Facts About the Oort Cloud
Before we dive into the detail, let’s take a look at some bite size Oort Cloud facts!
- The Oort cloud marks the boundary of our solar system. This is the limit of the Sun’s gravitational influence.
- Objects found in the Oort Cloud are known as “trans-Neptunian objects” or TNOs. This applies to all objects beyond Neptune’s orbit. It includes the Kuiper Belt objects (KBOs) as well.
- One theory about its formation is that the Sun captured cometary material from outer disks of other stars which were forming in the same nebula as the Sun.
- The Oort cloud comprises two regions. There is a spherical outer Oort cloud, and a disk-shaped inner cloud called the Hills cloud.
- Objects in the cloud are mostly composed of water ice, ammonia, and methane.
- The estimated number of objects believed to be in the Cloud is around 2 trillion. Although, there is currently no way to verify this theory.
- Long-period comets (with orbit paths of over 200 years) are believed to originate from the Oort Cloud, according to many astronomers.
- The planetoid Sedna, also known as 90377, was discovered in 2003. It is thought to be from the inner Oort Cloud. As of 2012, this planetoid was about three times the distance of Neptune from the Sun.
- The Oort Cloud is so distant that it is already influenced by outside tidal forces, particularly that of our galaxy. Aside from that, it can also be affected by passing stars.
- No spacecraft has reached the Oort cloud yet. It will take Voyager 1 roughly 300 years to enter the inner edge. If it does, it will exit around 30,000 years.
What Is the Oort Cloud?
The Oort Cloud is a theoretical spherical cloud that encircles the solar system. It is the farthest point of the gravitational influence of the Sun.
Simply speaking, it is the edge of the solar system. If we take a look closer, Neptune is the most distant planet from the Sun. Beyond it is the Kuiper belt, the scattered disk, and lastly the Oort Cloud.
Unlike the disk-shaped Kuiper belt, the Oort cloud is believed to be spherical in shape. It surrounds the Earth and everything in our solar system. Since it is far from the Sun, this big bubble is composed of icy pieces of all sizes. Billions or even trillions of them make up this spherical shell.
The Kuiper Belt and the scattered disk are about one thousand times closer to the Sun than the Oort cloud is. It is believed that this cloud of particles is the remains of the disk material which formed the Sun and the eight planets.
Astronomers have theorized that the matter composing the cloud formed close to the Sun. However, it was likely scattered out into space by the powerful gravity of the gas giants early in the solar system’s evolution.
The outer Oort cloud is only loosely bound to the solar system. This means that it is easily affected by the gravitational pull from molecular clouds, passing stars, and the Milky Way galaxy. In fact, the outer limit of the Oort cloud defines the cosmographical boundary of the solar system. It marks the edges of the Sun’s gravitational dominance.
Where Is the Oort Cloud Located?
It is believed that the Oort Cloud occupies an enormous space. It is very large and distant that it would take a lot of numbers if we measured it in kilometers. A better unit to use is the astronomical unit (AU)—the distance between the Sun and Earth. One AU equals about 150 million km (93 million miles).
The inner edge of the Oort Cloud is thought to be between 2,000 and 5,000 AU from the Sun. It is so big that the outer edge extends to around 10,000 to 100,000 AU away. This is about a quarter of the distance to the nearest star to us, the Proxima Centauri.
The Oort Cloud marks the limit of our Sun’s Hill Sphere. The Hill sphere defines the gravitational influence of an object against the perturbations of a larger body. This concept was named after the American astronomer and mathematician George William Hill.
Since it defines the Hill sphere limit, the Sun’s influence no longer dominates the Oort Cloud. It is often affected by the gravitational influence of the more massive Milky Way or that of a passing star.
We have not directly observed the Oort Cloud which is why it is still “theoretical.” The idea was first proposed by the Estonian astronomer Ernst Öpik in 1932. Many years later, in 1950, the Dutch astronomer Jan Oort also had the same idea. The cloud was named after him.
