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| Venus and Mercury don't have Moons |
Structure of the solar system
Our Sun is orbited by nine planets, more than 120 moons, millions of asteroids and other rocky bodies, and countless comets. Mercury is closest to the Sun, followed by Venus, Earth and Mars. These planets are known as the inner planets of our solar system. All the planets in our solar system are unique in an unusual way and each of them has a remarkable feature unique to them. Earth sustains life growth, while Venus receives highly corrosive sulfuric acid rain. All the planets in our solar system are surrounded by their minor planets, or better said, their satellites, which revolve around them in certain fixed orbits. Orbits can be distinguished and recognized using Kepler's laws of motion. Earth has only one moon, Mars has two, and Jupiter has about 80 moons, including the largest moon in our solar system, Ganymede. Saturn has two more than eight moons. Uranus has twenty-seven moons, while Neptune has about fourteen. Even our dwarf neighbor Pluto has five orbiting satellites. However, the first two planets in our solar system, Mercury and Venus, surprisingly, do not have moons. But why? Why are only the first two planets an exception to this characteristic? And how exactly were the satellites formed? In this video, we will take a detailed study of satellites and the factors that affect satellite formation.
What is a satellite?
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| What Is Satellite? |
Before we proceed, let's discuss what exactly satellites are and how satellites are made. (Spoiler alert, all our answers are after Solar System Formation and Solar System Formation.) Satellites are celestial bodies that orbit larger cosmic objects than themselves, usually a planet or a dwarf planet. However, they can also orbit larger astronomical bodies such as large asteroid fragments and planets. In scientific terms, any cosmic object that has a mass more than a thousand times that of its parent planet and has a defined orbit around it can be called a satellite. Don't confuse these satellites with the ones Elon Musk and others are launching into space to help you with your Wi-Fi. These satellites are different from our discussion.
How are satellites formed?
Fundamental Ways (Giant Impact and Capture Theory)
The formation of these satellites is a by-product of the formation of the planets. This can occur mainly in four ways, although many others are still undiscovered (we may discover more ways to form them). Satellite formation can occur through the collision of two or more large celestial bodies. The most famous example you've heard of is our moon. Many theories predict the formation of our Moon. The most plausible theory, however, describes the formation of the Moon as a by-product of a collision between Earth and another Mars-sized planet, ejecting material into space that coalesced under the influence of their gravity, thus forming our Moon. Well, we'll get into the formation theory of our moon later in this video, because that's a whole new topic in itself. Another theory explaining the formation of satellites is known as "The Capture Theory".
The Capture theory
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| The Capture Theory |
This theory was proposed by an American astronomer, Thomas Jefferson, who was studying theories of moon formation at the time. This theory describes the apparent capture of objects by small planetary satellites formed elsewhere by large planets. Most of these satellites are irregularly shaped objects because they do not have enough mass to maintain a spherical shape. Phoebe, a moon of Saturn, is believed to have been captured by Saturn under certain circumstances.
Fission Theory
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| Fission Theory |
Condensation Theory
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| Condesation Theory |
Another method of Satellite Formation is “Condensation Theory”, also known as the “co-accretion theory”. When massive gas giant planets, like Saturn or Jupiter, are formed, a lot of rocky mass is also left as a remnant. This remnant mass too can come under influence of their self-gravity, stick together and finally coalesce leading to Satellite formation. Similarly, satellites can also be formed during the early stages of the Solar System’s Formation when excess gas and dust come together under the influence of their gravity, creating a tiny planetary Satellite. Necessary Conditions for satellites to remain stable. Satellites revolve around their parent Planet in fixed circular orbits, which can be determined using the energy equation and Newton’s Laws of Gravitation.
Description of Mercury and Why doesn’t Mercury have Satellites?
Mercury is the smallest planet in our Solar System, only slightly larger than our moon. It is also the closest planet to the Sun. It is named after the Roman God, Mercury, the swift messenger of Gods. Taking a mere 88 Earth days to complete one revolution around the Sun, it also claims the title of The Fastest revolving Planet in our solar system. However, due to its closeness to the Sun, mercury could not sustain a Satellite in a stable orbit around it. The Sun’s gravitational force is so strong in this region that any Satellite orbiting Mercury would be engulfed by the Sun in mere hours. So, did Mercury ever have a moon during any particular time?
