On Earth, you possibly can search for at evening and see the moon shining vibrant from a whole lot of hundreds of miles away. However if you happen to went to Venus, that would not be the case. Not each planet has a moon – so why do some planets have a number of moons, whereas others have none?I am a physics teacher who has adopted the present theories that describe why some planets have moons and a few do not.First, a moon is known as a pure satellite tv for pc. Astronomers consult with satellites as objects in area that orbit bigger our bodies. Since a moon isn’t human-made, it’s a pure satellite tv for pc.At the moment, there are two foremost theories for why some planets have moons. Moons are both gravitationally captured if they’re inside what’s referred to as a planet’s Hill sphere radius, or they’re shaped together with a photo voltaic system.The Hill sphere radiusObjects exert a gravitational pressure of attraction on different close by objects. The bigger the thing is, the larger the pressure of attraction.This gravitational pressure is the rationale all of us keep grounded to Earth as a substitute of floating away.The photo voltaic system is dominated by the solar’s massive gravitational pressure, which retains all the planets in orbit. The solar is probably the most huge object in our photo voltaic system, which suggests it has probably the most gravitational affect on objects akin to planets.Breaking area information, the most recent updates on rocket launches, skywatching occasions and extra!To ensure that a satellite tv for pc to orbit a planet, it must be shut sufficient for the planet to exert sufficient pressure to maintain it in orbit. The minimal distance for a planet to maintain a satellite tv for pc in orbit is known as the Hill sphere radius.The Hill sphere radius relies on the mass of each the bigger object and the smaller object. The moon orbiting Earth is an efficient instance of how the Hill sphere radius works. The Earth orbits across the solar, however the moon is shut sufficient to Earth that Earth’s gravitational pull captures it. The moon orbits across the Earth, moderately than the solar, as a result of it’s inside Earth’s Hill sphere radius.Smaller planets like Mercury have a tiny Hill sphere radius, since they can not exert a big gravitational pull. Any potential moons would doubtless get pulled in by the solar as a substitute.Many scientists are nonetheless trying to see whether or not these planets might have had small moons up to now. Again through the formation of the photo voltaic system, they might have had moons that obtained knocked away by collisions with different area objects.Mars has two moons, Phobos and Deimos. Scientists nonetheless debate whether or not these got here from asteroids that handed shut into Mars’ Hill sphere radius and obtained captured by the planet, or in the event that they had been shaped similtaneously the photo voltaic system. Extra proof helps the primary concept, as a result of Mars is near the asteroid belt.Jupiter, Saturn, Uranus and Neptune have bigger Hill sphere radii, as a result of they’re much bigger than Earth, Mars, Mercury and Venus they usually’re farther from the solar. Their gravitational pulls can appeal to and maintain extra pure satellites akin to moons in orbit. For instance, Jupiter has 95 moons, whereas Saturn has 146.Earth has a bigger Hill sphere radius than Mercury. (Picture credit score: Nicole Granucci)Moons forming with a photo voltaic systemAnother concept means that some moons shaped similtaneously their photo voltaic system.Photo voltaic programs begin out with an enormous disk of gasoline rotating round a solar. Because the gasoline rotates across the solar, it condenses into planets and moons that rotate round them. The planets and moons then all rotate in the identical course.However just a few moons in our photo voltaic system had been doubtless created this manner. Scientists predict that Jupiter’s and Saturn’s inside moons shaped through the emergence of our photo voltaic system as a result of they’re so previous. The remainder of the moons in our photo voltaic system, together with Jupiter’s and Saturn’s outer moons, had been in all probability gravitationally captured by their planets.Earth’s moon is particular as a result of it doubtless shaped otherwise. Scientists imagine that way back, a big, Mars-sized object collided with the Earth. Throughout that collision, an enormous chunk flew off the Earth and into its orbit and have become the moon.Scientists guess that the moon shaped this manner as a result of they’ve discovered a kind of rock referred to as basalt in soil on the moon’s floor. The moon’s basalt seems the identical as basalt discovered contained in the Earth.In the end, the query of why some planets have moons continues to be extensively debated, however elements akin to a planet’s measurement, gravitational pull, Hill sphere radius and the way its photo voltaic system shaped might play a job.