On Earth, you can look up at night and see the moon shining brightly from hundreds of thousands of kilometers away. But if someone were to find themselves on Venus, that wouldn’t be the case. Not every planet has a moon—so why do some planets have several moons while others have none? First, a moon is called a natural satellite. Astronomers call objects in space that orbit larger bodies moons. Since the moon is not man-made, it is a natural satellite.
There are currently two main theories as to why some planets have moons. Moons are either gravitationally captured, if they are within what is called the radius of the planet’s Hill sphere, or they formed along with the solar system.
The Hill Sphere
Objects exert a gravitational pull on other nearby objects. The larger the object, the greater the pull.
This gravitational force is what keeps us all grounded to Earth, instead of floating away.
The solar system is dominated by the Sun’s massive gravitational force, which keeps all the planets in orbit. The Sun is the most massive object in our solar system, which means it has the greatest gravitational influence on objects like planets.
In order for a satellite to orbit a planet, it must be close enough for the planet to exert enough force to keep it in orbit. The minimum distance for a planet to keep a satellite in orbit is called the Hill sphere radius.
The Hill sphere radius is based on the mass of both the larger and smaller objects. The Moon orbiting the Earth is a good example of how the Hill sphere radius works. The Earth orbits the Sun, but the Moon is close enough to the Earth that the Earth’s gravitational pull can capture it. The Moon orbits the Earth, not the Sun, because it is within the radius of Earth’s Hill sphere.
Smaller planets like Mercury have small Hill sphere radii because they cannot exert much gravitational pull. Any potential moons would likely be pulled by the Sun instead.
Many scientists are still looking into whether these planets may have had small moons in the past. During the formation of the Solar System, they may have had moons that were knocked off by collisions with other space objects.
Mars has two moons, Phobos and Deimos. Scientists are still debating whether they are asteroids that passed close to Mars’ Hill sphere radius and were captured by the planet, or whether they formed at the same time as the Solar System. More evidence supports the former theory because Mars is close to the Asteroid Belt.
Jupiter, Saturn, Uranus, and Neptune have larger Hill sphere radii because they are much larger than Earth, Mars, Mercury, and Venus and are farther from the Sun. Their gravitational attraction can capture and hold more natural satellites in orbit. For example, Jupiter has 95 moons, while Saturn has 146.
Moons that formed with their system
Another theory suggests that some moons formed at the same time as their star system.
Photo: The contours depict the effective gravitational potentials of a two-body system (in the figure, the Sun and Earth) and the centrifugal forces in a rotating frame of reference. Hill spheres are regions bounded by circles around the Sun and Earth. In celestial mechanics, Lagrangian points (also libration points; L-points) are equilibrium points for objects of low mass under the gravitational influence of two massive orbiting bodies. NASA / Xander89 / CC BY 3.0
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First published in this link of The European Times.
