Oort cloud

The Oort cloud is a huge, distant area around our Sun, filled with billions of icy objects. It is thought to stretch from about 2,000 to 200,000 astronomical units from the Sun, much farther than where the planets are. This cloud was first suggested by a Dutch astronomer named Jan Oort in 1950. He believed it could be the source of long-period comets that sometimes visit the inner parts of our Solar System.

Scientists think the Oort cloud has two parts: an inner part shaped like a disk and an outer part that is round. Both of these parts are far beyond where the Sun’s influence ends, floating in the space between stars. Even though we cannot see the Oort cloud directly with our telescopes, it plays an important role. When the gravity of passing stars or our galaxy nudges these icy objects, some may fall toward the Sun, becoming the comets we sometimes see in our night sky.

The Oort cloud is believed to be made of material that once formed closer to the Sun but was pushed far out by the gravity of the giant planets. Today, it may be the main source of many comets that enter the inner Solar System.

Development of theory

By the early 1900s, astronomers knew about two kinds of comets: short-period comets and long-period comets. Short-period comets have small orbits close to the ecliptic plane and stay near the Kuiper cliff, about 50 AU from the Sun. Long-period comets have huge orbits, thousands of AU from the Sun, and can appear from any direction in the sky.

In 1932, the astronomer Ernst Öpik suggested that long-period comets might come from a cloud at the edge of the Solar System. Later, in 1950, the Dutch astronomer Jan Oort expanded this idea. He noticed that comets with orbits very close to the Sun couldn’t have been that way since the Solar System formed. He thought these comets must come from a faraway place. Oort also found that many long-period comets seem to come from about 20,000 AU from the Sun, suggesting a spherical cloud there. He believed that comets with orbits around 10,000 AU might have been pulled inward by the gravity of planets.

Structure and composition

The Oort cloud is a huge area around the Sun. It stretches from about 2,000 to 200,000 AU away. It has two parts: an outer round part and an inner torus-shaped part, called the Hills cloud.

The inner Oort cloud is denser and helps fill the Oort cloud. The outer Oort cloud may have trillions of icy objects larger than 1 km, made of water ice and frozen gases. These objects are very far apart. Their total mass is about five times that of Earth.

Origin

The Oort cloud formed after the formation of planets from a cloud of dust and gas around 4.6 billion years ago. Scientists think these objects first formed closer to the Sun, along with the planets. The gravity of big planets like Jupiter changed their paths, sending them far out into space.

Studies suggest many of these objects may have come from other stars near our Sun. Over time, their paths became more circular, giving the Oort cloud its round shape. This cloud likely grew to its biggest size about 800 million years after the Solar System began.

Comets

Further information: Halley-type comet and List of Halley-type comets

Further information: Jupiter-family comet and List of periodic comets § List of unnumbered Jupiter-Family comets

Further information: List of centaurs (small Solar System bodies)

Comets are pieces left over from when our Solar System formed about 4 billion years ago. They are found in two main places: the Kuiper belt and the Oort cloud. Comets that return to Earth’s neighborhood every 200 years or less usually come from the Kuiper belt or the scattered disc, two areas of icy material beyond the planet Neptune. Very long-period comets, whose orbits take millions of years, are believed to come from the Oort cloud. Examples include C/1999 F1 (Catalina), C/2006 P1 (McNaught), and C/2010 X1 (Elenin).

There are two main types of short-period comets. Jupiter-family comets have orbits smaller than 5 AU, and Halley-family comets, named after Halley's Comet, are thought to come from the Oort cloud. The giant planet Jupiter may pull comets into it. An example of a comet from the Oort cloud is C/2018 F4.

Sedna and similar objects

See also: Sedna (dwarf planet)

Scientists have found some objects that might be part of the inner Oort cloud. Sedna, discovered in 2004, has an orbit that takes it very far from the Sun and then very close. Another object, 2012 VP₁₁₃, found in 2012, also moves far from the Sun but not as far as Sedna when it is at its farthest point. Other possible members of this group include 2010 GB₁₇₄ and 474640 Alicanto (originally 2004 VN₁₁₂).

Tidal effects

Further information: Galactic tide

Many comets near the Sun come from the Oort cloud because of the tidal force from the Milky Way. Like the Moon changes Earth’s tides, the Milky Way’s gravity changes paths of objects in the outer Solar System. In the outer Solar System, the Sun’s gravity is weaker. So the Milky Way’s gravity can push objects from the Oort cloud closer to the Sun.

Some think the Milky Way’s gravity may have helped create the Oort cloud by moving icy objects closer to the Sun. This effect is complex and depends on each object, but it can be important. It is thought many comets from the Oort cloud reach us because of the Milky Way’s gravity.

Stellar perturbations and stellar companion hypotheses

Besides the pull of the galaxy, comets come into the inner Solar System when nearby stars or big clouds of gas and dust pass close to the Sun’s Oort cloud and change the paths of the comets. Sometimes the Sun moves through the Milky Way and comes near other stars. For example, about 70,000 years ago, a star called Scholz's Star passed through the outer Oort cloud, but it didn’t change much because it was small and moving fast.

In 1984, a scientist named Richard A. Muller suggested the Sun might have a hidden companion, either a warm, dim star or a bigger type of object, orbiting far away in the Oort cloud. This idea, called Nemesis, was thought to send comets toward us every 26 million years. But we haven’t found any proof of Nemesis, and many reasons make scientists think it probably doesn’t exist.

Another idea from 2002 by John J. Matese suggests that a big planet, about the size of Jupiter, might be far out in the Oort cloud, sending more comets our way than we can explain otherwise. A space mission called the WISE mission looked for this, but in 2014, NASA said they didn’t find any object like that.

Future exploration

The Voyager 1 spacecraft, the farthest human-made object, will reach the Oort cloud in about 300 years. It would take around 30,000 years to pass through it. In the 1980s, scientists thought about a probe named TAU. This probe could travel 1,000 AU in just 50 years to search for the Oort cloud.

In 2014, a mission called the "Whipple Mission" was suggested as part of the Discovery program. This mission would watch distant stars to find objects in the Oort cloud and the Kuiper belt. It would orbit a special point in space for five years. Some believe the Kepler space telescope might also have been able to spot objects in the Oort cloud.