Asteroid Belt: A Planet That Never Formed

~6 min
Asteroid Belt: A Planet That Never Formed

Fifty years ago, on July 15, 1972, Pioneer 10 became the first spacecraft to reach the asteroid belt. We’d like to use this occasion to talk about this curious region of the Solar System. How did the belt form? Could it be the remnants of a planet disrupted billions of years ago? Read this article to get answers to these and many other questions.


What is the asteroid belt?

The asteroid belt is a region between Mars and Jupiter that hosts most of the Solar System asteroids and marks the boundary between the inner rocky planets and the outer gas giants. It is also sometimes called the main asteroid belt to distinguish it from the Kuiper belt. The main belt contains four large bodies — Ceres, Vesta, Pallas, and Hygiea — and millions of smaller ones.

Who discovered the asteroid belt?

The existence of the asteroid belt wasn’t known until the mid-19th century. However, the area between Mars and Jupiter had attracted astronomers’ attention long before that — they had been looking for a planet there.

In 1766, German astronomer Johann Daniel Titius proposed the following hypothesis: extending outward from the center of the Solar System, each planet should be located approximately twice as far from the Sun as the planet before it. According to this hypothesis (now known as the Titius–Bode law), there was a yet undiscovered planet between the orbits of Mars and Jupiter!

Many people became obsessed with this idea. For example, a group of German astronomers called the Celestial Police organized a large international project to find the missing planet. However, they were outrun by an Italian astronomer Giuseppe Piazzi who discovered Ceres in 1801. The celestial body was located at the almost exact distance predicted by the Titius–Bode law.

Of course, Ceres was initially considered the missing planet. However, other similar objects were very soon found in the same area. In 1802, fortune smiled upon the Celestial Police: its member Heinrich Olbers (the author of Olbers’ paradox) discovered Pallas. After that, the Police were on a roll: in 1804, Karl Harding observed Juno, and in 1807, Heinrich Olbers made his second discovery by observing Vesta.

As more and more celestial bodies were found between Mars and Jupiter, it became evident that they were too small to be considered planets. William Herschel, the discoverer of Uranus, coined the term “asteroid”, and it stuck. Around the 1850s, the expression “asteroid belt” came into use.

So, there was no single discoverer of the asteroid belt. Giuseppe Piazzi observed the first object in the belt, and then other astronomers contributed by finding more celestial bodies in this region.

One interesting footnote about the Titius–Bode law that started the whole thing: when Neptune was discovered in 1846, its location didn’t correspond with Titius’ predictions. It seems the Titius–Bode law was just a mathematical coincidence rather than an actual physical law!

How did the asteroid belt form?

Initially, astronomers believed the asteroid belt was formed after the destruction of a large planet. This theory belongs to Heinrich Olbers, who we already mentioned above. The hypothetical planet was given the name Phaeton. The “disrupted planet hypothesis” was supported by many astronomers around the world and remained influential until the end of the 20th century.

According to modern studies, the asteroid belt is more likely a planet that never formed. About 4.6 billion years ago, in the Solar System's early days, small chunks of cosmic dust (called planetesimals) formed through a process of accretion. Some of the planetesimals eventually became the planets we know today. However, in the region between Mars and Jupiter, gravitational influence from Jupiter didn’t let the planetesimals accrete into a planet — instead, they collided and fragmented. That’s why we now see the asteroid belt in this area.

What makes up the asteroid belt?

The asteroid belt consists primarily of C-type or carbonaceous asteroids. The other common types are S-type or silicate asteroids and M-type or metallic asteroids.

The vast majority of asteroids are relatively small — only about 30 asteroids are larger than 200 km (124 mi) in diameter. The largest objects in the asteroid belt are Ceres (940 km or 580 mi), Vesta (525 km or 325 mi), Pallas (510 km or 320 mi), and Hygiea (410 km or 250 mi). These four celestial bodies make up about half of the mass of the entire asteroid belt. It’s important to note that Ceres is now considered a dwarf planet, which makes Vesta the largest asteroid in the belt.

Ceres is the only body in the asteroid belt that’s large enough to maintain a spherical shape. Most asteroids look like lumpy potatoes, though some of them come in more unusual shapes — like 216 Kleopatra that looks like a dog bone.

