Between the orbits of Mars and Jupiter we find one of the most interesting regions of the solar system: the asteroid belt. Millions of rocks of very different sizes orbit in this region, from dust grains millimeters or centimeters thick to several asteroids and a dwarf planet several hundred kilometers in diameter.
In fact, there is such a disparity between their sizes that only 5 asteroids accumulate more than half of the total mass of the belt. The largest of these is Ceres, the only one with enough mass to have taken on a roughly spherical shape. This is an essential requirement to be considered a dwarf planet, so Ceres is the only dwarf planet in the asteroid belt and the closest to the Sun. Despite having the same designation, Ceres is considerably smaller than Pluto. It is even smaller than Charon, Pluto’s binary partner/moon. The fourth largest asteroid, Hygiea, also appears to be spherical in shape, which would earn it the title of dwarf planet, even though it is smaller and less massive than Vesta and Pallas. However, it is likely that this configuration was obtained after a large collision, rather than during the formation of the asteroid itself.
But how did these asteroids form? And why are there so many and so close together? During the early solar system, when the great cloud of gas and dust that gave rise to our sun was contracting, a disk formed around the stellar embryo . From this protoplanetary disk the different planets would come out. After countless slow collisions between bodies of centimeters or meters in size, some planetesimals were formed , capable of gravitationally attracting nearby material. Little by little this disc was cleaned, falling all the material to some of these planetesimals, or their moons, forming the planets that we have today. However, in the region between Mars and Jupiter something prevented the formation of a new planet .
The farther a planet is from the Sun, the longer it takes to complete one orbit around it. So there will be a certain distance from the Sun at which a stable orbit will take half the time it takes for Jupiter to complete its orbit. This will mean that every two orbits of this supposed planet will coincide with one orbit of the gas giant. This periodicity will eventually cause Jupiter to perturb the planet’s orbit, making it unstable and altering it . This not only happens when the orbit of the planet or asteroid is exactly half that of Jupiter, but when it is an exact fraction of it. The simpler the fraction, the more often they will coincide in their orbits, and the more the inner orbit will be affected.
In the region between Mars and Jupiter there are several of these resonances (as they are called), so any body that tries to maintain a stable orbit in this region will end up seeing its orbit altered directly or indirectly. All these planetesimals that would end up merging to form a larger planet, could not do so due to the presence of Jupiter in the neighborhood. The maximum that could form is a dozen bodies of a few hundred kilometers in diameter , and thousands and millions of much smaller bodies.
In fact, it is believed that the initial mass of all objects in that region must have been similar to the mass of the Earth during the early solar system. However, after a few million years, just a breath on astronomical scales, 99% of the mass had been removed from its orbit by Jupiter, leaving only a small sample, which has barely survived to this day. At present, the total mass of the asteroid belt is estimated to be around 3% of the mass of our moon .
This has made the asteroid belt today a tremendously empty region , considerably more so than what we are shown in the movies. In the movie The Empire Strikes Back, from the Star Wars saga, there is a scene in which Luke, Leia, Han and Chewbacca escape from an imperial destroyer aboard the Millennium Falcon and to mislead several TIE Fighters they enter a field of asteroids, in which they barely manage to avoid rocks several dozen in size (one of which even harbors life inside). The reality, at least in our asteroid belt, is that the typical distance between objects that size would be at least thousands of miles , so Han and Chewbacca would have no problem dodging them. In fact, they most likely had trouble deducing that they had entered an asteroid field.
Fiction aside and back to science, several space probes have passed through this region in recent decades, without ever colliding (or even coming close to colliding) with any bodies of any size . NASA’s Dawn mission, which visited the two largest bodies in the belt, Ceres and Vesta, traveled (according to NASA itself) more than a billion kilometers within this region. During its entire journey it did not pass within 1 million kilometers of the nearest known asteroid.
REFERENCES:
Krasinsky, G. A. et al, 2002, Hidden Mass in the Asteroid Belt, Icarus. 158 (1): 98–105.
