Moons are common in our solar system, and the large outer planets (especially Jupiter and Saturn) have many dozens of them. Even dwarf planets like Pluto have satellites.
Still, two planets have none at all.
In brief: the two planets with no moons
Venus and Mercury are the only planets in our solar system with no moons. That’s primarily because they’re too small and too close to the Sun to retain a moon, and Venus’s motion is too unusual. The other planets and dwarf planets in our solar system have at least one moon, but Mars may lose one in 10-50 million years.
But how does a planet get a moon in the first place, and what exactly went “wrong” with these exceptions?
As we can see, a moonless planet is a rarity, and there are three possible ways for a planet to get a moon.
Let’s see what these conditions are and why not all planets have moons.
Why don’t all planets have moons?
There are basically three factors that influence the numbers of moons each planet has, and those are the following:
- The size (mass) of the planet – ability to capture objects.
- Collision – just like our Moon formed.
- Co-formation – formed at the same time as the planet.
Without just the right size, collisions, and co-formation, a planet almost certainly won’t capture and retain a moon.
A planet’s size is important when it comes to moons since their gravitational pull is strong. Planets that are closer to asteroid belts, for example, may get many moons, or if they are too close to the Sun, the chances are slim that they would have any.
An example, in this case, are the planets, Mars and Pluto. On the other hand, some planets are smaller and closer to the Sun, like Mercury, Venus, or Earth, and because the Sun absorbed most of the debris, there were no leftovers.
In our solar system, the planets with more moons are the biggest planets – Jupiter, Saturn, Uranus, and Neptune. They can absorb comets and grab asteroids and so all that material will start to condense into lots of moons that will orbit each planet.
As such, the two planets closest to the Sun are moonless, while Earth has only one moon.
Collision is also responsible for the creation of moons. For example, our Earth suffered a massive collision with a Mars-sized planet, which resulted in the creation of our Moon.
The third case in which a planet can get a moon is co-formation.
Co-formation is the simplest way since it involves the moon’s formation at around the same time as the planet, and the planet’s gravity captures it.
Why Mercury and Venus have no moons
Why are Mercury and Venus the lone exceptions that are moonless, but some planets have plenty of them? It’s primarily because of both planets’ size and proximity to the Sun, as well as Venus’ rotation.
To get a better idea of this, we need to go back to the solar system’s beginning.
When the Sun was born, it began to release powerful solar winds that swept away all the lighter materials, such as hydrogen, methane, and ice, into the farthest reaches of the galaxy.
Through accretion, Mercury, the closest planet to the Sun and the smallest in our solar system, formed itself accumulating the heavier elements that withstood the solar winds.
The probability of forming a moon was almost impossible given that there was basically no material leftover, and even if some left would have created a moon, it is unlikely that Mercury could hold on to it very long.
One of the reasons is that this planet’s hill sphere is much smaller than the hill spheres of other planets. Also, any strong gravitational pull exerted by the Sun would grab the moon away from it. As such, even if Mercury had moons, the Sun would take them.
Venus is the planet that most closely resembles the Earth in terms of size and mass, and it is not the closest planet to the Sun, so it should have a better chance of having a moon.
But there are two strange things about its rotational dynamics. The first one is that Venus moves in a retrograde motion: it rotates in the opposite direction to the rotation of most other planets. The second one is that it’s a very slow rotator.
It takes less time for Venus to make one revolution around the Sun than it does to complete one spin on its axis. In other words, one year on Venus is shorter than a day.
The retrograde motion, combined with its prolonged rotation, makes it almost impossible for any moon to remain on a stable orbit around this planet.
It’s very possible that Venus had moons, but they slowly creeped away due to its slowness, while some believe that Mercury was actually Venus’s moon. However, this is still debated.
So, one of the main reasons why Mercury and Venus are moonless is because of their proximity to the Sun. The Sun’s hostile solar radiation and intense gravitational pull either grabbed or blown off any moons.
Why Mars is losing a moon
Mars, the second-smallest planet in the Solar System (the first one being Mercury) and the fourth planet from the Sun, has two moons, named after Ares’ mythological sons – Phobos, meaning fear, and Deimos, representing panic.
But in a few 10s of millions of years, Mars will most likely lose Phobos, which is slowly moving inward.
Asaph Hall was the one who discovered the moons of Mars in 1877, and ninety-four years later, NASA’s Mariner 9 spacecraft got a much better look at them. A fascinating characteristic of the moon Phobos is a crater 10 kilometers / 6.2 miles wide – almost half the moon’s width.
Phobos measures just 22.7 km / 14.1 mi across, and it’s the moon that orbits closest to the planet (only 6.000 km / 3,728 mi above the Martian surface). Deimos, Mars’s second moon, is even smaller, measuring just 12.6 km / 7.8 mi across.
It orbits at a much greater distance from Mars, which means that Deimos will make a complete orbit around Mars in 30.35 hours. Compared to Phobos, it revolves around the planet three times in a single day, crossing the sky in only 4 hours or so.
Phobos and Deimos are very similar to Earth’s Moon, always presenting the same face to their planet, and they are both lumpy, heavily-cratered and covered in dust and loose rocks,
They have the size of an asteroid and misshaped appearance because they are not large enough to become round spheres like the vast majority of moons in our solar system (similar to our Moon, for example).
However, the two moons won’t follow this circular path around Mars forever because they orbit Mars faster than the planet itself rotates. Also, since the closest moon, Phobos actually has a slightly inward spiral motion – it will make the moon get closer to Mars at a rate of 1.8 meters every century.
Therefore, this means that within 50 million years, Phobos will either collide with Mars and become a ring of rubble around it. When the impact occurs, because there is not enough gravitational pull to keep the wastes, the dust and debris will leave the surface of Mars.
Nevertheless, given the Red Planet’s gravity, it will be able to keep a ring of debris around itself, approximately in the same region where Phobos orbits now.
Background: How moons form in our solar system
There are basically three possible ways of how moons form in our solar system. The first one is co-formation, meaning that the moons are formed simultaneously with the planet.
Examples in this regard are the gas giants like Saturn and Jupiter. Billions of years ago, clouds of gas and debris formed a ring shape and started orbiting around the planet.
After a while, a process called accretion begins, and particles slowly get closer to one another, stick and then merge into a single body. And so, the accumulation continues, gradually taking the form of a sphere and ultimately developing into a moon.
The second possibility a planet can get a moon is by capturing one. Almost every planet has a region around it that is capable of holding a moon and keeping it in a stable orbit.
This notion is named the hill sphere. The two moons of Mars, Phobos, and Deimos are considered by many astronomers to be captured objects by the Red Planet when they transgressed into the hill sphere from the nearby asteroid belt.
The last way a moon can be created is from a high-impact collision, and it’s probably also the most unpleasant way a planet can get its natural satellite.
As a matter of fact, our own moon is considered to have formed in such a collision that happened a few billion years ago. A Mars-sized planet collided with Earth and resulted in the formation of the moon.