The formation of the planetary bodies was not a peaceful one. Planetesimals, effectively “planet seeds” that would accrete material to gain mass, would frequently collide with each other at violent speeds. A collision between two planetesimals of similar sizes would spell the doom for both – all of their hard work accreting mass would be gone, shattering into smaller fragments and flying through the solar system.

The fragments don’t simply disappear, however. They remain the source of millions of other collisions later on through the solar system’s life. Most people know them, perhaps even seen one – asteroids.

Asteroids, however, are relatively boring. They’re simply chunks of metal or rock that happened to get caught in a planet’s gravity, pulled in, and collided, creating a crater in the process. It’s an energetic process for sure, but the asteroids themselves floating in space are nothing special.

An example of an asteroid – a “space potato”

Fortunately, asteroids aren’t the only type of “failed planetesimal” that exist in our solar system. Much further out, past the orbit of Mars but before the orbit of Jupiter, lies the frost line, an imaginary line where hydrogen-based compounds can solidify and become planetesimal cores. A similar process of accretions and collisions occurs past this frost line, except these planet seeds aren’t made of rock and metal, but water and ice.

Destroyed planetesimals no longer form plain space potatoes that litter the solar system, but something far more intriguing – comets.

What is a comet?

Characterized by their long, glowing “tail,” comets are balls of ice and rock that orbit the Sun in highly elliptical fashions. As the icy comet passes by the Sun, it warms up significantly, and melts the solidified gases and liquids in a process known as outgassing. The released gases give the comet its characteristic tail. Since they formed in the outer solar system, they’re a rare sight on Earth, but do occasionally streak by, acting as a unique spectacle for onlookers to enjoy.

Where are all the comets?

Planetesimals nearly the size of current planets must have shattered into each other just as violently in the outer solar system as anywhere else. So where are all the comets?

We currently do not have direct evidence, but it is highly theorized that all of the leftover ice planetesimals (that would have become jovians had they not been obliterated!) exist in a spherical “shell” surrounding the solar system, known as the Oort Cloud.

In fact, most comets exist in this Oort Cloud, and will generally stay there until the gravitational influence of a nearby star kicks one into a closer orbit. This is where we get the comets that are observable from Earth, such as the well-known Halley’s Comet. Unfortunately, all of the other comets in our solar system probably exist in an area far too distant to be detected directly. It is estimated that up to 2 trillion comets could exist in this shell.


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