Explore the Oort Cloud, Home of Long-Period Comets

~6 min

Have you ever wondered what lies at the farthest reaches of our Solar System? Scientists believe that a massive “bubble” of ice and dust surrounds the Solar System – this region of space is called the Oort cloud. The Oort cloud is so far away and so hard to reach that most of its secrets remain hidden. But we need to keep studying it to understand what our Solar System actually looks like and how it was formed. In this article, we’ll take a look at what we know so far about this fascinating region of space.


What is the Oort cloud?

The Oort cloud is a theoretical spherical cloud composed of small icy bodies that surrounds the Solar System. The existence of the Oort Cloud hasn’t yet been proven by direct observation, but it is widely accepted by the scientific community.

What is the Oort cloud made of?

The Oort Cloud is made of trillions of small icy objects that travel in different orbits. These objects, most of which are less than 100 km (62 miles) in size, contain a variety of icy substances like water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia. Together, they form a celestial cloud with a total mass estimated to be 10-100 times that of the Earth.

Objects in the Oort cloud

The Oort cloud is believed to be home to long-period comets – those that take from 200 to thousands of years to orbit the Sun. In fact, these comets are evidence that the Oort cloud exists.

As the Dutch astronomer Jan Oort, after whom the Oort Cloud is named, suggested, long-period comets couldn’t survive in their orbits close to the Sun. The gravitational pull would soon cause them to collide with the Sun or one of the planets. Moreover, comets burn out relatively quickly as they pass through the Solar System, so there must be a “fresh supply” of comets in a colder, distant region; otherwise, we wouldn’t see so many comets in our era. Based on these observations, Jan Oort concluded that there is a spherical reservoir of comets at the edges of the Solar System.

Long-period comets spend most of their life within the Oort cloud. However, they can be occasionally “knocked out” of their orbits by passing stars, molecular clouds, or galactic tides. As a result, they begin to fall into the inner Solar System and become visible as they approach the Sun. It’s supposed that there are numerous potential long-period comets in the Oort cloud that have yet to visit the Sun.

It’s also thought that the Oort Cloud may contain dwarf planets. One notable example is Sedna, discovered in 2005. With its remarkably elongated orbit, Sedna takes around 11,400 years to return to its closest point to the Sun, at a distance of 76 astronomical units (AU).

Measuring Distances in Space
What’s bigger: lunar distance, astronomical unit, or light-year? How are these units used? Check out our infographic to find these out!
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How did the Oort cloud form?

Scientists believe that the Oort cloud appeared together with the Sun and the planets of the Solar System approximately 4.6 billion years ago. As young giant planets (such as Jupiter and Neptune) formed, their gravitational pull started to influence the trajectories of smaller objects, called planetesimals. Some of the planetesimals collided with larger objects, some were captured as moons, and others were pulled far away from the Sun into the forming Oort cloud. After that, the gravity of the galaxy probably caused them to settle in the spherical cloud at the edge of the Solar System, where the planets and the Sun couldn’t disturb them anymore.

The Oort cloud is still not stable. Some of the cloud’s inhabitants may get pulled away into the vastness of space, and some objects may also be collected from the neighboring star systems.

Oort cloud distance & size

The Oort cloud is a massive and most distant region of the Solar System, but what is its actual size and location? Let’s try to clear this up.

Where is the Oort cloud located?

The Oort cloud is located in the interstellar space at the very edge of the Solar System. At such a distant location, the Oort cloud isn’t affected by the Sun’s magnetic field and the planets’ gravitational forces.

The inner edge of the Oort cloud is located at around 2,000 AU from the Sun. This means the Oort cloud begins a whopping 2,000 times farther from the Sun than the Earth! Just to give you an idea, Neptune, the farthest planet in our Solar System, is only about 30 AU away from the Sun.

How big is the Oort cloud?

As noted above, the inner boundary of the Oort cloud is around 2,000 AU from the Sun. Its outer boundary lies somewhere between 10,000 and up to 200,000 AU from the Sun. The outer boundary of the Oort cloud can reach halfway to our nearest neighbor, Proxima Centauri. With our current technology, it would take approximately 30,000 years for a human-made spacecraft to pass through this vast space region.

