Tenth Planet: The Next World in the Solar System

Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)

Before 1930, there were 8 planets in the Solar System. And then with the discovery of Pluto in 1930, the total number of planets rose to 9. Although astronomers kept searching for more planets, it wasn’t until 2005 that an object larger than Pluto was found orbiting in the distant Solar System. This new object was known as Eris, and many considered it to be a tenth planet; but it actually created a controversy that ended up with Pluto being kicked out of the planet club and becoming a dwarf planet. There really is no 10th planet, in fact, we don’t even have a ninth planet any more.

Discovery of Eris

Eris, originally named 2003 ub 313 was discovered by Palomar observatory researcher Mike Brown; Mike has been behind many of the trans-Neptunian discoveries in the last decade. Mike and his team discovered Eris by systematically scanning the sky for objects moving at the right speed in the right object to be in the outer Solar System.

Further observations of Eris showed that it was actually larger than Pluto by a significant amount; it measured 2,500 km across, compared to Pluto’s 2,300 km diameter. And it orbited at a distance of 67 astronomical units, compared to Pluto’s 39 AU (1 AU is the average distance from the Earth to the Sun).

Tenth Planet, Dwarf Planet

Because there was now a larger object than Pluto found orbiting the Sun, astronomers needed to decide whether this would be come the tenth planet. At a meeting of the International Astronomical Union in 2006, astronomers decided to redefine their classification of a planet. And these new rules excluded Eris. Instead of becoming the tenth planet, Eris became a dwarf planet; the same fate as Pluto.

We’ve written many articles about Eris for Universe Today. Here’s an article about how Eris is changing, and here’s an article about how Xena was renamed to Eris.

If you’d like more info on Eris, check out NASA’s page on Eris.

We’ve also recorded an episode of Astronomy Cast that explains why Pluto isn’t a planet any more. Listen here, Episode 1: Pluto’s Planetary Identity Crisis.

Solar System Coloring Pages

Want to find some cool Solar System coloring pages? Here are some links to resources we’ve been able to dig up from around the Internet.

Check out the offerings from Coloring Castle. I find it cool that they offer a version with Pluto, and then another without Pluto.

And one of the best resources on the internet for this kind of thing is Enchanted Learning. They’ve got a page just for Solar System coloring pages.

Windows on the Universe has coloring pages for all the planets in the Solar System. They even have an entire PDF book that you can print off with all the planets (including Pluto).

Coloring Fun has some more solar system pages for coloring.

And here are some resources from About.com.

Here are some resources from NASA. And here’s a link to a 3d Solar System.

We have written many articles about the Solar System for Universe Today. Here’s an article about Solar System projects for kids, and here’s an article about how to build a model of the Solar System.

We have also recorded an audio tour through the Solar System for Astronomy Cast. Start here at Episode 49: Mercury.

List of Planets

Mercury is the closest planet to the Sun but not the hottest. That distinction goes to Venus. The planet was named after the Roman messenger of the gods because it orbits the Sun so quickly. Mercury is a small, grayish planet that is often said to resemble the Earth’s Moon.

Venus, the second planet from the Sun, is the hottest planet because its atmosphere tends to trap heat. Named after the Roman goddess of beauty, Venus is the brightest planet. In fact, the only celestial body that is brighter¬†is the Moon. Venus is around the same size as Earth with similar gravity, causing it to be referred to as Earth’s twin.

Earth is the third planet from the Sun. It is the only planet where life has been confirmed to exist. Roughly two-thirds of Earth’s surface is covered with oceans, and so far Earth is the only place where liquid water is known to exist.

Mars was named after the Roman god of war because of its red color, which is caused by rust in the rocks on the surface. Since it is the closest planet to Earth, people have long wondered if life could exist on Mars. Although no life has been discovered so far, some people still think that there may be life on Mars.  

Jupiter, a gas giant, is the largest planet in this solar system. It was named after the Roman king of the gods, probably because of its size. Jupiter has 63 moons, one of which, Ganymede, is the solar system’s largest moon. Jupiter is also home to an enormous storm, the Great Red Spot, which has been raging for over two hundred years.

Saturn, the sixth planet from the sun, was named after the Roman god of agriculture and harvest, Saturnus. It is also a gas giant and therefore does not have a solid surface. One distinctive feature of the planet is its rings, which are composed of small pieces of rock and ice.

Uranus, the third largest planet, is also a gas giant. One interesting fact is that its moons were named after characters from works of literature by Shakespeare and Alexander Pope. Uranus orbits very slowly; it takes the planet 84 years to circle the sun.

Neptune is the furthest planet from the Sun. It was named after the Roman god of the sea; this is not surprising because it is bright blue, reminding one of a beautiful ocean. Neptune has four rings, although they are difficult to see. When Pluto was reclassified as a dwarf planet, Neptune became the eighth and last planet in the solar system.

Universe Today has a number of other articles about this including the planets and the solar system for kids.

If you are looking for more information check out this overview of the planets and article on planets in our solar system.

Astronomy Cast also has numerous articles on the planets so take a look at this one for starters: the planet Earth.