A few days ago, I was in Cambridge attending a talk by the great Jonathan Shanklin on the history of comets. While there, I started wondering how exactly we classify bodies such as comets and asteroids; more specifically, I thought about the point at which we'd start considering them as a planetary body. Is it down to size, density, orbit, or maybe something else?

Asteroids are small, rocky bodies that are found mainly in the asteroid belt -- a region of the Solar System between Mars and Jupiter. They are thought to be remnants from the early Solar System and some of them may be fragments of larger objects that were broken apart through collisions. They vary in size and shape, with many having craters (like the ones on the Moon) from impacts.

Comets are small, icy bodies which originate from the outer Solar System. They are made up of rock, dust, and ice. To learn more about the science behind comets, you can check out my last post. Comets can have highly elliptical orbits around the Sun, with some having an orbital period of many thousands of years.

So what exactly is a meteor? Let's start with a meteoroid: a meteoroid is a small piece of asteroid or comet, typically pebble-sized, usually created from some sort of a collision. When a meteoroid enters the Earth's atmosphere, it's referred to as a meteor. Meteors enter the Earth's atmosphere at very high speeds; due to the friction with the air, they burn up and produce a streak of light called a shooting star. Meteors are usually only visible for a few seconds and are best seen during meteor showers, which occur when the Earth passes through a stream of debris left behind by a comet or asteroid. A meteor which makes it all the way to the ground without burning up is called a meteorite.

As it turns out, an asteroid or meteor would not be considered a planet, even if very large in size. Planets are defined as celestial bodies that meet three criteria:

• They orbit a star.

• They are roughly spherical in shape due to their own gravity.

• They have cleared their orbit of other debris -- i.e., they are the dominant gravitational force in their orbit and have either absorbed or ejected all other debris in their path.

Asteroids and meteors are typically much smaller than planets and do not meet the critetia to be considered a planet. However, there is a type of asteroid called a 'dwarf planet' which is similar in size to a planet, but do not fulfil the third planetary criteria of clearing out their orbit. Some examples of dwarf planets in our Solar System are Pluto, Ceres, Haumea, Makemake, and Eris. 

Also, we can plot inclination against logarithmic mass, making the discrepancy between asteroids and planets slightly clearer:

Planet 9 (aka "the perturber") is a hypothetical planet in our Solar System which has not yet been observed directly. It was hypothesised by Caltech astronomers, Konstantin Batygin and Mike Brown, who suggested that the unusual orbits of some trans-Neptunian objects (TNOs - an object orbiting the Sun beyond Neptune) could be explained by the gravitational influence of a large, undiscovered planet. It's estimated to be 5-10 Earth masses, with an orbital period of 10000-20000 years and possibly as far as 1000AU from the Sun.

Despite years of searching, no direct evidence for Planet 9 has been found yet. Some scientists are skeptical of its existence, suggesting that the unusual orbits of TNOs could be explained by some other phenomena such as the gravitational influence of multiple smaller objects (i.e., a swarm of asteroids). Only time will reveal the true identity of "the perturber".

published: 24/02/23 by kaan evcimen