Oort Cloud - Home of Long Period Comets

The Oort Cloud is located far beyond Pluto and the Kuiper Belt's furthest reaches. The Oort cloud is thought to form a massive spherical shell enclosing the Sun, planets, and Kuiper Belt Objects, while the planets of our solar system circle in a flat plane. It resembles a large, thick bubble made of icy, comet-like particles that surround our solar system. The frozen bodies of the Oort cloud can be as big as mountains – and occasionally even bigger.

Figure 1 Oort Cloud formation.

The Oort cloud is our solar system's most remote part, extending around one-quarter to halfway between our Sun and the next star, and it's awe-inspiringly far away.

To understand the distance to the Oort Cloud, put miles and kilometers aside and instead use the astronomical unit, or AU – a number defined as the distance between Earth and the Sun, with 1 AU equaling around 93 million miles or 150 million kilometers.

Pluto's highly elliptical orbit, by comparison, takes it between 30 and 50 astronomical units from the Sun. The inner boundary of the Oort Cloud, on the other hand, is estimated to be between 2,000 and 5,000 AU away from the Sun, while the outer border is thought to be between 10,000 and 100,000 AU away.

If visualizing those distances is difficult, you might use time as your ruler instead. NASA's Voyager 1 probe will take around 300 years to enter the Oort Cloud at its current speed of about a million miles per day. And it won't leave the rim for another 30,000 years or more.

Even if you could travel faster than light (671 million miles per hour, or 1 billion kilometers per hour), a voyage to the Oort Cloud would necessitate packing for a long journey.

When light leaves the Sun, it takes around eight minutes to reach Earth and 4.5 hours to reach Neptune. The Sun's light goes beyond the outer border of the Kuiper Belt just under three hours after passing Neptune's orbit.

After another 12 hours, the sun reaches the heliopause, where the solar wind smooshes up against the interstellar medium. The solar wind is a torrent of charged particles moving away from the Sun at nearly a million miles per hour (400 kilometers per second). Interstellar space lies beyond this line, where the Sun's magnetic field has no influence. For the past 17 hours, the Sun's light has been going away from it.

The sunlight has already traveled farther from the Sun than any human-made spaceship in less than one Earth day after leaving the Sun. However, it will take another 10 to 28 days for that same sunlight to reach the Oort Cloud's inner edge, and maybe up to a year and a half before it travels beyond the Oort Cloud's outer edge.

According to the most popular theory for the creation of the Oort Cloud, these frozen objects were not always so far from the Sun. After the planets formed 4.6 billion years ago, there were still a lot of leftover fragments called planetesimals in the region where they originated. Planetesimals are made of the same substance that planets are made of. The planetesimals were subsequently distributed all over the place due to the gravitational pull of the planets (particularly Jupiter).

Some planetesimals were completely ejected from the solar system, while others were blasted into eccentric orbits where they were still kept in place by the Sun's gravity but far enough away from it that galactic effects pushed on them. Our galaxy's tidal force was most likely the most powerful influence.

Figure 2 In 2014, the comet Siding Spring makes a close approach to Mars. NASA, ESA, J.-Y. Li (PSI), C.M. Lisse (JHU/APL), and the Hubble Heritage Team (STScI/AURA) Credit: NASA, ESA, J.-Y. Li (PSI), C.M. Lisse (JHU/APL), and the Hubble Heritage Team (STScI/AURA).

In brief, the planets' gravity pushed numerous ice planetesimals away from the Sun, and the galaxy's gravity likely forced them to settle in the solar system's borderlands, where the planets couldn't bother them any longer. They eventually became known as the Oort Cloud. Again, this is the most popular theory, but the Oort Cloud might possibly collect things that did not develop in our solar system.

Objects in the Oort Cloud, unlike planets, the main asteroid belt, and many objects in the Kuiper Belt, do not always travel in the same direction in a shared orbital plane around the Sun. Instead, they can travel around the Sun as a thick bubble of distant, frozen material, passing beneath, over, and at various inclinations. As a result, they are referred to as the Oort Cloud rather than the Oort Belt.

The existence of the cloud was hypothesized by Dutch astronomer Jan Oort to explain (among other things) where long-period comets come from and why they appear to arrive from all directions rather than the orbital plane shared by the planets, asteroids, and the Kuiper Belt.

The Oort Cloud could contain hundreds of billions, if not trillions, of ice bodies. Something occasionally upsets one of these frozen worlds' orbit, and it begins a protracted fall into our Sun. Comets C/2012 S1 (ISON) and C/2013 A1 Siding Spring is two recent instances. When ISON got too close to the Sun, it disintegrated. Siding Spring, which came within a hair's breadth of passing by Mars survived its trip to the inner solar system, but it won't be back for another 740,000 years.

Because of their extended orbital periods, the majority of known long-period comets have only been spotted once in recorded history. (Thus the moniker.) Many more undiscovered long-period comets have never been sighted by humans. Some of them have such lengthy orbits that our species did not exist when they passed through the inner solar system the last time. Others, in the billions of years since their formation, have never come close to the Sun.