Why do planets have moons
Well, most moons are created by gravity, either coalesced dust or other rocky bodies “grabbed”. So generally the larger the body the more moons.
There are various ways in which, theoretically, planets can “acquire” moon. The most popular ones are
1) The moon is formed of nearly same substance as the planet and during the birthing time of the planet.
2) Relatively foreign object gets into orbit around planet.
Our moon is formed by Theory-1 and only variation is that a Mars sized object named Thea collided with Earth and the moon was formed within 1 year of collision form the derbies of Earth and Thea.
To understand how the moons formed, we need to know how planets formed.
To understand how planets formed, we need to know how solar system was formed.
To understand how solar system was found, we need to know the starting point where it all happened.
Initially entire solar system was just a cloud of dust, Quite literally.
Some cloud of dust that was a result of aftermath of some ancient supernovae around our giant neighborhood.
Then a similar event, supernova, is believed to have set this cloud of dust and gas spinning.
As a result, the relatively flat disc of dust was formed. As materials collided, they heated up.
The coalesced into rocks. Rocks coalesced into bigger rocks. At one point, the density of material was so high in the dust cloud that the Sun was born!
There were no planets formed yet. There were no moons yet. There was nothing except the sun that you had at that point.
Yes there are small rocks, that will grow eventually to a bigger one.
Eventually they become big lumps of rocks which have their own substantial mass.
Sun was nicely doing his gig far away meanwhile, exerting overall gravitational field on the dirt and derbies around it in solar system.
They were in orbit around sun. Eventually these rocks come together in innumerable violent clashing and colliding of smaller and smaller objects becoming bigger mass of rocks.
Things are so violent that these rocks turn liquid during collision. Since they were malleable, and spinning, they eventually sphere out!
You start seeing first signs of planets being formed!
Even after this, there was lots of derbies that were left over in Solar system and these rocks were hurtling towards the infant planets and dive bombing the planets in a Kamikaze style (Google it)!
Sometimes bigger rocks, as much as half as big as planets would think of planets as pins and themselves as a bowling bowl and aim straight down for the planet to crash dive into it!
The derbies of these gigantic collisions, coming from both objects, are hurtled into space to enormous heights.
The core of 2 bodies may unite and become one body with litter all around the planet.
These derbies may get into orbit around the planet. The result? You have got a miniature version of “Sun with debris of gas and dust orbiting around sun”.
Just like how planets forming from this Sun and debris of gas and dust, there will be moon(s) from from the orbiting debris field around planets.
The debris may collide and coalesce into what we have as Earth’s moon, or may become ring round the planet like Jupiter if the planet is so large that it constantly keeps tearing its moon apart!
And this is one of the most prominent ways the moons are formed.
What would happen if moons were not there
Now, what would happen if moons were not there is a tricky question to evaluate.
Every moon is different. Every planet is different. Every Planet and its Satellite system is different.
If you are talking about earth, there wont be tidal forces to ocean.
Forget that, terrestrial life MAY have never evolved. During low tides, water recedes and the amphibians would get a chance to go to land and hang out there for a while.
They started adopting towards living on land and that’s how terrestrial life started.
If that’s not enough, our Moon gives us a stable orbit without a wobble.
This means controlled seasons and no extreme changes in the temperature across different part of world.
This means, if there was no moon and this conditions prevailed, life as we know it may not have formed at all.
Moon also protect earth from asteroids and other foreign bodies.
If you are talking about other planet’s moon, effects might not be THAT dramatic, since all the rest 7 planets are lifeless.
We wont have any dramatic story to tell of life and dangers to it.
There might be other physical phenomenon (related to their orbits and wobble) that may not be prevalent in the planets like how we see today!
Why Mercury and Venus has no moon
The planets in the Solar System that doesn’t have a moon are Mercury and Venus.
Most likely because they are too close to the Sun. All the other planets have at least one moon.
Any moon with too great a distance from these planets would be in an unstable orbit and be captured by the Sun.
If they were too close to these planets they would be destroyed by tidal gravitational forces.
Not only is Mercury the smallest planet, but it’s also the closest planet to the Sun, which makes its Hill Sphere (a spherical area where a planet can have moons) very small.
Any Mercurian moons will be pulled away by the Sun.
Venus is interesting, though. It’s mass is similar to Earth, and Earth has a moon.
Sure, Venus is also closer to the Sun than Earth is, but its Hill Sphere should be big enough for it to have a moon or two.
There’s a possibility that, when the Solar System was very young, Venus actually had a moon from colliding with another proto-planet, which is exactly how Earth got its moon as well.
However, unlike Earth, Venus received a second huge collision. This collision caused its rotation to reverse to a retrograde direction.
Assuming the moon’s orbit to be pro-grade, and due to tidal deceleration, the moon’s orbit will decrease in size, and Venus’ rotation will slow down (because it’s retrograde; if the direction of the rotation is the same as the moon’s orbit but rotates slower than the moon’s orbital period, it will go faster).
The moon crashes to Venus, and Venus ended up with a slow, retrograde rotation and without any moons.
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