Hills cloud

The Hills cloud is a theoretical, vast circumstellar disc located within our Solar System.

It is positioned inside the Oort cloud and sits roughly along the solar ecliptic plane. This plane is the flat path that most planets follow as they orbit the Sun. Scientists believe the outer border of this cloud reaches about 20,000 to 30,000 astronomical units (AU) from the Sun. One AU is the distance between the Earth and the Sun. The inner border is harder to define, but it sits well beyond the orbits of the planets and the Kuiper Belt. If it exists, the Hills cloud might contain five to ten times as many comets as the Oort cloud.

This hypothesis exists to solve a major problem regarding comet dynamics. Comets in the Oort cloud are constantly being perturbed, or nudged, by their environment. These disturbances cause many comets to leave the Solar System entirely. Others may fall into the Sun, collide with planets, or be ejected by giant planets. Because of this constant loss, the Oort cloud should have been depleted long ago. However, we still see a steady supply of comets. The Hills cloud acts as a reservoir to solve this mystery. It is a densely populated region that sends objects outward to resupply the Oort cloud.

Astronomers have been studying comet reservoirs for many decades. In 1932, Ernst Öpik hypothesized that comets lived in a cloud at the edge of the Solar System.

Jan Oort revived this idea in 1950 to explain why comets had not all been destroyed. Oort studied 46 comets to find patterns in their orbits. In 1981, Jack G. Hills, an astronomer from the Los Alamos Laboratory, proposed the existence of an internal section. He suggested this inner region could contain tens or even hundreds of times more comet nuclei than the outer halo. This discovery gave the region its name, the Hills cloud. Other scientists like Sidney van den Bergh and Mark E. Bailey also studied its structure in the early 1980s.

The structure and composition of the Hills cloud are subjects of intense study. The cloud is expected to be much denser than the outer Oort cloud. While the Oort cloud is located between 50,000 and 100,000 AU, the Hills cloud is thought to sit between 5,000 and 20,000 AU. The total mass of the cloud is not yet known. However, Mark E. Bailey estimated the mass could be 13.8 Earth masses if most bodies are at 10,000 AU. Most objects in this region are likely made of various ices. These include water, methane, ethane, carbon monoxide, and hydrogen cyanide. Some evidence suggests the cloud might also contain rocky objects.

Scientists have different ideas about how this cloud formed. One theory suggests the Hills cloud formed during a close encounter with another star. This encounter might have happened within the first 800 million years of the Solar System. Such an event could explain the strange, eccentric orbits of certain objects like the dwarf planet Sedna. Sedna is a unique object because its orbit is not influenced by Neptune or Jupiter. This makes it a "detached object." Some researchers believe the Hills cloud might actually be younger than the Oort cloud because of this formation history.

We can find clues about the Hills cloud by looking at specific comets and dwarf planets. Several famous comets have orbits that suggest they come from this distant region. Comet Hyakutake has an outbound aphelion of 3,500 AU. Comet McNaught was a very bright comet that reached an aphelion of 4,100 AU.

Comet Machholz also came from about 5,000 AU. Additionally, the dwarf planet Sedna provides a significant clue. Discovered in 2003, Sedna has a very red surface made of water ice, methane, and nitrogen. Its orbit is so far out that it is not considered a Kuiper Belt object.

Understanding the Hills cloud helps us connect different parts of our Solar System. It links the behavior of long-period comets to the larger structure of the outer Solar System. By studying these icy bodies, astronomers can learn how the protoplanetary disk formed. This disk is the original ring of gas and dust that created our planets. Even though the Hills cloud remains theoretical, it provides a vital framework for understanding how comets are supplied to our inner Solar System. It helps explain why the space around us remains active and full of moving objects.