Astronomy An Introduction to the Solar System

Published on July 24th, 2011 | by Carl Mundy


An Introduction to the Solar System (Part 5)

Carl introduces you to the planets, asteroids and dwarf planets that make up our solar system and that have surprised us with their unexpected properties and beauty since we sent eyes to visit them…

Depending on when you went to school, or indeed how conservative your school is on updating their textbooks, your picture of our solar system may be a little out of date or even incomplete. Our solar system is full of wonders and surprises that you probably didn’t even know were there.

If you want to skip to a particular part of our backyard, click on any of the links below and you will be taken directly to the information on that body.


We finally arrive at the furthest known planet from the Sun. A planet which orbits at an average of 30 times the distance between the Earth and the Sun. The first planet to be discovered by mathematical calculation rather than direct observation, Neptune was discovered in 1846 after the orbit of Uranus did not match up to prediction by astronomers here on Earth.

Although Neptune has a very similar composition to Uranus, Neptune’s atmosphere is far from featureless. The strongest sustained winds known in the solar system drive the fast changing weather seen on the furthest planet from the Sun. When NASA’s Voyager 2 spacecraft visited in 1989, it observed a giant storm much like Jupiter’s Great Red Spot which has been designated the imaginative Great Dark Spot. While similar in many ways, Neptune’s storms appear to last only a few years each compared with centuries on Jupiter. Follow up observations by the Hubble Space Telescope revealed that the storm seen by Voyager 2 had completely disappeared although a new storm had formed towards the north of the planet.

Neptune has 13 known moons but the only moon massive enough to become spherical under its own gravitational forces is Triton, which accounts for over 99% of the mass in orbit around the planet. What makes this moon interesting is that it is the only large planetary moon to orbit in the opposite direction to Neptune’s direction of rotation. This has led astronomers to believe that Triton was captured by Neptune, rather than forming with the planet. It is thought Triton was once a dwarf planet in the Kuiper Belt – a belt of asteroids and dwarf planets beyond the orbit of Neptune. With observations by Voyager 2, Triton was added to the small list of bodies in the solar system where eruptions had been observed. Geysers of nitrogen gas and dust reached up to 8km high and are thought to be the main source for the thin nitrogen atmosphere also observed by Voyager 2.

The Trans-Neptunian Dwarf Planets & Beyond

Beyond Neptune lies a belt of material left over from the solar system’s formation that lies between 30 and 55 AU (1 AU = 1 Earth-Sun distance). Within this belt are numerous dwarf planets, asteroids and other small objects all mostly made up of ices such as hydrocarbons, ammonia and water ice. Since its discovery in 1992, there are now over a thousand confirmed Kuiper belt objects (KBOs) with at least 70,000 believed to exist. Beyond the Kuiper belt is a region of space known as the scattered disk. An object part of the scattered disk can have an orbit that can come as close as Neptune’s orbit but reach distances many times this.

The most well known object in these trans-neptunian regions is Pluto, now officially designated a dwarf planet. Discovered in 1930, Pluto has never been visited by any spacecraft and the best imagery and data astronomers have is from the Hubble Space Telescope – the surface of the dwarf planet was observed to change over the nine year period between 1994 and 2003. With the launch of the New Horizons spacecraft in early 2006, Pluto will be unvisited for only a few years longer when, in 2015, the craft is expected to make a flyby of the former planet and its 3 known moons 4 known moons (Pluto has a fourth moon!) 5 known moons. While it is unfortunately only a flyby, it may be our only opportunity to study this illusive and unexplored member of our solar system that is still surprising us.

Another member of the family is another dwarf planet called Eris which was only discovered a few years ago in 2005. Observations suggest its surface is methane ice, joining only Pluto on the list of trans-neptunian objects with a surface of methane ice. It is also thought that this dwarf planet may actually be larger than Pluto which was once a planet! Shortly after its discovery,  a moon was found to be orbiting this remote icy outpost which was later named Dysnomia.

On the same day that the discovery of Eris was announced, the discovery of another dwarf planet was also announced. Discovered shortly after Eris, Makemake is currently the third largest dwarf planet in the solar system as is unique among the large KBOs in that it has no known moons orbiting it, although this may change with future missions and observations.

After Makemake, the next largest dwarf planet in the solar system is Haumea, discovered in 2004. By now you can hopefully see that the names are becoming ever more exotic as ever more distant and strange worlds are discovered. Thought to be a member of a family of objects born from a giant collision, Haumea has an elliptical shape and two moons that are known to orbit it, called Hi’iaka and Namaka.

The final object we are to visit is Sedna. One of the most distant known objects in the solar system, Sedna’s orbit takes it to over 32 times Neptune’s distance from the Sun and is thought to be rounded by its own gravity and so qualify as a dwarf planet, although it is not officially designated as so yet. Its unusual, comet-like orbit will next bring it closest to the Sun in 2075 which will be a chance to study the remote object and the only opportunity for another 12,000 years. Despite this, no missions from any space agency are currently planned to visit one of the most remote outposts of the solar system.

There are many other objects in these regions that have so far been discovered, and no doubt that many more will surprise us in the future. These objects are of particular importance because of what they can teach us about the formation of the solar system and how it has evolved over time.

The Edge and Beyond…

The family that is our solar system is always growing with new objects discovered, objects promoted to different statuses (and demoted) and increasing observations from Earth and robotic missions uncovering the amazing secrets that these worlds hold. In our solar system alone there is an unbelievable variety of planets, dwarf planets, moons and asteroids. Who knows what else the universe holds…

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About the Author

Astronomy PhD student from the UK with a passion for astronomy and science outreach projects. Involved with weekly science-based radio programme The Science Show on University Radio Nottingham (URN).

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