Why are there so many rocks floating in the vast universe? How many planets are made of rocks?

This article is based on answering a question from netizens: Most planets in the universe are made of stone, so the question is, where do these stones come from?

The argument for this question itself is wrong, so the so-called "then the question arises" has nothing to do with the previous argument.

The "so here comes the question" is based on the cognition that "most planets are rocky". It is just a conclusion drawn without any basis, so the subsequent "problem" is unclear.

In order to clarify such misconceptions, let’s talk about some basic common sense here.

First, most planets are not rocky.

Take the solar system for example. There are 8 major planets, 4 of which are terrestrial planets, that is, planets with a solid rocky shell. They are arranged from near to far from the sun: Mercury, Venus, Earth, and Mars. In addition to these planets with rocky shells, there are 4 Jovian planets, that is, planets mainly composed of gas. They are all outside the orbit of Mars, and arranged from near to far: Jupiter, Saturn, Uranus, and Neptune.

Thus, we can see that even in the solar system, not all planets are rocky. In terms of quantity, it is a tie of 4:4. But what about mass? The rocky planets are dwarfed by the giants. The total mass of the four terrestrial planets is not as large as the smallest of these gas planets.

The smallest Jovian planet is Uranus, with a mass of 8.681*10^25 kg. The masses of the four terrestrial planets are: Mercury is about 3.3011*10^23 kg, Venus is about 4.8675*10^24 kg, Earth is about 5.97237*10^24 kg, and Mars is about 6.4171*10^23 kg. The total mass of the four terrestrial planets, with Earth as the largest, is only about 1.18*10^25kg, which is only about 13.6% of the mass of Uranus.

The combined mass of the four Jovian planets is about 2.65576*10^27kg, which is 225 times the combined mass of the four terrestrial planets. Jupiter is the largest planet in the solar system, with a mass 318 times that of the Earth and 2.5 times the combined mass of all the planets. In the solar system, the Sun, a plasma gas planet, accounts for 99.86% of the total mass, Jupiter accounts for 0.1% of the total mass of the solar system, and the Earth accounts for only 0.0003% of the mass of the entire solar system.

Are there rocks among the Jovian planets?

Of course, but not many.

Jovian planets or gas planets are planets without a solid surface, and they also have a rocky core at their core. However, this core is relatively small, generally about 1 to 3 times the diameter of the Earth, and only accounts for a very small part of the mass of the gas planet. The gas planets are not completely in a gaseous state from the atmosphere to the core, because these planets have huge mass and volume. As the atmosphere continues to go deeper into the core, the pressure will become greater and greater, and the temperature will also become higher and higher. Under this high temperature and high pressure, the gas will also phase into a liquid or metallic state.

Jupiter has a radius of about 70,000 kilometers. Above its core, there is tens of thousands of kilometers of metallic hydrogen, and above that, tens of thousands of kilometers of liquid hydrogen. Its core temperature is as high as 30,000 degrees Celsius; Saturn's interior is similar to Jupiter's; deep in the atmosphere of Uranus and Neptune, there are oceans of methane, water ice, and diamonds. These substances, even in liquid form under high pressure, are extremely dense and hot.

Therefore, in the universe, rocky planets are only a very small minority.

Among the satellites of the Jovian planets and the Kuiper Belt far from the sun, many planets are in an icy state due to extremely low temperatures, such as Europa, Tethys, Pluto, etc., which are all ice balls covered with thick ice layers. However, these ices are not necessarily all water ice. For example, the surface plains of Pluto are covered with more than 98% nitrogen ice, trace amounts of methane and carbon monoxide.

Therefore, not only planets, but also satellites and asteroids, are composed of very few rocks. This is related to the abundance of elements in the universe. Hydrogen and helium account for more than 98% of the entire universe, and other heavy elements account for only a little more than 1%. This determines that the composition of celestial bodies in the universe is composed of very few heavy elements.

Stars are the mother of all elements

All matter is made up of elements, so we have to talk about where the elements in the universe come from. It is now known that there are 118 elements in our universe, all of which are generated by the process of stellar evolution.

