Pluto

Properties:

Mass: 1.3090 * 10²² kg

Mean Radius: 1,151 km

Semi-major Axis: 39 AU

Orbital Period: 247.94 years

Rotational Period: 6.39 days(retrograde)

Surface Gravity: 0.620 m/(s²)

Etymology: Pluto(Roman God of the Underworld)

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Pluto is a trans-Neptunian dwarf planet located in the Kuiper Belt. The Kuiper Belt is a ring of bodies(like asteroids, moons, and dwarf planets) that are located outside Neptune's orbit. Pluto used to be considered an actual planet until it was later re-classified as a dwarf planet in 2006.

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So, why can't Pluto be considered a planet? To answer this, we need to lay out the criteria that planets meet.

The criteria laid out officially by the International Astronomical Union(IAU) are:

1) The object orbits the Sun

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2)The object must have enough mass to be rounded by its own self-gravity(in more precise terms, the thermal pressure from within the planet is balanced by its gravitational forces<-- Remember that planets don't just exert gravity on other objects, they also exert gravity on their own cores)

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3)The object must "clear the neighborhood" in its orbit(there are no combinations of other bodies within proximity of the object in question that exert significant gravitational forces on the object)

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Pluto fully satisfies the first two conditions of this definition but fails to satisfy the third. This is because the objects orbiting Pluto have a combined mass(not individual masses) far greater than Pluto's. This means that Pluto doesn't satisfy the third criterion. If an object satisfies only the first two criteria given by the IAU, then the object is considered a dwarf planet.

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Pluto's surface consists mainly of nitrogen ice mountains and frozen plains and the surface temperatures go near -240 C, making it far too cold for any significant amount of life.