What Makes a Planet and How Many Are There in Our Solar System?

How many planets are there in our solar system? 9? 8? 12? More? The answer might actually surprise you.

Here we explore the answer to this apparently simple question and take a quick tour of the main primary planets of our home solar system.

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What are the 3 criteria for a planet?

What is a planet? An enormous chunk of stuff (rock or gas), roughly spheroidal in shape that orbits a star and may or may not have a moon, right?

While this is in part true, there are some issues with such a simple definition. What about asteroids? When does an object become large enough to be considered a planet?

Why are Venus and Mercury planets (according to current classifications) but not Pluto? As it turns out, the reasons are pretty straight forward. 

Back in 2006, The International Astronomical Union (IAU) decided to define what makes a planet. The IAU is a worldwide organization of astronomers. 

They passed a resolution setting out the formal definition of a planet. According to the resolution, in order for an object to be called a planet, it must meet the following criteria (courtesy of Cornell University): - 

1. It must be a celestial object which independently orbits the Sun (this means moons can't be considered planets since they orbit planets).
2. It must have enough mass that its own gravity pulls it into a roughly spheroidal shape.
3. It must have cleared the neighborhood around its orbit. That is, it be large enough to "dominate" its orbit (i.e. its mass must be much larger than anything else which crosses its orbit).

Because Pluto is not actually large enough to "dominate" its orbit, it can not be called a planet according to the IAU. Other, smaller planets like Neptune are actually somewhere in the order of 8000 times more massive than Pluto and do indeed "dominate" their own orbits.

For this reason, Neptune comfortably fits the above criteria and can be called a planet. Pluto, on the other hand, is now commonly referred to as a dwarf planet. 

Are there actually 12 planets in our solar system?

The very same organization IAU, mentioned above, have proposed that there might be more than the commonly recognized 8 planets in our solar system. In the IAU meeting in Prague in 2006, resolutions were also passed for a new planet classification system which might actually boost our current yield of planets to 12.

Under this new classification system, some objects in our solar system could technically be considered to be planets. For example, the asteroid Ceres, Pluto's moon Charon, and a newly discovered object UB313 (Xena) could be considered candidates for planet status. 

"The world’s astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the difference between “planets” and the smaller “solar system bodies” such as comets and asteroids. If the definition is approved [which it was] by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, our Solar System will include 12 planets, with more to come: eight classical planets that dominate the system, three planets in a new and growing category of “plutons” – Pluto-like objects – and Ceres. Pluto remains a planet and is the prototype for the new category of “plutons.” - University Today. 

But, as you may have noticed, the newer additions to the ranks of planets are actually dwarf planets or plutons, not traditional planets (called primary) like Earth and Mars. So Ceres, Xena and Pluto's moon Charon would be classified as planets, but very small ones. Pluto would also remain a planet, but would become the basis for a new 'pluton' category. 

The IAU also passed a resolution that established and defined the new category of dwarf planet. According to NASA, "a dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite."

The meeting recognized five dwarf planets - Eris, Ceres, Pluto, Haumea and Makemake. But there may be another 100 dwarf planets in the solar system and hundreds more in and just outside the Kuiper Belt.

• Makemake - This object orbits the Sun every 310 Earth years.
• Haumea - This is one of the fastest rotating objects in our solar system.

• Eris - This is one of the largest known dwarf planets in our solar system. It is roughly 3 times bigger than Pluto and is also about three times farther away from the Sun. It takes around 557 Earth years to orbit the Sun.

What are the 9 planets?

Prior to the IAU's pronouncements about the definition of a planet the traditional list of primary planets in our solar system were as follows: -

• Mercury,
• Venus,
• Earth,
• Mars,
• Jupiter,
• Saturn,
• Uranus,
• Neptune, and;
• Pluto.

But as we now know, Pluto was stripped of its title as one of the 9 primary planets of our solar system at the 2006 IAU meeting. It now joins the ranks of smaller, large objects called dwarf planets (or plutons).

What are the characteristics of the 8 "classical" planets?

Of our remaining primary planets, they each have their own characteristics and features. 

1. Mercury is a place of extremes

Mercury is the smallest planet in our solar system and the one closest to our parent star, the Sun. It is just slightly larger than our Moon.

It is also one of the fastest orbiting bodies in our solar system and completes an orbit every 88 Earth days. It rotates very slowly and has an enormous temperature difference between its surfaces that face the Sun and are in shadow. 

On the Sun's side, temperatures can reach 426.7 degrees Celsius and plunge to -173 degrees celsius on its dark side. 

2. Venus would not be a pleasant place to live

Venus is slightly smaller than Earth and can be seen with the naked eye here on Earth. Surface temperatures can reach 482 degrees celsius, which are the product of a runaway greenhouse effect. 

It has a thick atmosphere, about 90 times that of Earth, that is rich in sulfuric acid and carbon dioxide. 

3. Earth is the only planet known to support life

Mother Earth is the only planet known to be habitable to life. It is also one of the largest terrestrial planets in our solar system. Conditions are such that water can exist in liquid form.

Enough said, we are sure you are more than aware of the characteristics of our home planet, since you live on it. 

4. Mars: God of War!

Named after the Roman God of War, Mars is the fourth planet from the Sun. Its most distinguishing feature is its red surface coloration, formed by large amounts of iron oxide.

Mars' surface has many large volcanoes and deep valleys, and frequently experiences planet-wide wind storms. Its atmosphere is very thin, about 1/100th that of Earth, and it may have once had liquid water.

Mars is colder than Earth, with temperatures fluctuating between -113 and 0 degrees celsius.

5. Jupiter is the king of the solar system

As the largest planet in our solar system, Jupiter is named after the king of the Roman gods.

This is the first of the gas giants, and has a characteristic turbulent atmosphere of hydrogen, helium, methane, ammonia and frozen water. The largest and most distinctive of its storms, the "Great Red Spot", is larger than Earth. 

Jupiter has 53 named moons and another 26 awaiting official names.

6. Saturn is the gem of the solar system

Saturn is the 6th planet from the Sun and is another of our solar system's gas giants. By far its most characteristic feature is its set of magnificent rings. 

Each ring orbits the planet in a thin band roughly 1-mile (1.6 km) thick in places. Saturn is 9.5 times the size of Earth and has 62 confirmed moons. 

Its atmosphere is similar in composition to Jupiter and at its core pressures are so high that gases may condense to form liquids, and even metals. 

7. Uranus is very strange indeed

Uranus is the 7th planet in our solar system and also one of the strangest. It's spinning axis is so extreme that it is actually almost parallel to its orbital plane around the Sun. While the rest of the planets in the Solar System can be thought of like spinning tops, Uranus is more like a rolling ball going around the Sun.

Uranus is about four times the size of the Earth and is made up of mostly methane. It has a faint ring system and 27 moons.

8. Neptune takes a long time to orbit the Sun

Neptune is the last of the primary planets of our solar system. It is also the furthest away from the sun. 

Surface temperatures average about -214 degrees celsius and it takes 165 Earth years to orbit the Sun. Like Uranus, its atmosphere is mostly methane, and the planet is so cold that the interior of the planet is mostly frozen methane. 

Neptune is about 4 times the size of Earth, has fourteen moons and a faint ring system in orbit.