The origin of the interstellar "hula hoop": Why do some planets have a "ring"?

In 2012, scientists discovered an extrasolar object called J1407B, which instantly attracted the attention of astronomy enthusiasts around the world. It is located in the constellation Centaurus, about 434 light-years away from the Earth. Shockingly, it has an unimaginably large star ring system, with a diameter of 180 million kilometers, which is more than 200 times the diameter of Saturn's rings. Such a large star ring system can be said to be beyond imagination. Although the scientific community has not yet reached a conclusion on the formation mechanism of the J1407B star ring, such a shocking scene can't help but make us full of curiosity about the "star ring" as a cosmic wonder.

Planetary rings, in simple terms, are ring-shaped structures made up of materials orbiting around a planet. These materials are mainly composed of countless tiny particles and some blocky materials, ranging from the size of dust to the size of tens of meters. They rotate around the equatorial plane of the planet, forming a flat and wide ring belt.

Take the solar system as an example. The most familiar thing to us is Saturn's rings. When Galileo first observed Saturn through a telescope in the 17th century, he saw two "ears" on both sides of Saturn, which are actually Saturn's rings. With the continuous advancement of observation technology, we can now clearly see the exquisite structure of Saturn's rings, which are composed of countless ice and rock fragments. They sparkle charmingly under the sunlight, as if a brilliant Milky Way surrounds Saturn.

In the solar system, in addition to Saturn, gas giants such as Jupiter, Uranus and Neptune also have their own planetary rings, which are either obvious or hidden. Although the rings of these different planets have their own characteristics, they all follow similar basic composition and movement laws. They are unique marks in the process of planetary evolution and witness the vicissitudes of the solar system for billions of years.

So how are planetary rings formed?

The formation mechanism of planetary rings is a complex scientific issue that is still being explored. Scientists have proposed several mainstream hypotheses, each of which attempts to explain the reasons for the birth of different types of planetary rings.

Satellite fragmentation hypothesis

This hypothesis holds that planetary rings may originate from the breakup of satellites. In the long years after a planet is formed, satellites orbiting it may be destroyed for various reasons. For example, a satellite that was originally operating stably is suddenly hit by an asteroid, and the huge energy generated by the impact instantly shatters the satellite. Under the influence of the planet's gravity, the satellite fragments begin to disperse, but they cannot escape the planet's gravitational pull and can only gradually spread near the planet's equatorial plane to form a ring structure.

Taking Saturn as an example, some scientists speculate that some of Saturn's satellites may have experienced such a disaster in ancient times. These broken satellite fragments contain rocks and ice of different compositions. They inherit the original orbital characteristics of the satellites and continue to revolve around Saturn. Over time, they eventually formed the magnificent Saturn rings we see today.

Roche limit hypothesis

The Roche limit is a crucial concept in celestial mechanics. When a small celestial body approaches a large celestial body, if the distance is less than a certain value (i.e., the Roche limit), the tidal force of the large celestial body will exceed the gravity of the small celestial body itself, causing the small celestial body to be torn apart. In a planetary system, some small celestial bodies that originally orbited the planets may gradually approach the planets due to changes in their orbits or interactions with other celestial bodies, and eventually enter the Roche limit range. As a result, these small celestial bodies were ruthlessly torn apart by the tidal forces of the planets, and their remains were scattered around the planets, forming planetary rings. For example, scientists speculate that some comets accidentally entered the Roche limit when approaching gas giants such as Jupiter, and the fragments of the disintegrated comets contributed new sources of material to the planetary rings, allowing the planetary rings to continue to update and evolve.

Protoplanetary disk remnant hypothesis

During the planet formation process, planets condense from protoplanetary disks. Protoplanetary disks are accretion disks composed of gas, dust, ice and other materials. When a planet is formed, there may still be some materials that have not been completely absorbed or gathered to form satellites. These materials form planetary rings near the equatorial plane of the planet.

It is worth noting that these hypotheses are not mutually exclusive. The formation process of different planetary rings may be the result of the combined action of multiple factors.

Although scientists have conducted many in-depth studies on planetary rings, there are still many unsolved mysteries waiting to be revealed in the field of planetary rings.

First, the fine distribution and evolution of matter in planetary rings remains a difficult problem. Although we roughly understand that planetary rings are composed of particles of various sizes, there is still a lack of a complete theoretical model for how these particles achieve long-term stable distribution and slow evolution under the influence of multiple factors such as the planet's gravity, electromagnetic force, and its own collisions.

Secondly, the interaction between planetary rings and planetary magnetic fields also hides many mysteries. The strong magnetic field of a planet will affect the charged particles in the planetary rings, triggering a series of complex electromagnetic phenomena. For example, in Jupiter's rings, scientists have observed some radiation characteristics related to the magnetic field, but the specific energy transfer, particle acceleration and other processes are still unclear. Whether these electromagnetic interactions play a key role in the structure, stability and evolution of planetary rings remains to be further explored.

Finally, the discovery of an exoplanetary ring system may open up broader ideas for us. For example, a huge exoplanetary ring system like J1407B, according to scientists, may be a very young celestial body, and its rings may be formed because the protoplanetary disk has not completely condensed into satellites, so many of its ring fragments may slowly merge into satellites in the future. Studying its evolution process will help us better understand the evolution of the rings in our solar system.

As a unique and fascinating celestial structure in the universe, planetary rings carry the history of planetary evolution and contain endless scientific mysteries waiting for us to unravel one by one.

Author: Stars Become Light

Reviewer: Li Mingtao, researcher at the National Space Science Center of the Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.