Is it possible to see the Oort cloud?

The ice particles that form the Oort cloud move too slowly and don’t reflect much light; they are also just too far away from us – so, for now, the Oort cloud can’t be seen even with the most powerful telescopes. But we can explore it indirectly through the objects that escape the cloud and “fall” into the inner Solar System. Most of such objects are long-period comets.

You can use the free astronomy app Sky Tonight to locate the comets reaching us from this distant region of space. For example, the newly discovered long-period comet C/2023 A3 (Tsuchinshan-ATLAS) is expected to reach a magnitude of -3.5 by October 2024, clearly visible to the naked eye. For now, it’s not a notable sky object, but you can already track it in the sky with Sky Tonight. To find out more about the comet C/2023 A3 and how to locate it via Sky Tonight, read our dedicated article.


Who discovered the Oort cloud?

In 1950, the Dutch astronomer Jan Hendrik Oort hypothesized the existence of a distant cloud surrounding the Solar System, from which long-period comets originate. His idea was based on that of the Estonian astronomer Ernst Julius Öpik, who, in 1932, proposed that there might be a distant reservoir of comets in the Solar System.

How far is the Oort cloud from the Sun?

The Oort Cloud is located at distances ranging from 2,000 to 200,000 AU away from the Sun. AU (astronomical unit) equals the mean distance between the Earth and the Sun. If you’re curious to explore more about astronomical distances and how they are measured, be sure to check out our colorful infographic on the subject.

Measuring Distances in Space
What’s bigger: lunar distance, astronomical unit, or light-year? How are these units used? Check out our infographic to find these out!
See Infographic

How do comets get away from the Oort cloud?

Long-period comets are ejected from the Oort cloud by the gravitational pull of passing stars, molecular clouds, or the Milky Way’s tidal force, which can shift the orbits of the objects in the Oort cloud and send them into the inner Solar System. Sometimes comets that originate in the Oort cloud can be captured by gravitational forces and given a new “home.” For example, Halley’s Comet is believed to have been born in the Oort Cloud, but now it’s in the Kuiper belt.

What is the difference between the Kuiper belt and the Oort cloud?

The Kuiper belt has a disk shape, while the Oort cloud is roughly spherical. It is also beyond the orbit of Neptune, but closer to the Sun than the Oort cloud. In the Kuiper belt, objects tend to orbit the Sun near the ecliptic plane, while the Oort cloud hosts objects with a wide range of orbital paths. Finally, the Kuiper belt was studied by a spacecraft (NASA’s New Horizons), and the Oort cloud hasn’t been visited yet.

Is it possible to travel through the Oort cloud?

Human-made spacecraft can travel through the Oort Cloud, but conducting scientific operations or receiving signals from it is not currently possible. Voyager 1, for instance, will take around 300 years to reach the Oort Cloud and another 30,000 years to pass through it, but it will already lack the fuel for scientific exploration by 2025. Likewise, Voyager 2, Pioneer 10 and 11, and New Horizons will become non-operational before reaching the Oort Cloud. Check your knowledge of the famous spacecraft with our quiz!

Famous spacecraft
What spacecraft was the first to land on a comet? What was the first artificial satellite in Earth’s orbit? Take this quiz to test your knowledge about famous spacecraft!
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What is beyond the Oort cloud?

The Oort cloud is located in the interstellar space at the very edge of the Solar System. Beyond it are the other star systems and all the vastness of space.

Bottom line

The Oort cloud is a huge and distant cloud made up of trillions of icy objects surrounding our Solar System. Stretching from 2,000 to 200,000 AU away from the Sun, it remains hidden from direct observation. However, by observing long-period comets that originate from the Oort cloud, we can gain insights into this enigmatic region. Track these cosmic visitors with the help of stargazing apps like Sky Tonight, and marvel at the wonders of our vast and fascinating universe.