Well, if it ever had one, the consequence would still be the same. The immense Gravitational pull of the sun around Mercury would make the orbit of its Satellite highly unstable, gradually pulling the Satellite towards the Sun and finally engulfing, and hence, destroying it. Mercury’s small size compared to the sun, would make it practically impossible for it to create a Gravitational field large enough to hold its moon in a fixed orbit around it.
Hence, out of all the Planets, having a Satellite to orbit around it would be the most challenging task for Mercury. If we follow the capture theory, a Satellite that did come around Mercury and be captured by its gravitational presence would eventually be drawn into a solar orbit, gradually being engulfed by it. Taking the distance factor into account, if a Satellite is too far from Mercury, it won’t possess a stable orbit around it.
Conversely, if it is close to Mercury, it may crash into it or may crash into the Sun. The final results will always be the same for Mercury. Hence, sealed by fate, Mercury continues to revolve around the Sun, alone, without a companion. Well, of course, we’re always there for you Mercury. Description of Venus + Why doesn’t Venus have Satellites? So, does the same reason stand true for Venus too? Is Venus too Influenced by the Huge Gravitational Force of the Sun? Named after the Roman Goddess of Love, Venus is the second Planet from our Sun and the third brightest object in the night sky.
It also claims the title of the hottest Planet in our Solar System.
Venus is one of the rarest cases among all of the planets itself. The reason why Venus does not has a natural Satellite in orbit around it is that no cosmic remnant managed to make it to the belt where the gravitational pulls of Venus and the Sun were balanced. Venus may have had a moon too at some point. But unlike the Earth, Venus was closer to the Sun, causing the moon to leave its original orbit around the planet and wander elsewhere.
Taking its proximity to the Sun, it would be extremely rare for Venus to have a moon around it, such that the orbit of the Satellite was the exact place of Gravitational Neutrality between Venus and the Sun, all thanks to this extra slim band. Hence, the sun greatly influences the chances of Venus having a Satellite in orbit around it.
How did the Earth sustain having Satellites? Even our Earth was lucky enough to have a Satellite. Several factors were responsible for the survival of our moon. The presence of Jupiter was the most important of all. If it wasn’t for Jupiter, our moon too would have been dragged towards an orbit around the sun, and slowly be destroyed by it.
The sun’s presence near our Earth is still strong enough to cause tides as high as a building. However, the sun’s presence gradually weakens as we go away from Venus. The moon is Earth's only natural satellite. It is thought to have formed about 4.5 billion years ago, not long after Earth itself. As we discussed earlier in this video, there are several theories about how the moon came to be, but the most widely accepted one is that a Mars-sized body collided with Earth, causing part of our planet's mantle to be hurled into space.
How did the Earth manage to keep its Satellite? The pull of gravity between the two bodies meant that the newborn moon was drawn into orbit around Earth. Although our planet has changed a great deal since then, the moon has remained relatively unchanged. It is thought that the moon's orbit has become gradually wider over time, but it will continue to circle Earth for many billions of years to come. Thanks to the stability provided by the moon, life on Earth has been able to flourish and evolve.
Without our loyal satellite, things would be very different indeed. Conclusion Whatever the reasons may be, we still wouldn’t ever be able to confirm the real reasons behind the absence of moons for the first two planets.
Is there still a possibility for Mercury or Venus to have moons? And if it is possible, then how? What if a large asteroid collided with either of them in the present time? Will it get a Satellite then, as the Earth got? The Universe is vast. We, humans, are just mere accidental happenings, with a sense of judgment.
There is a famous saying, Judging the happenings of our universe based on mere notable facts would be just like examining a teaspoon of ocean water and commenting on the existence of sharks and other humongous creatures, lurking in the sea. It motivates us to further release more interesting topics like this. If you feel like any other topic confuses you, do comment to help us know about it.