neptunian object which was discovered only after sixty years. By July 2014 about 1500 trans-Neptunian objects became known.

We have already mentioned the largest trans-Neptunian objects. They are also dwarf planets: Pluto, Eris, Makemake and Haumea. Other trans-Neptunian objects are candidates for the dwarf planet status: 2007 OR10, Charon (Pluto’s moon), Quaoar, Sedna, 2002 MS4, Orcus and about twenty more celestial bodies.


The largest trans-Neptunian objects

The Asteroid Belt

And where is the asteroid belt? It is located between the orbits of Mars and Jupiter and there is a large number of objects of different sizes. These objects are mostly irregular in shape and are asteroids or minor planets.

The asteroid belt is also called the “main belt” to emphasize its difference from other similar areas of accumulation of celestial objects such as the Kuiper belt, the scattered disc and the Oort cloud.


The asteroid belt

The mass of all asteroids in the belt is about four percent of the mass of the Moon. Noteworthy most of the belts’ mass is concentrated in the four largest objects. Ceres is the largest and the most massive body in the asteroid belt which exceeds many of the major satellites of outer planets and contains almost a third (32 %) of the total weight of the belt. Together with the three largest asteroids: Vesta (9 %), Pallas (7 %) and Hygiea (3 %) the weight of these four asteroids comes to 51 % and that means that the vast majority of asteroids have a tiny, in astronomical terms, mass.

The diameter of most asteroids (which are about several millions) is only a couple of tens of meters. These asteroids are so scattered in this area of outer space that at this moment no spacecraft was damaged while crossing the asteroid belt.

The “Dawn” spacecraft was send to the Vesta and Ceres asteroids in 2007 and so now we have the exact data of these bodies (mass, diameter and so on) as well as their pictures.


Vesta rotation animation


A snapshot of Ceres in a natural color made by the “Dawn” on May 4, 2015


The orbit of Ceres

In spite of the fact that the Earth is much larger than all known asteroids, a collision with an asteroid more than three kilometers in size will lead to the extinction of mankind. Even medium-sized space objects can lead to irreparable consequences. For example, the object which caused the Tunguska event had a size of just 50–80 meters!

The Kuiper Belt

The region of the Solar System from the Neptune’s orbit (thirty astronomical units (au) from the Sun) to a distance of about fifty-five au from the Sun is called the Kuiper belt. It consists mainly of small bodies. Although it is similar to the asteroid belt, its mass is about 20–200 times larger and it is approximately twenty times wider. Four out of five dwarf planets are located in this area. The outer edge of the belt might be called the boundary of the Solar System but it is not. The Oort cloud is located next.


The Kuiper belt

The Oort Cloud

The boundaries of the Solar System are still poorly understood but it is already known that beyond the Neptune’s orbit there are trillions of comets and asteroids left from the formation of the Solar System. The area where they are located is called the Oort cloud.

It has supposedly a spherical shape and is a source of long-period comets. The existence of the cloud is not yet confirmed but many factors indirectly indicate its existence. Observations of similar Solar Systems could be an example.

The research of the Oort cloud is complicated by the fact that the Sun barely illuminates objects at a such distance. The distance to the outer boundary of the cloud is about one light year which is equal to 9,460,000 million kilometers. It is so far away that makes up to one-fourth the distance to Proxima Centauri, the nearest star to the Earth after the Sun!



The Oort cloud

If we get to the border of our Solar System, we will find out that it is a very cold, distant place and the Sun will appear as a little bright star. Is it worth studying such distant places? Of course it is as this knowledge will help us understand how our Solar System and the Universe were formed.

Today we learned that the Solar System is not only the Sun and the eight planets but a large system with many objects that influence each other and our planet.


The map of the Solar System which allows to better visualize the objects of it

The next article will focus on satellites of planets and comets.