Although Neptune is far away from Earth, it has never touched the true edge of the solar system. In the history of human astronomy, Pluto, the ninth largest planet for decades, has a perihelion distance of about 4.5 billion kilometers from the solar system, only slightly farther than Neptune.

So, many astronomers believe that Pluto was once a moon of Neptune. It's as if the moon is now a satellite of the Earth, but in billions of years, it will leave the Earth.

The distance between Pluto and the Sun at aphelion is 7.4 billion kilometers! According to Pluto's orbit when Thunder arrived at Neptune, Thunder would need to fly at least another 2 billion kilometers to reach Pluto. Pluto is already in the Kuiper Belt. There, it's the inner layer at the edge of the solar system.

As for the outer layer at the edge of the solar system, it is the Oort cloud, which has existed since the beginning of the solar system and is now 5 billion years old. Most of the material in this Oort cloud is the remnants of the formation of the sun and many planets in the solar system five billion years ago.

The total amount of this matter is roughly equivalent to 100 Earths. However, because it is so far away from the sun, although these materials generally surround the sun and form an Oort nebula, their orbits are very peculiar. It is completely in a chaotic state of random combination and collision. But it can't form a large-scale planet.

This Oort cloud is extremely vast. The inner side of the Oort cloud is almost two billion kilometers away from the Sun. That is, the distance from Uranus to the Sun. The Oort cloud in this area, that is, the nebula that formed the solar system in ancient times, is not too dense because of the clearance of Uranus and Neptune.

The outer side of the Oort cloud, on the other hand, is near the edge of the universe, and the distance between it and the sun is two light-years! One light-year is equivalent to almost 9,460 billion kilometers, and two light-years is equivalent to 18,920 billion kilometers! It is a huge and extremely large nebula cluster. This huge Oort cloud envelops the entire solar system.

This cloud may be a material remnant of a previous generation of stars. According to the Big Bang theory, the first generation of stars was formed not long after the formation of matter. These stars are huge, and most of them are supernovae.

However, the lifespan of this first generation of stars, also known as supernovae, was very short. It will quickly explode and form a nebula. And this nebula changes and collapses again, and a second batch of stars is formed. And so on and so forth. Generations of stars have been formed in the universe.

As for the existence of white dwarfs, neutron stars, etc., although they are extremely massive, they are actually only a part of the star. When a red giant star erupts, it throws all the outer material away, leaving behind the core of the white dwarf. And the matter that is thrown away is the root of the flow of matter between the stars in the universe.

The solar system is not the first generation of stars. Based on its size and time of formation, it is not known how many generations of stars it is.

However, it is very likely that the material in the Oort Nebula was left behind by the explosion of a previous generation of stars. Aggregate into individuals of various sizes, occasionally existing, becoming a giant cloud. Among them, the main substances are water, carbon monoxide, carbon dioxide, ammonia and methane, which, after solidification, form ice and are mixed with various dusts. Even if it was taken to the earth, it was a gray-black mass. Like a dirty snowball.

There is no chance that these dirty snowballs of different sizes will melt. Two light-years away, the sun is not as bright as Proxima Centauri's three-star system. And it is these materials, after polymerization, that form comets with strange orbits. Almost every comet has a long circular trajectory.

The most famous of these comets is Halley's Comet, which orbits the Sun once every 76 years and is observed by earthlings. And its apogee, according to calculations, is in the Oort Nebula. And the Iketani-Seki comet has a longer period, up to a thousand years!

At a speed of 500 kilometers per second, Comet Iketani-Seki can cover half of the solar system in a short time, but the rest of the distance is far farther than the solar system, and its return is also the Oort Nebula.

Because of its remoteness, many astronomers do not believe that the Oort Nebula is part of the solar system, but it does form around the solar system. Although it is two light-years away from the sun, it is even more immeasurably far away from other stars. So, where the Oort Nebula is located, is the actual edge of the solar system, formed before the formation of the sun and the solar system!

Pluto's Kuiper Belt is just a little farther away from the Oriort Nebula. The scientists on the Thunder, although very interested in publishing papers in top journals, are very interested in reading articles. Although he studies academic issues, he is often crazy to the extreme, and he doesn't even want to die.

However, there is enough to gain from exploring the solar system this time.

Moreover, no one thought that the Thunder would have to cross a distance of about a light year for the first time!

Therefore, whether it is those scientists who make corresponding exploration plans before departure, or these top scientists on Thunder, they have a tacit understanding to limit this exploration of the solar system to the Kuiper belt, that is, the orbit of Pluto. After the Thunder flew past the two ice giants of Uranus and Neptune, no one proposed to enter the densely populated Oort Nebula.

And Yun Lei borrowed the Lingbao of the Sea Mirror from the Yushan River that the two factions of Shushan and Kunlun that day, and spent a lot of effort to take out the Speculative Sea Mirror Spiritual Treasure from Daiyu Mountain. This spiritual treasure, when it is on the earth, has an extremely weak effect. Many advanced scientific instruments of human beings can replace their role in finding and locating.

However, in the stars of the universe, the role of this sea mirror has been magnified a lot.

According to the classics left by the merman clan and the clues in the classics of the two factions of Shushan and Kunlun, Yun Lei roughly deduced the location of the inheritance left by the ancient immortal Buddha when he left the solar system.

On this day, the Thunder traveled to the vicinity of Pluto, and with the help of a telescope, it was already able to see Pluto orbiting in the dim cosmic background space. On this sea mirror, a flash of spiritual light suddenly flashed!