At the moment when the Big Bang began, there was only energy, no elements. All the elements now are converted from energy, which strictly conforms to the description and calculation of Einstein's mass-energy equation. At the beginning of the Big Bang, matter began to form, but due to the extremely high temperature and high density, only some basic particles could be gradually generated, and atoms could not be formed.

It was not until 380,000 years after the Big Bang that the universe expanded and cooled, and the temperature dropped below 3000K. Electromagnetic waves (or "light") then decoupled from the high-temperature and high-density universe, and the universe began to become transparent and observable. The cosmic microwave background radiation observed now was emitted at that time.

At this time, the basis of all matter, neutral atoms, finally began to form. But at that time, only the lightest and simplest elements could be formed, namely hydrogen and helium, which are numbered 1 and 2 in the periodic table, and a very small amount of lithium, number 3. The particles composed of these elements floated in the universe, and under the action of gravity, they moved closer to each other and gradually formed the original nebula.

These nebulae continued to shrink under the influence of gravity, and the first stars were born in the nebulae. Then the universe continued to emerge, and the universe was full of stars. The stars gathered into galaxies (some people call them cosmic islands), and gradually the universe took on the shape we see today.

Stars are spheres of plasma that emit light and heat. Their huge mass creates enormous centripetal pressure, resulting in high core pressure and temperature, which stimulates hydrogen nuclear fusion. Stars continuously form heavier elements through nuclear fusion, such as from hydrogen to helium, and then to beryllium, boron, carbon, etc. The temperature and pressure of small and medium mass stars can only end here.

The temperature and pressure in the core of a massive star are extremely high, and it will stimulate nuclear fusion of heavier elements along the way, generating nitrogen, oxygen, fluorine, neon, sodium, magnesium, silicon, phosphorus, sulfur, calcium and other heavier elements in sequence, all the way to iron, the 26th element.

No matter how big a star is, nuclear fusion in its core can only produce the heaviest element, iron. As for why, it's a long story that has been explained many times in the past, so I won't go into the details here.

Where do elements heavier than iron come from?

If no more powerful events occurred in the universe, our world would only have 26 elements. Although calcium and silicon, the elements for making stones, were already available at that time, all creatures on Earth are composed of dozens or hundreds of elements. Without the subsequent generation of heavy elements, life would not have been possible, and there would have been no humans like us who observe the universe and comment on it.

Fortunately, there are events such as supernova explosions and collisions of massive dense celestial bodies in the universe. These events can lead to even greater pressure and temperature. In this way, 92 elements above iron are born in these extreme cosmic events.

These elements generated by these extreme events attract, collide and condense with each other in the universe to form various cosmic bodies, including rocks, ice, iron, gold, etc. Of course, many times these substances are not pure single substances, but mixtures or compounds.

This is the origin of stones in the universe and the origin of the earth.

The solar system was not born in a pure primordial nebula

From the previous introduction, we know that the elements in the original nebula of the universe are only hydrogen and helium, and there is a very small amount of lithium. However, the solar system has a complex composition and contains all 118 elements in the universe, so it is impossible for it to be born from such a primitive and pure nebula.

Nebulae are the mother of stars. All star systems are formed in huge nebulae of light years. From the generation and origin of cosmic elements, we can infer that the solar system was not born from a primordial nebula with only hydrogen and helium. If there were only hydrogen and helium, there would be no earth, let alone life.

Therefore, scientists have inferred that the solar system was born from a regenerated nebula formed by the re-condensation of elementary particles that were scattered into space by extreme events such as supernova explosions or collisions of massive celestial bodies. Moreover, such explosions may have occurred many times before the solar system was born.

At the beginning of the formation of any star system, huge radiation pressure, or stellar wind, is generated when stars are formed. These powerful stellar winds will blow nearby interstellar matter away. The lighter matter is blown farther, while the heavier matter is not blown far enough.

Therefore, the several planets in the solar system that are close to the sun, also called inner planets, are composed of heavier materials that are not blown away by the solar wind and appear as rocky planets; the farther outer planets are mainly composed of lighter elements that are blown away by the solar wind and appear as gaseous planets.

Having said so much, I wonder if those who have these questions have been answered? If you still have questions, please leave a message for discussion. Thank you for reading.

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