Scientists believe that the Oort Cloud is the origin of the comets that sometimes pass close to the Sun. Because of that, some people consider it a “cometary reservoir.”
One in a while, the inner solar system is visited by comets. Some of them are considered short-period comets because they come around in less than 200 years. They follow the ecliptic or the plane of the planets. An example of that is the periodic Halley’s Comet which passes every 75 years.
Long-period comets are even more interesting. They take more than 200 years to orbit the Sun, with some even up to 30 million years. Their very long orbital period means that they travel from a very far place. This far place is what scientists call the Oort Cloud. Its existence is the most probable explanation for the origin of long-period comets.
How It Came To Be
The Oort Cloud is made up of space debris from the formation of the solar system 4.6 billion years ago. These accumulated bodies are collectively called planetesimals. The asteroid belt and the Kuiper belt are also composed of these leftovers.
It is believed that these planetesimals are not always as far away as they are now. As the planet formed, their gravity pushed and scattered these bodies away. The giant planets, particularly Jupiter and Saturn, had the most gravitational effects on these objects.
Some objects flew out of the solar system. There are also others that cling to the outer edges where the planets cannot affect them anymore. They are still influenced by the Sun but are also pulled by outside influences. As a result, they have very elliptical orbits.
A different theory suggests how the Oort Cloud came to be. It states that the early solar system may have been a part of a star cluster with around 200 to 400 stars. Stellar encounters resulted in the mixing of materials from different protoplanetary disks.
If that is the case, then it means the Oort Cloud did not form close to the Sun as commonly believed. This also implies that it contains material from other stars in the said cluster. A computer simulation in 2010 showed that around 90% of the cloud population originated from other stars.
Unlike the rocky asteroid belt, the Oort Cloud is populated with icy bodies. These objects can be very big, with some even being the same size as mountains, or even more!
Objects here do not have a common orbital plane like the planets do. Instead, they follow orbits that go around the Sun from all directions. They travel above, below, and from all sides like a very big knitting ball. That is why it is called a “cloud” and not a “disk.”
The Oort Cloud is believed to be largely populated by icy objects composed mostly of water ice, ethane, methane, and other ices. This long-standing belief was challenged in 1996 when an unknown object called 1996 PW was discovered.
1996 PW came from the Oort Cloud but did not have a tail and other characteristics of a comet. In fact, it looked more like an asteroid. Its classification, however, is not fully confirmed as it can also be an extinct comet.
The total mass of the Oort Cloud is hard to determine but its outer region is believed to be around five Earth masses. Similarly, it is still impossible to know the exact number of Oort Cloud objects. However, it is believed to be in the billions or even trillions. They are dark and cold and life as we know it cannot thrive there.
Home Sweet Home
The Oort Cloud is believed to be the home of long-period comets. They are catapulted into the inner solar system when perturbed by galactic influences or a nearby star.
These objects travel around the Sun in 200 up to millions of years. Because of that, we will only see a few of them in our lifetime. Some examples include Comet ISON, Siding Spring, Comet Kohoutek, and C/2011 L4. The minor planet Sedna is believed to be from this cloud too.
A strange object called C/2013 P2 Pan-STARRS was discovered in 2013. It lacked the common comet tail and did not behave like one either. If this is not an asteroid, then it is proof of Jan Oort’s previous theories.
According to Oort, objects would look very bright on their first journey because they still contain a lot of material on their surfaces. C/2013 P2 is likely on its return visit that is why it appeared tailless. Most of the ices on its surface may have sublimated already on its first trip to the solar system.
During its approach in 2014, C/2013 A1 (Siding Spring) was thought to collide with Mars. It missed the red planet by roughly one-third of the Earth-Moon distance. This was initially seen as a threat to the orbiters outside Mars.
The good news is that it never caused any damage. Instead, it became a good opportunity for the spacecraft to capture such a rare event. Spring Siding will take another 740,000 years to visit us again.
Jan Oort discovered that long-period comets sometimes disappear. He developed a model to predict the return of Oort Cloud comets only to learn that only a few of them revisit the inner solar system.
