Why don’t Venus and Mercury have Moons?
(Formation of Solar System)
Spinning around our Sun are nine planets, more than 120 moons, millions of asteroids
and other rocky objects, and countless comets. Together, they make up the Solar System,
our home in this vast universe. Closest to the sun lies Mercury, 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 notable
characteristic feature unique to them. The Earth sustains life to grow, while venus receives rain
of extremely corrosive Sulphuric Acid. All Planets in our Solar System are surrounded by their mini
planets, or better to say, their Satellites, which revolve around them in certain fixed orbits.
The orbits can be distinguished and recognised by using Kepler's Laws of motion. The moon is
the closest celestial body to the Earth. It is Earth’s only natural Satellite. Most satellites
are much smaller than their parent planet. Earth has only one Satellite, Mars has two Satellites,
and Jupiter has about 80 of them, including the largest satellite in our Solar System, Ganymede.
Saturn has more than eight two moons. Uranus has twenty-seven Satellites, while Neptune has about
fourteen of them. Even our dwarf neighbour, Pluto, has five Satellites revolving around it. However,
the first two planets of our Solar System, Mercury and Venus, surprisingly, do not possess moons.
But, Why? Why are only the first two Planets exception to this characteristic feature? And
How exactly were the Satellites formed? In this video, we will have a detailed study of
Satellites and the Factors affecting the formation of Satellites. Before we continue, make sure you
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What are Satellites? Before we move on, Let’s discuss what exactly are
Satellites and how are Satellites formed. (Spoiler alert, All our answers lie in the formation of the
Solar System and the aftermath of the Solar System’s formation.) Satellites are celestial
bodies that revolve around a much larger celestial body than them, usually a planet, or a dwarf
planet. However, they can also orbit large massive astronomical bodies such as large asteroid chunks
and Planets. In scientific terms, any celestial body that has a mass of about a thousand times
that of its parent planet and has an identified orbit around it can be termed to be a Satellite.
Do not confuse these satellites with the ones Elon musk and others are launching into space to
help you with your Wi-Fi. Those satellites are way different from the one in our discussion.
How are Satellites formed? -> Basic ways (Giant Impact and Capture theory)
The formation of these Satellites is a by-product of the formation of planets. It can occur in
mainly four ways, although many other ways are still undiscovered (we might discover
more ways of their formation someday). Satellite formation can occur through "the Giant Impact",
which is the collision of two or more large Celestial bodies.
The most famous example that you may have heard of is that of our moon. Many theories are predicting
the formation of our moon. Although the most suitable theory describes the formation of the
moon as a by-product of collision between Earth and another Mars-sized planet, ejecting matter in
space which came together under influence of their gravity, hence, forming our moon. Well, we will
come to our moon’s formation theories later in this video, as it is a whole new topic in itself.
Another theory describing Satellite formation is known as “The Capture Theory”.
This theory was proposed by an American Astronomer, Thomas Jefferson, who at that time was
studying the moon formation theories. This theory describes the capture of tiny Planetary Satellite
seeming objects which were formed elsewhere by Large Planets. Most of these satellites
are irregularly shaped objects as they don’t have enough mass to sustain a spherical shape.
Phoebe, a moon of Saturn, is believed to have been captured by Saturn under some circumstances.
How are Satellites formed? -> Other ways (Condensation and Fission theory)
Another theory explaining Satellite formation is the “Fission Theory”, or as we may say,
“the wear and tear theory”. During the early stage of Planet formation, that is, when the Planet
still existed in a molten state, if the Planet rotates at a very high angular velocity, there
is a high possibility that a part of the Planet may be ripped apart leading to the formation of
a tiny Planetary satellite, as proposed by the angular momentum conservation 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 + 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. This material then coalesced to form the moon.
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. Haha,
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