Asteroid belt

An asteroid belt is a ring of small rocky and icy bodies orbiting a star where no planet managed to form. The Sun's Main Belt, between Mars and Jupiter at 2.06 to 3.27 astronomical units (AU), holds over a million bodies larger than a kilometre, yet their combined mass is only about 3 percent of the Moon's ^{[1] [2]}. Its edges and gaps are set by resonances with Jupiter, its make-up records how material moved around the young Solar System, and it is the source of most meteorites that reach Earth.

Where the Main Belt is, and why there

The belt's inner edge at about 2.06 AU coincides with Jupiter's 4:1 mean-motion resonance, where an asteroid would complete four orbits for every one of Jupiter's, together with the $\nu_6$ secular resonance with Saturn. Its outer edge, near 3.28 AU, is Jupiter's 2:1 resonance ^[3]. Kepler's third law turns any resonance into a distance. An asteroid making $p$ orbits for every $q$ of Jupiter's has a period $q/p$ of Jupiter's, and so a semi-major axis

$$a = a_J \left(\frac{q}{p}\right)^{2/3}$$

With Jupiter at $a_J = 5.20$ AU, the 3:1 resonance falls at 2.50 AU, the 5:2 at 2.82 AU and the 7:3 at 2.96 AU. Because the rule depends only on period ratios, the same fractions of a giant's orbit mark the same features around any star.

The Kirkwood gaps. In the 1860s Daniel Kirkwood noticed that asteroids avoid exactly these distances ^[3]. At a resonance, Jupiter tugs on an asteroid at the same point of its orbit again and again, and the kicks build up: its eccentricity grows until it crosses the orbit of Mars or Earth, falls into the Sun or is thrown out by Jupiter. Numerical experiments show that bodies pushed into the 3:1 or $\nu_6$ resonances typically last only a few million years, and some of the survivors reach Earth as meteorites ^[4]. The gaps are the emptied lanes.

Not every resonance empties. The Hildas, at the 3:2 resonance near 3.97 AU, are a stable clump whose orbits keep them away from Jupiter, and the Trojans share Jupiter's orbit at its L4 and L5 points, 60 degrees ahead and behind. The Trojans are dark, primitive bodies that were probably captured when the giant planets' orbits reshuffled early in the Solar System's history ^[5]. NASA's Lucy spacecraft, launched in 2021, is due to make its first Trojan flyby in August 2027 ^[6].

A belt that is mostly empty

Pictures of spaceships dodging tumbling boulders get the belt badly wrong. Its total mass, measured from the pull it exerts on the planets and on spacecraft, is $4.0 \times 10^{-4}$ Earth masses, or $2.4 \times 10^{21}$ kg ^[1]. Ceres alone, at 939 km across, holds about 39 percent of that ^[7]. The number of bodies larger than a kilometre is 1.1 to 1.9 million ^[2], spread through a doughnut-shaped volume roughly $10^{26}$ cubic kilometres in size. Dividing one by the other puts neighbouring kilometre-sized asteroids some 3 million km apart on average, about eight times the distance from Earth to the Moon. Standing anywhere in the real belt you would almost never see an asteroid as more than a point of light, and spacecraft cross it without having to steer around anything.

What asteroids are made of

Asteroids are sorted into classes by their colour and reflectance spectra. The stony S types dominate the inner belt, the dark carbon-rich C types the outer belt, and the even darker P and D types the Hildas and Trojans. Smaller groups include the basaltic V types, chips off Vesta's crust, and the metal-rich M types ^[8]. The gradient from S to C roughly tracks distance from the Sun, and from the snow line, beyond which water ice could condense. DeMeo and Carry's survey of asteroid colours found far more mixing than a smooth gradient allows, especially among smaller bodies, evidence that material was stirred and moved around after it formed ^[8].

Meteorites tell a matching story. They fall into two isotopically distinct families that formed in separate reservoirs, one inside and one outside the orbit of Jupiter, and the separation appears to have been set by Jupiter's early growth within the first million years or so ^[9]. Both families are now mixed in the belt.

Why no planet formed here

The idea that the belt is the rubble of an exploded planet dates from Heinrich Olbers in 1802 and does not survive the numbers: the whole belt would make a body less than half the size of the Moon, and its material comes from at least two different reservoirs with different formation ages. The accepted picture is that a planet never formed. Several models reproduce the belt we see. In one, Jupiter's resonances and wandering planetary embryos stirred up the original belt and cleared out most of its mass ^[10]. In the "Grand Tack", Jupiter migrated inward toward Mars's present orbit and back out again, truncating the disk and refilling the belt with S types from inside and C types from outside ^[11]. In a third, the belt formed nearly empty and was later seeded with material scattered in from both sides ^[12].

The Kuiper belt and belts around other stars

Beyond Neptune lies a second, larger belt. The Kuiper belt's main population orbits between Neptune's 3:2 resonance at 39.4 AU, where Pluto and the other Plutinos sit, and its 2:1 resonance at 47.8 AU. Its mass, about $2 \times 10^{-2}$ Earth masses, is roughly 50 times the Main Belt's ^[1]. The first Kuiper belt object after Pluto was found in 1992 ^[13], and in 2019 New Horizons flew past Arrokoth, a primitive body about 36 km long made of two lobes that joined gently ^[14]. Kuiper belt objects are ice-rich, and the belt feeds the short-period comets.

Other stars have belts too, seen as the infrared glow of dust ground off colliding bodies. Herschel detected such debris discs around 17 percent of nearby Sun-like stars ^[15]. Those are only the dustiest: the Sun's own Kuiper belt would sit close to the median of the population, so perhaps half of all stars have a belt at least as bright as ours, and most belts as faint as the Sun's Main Belt are far below any survey's reach ^[15]. The James Webb Space Telescope resolved three nested belts around the young star Fomalhaut, reaching about 23 billion km (some 150 AU) from the star, with gaps that may be carved by unseen planets ^[16].

How we know

Most of what is known comes from ground surveys that find asteroids and track their orbits, now numbering in the hundreds of thousands, and from the belt's gravitational pull on the planets and spacecraft, which gives its total mass ^[1]. The Gaia mission added precise astrometry for more than 150,000 Solar System objects and reflectance spectra of 60,518 asteroids in its third data release ^[17]. Spacecraft have visited individual bodies. Dawn orbited Vesta, a differentiated protoplanet with an iron core ^[18], and Ceres, which turned out to be partly differentiated, with a rocky interior under a volatile-rich shell ^[7]. NASA's Psyche spacecraft, launched in October 2023, is due to begin orbiting the metal-rich asteroid Psyche in August 2029 ^[19].

Notable members

Orbital and size values from the JPL Small-Body Database ^[20].

See also

• Orbit
• Natural satellite
• Planetary system archetypes
• Star system generation
• Sol
• Gas giant
• Planet occurrence

References

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19. 19NASA. Psyche. NASA Science. science.nasa.gov/mission/psyche/
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