Podcaster: Avivah Yamani

Title: Why do Giant Planets Have More Satellites?

Organization: 365 Days Of Astronomy

Link :

Description: Why do giant planets have more moons compare to terrestrial planets?

Bio: Avivah is a project director of 365 Days Of Astronomy and astronomy communicator at langitselatan in Indonesia

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Hi, Welcome to the 365 Days Of Astronomy. This is Avivah your host today. Today’s Q&A topic is

Why do giant planets have more satellites?

Before we answer the question, please do remember that the force of gravity is directly proportional to the mass of the two bodies in question and inversely proportional to the square of their distance.

This means that the more massive a body is, the more gravity it has. But also, the farther you go away from it, the less gravity you experience.

For a short answer: The giant planets have more moons than the terrestrial planets because of their great masses which result in stronger gravitational fields. And giant planets also occupy greater space and hold larger volumes of mass in their surrounding atmospheres.

But before we jump into conclusions, lets explore a little bit about those planets in our Solar System. We usually divide all the planets in the Solar System into two groups. The terrestrial planets and the jovian planets. 

The inner planets, often classified as terrestrial planets, are Mercury, Venus, Earth and Mars. The terrestrial planets are closer to the Sun and are smaller and rockier.   

Jupiter, Saturn, Uranus and Neptune collectively make up the group known as the jovian planets. The general structures of the jovian planets are opposite those of the terrestrial planets.

Rather than having thin atmospheres around relatively large rocky bodies, the jovian planets have relatively small, dense cores surrounded by massive layers of gas.

Made almost entirely of hydrogen and helium, these planets do not have solid surfaces. Jovian planets also known as giants planets and are located farther away from the Sun than the rocky planets.

Why do objects that formed from the same cloud of gas have different compositions? The answer lies in where these objects formed in relation to the parent star, our Sun.

After the solar nebula collapsed to form our Sun, a disk of material formed around the new star.

The temperature across this protoplanetary disk was not uniform. Since different materials condense at different temperatures, our solar system formed different types of planets.

The frost line marked an important dividing point in the solar nebula.

All terrestrial planets formed inside the frost line and were closer to the Sun, where temperatures are high.  In this region, hydrogen compounds, such as water and methane remains gaseous.

Thus, the terrestrial planets accreted from planetesimals made of rocks and metals, they ended up too small to capture significant amounts of the abundant hydrogen and helium gas in the solar nebula.

Terrestrial planets have smaller mass and weaker gravitational force, which leads to smaller volume of space for a planet’s gravitational influence to pull more material and create a moon or just capture a moon.

The Sun’s gravitational force also plays a significant role which leads inner planets to have fewer moons. Any moon in a planet near the Sun will have an unstable orbit and later may be captured by the Sun.

The Earth’s moon formed billions of years ago when our planet collided with a Mars-sized object During the collision some material was ejected and was trapped in Earth’s gravity.

This ejected material then merged and formed the Moon. As for Mars, Phobos and Deimos are asteroids captured by the planet’s gravitational influence and later on, both objects were trapped as satellites.

Mercury and Venus are moonless.  One theory said both planets should have at least one small satellite.  The problem is they are located really near to the Sun and the Sun’s gravitational force dominates the region.

So even if those planets had a moon, it won’t be staying for long as it will have an unstable orbit and would be captured by the Sun.

Now how about the Jovian planets?

All four jovian planets have rings composed of countless small pieces of rock and ice, each orbiting its planet like a tiny moon. Why do they have so many moons?

The jovian planets formed outside the frost line, farther from the Sun, where the temperature is much colder. In this region, ices and rocks were plentiful. Rock and metal are still present in the outer solar system, but both are outnumbered and outweighed by the hydrogen compounds.

Thus, the planetesimals that formed in the outer solar system are composed primarily of hydrogen compounds with traces of rock and metal. The cores accreted rapidly into large clumps of ice and rock. Eventually, they got so large that they captured a large amount of hydrogen and other gasses from the surrounding nebula with their enormous gravity.

All jovian planets are massive and they do have stronger gravitational force. Since jovian planets are farther from the Sun, they will experience less influence from the Sun’s gravity.  So they will also have a larger volume under the planet’s gravitational influence.

Thus, they can capture all leftover material and had their own protoplanetary disk of gas, dust and ice surrounding them. All this material coalesced into regular moons orbiting the planet.

Because the giant planets gravitationally dominate a very large area, they also more easily capture passing objects as moons.

Well that’s the answer to why giant planets have more moons compared to terrestrial planets.

Thank you for listening.

This is 365 days Of Astronomy

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