O is for the Oort Cloud

I know, I know, I’m way behind on my A to Z posts. Life hit me hard this month but I’m doing my best to start catching up. I plan to post for two letters a day until I’m back on track. Thank you for reading :) 

Most people think that our Solar System ends out there with Pluto, the sad, demoted dwarf planet. Most people don’t really think about what lies between us and our nearest neighbor Proxima Centauri. We assume it’s all just empty space but we might be wrong, there could be a cloud of dust and icy planetesimals, the stuff planets are made of, called the Oort cloud.

Sometimes called the Öpik–Oort cloud, it is named after Dutch astronomer Jan Oort, who made significant contributions to the understanding of the Milky Way, and Estonian astronomer Ernst Öpik, who postulated the theory that comets in our solar system  originated in a cloud orbiting far beyond the orbit of Pluto in 1951.

It is thought to orbit the sun from about 2 light-years out, which is about half the distance to the nearest star Proxima Centauri, and marks the outer boundary of our Solar System and the end of the Sun’s gravitational dominance. It’s hard to picture it but the Oort cloud, if it exists, is really, really far out there. Well beyond Pluto!

The presumed distance of the Oort cloud compared to the rest of the Solar System

The are two regions to the Oort cloud, the outer, spherical one, and a donut-shaped inner one. The outer one is barely held gravitational to the sun and may supply all of the longer term, Halley-type comets. The inner cloud is sometimes called the Hills cloud, named after Jack G. Hills, who proposed its existence in 1981. The outer Oort cloud may have trillions of objects, The inner cloud is thought to contain tens or hundreds times as many comets as the outer one.

The vast majority of the objects are made of rock and ices such as water, methane, carbon monoxide, and hydrogen cyanide.

So the Oort cloud is made mostly of comets, but how do they get from the Oort cloud into the obits of the planets so that we can see them. It is thought that there is a tidal force the Milky Way Galaxy exerts on the Oort cloud much like the moon works on the Earth’s oceans. The Milky Way exerts this effect on all objects in the Solar System but the Sun dominates. Far out in Oort cloud the Sun’s gravity is weak and the Milky Way’s can be felt more strongly.

The Galaxy stretches and compresses the cloud and once an object gets a little closer to the inner Solar System the Sun’s gravity takes over and the object begins to free fall toward the inner planets and the sun. Once it goes around it has picked up a lot of speed and is shot back out to the outer Solar System where the process begins again.

To me the Oort cloud is a sort of Wild West of our tiny corner of the universe. Hell, we aren’t even sure it exists! It’s amazing that there is an area so (relatively) close to home that we know almost nothing about. Nothing for sure anyway.


Image:  In one of the most detailed astronomical images ever produced, NASA/ESA’s Hubble Space Telescope captured an unprecedented look at the Orion Nebula. … This extensive study took 105 Hubble orbits to complete. All imaging instruments aboard the telescope were used simultaneously to study Orion. The Advanced Camera mosaic covers approximately the apparent angular size of the full moon.

The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way south[b] of Orion’s Belt in the constellation of Orion. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of 1,344 ± 20 light years[3][6] and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light years across. It has a mass of about 2000 times the mass of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.[7]

The Orion Nebula is one of the most scrutinized and photographed objects in the night sky, and is among the most intensely studied celestial features.[8] The nebula has revealed much about the process of how stars and planetary systems are formed from collapsing clouds of gas and dust. Astronomers have directly observed protoplanetary disks, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula. By NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team [Public domain], via Wikimedia Commons


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Hello! My name is Lisa. I find the human condition fascinating and I often write stuff about that. I blog at zenandpi.com but you can also find me on Twitter, Tumblr, and Instagram, and if you like what I do, consider signing up for my newsletter. Thanks :)

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