By Emily Levesque, University of Washington
Besides the Sun, planets, and their respective moons, there are many other smaller members in the solar system. Asteroids, comets, and other little rocky and icy objects are everywhere. They’re an important part of the solar system, and studying them helps in learning about the history of the Sun, as well as how the planets, moons, and other objects form.
Asteroids and Planetesimals
Asteroids are rocky objects orbiting our sun. Their size can vary greatly, from just one meter across to nearly 600 miles across, and they’re typically composed of carbon, silicon, or nickel-iron composites.
Asteroids are generally thought of as being formed the same way as the Earth, and understanding this can take us back to the early days of the Sun’s life before there was a solar system to speak of.
While the Sun was still a young star it would have had something called a protoplanetary disk, a thick ring of gas and dust and other cosmic debris left over from the cloud that formed the Sun itself. Inside that disk, tiny cosmic dust grains constantly collide, and in some cases, the dust grains will stick together.
As these building blocks grow progressively bigger, they eventually have enough self-gravity to be held together, becoming small rocky bodies known as planetesimals.
From Planetesimals to Planets, Moons, and Asteroids
During the formation of our solar system, some planetesimals continued to coalesce, eventually becoming the planets that we see today. Others lasted a while but finally collided with larger objects or got nudged into very wide orbits around our Sun. A few planetesimals were captured as moons, like Mars’s Phobos and Deimos.
The rest remain in the solar system today as asteroids. The asteroid belt between Mars and Jupiter contains one to two million asteroids that are larger than a kilometer, and many more millions smaller than that; other asteroids trail behind planets or follow their own unique orbits around the sun.
How Are Asteroids Different from Comets ?
Comets have a similar history to asteroids, with one key difference. We think that asteroids formed in the inner part of the Sun’s protoplanetary disk, in the warmer region closest to our central star. Comets, by contrast, seem to have formed further out in the solar system, which gave them a crucial extra building block: ice.
The comets we see today began as planetesimals, amalgams of rocks, dust, ice, and frozen molecules traveling in wide and highly eccentric orbits around our Sun. This means that comets take on a distinctive appearance when they approach the Sun.
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Comet Tails
Comets that get close enough will begin to see their surfaces fried by the flux of photons coming from the Sun: their surfaces warm and begin to release gas and dust, giving comets their distinctive tails.
Most comets actually have two tails. One comes from the dust particles streaming off the comet as it approaches the Sun. It’s easy to imagine this dust trailing behind the comet as it streaks through the solar system, blown back by the comet’s motion, but in fact, the direction of the tail is dictated by the position of the Sun!
The dust will always move away from the Sun and winds up appearing as a curved white tail, reflecting the Sun’s light and leaving a trail that indicates where the comet used to be in its orbit.
The other tail comes from the gas pouring off of the comet. This tail appears bright blue thanks to glowing gas particles and is swept in a straight line away from the Sun by the strength of the solar wind—a stream of plasma and charged particles speeding away from the Sun’s upper atmosphere.
One might recognize these two tails from photographs of especially bright comets in recent years, like the comet Neowise that passed close to Earth in July of 2020. In astrophotography, they look spectacular, but to naked-eye observers, the two tails tend to give comets a faint, fuzzy, sometimes slightly asymmetric, appearance.
Observing Comets
Many of the solar system’s asteroids and comets lie along the ecliptic, and all of them move in a way that looks a bit peculiar relative to the stars.
This means that if an astronomer is able to spot one of these small faint objects and keep an eye on it for long enough to watch it move through the sky, they can identify it as a small solar system member and begin to make guesses about what it is, how far away it is, how big it is, and even where it might be headed.
Discovering new comets and asteroids can be a challenging proposition. This is because unlike stars these objects don’t glow. Any light they emit comes from reflected sunlight or latent heat, making them incredibly dim objects. Spotting them requires deep and detailed observations and confirming that a tiny point of light is in fact a member of our solar system comes from studying its motion.
Common Questions about Understanding Asteroids, Meteoroids, and Comets
Asteroids are rocky objects orbiting our sun. Their size can vary greatly, from just one meter across to nearly 600 miles across, and they’re typically composed of carbon, silicon, or nickel-iron composites.
Discovering new comets and asteroids can be challenging. This is because unlike stars these objects don’t glow. Any light they emit comes from reflected sunlight or latent heat, making them incredibly dim objects. Spotting them requires deep and detailed observations and confirming that a tiny point of light is in fact a member of our solar system comes from studying its motion.
Asteroids formed in the inner part of the sun’s protoplanetary disk, in the warmer region closest to our central star. Comets, by contrast, seem to have formed further out in the solar system, which gave them a crucial extra building block: ice.
