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                the Sun has 24 planets including Pluto

                                        |||

What is a planet? a moon? a star?

The definition of a planet?
Any (round) star can be orbited by 4 solid things:
1. round “planets”
2. non-round “little planets” like asteroids + comets
3. “meteoroids”
4. “dust”.

The definition of a moon?
Any (round) planet can be orbited by 4 solid things:
1. round “moons”
2. non-round “little moons” like asteroids + comets
3. “meteoroids”.
4. “dust”.

(*Pluto is a planet. And a black hole is not a singularity; it is real.
 And a circle is a polygon, Ringo is a Beatle and infinity is a number.)

The fewer the rules, the better.

* Earth has 1 (tidally locked) moon (+ counting)
* Mars has 2 (tidally locked) little moons (+ counting)
* Jupiter has 4 (tidally locked) moons + 88 little moons(+ counting)
* Pluto has 1 (mutually tidally locked) moon + 4 little moons (+ counting)

A meteoroid is a small asteroid: 1 cm to 1km (I see a fight from you here).
Meteoroids entering + burning in an atmosphere are called meteors.
If they survive + hit our planet (maybe 10 per day) they are then meteorites.

We now have 4 distinct sizes:
 planets+moons…asteroids+comets…meteoroids…dust.
Dust is smaller than meteoroids

which are smaller than little planets + little moons (asteroids + comets)
which are smaller than planets + moons.

ivica pix

Gravity in a vacuum of gases determines the sizes.
Momentum: your weight x your speed.
(Oops, I meant your mass x your velocity.)
And everybody has inertia: you don’t want to change unless forced to.
Planets + moons have inertia too.

A double (or more) star/planet (Pluto) is also possible. Double moon?

”Round” includes any elliptical shape (like Earth).
(Round orbits or round natural solid things are not “round”:

they are almost always somewhat elliptical. The Sun is very round.)

How elliptical is something?
An ellipse has an eccentricity between zero and one.
Zero = a circle. One = a parabola.

More than one = a hyperbola which is not a hyperbole.
(An oval is an elongated circle without an equation. In 3D it’s an ovoid.)

(I can hear your next question: negative eccentricity?)


The definition of round is a problem. These things can become round
(a roughly hydrostatic equilibrium) because of the pull of their own gravity
on themselves…if a rocky body has a diameter > ~600 km
or an icy body has a diameter > ~400 km.
(*A distorted planet/moon can be non-round: Haumea because of spin.)

Planet or little planet? Moon or little moon? arguments will happen. 
(I suggest the Pluto Solution: give them a break: round it

So Pluto is (ipsō factō) a planet. (You can argue with me,
but you can’t argue with Pluto. It is round and it orbits a star. Simple.)

According to the International Astronomical Union (IAU)
which is a member of the International Science Council (ISC):
there are 5 “official” dwarf planets that are round but NOT planets:
Ceres Pluto Haumea Makemake Eris (in Sun order).

There are also 11 unofficial dwarf planets + counting: Orcus 208996 Ixion 90568 55636 Quaoar Salacia Varuna 55565 Gonggong Sedna
(They (the IAU) are entitled to an opinion.)
Quaoar has a ring around it. Haumea is an ellipsoid with a ring around it.

ivica pix

So the Sun has 24 planets: 8 + 5 + 11 (+ counting).
If you are round + orbiting a star: you are a planet.
If you are round + orbiting a planet: you are a moon.

(*You can also have rogue stars + planets + IAUs.)

Some people claim the Sun might have more than 200 (round) planets.

A star is a celestial body that produces light and heat because of nuclear fusion. One night, our Sun will become a white dwarf which is not a star. A neutron star or a black hole is not a star. A brown dwarf is not a star. (*A star-like mass or bigger can also have planets with moons.)

Feel free to correct me. (I’d sure correct you.) This stuff is out of date constantly and that is a good thing:
August 1, 2024.