This was the case in 2013 when sky enthusiasts waited for Comet ISON. It came a long way from the Oort Cloud. It was expected to produce a magnificent sky show because it seemed to be big enough to do so. However, as it approached the Sun, scientists were not able to see anything.
Where did it go then?
The most likely explanation for this is that Comet ISON disintegrated before it made its closest approach. Scientists likely overestimated its size because it looked brighter before. This sungrazing comet was just too small to survive the encounter. It might have broken down into dust and rubble when it reached perihelion. And because of that, the expected light show was a fizzle.
The Oort Cloud is important because it holds clues to what our solar system was like in the beginning. How so?
Comets contain primitive molecules and materials. Some of these have been around even before the Sun formed. Unlike the planets, comets are small and far from the Sun that they have not undergone many changes after they formed.
In short, they are able to preserve their initial state and composition. These ancient traces will not survive here on Earth. Our planet has gone through many changes and developments since its formation.
Studying comets allows scientists to understand what it was like when the solar system formed. With this, we will better understand how life started and evolved. Tracing our roots is important because it also gives us an idea of how things will develop in the future.
What Is Outside the Oort Cloud?
No one has exited the Oort Cloud yet to see what is beyond our solar system. Our Sun is just one of the many stars in our galaxy and our galaxy is also just one of many.
We can expect other star systems outside our own. Our closest neighbor is Proxima Centaurus which lies about 4.2 light-years away. It is part of the multiple star system of Alpha Centauri. Two exoplanets are found orbiting Proxima Centauri. One of them is roughly similar in size to Earth and is also in the habitable zone.
The influence of other star systems is generally felt in the Oort Cloud. In fact, stars pass through the Oort Cloud around every 100,000 years. Every nine million years, an approach as close as 52,000 AU is expected.
The orange star Gliese 710 is predicted to come as close as 8,800 to 13,700 AU in roughly 1.4 million years. This will perturb icy bodies in the Oort Cloud and send them inwards as comets.
Explorations to the Edge of the Solar System
Venturing to the outer edges of the solar system is no small feat. It is a very long journey that takes thousands of years. Even if you travel at the speed of light, it would still take about a year to pass the outer edge of the Oort Cloud.
Let’s take the journey of sunlight for example. Sunlight takes about 8 minutes to reach us here on Earth. It reaches Jupiter in 43 minutes and roughly 4.5 hours to Neptune.
At the same speed of 1 billion km per hour (around 671 million miles per hour), it will pass through the Kuiper belt, the heliopause, and interstellar space. It will take around 17 hours to go this far. However, it will be about 1.5 more years for this same sunlight to exit the Oort Cloud.
No spacecraft has reached and gone past the Oort Cloud. However, five of them are heading out to the edges of the solar system. With their launch dates, these are:
- Pioneer 10: March 2, 1972
- Pioneer 11: April 5, 1973
- Voyager 2: August 20, 1977
- Voyager 1: September 5, 1977
- New Horizons: January 19, 2006
These spacecraft will eventually reach the Oort Cloud if their power sources would allow it. Leading the group is Voyager 1 which travels more than one million kilometers a day. The two Voyagers and New Horizons are still sending data back to Earth. Pioneer 1 and 2 are, however, in a derelict state and are not communicating anymore.
Voyager 1 entered the interstellar medium on August 25, 2012. Its twin, Voyager 2, followed on November 5, 2018. They are the farthest and the only spacecraft that have left the heliopause.
Though they are already very far away, they are still so far from the Oort Cloud. Voyager 1 will enter the cloud in 300 years and exit it in 30,000 years. If it does reach that milestone, it will say goodbye to our Sun and become closer to another star in 40,000 years.
Past and Future Plans
The Thousand Astronomical Units (TAU) probe was proposed in 1987 to study the outer solar system. Around the 2010s, the Whipple Mission was proposed to search for Kuiper belt objects (KBOs) and Oort Cloud objects. However, it was not selected for the NASA Discovery Program.
