Astronomical definitions

There are many different objects and nowadays a good classification is starting to mix up things. The definitions here are mine.

Star

A star is a massive, compact body of plasma in outer space that is held together by its own gravity and, unlike a planet, is sufficiently massive to sustain nuclear fusion in a very dense, hot core region. To do so it needs a minimum mass of 0.083 solar masses.

White Dwarf

This is no longer a star by definition since it no longer produces energy through nuclear fusion and is defined as the product of the death of a low- or medium-mass star. It is supported by electron degeneracy pressure and extremely dense.

Neutron Star

This is no longer a star by definition since it no longer produces energy through nuclear fusion and is defined as the product of the death of a high-mass star. It distinguishes itself from a white dwarf in that it is past the point of being supported by electron degeneracy and is supported by neutron degeneracy pressure.

Black Hole

Any object with an escape velocity larger than the speed of light. One can speak from stellar black holes with the mass of a typical star, usually the product of the death of a massive star, and galactic black holes at the center of a galaxy.

Brown Dwarf

This is an object that forms like a star but has insufficient mass to burn hydrogen (less than 0.083 Solar masses) but heavy enough to have produced energy by Deuterium fusion at some stage of their evolution, which requires a minimum mass of 0.013 Solar masses. They may be free-floating or orbit a star or another Brown Dwarf.

Planet

A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. That's not a good definition at all, what is relatively large? And does it have to be around a star?

Here's my own definition: A planet is an object that is formed by core accretion that is not part of a larger group of objects sharing similar orbits. This includes free floating planets. A similar definition was adopted by the IAU in 2006.

This means there are 8 planets in our Solar System. Pluto and Charon belong to the Kuiper Belt, Ceres belongs to the asteroid belt.

Planet types

Gas giants. These planets are gaseous on the outside and consist mostly of hydrogen and helium. In our Solar System there are 2: Jupiter and Saturn.

Ice giants. These planets are mostly made up of ices and methane. In our Solar System there are 2: Uranus and Neptune

Rocky planets. These planets are well, rocky. They are mostly made up out of metals, carbon and silicates. In our Solar System there are 4: Mercury, Venus, Earth and Mars.

Dwarf planet

These are objects orbiting the a star that are not a star or a planet, but large enough to be spherical in shape. Some examples are:

Name	Diameter (km)
Eris	2600	(Previously known as Xena / 2003 UB₃₁₃)
Pluto	2300	(Previously classified as planet)
Sedna	1500	(Dwarf planet candidate)
Charon	1270	(Dwarf planet candidate)
Ceres	930	(Previously classified as asteroid)

Asteroid

These are all natural rocky objects orbiting a star that are not large enough to be spherical in shape but that are larger than meteoroids (about 10 metres in size).

Comet

These are all natural icy objects orbiting a star that are not large enough to be spherical in shape but that are larger than meteoroids (about 10 metres in size). This includes some of the smaller Trans-Neptunian objects.

Moon

In short: Natural objects orbiting a planet that are not part of a ring. These fall into two groups: Major Moons, that are those that would be (Dwarf) Planets had they orbited the star, and the rest.

The Major Moons in our Solar System are:

Name Planet Diameter (km)

Luna Earth 3840

Io Jupiter 3630

Europa Jupiter 3140

Ganymede Jupiter 5260

Callisto Jupiter 4800

Tethys Saturn 1060

Dione Saturn 1120

Titan Saturn 5150

Ariel Uranus 1160

Umbriel Uranus 1170

Titania Uranus 1580

Oberon Uranus 1520

Triton Neptune 2700

Objects orbiting Dwarf Planets or Asteroids fall in the same category as had they orbited the central star. Using this criterion, Pluto and Charon are a double Dwarf Planet system.