Astronomers discovered Uranus in 178 1 year, Neptune in 1846 and Pluto in 1930.

But Pluto is only 2322 kilometers in diameter, smaller than the moon, and its mass is not enough to affect Neptune's orbit.

Subsequent observations show that Neptune's orbit is completely in line with expectations, and there is no need for a large mass perturber.

So far, there are many icy celestial bodies outside Neptune's orbit, and we have only detected a small part of them.

There are about 2000 celestial bodies moving near Pluto.

That area is called the Kuiper Belt. Its boundary is roughly located at 48 astronomical units (1 astronomical unit is equal to the distance between the sun and the earth), because the number of celestial bodies has decreased rapidly since then.

The Kuiper Belt is the remains of the primitive solar nebula (the nebula that gave birth to the solar system). That area is vast, the density of matter is very low, and it is so far away from the sun that matter can't collapse to form a planet.

In fact, it is difficult to form any celestial bodies in the outer solar system. Even Uranus and Neptune were probably not formed in situ, but were kicked to their present positions when interacting with Jupiter and Saturn.

Short-period comets (the semi-long axis of the orbit is only a few dozen astronomical units, and the orbital inclination is very small) may be celestial bodies that have just escaped in the Kuiper Belt.

Although the main part of the Kuiper Belt ends at about 48 astronomical units, there is another celestial gathering area at the edge of the solar system, where long-period comets come from.

The length of the semi-major axis of the long-period comet orbit can usually reach tens of thousands of astronomical units. The orbits of the planets in the solar system are almost all on the same plane (the planets in the solar system are actually on the ecliptic plane).

Oort cloud may have formed at the same time as the planets in the solar system.

During the formation of the solar system, there were also many celestial bodies in the area where giant planets were located. Most of them were swallowed up by giant planets.

However, the growing planets also throw out some celestial bodies.

Most of them were thrown out of the solar system and flew into interstellar space.

However, there are still 1%- 10% celestial bodies that can't get rid of the shackles of the solar system due to lack of energy, and finally can only wander in the distant outer solar system.

Celestial bodies that have been thrown out but failed to escape the gravity of the sun all move in elliptical orbits, which may move to thousands or even tens of thousands of astronomical units.

However, they can still get close to the sun (perihelion in its orbit) and pass where they are kicked out.

So the orbits of these celestial bodies are still partial.

It is located in the giant planet area and has the opportunity to interact strongly with massive planets again.

The result of the action is either a final collision or they are completely kicked out of the solar system.

Celestial bodies in elliptical orbits gather loosely together to form Oort clouds.

There, the sun's gravity is already weak, but the forces of neighboring stars, the center of the Milky Way and the disk of the Milky Way have begun to dominate.

These forces, similar to tidal force, can pull the perihelion of celestial bodies outward so that they will not collide with giant planets like Jupiter again.

With the passage of time, these forces randomly change the orbit and inclination of Oort cloud objects, causing some of them to escape from the solar system and enter interstellar space.

Other celestial bodies are thrown back near the planet and become long-period comets we see.

In fact, the inner solar system may have experienced comet rain due to the accidental passage of other stars.

Scientists believe that the convergence of stars causes comets to hit the earth frequently, which may lead to the extinction of life on earth. However, the consequences caused by the rendezvous of stars are very unpredictable.

So, what celestial bodies are there in the area between Oort Cloud and Kuiper Belt?

Astronomers once thought that there was no celestial body, and its entire orbit was in that region, because the tidal force of the Milky Way there was not enough to completely move the orbit perihelion of the celestial body out of the planetary region of the solar system.

In 2003, astronomers made a shallow survey of almost all the observable sky areas in the northern hemisphere by using the Samuel Oshin telescope with a diameter of 0/0.2 meter at Paloma Observatory in the United States.

At the same time, they found Sedna. Its head is about 1000 km, the perihelion is located at 76 astronomical units, and the semi-major axis is 532 astronomical units.

It is the first celestial body whose orbit lies entirely in this region.

Sedna is so unexpected and unusual that astronomers have to rethink the formation of the solar system.

10 years later, Gemini Observatory discovered 20 12 VP 1 13. Its orbital perihelion is located at 80 astronomical units, and Viced's orbital perihelion is far away.

Surprisingly, the semi-long axis of its orbit is smaller than Viced, with only 265 astronomical units.

The orbits of these two celestial bodies are very stable. At present, they have no strong interaction with any known celestial bodies in the solar system.

Nevertheless, the fact that they have extremely elliptical orbits shows that they must have collided with some celestial bodies at some time.

Some astronomers call them inner Oort clouds because they are not as easily manipulated by the tidal force of the Milky Way as the farther outer Oort clouds.

In other words, the orbit of Oort Cloud has remained stable since ancient times, so it is essentially a "fossil" that preserves the formation information of the solar system.

Sedna was discovered with the largest pixel camera during an effective survey of the sky.

When astronomers installed this camera on a telescope with a larger aperture, they found 20 12 VP 1 13.

The dark energy camera on the 4-meter-diameter Blanco telescope can capture an area of about 2.7 square degrees at the American Observatory in Tololo Mountain, Chile.

Such a large image is equivalent to the sum of 1 1 full moon area, which is several times larger than the sky area taken by all cameras on the previous 4-meter or larger telescope.

We continue to look for distant places and hope to find more IOC objects in the next few years.

The farther away a celestial body is from us, the darker it looks. Therefore, there are probably many large celestial bodies hidden in the outer solar system.

What we see is the sunlight reflected from their surfaces.

Sunlight first reaches the celestial body, reflects on its surface, and then reaches the earth. When the distance between the celestial body and us doubles, its brightness will decrease by 16 times. Therefore, we can only see Sedna and 20 12 VP 1 13 near perihelion.

Besides, we can't see them most of the time.

In the same way, we can't see those celestial bodies that are about the size of Mars and have similar orbits, because they are too far away from us and very dim.

There may be no more big planets in the solar system, otherwise NASA's wide-field infrared sky survey probe will detect their warm atmosphere in the infrared band.

Giant planets emit more heat than they absorb from the sun because the energy accumulated during the formation of planets has not been exhausted.

On the celestial bodies on the edge of the Kuiper Belt, we noticed the similarity between them: these 12 celestial bodies have similar perihelion amplitude angles.

The amplitude angle of perihelion is the angle between the perihelion of orbit and the rising intersection of orbit on the ecliptic plane. The amplitude angle of the perihelion is 0, which means that the orbital perihelion of the celestial body is in the ecliptic plane, and 90 degrees means that the celestial body deviates farthest from the ecliptic plane when it moves to the perihelion.

None of these distant celestial bodies has a perihelion angle of more than tens of degrees. This is completely unexpected.

We think their perihelion angles should be randomly distributed. One possible explanation is that an unknown massive celestial body is manipulating them to orbit with an amplitude similar to perihelion.

The formation process of celestial bodies on the edge of 10 Kuiper Belt may be similar to Sedna, 20 12 VP 1 13.

However, another possible explanation is that they once interacted with Neptune because their orbital perihelion is closer to Neptune's sphere of influence.