A new player will join the five spacecraft in the outer solar system. The Interstellar Heliosphere Probe or Interstellar Express is a proposed program that aims to explore the heliosphere and, eventually, interstellar space. This mission is under the China National Space Administration (CNSA). It features two probes that will launch in 2024.
More Interesting Oort Cloud Facts
- To compare, the orbit of Pluto takes it as far as 30 to 50 astronomical units from the Sun. However, the outer edge of the Oort Cloud is thought to extend as far as 100,000 AU from the Sun.
- If there are indeed asteroids in the Oort Cloud, then they likely make up around 4% of the cloud. That is around 8 million objects and more populous than the asteroid belt!
- Since the Oort Cloud is influenced by both the Sun and our Milky Way galaxy, it is often stretched and compressed in all directions. This gravitational tug-of-war perturbs the relatively stable bodies. As a result, they are slingshotted towards the Sun as long-period comets.
- Many objects were thrown out of the solar system when the planets formed. However, it is also believed that the Oort Cloud also contains captured objects from outside the solar system.
- Because of the great distance, long-period comets take a very long time to orbit the Sun. Some of them are so far that they have never even approached the Sun since they formed.
- Passing stars can traverse through the Oort Cloud. In 2015, astronomer Eric Mamajek and team revealed that a star passed through it about 70,000 years ago. It is called Scholz’s Star, a binary star system located in the constellation of Monoceros.
Oort Cloud: https://static.wikia.nocookie.net/verse-and-dimensions/images/4/40/Oort-cloud.jpg/revision/latest/scale-to-width-down/700?cb=20180102211432
Kuiper Belt and Oort Cloud: https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/12/kuiper_belt_and_oort_cloud_in_context/15106869-1-eng-GB/Kuiper_Belt_and_Oort_Cloud_in_context_pillars.jpg
Oort Cloud location: https://upload.wikimedia.org/wikipedia/commons/thumb/d/da/PIA17046_-_Voyager_1_Goes_Interstellar.jpg/1280px-PIA17046_-_Voyager_1_Goes_Interstellar.jpg
Ernst Öpik: https://upload.wikimedia.org/wikipedia/commons/8/85/ErnstJuliusOpik.jpg
Jan Oort: https://upload.wikimedia.org/wikipedia/commons/thumb/a/a2/JanOort.jpg/1024px-JanOort.jpg
Solar nebula: https://www.nasa.gov/images/content/149890main_BetaPictDiskbMac.jpg
1996 PW: https://upload.wikimedia.org/wikipedia/commons/1/13/1996PW_NEAT-discovery-crop.gif
Siding Spring: https://cdn.spacetelescope.org/archives/images/screen/opo1419c.jpg
Siding Spring and Mars: https://upload.wikimedia.org/wikipedia/commons/thumb/e/e9/Comet-C2013A1-SidingSpring-NearMars-Hubble-20141019.jpg/900px-Comet-C2013A1-SidingSpring-NearMars-Hubble-20141019.jpg
Comet ISON: https://upload.wikimedia.org/wikipedia/commons/thumb/6/65/Comet_ISON_%28C-2012_S1%29_by_TRAPPIST_on_2013-11-15.jpg/800px-Comet_ISON_%28C-2012_S1%29_by_TRAPPIST_on_2013-11-15.jpg
Solar nebula: http://sservi.nasa.gov/wp-content/uploads/2013/12/Hartmann_Stardust2Planets.jpg
Milky Way galaxy: http://4.bp.blogspot.com/-caLZdLGjjEQ/VeEhehZAgUI/AAAAAAABARo/nVY8CIIRHLY/s640/MilkyWay-665×365.jpg
Voyager 1: https://upload.wikimedia.org/wikipedia/commons/thumb/2/29/Voyager_spacecraft.jpg/1024px-Voyager_spacecraft.jpg
TAU concept art: https://upload.wikimedia.org/wikipedia/commons/f/f5/Thousandau1_space_probe.jpg