太阳系直径多少光年（测算及特点分享）

The boundary of the solar system extends beyond the eight recognized planets and beyond the dwarf planets as well.

The solar system can be defined as a system constrained by gravity, which includes the Sun and the objects that orbit it, either directly or indirectly. Among the objects that orbit the Sun directly, the largest eight are planets, while the rest are minor planets such as dwarf planets and small solar system bodies.

The Sun and the solar system are located within the Milky Way, which is a barred spiral galaxy with a diameter of approximately 100,000 light years and contains billions of stars. The Sun is located in one of the outer spiral arms of the Milky Way, known as the Orion–Cygnus Arm or the Local Arm. The Sun is approximately 25,000 to 28,000 light years away from the center of the Milky Way.

The image below shows the position of the Sun in the Milky Way. The angles represent the longitudes in the galactic coordinate system.

An astronomical unit (AU) is the average distance between the Earth and the Sun, rounded to approximately 150 million kilometers. More precisely, one AU is equal to 149,597,870,700 meters, which is approximately 1.5813×10-5 light years (ly).

The inner planets are the closest to the Sun. Mercury is approximately 0.39 AU from the Sun, followed by Venus, Earth, and Mars. The asteroid belt occupies the region between Mars and Jupiter, with distances from the Sun ranging from 2.3 to 3.3 AU.

The four outer planets or gas giants (Jupiter, Saturn, Uranus, and Neptune) account for 99% of the known mass orbiting the Sun.

Centaurs are small bodies in the solar system with orbits located between Jupiter and Neptune. Their orbits are unstable as they cross or have crossed the orbits of one or more gas giants, and almost all centaurs have dynamic lifetimes of only a few million years. Centaurs exhibit characteristics of both asteroids and comets.

Comets have highly eccentric orbits, usually with a perihelion within the orbits of the inner planets and an aphelion beyond Pluto. Short-period comets are believed to originate from the Kuiper Belt, while long-period comets, such as Hale–Bopp, are considered to originate from the Oort Cloud.

Outside the orbit of Neptune, there is a region known as the "trans-Neptunian region," where the Kuiper Belt, shaped like a doughnut, is located. It is also the home of Pluto and some other dwarf planets.

Pluto's average distance from the Sun is approximately 39.5 AU. It is estimated that the Kuiper Belt extends from approximately 39.4 AU to 47.7 AU. The scattered disk overlaps with the Kuiper Belt but extends farther out and is considered to be the source of short-period comets.

The Oort Cloud is a hypothetical spherical cloud composed of as many as one trillion icy bodies, believed to be the origin of all long-period comets. It is located at distances ranging from approximately 50,000 AU to potentially as far as 100,000 AU (approximately 1 light year to less than 2 light years) from the Sun. It is believed to be composed of comets that were ejected from the inner solar system due to gravitational interactions with the outer planets.

The extent of the solar system does not end here.

The boundary of the solar system and the beginning of interstellar space has not been precisely defined, as its outer boundary is formed by two different forces: solar wind and solar gravity. The limit of solar wind influence is approximately four times the distance from Pluto to the Sun, while the solar apex represents the outer boundary of the solar layer and is considered the beginning of interstellar matter.

The solar wind layer is a stellar wind bubble, a space region dominated by the Sun that emits solar wind at approximately 400 kilometers per second, which is a stream of charged particles, until it collides with the stellar wind of interstellar matter.

The collision occurs at the termination shock, approximately 80-100 AU from the Sun in the direction against the stellar wind and approximately 200 AU in the direction of the stellar wind.

There is speculation about an arch-shaped shock above the solar apex, but data from the Voyager spacecraft suggest that the speed at which the Sun moves through interstellar matter is too slow to form an arch-shaped shock. It may be a more gentle "bow wave."

We should also mention 90377 Sedna, a large minor planet located in the outer reaches of the solar system. As of 2015, it is approximately 86 astronomical units (AU) from the Sun, which is roughly three times the distance to Neptune. For most of its orbit, it is even farther from the Sun than it is now, and its aphelion distance is estimated to be 937 AU (31 times the distance to Neptune), making it one of the farthest known objects in the solar system apart from the long-period comets.

Sedna has an exceptionally long and elongated orbit with a period of approximately 11,400 years and a perihelion distance of 76 AU.

Here is an (artistic) image showing the inner heliosheath, with the inner region bounded by the termination shock and the outer region constrained by the solar apex:

This is an illustration on a logarithmic scale showing the layout and extent of the solar system, including the Kuiper Belt, the heliosphere, and the Oort Cloud:

References

1. Wikipedia

2. Glossary of Astronomical Terms

3. Quora - f2003911

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