Despite what you might have seen in sci-fi movies, the asteroid belt is not a crowded place. It is so huge that average spacing between two asteroids is about one million km (620,000 mi)! So spacecraft passing through the asteroid belt have virtually no chance of a collision.

Moreover, the asteroids in the belt aren’t uniformly distributed — there are certain areas where they are practically absent. These areas, called the Kirkwood gaps, are cleared of asteroids by the gravitational influence of Jupiter. The Kirkwood gaps were named after the American astronomer Daniel Kirkwood, who first observed them in 1866.

What spacecraft visited the asteroid belt?

Since the 1970s, multiple space probes have reached the asteroid belt and studied its objects. We’ll mention the three most notable missions of the past and one exciting future mission.

  • The first spacecraft to reach the asteroid belt was Pioneer 10; on its mission to Jupiter, it flew through the belt in 1972.
  • The Galileo spacecraft studied the asteroids Gaspra and Ida in 1989 and discovered the first moon around an asteroid — Ida’s moon Dactyl.
  • The Dawn space probe became the first to visit the asteroids Vesta (in 2011) and Ceres (in 2015).
  • The Psyche spacecraft, scheduled to launch in 2023, will visit asteroid 16 Psyche. Scientists believe this asteroid could be the iron core of a protoplanet the size of Mars.

Can you see the asteroid belt?

You can’t see the belt itself, but you surely can see some of its asteroids. The four largest bodies in the asteroid belt — Ceres, Vesta, Pallas, and Hygiea — can be observed through a small telescope or even large binoculars. The best time to observe an asteroid is during opposition when it appears at its brightest in the sky. Below we provide the upcoming opposition dates for the above-mentioned celestial bodies. In brackets, you can see the magnitude each of them will reach.

  • Vesta (mag 6.1): August 22, 2022
  • Pallas (mag 7.7): January 15, 2023
  • Ceres (mag 7.1): March 21, 2023
  • Hygiea (mag 9.7): August 10, 2023

To quickly find the desired asteroid in the sky, use the Sky Tonight app. Tap the magnifier icon at the bottom of the screen, type the asteroid’s name, and tap the blue target icon on the corresponding search item.


How far is the asteroid belt from the Earth?

The distance between the Earth and the asteroid belt’s edge that’s closest to it ranges between 1.2 to 2.2 astronomical units. One astronomical unit is the average distance from the Earth to the Sun, which equals 150 million km (93 million mi).

How many asteroids are there in the asteroid belt?

According to ESA, the asteroid belt contains between one to two million asteroids larger than one km in diameter. Plus, there are millions of smaller space rocks there. As of April 2022, astronomers discovered and numbered 598,053 asteroids in the asteroid belt.

What is the largest object in the asteroid belt?

The largest object in the asteroid belt is the dwarf planet Ceres, which is 940 km in diameter. Watch our video on Ceres to learn interesting facts about it.

Why didn't a planet form where the asteroid belt is now located?

The asteroid belt didn’t become a planet because it’s positioned too close to Jupiter. The gas giant’s immense gravitational influence prevented the asteroids from collecting together into one large body.

What are the two red objects found in the asteroid belt?

In July 2021, the Japan Aerospace Exploration Agency (JAXA) found two huge asteroids whose spectrum was much redder than that of any other object in the asteroid belt. The asteroids are called 203 Pompeja and 269 Justitia. Scientists believe they might have formed near the Solar System's outer edge and then migrated to the asteroid belt about 4 billion years ago.

Can the asteroid belt be mined?

Theoretically, it can be done. An ideal candidate for mining would be the asteroid 16 Psyche that’s possibly made of iron and nickel. According to some estimates, this asteroid may be worth hundreds of quintillions of dollars! However, mining an asteroid is an extremely hard task that requires technologies that we probably don’t have right now.

Bottom line: The asteroid belt between Mars and Jupiter is home to millions of space rocks of different shapes and sizes. Most probably, the asteroids are remnants from the creation of our Solar System that didn’t coalesce into a planet because of Jupiter’s gravitational influence. The next space mission to a main-belt asteroid is NASA’s Psyche, scheduled to launch in 2023.