4-4 Small chunks of rock and ice also orbit the Sun: asteroids, trans-Neptunian objects, and comets

Our solar system is more than a single star orbited by two categories of planets. In addition to the eight planets and their moons, countless smaller objects orbit the Sun. These fall into three broad categories: asteroids, which are rocky objects found in the inner solar system; trans-Neptunian objects, which are found beyond Neptune in the outer solar system and contain both rock and ice; and comets, which are mixtures of rock and ice that originate in the outer solar system but can venture close to the Sun.

Asteroids

Within the orbit of Jupiter are hundreds of thousands of rocky objects called asteroids. There is no sharp dividing line between planets and asteroids, which is why asteroids are also called minor planets. The largest asteroid, Ceres, has a diameter of about 560 mi (900 km). The next largest, Pallas and Vesta, are each about 300 mi (500 km) in diameter. Still smaller ones, like the asteroids shown in close-up in Figure 4-6, are increasingly numerous. Only the largest ones are round. Hundreds of thousands of kilometer-sized asteroids of widely differing shapes are known, and there are probably hundreds of thousands more that are automobile-sized or smaller. All of these objects orbit the Sun in the same clockwise direction as the planets.

Figure 4-6: RIVUXG Asteroid Family Portrait A collage of asteroids, shown to scale, from largest to smallest: Vesta, 330 mi (530 km) across; Lutetia, 81 mi (130 km); and Mathilde, Ida (and its moon Dactyl), Eros, Gaspra, 2867 Steins, and Itokawa. Most asteroids are too small to have enough gravity to pull them into spherical shapes.

Most but not all asteroids orbit the Sun between distances of 2 AU to 3.5 AU. This region of the solar system between the orbits of Mars and Jupiter is called the asteroid belt.

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CAUTION

One common misconception about asteroids is that they are the remnants of an ancient planet that somehow broke apart or exploded, like the fictional planet Krypton in the comic book adventures of Superman. In fact, the combined mass of the asteroids is less than that of the Moon, and they were probably never part of any planet-sized body. The early solar system is thought to have been filled with asteroidlike objects, most of which were incorporated into the planets. The “leftover” objects that missed out on this process make up our present-day population of asteroids.

Question

ConceptCheck 4-8: Is the largest asteroid, Ceres, about the same size as a metropolitan city, a large U.S. state, a large country, an entire continent, or Earth itself?

Trans-Neptunian Objects

While asteroids are the most important small bodies in the inner solar system, the outer solar system is the realm of the trans-Neptunian objects (TNOs). As the name suggests, these are small bodies whose orbits lie beyond the orbit of Neptune. The largest of these are known as dwarf planets. The first of these to be discovered (1930) was Pluto, with a diameter of only 1413 mi (2274 km). The orbit of Pluto has a greater average orbital radius (39.54 AU), is more steeply inclined to the ecliptic (17.15°), and has a greater eccentricity (0.250) than that of any of the planets (Figure 4-7). In fact, Pluto’s noncircular orbit sometimes takes it nearer the Sun than Neptune. (Happily, the orbits of Neptune and Pluto are such that they will never collide.) Pluto’s density is only 2000 kg/m3, about the same as Neptune’s moon Triton shown in Table 4-2. Hence, its composition is thought to be a mixture of about 70% rock and 30% ice.

Since 1992 astronomers have discovered more than 1200 new trans-Neptunian objects, and they are discovering more each year. All trans-Neptunian objects orbit the Sun in the same direction as the planets. The largest of these trans-Neptunian objects are comparable in size to Pluto. At least one dwarf planet, Eris, is even larger than Pluto, as well as being in an orbit that is much larger, more steeply inclined, and more eccentric (Figure 4-7).

Figure 4-7: Trans-Neptunian Objects Pluto and Eris are the two largest trans-Neptunian objects, small worlds of rock and ice that orbit beyond Neptune. Unlike the orbits of the planets, the orbits of these two objects are steeply inclined to the ecliptic: Pluto’s orbit is tilted by about 17°, and that of Eris is tilted by 44°.

Just as most asteroids lie in the asteroid belt, most trans-Neptunian objects orbit within a band called the Kuiper belt (pronounced “ki-per”) that extends from 30 AU to 50 AU from the Sun and is centered on the plane of the ecliptic. Certainly, many more trans-Neptunian objects remain to be discovered as telescope technology improves. Astronomers estimate that there are 35,000 or more such objects with diameters greater than 100 km. If so, the combined mass of all trans-Neptunian objects is comparable to the mass of Jupiter and is several hundred times greater than the combined mass of all the asteroids found in the inner solar system.

Trans-Neptunian objects are thought to be debris naturally left over from the formation of the solar system. In the inner regions of the solar system, rocky fragments have been able to endure continuous exposure to the Sun’s heat, but any ice originally present would have evaporated. Far from the Sun, ice has survived for billions of years.

Question

ConceptCheck 4-9: Is Pluto an asteroid, planet, dwarf planet, Kuiper-belt object, or trans-Neptunian object?

Question

CalculationCheck 4-3: Which has a larger diameter, the asteroid belt or the Kuiper belt?

Comets

Two objects in the Kuiper belt can collide if their orbits cross each other. When this happens, a fragment a few kilometers across can be knocked off one of the colliding objects and be diverted into a new and elongated orbit that brings it close to the Sun. Such small objects, each a combination of rock and ice, get a new name—comets. When a comet gets close enough to the Sun, the Sun’s radiation vaporizes some of the comet’s ices, producing long flowing tails of gas and dust particles (Figure 4-8). Astronomers can then deduce the composition of comets by studying the spectra of these tails created by reflected sunlight.

Figure 4-8: RIVUXG A Comet This photograph shows Comet Hale-Bopp as it appeared in April 1997. The solid part of a comet like this is a chunk of dirty ice a few tens of kilometers in diameter. When a comet passes near the Sun, solar radiation vaporizes some of the icy material, forming a bluish tail of gas and a white tail of dust. Both tails can extend for tens of millions of kilometers.

CAUTION

Science fiction movies and television programs sometimes show comets tearing across the night sky like a rocket. That would be a pretty impressive sight—but that is not what comets look like. Like the planets, comets orbit the Sun. And like the planets, comets move hardly at all against the background of stars over the course of a single night. If you are lucky enough to see a bright comet, it will not zoom dramatically from horizon to horizon. Instead, it will seem to hang majestically among the stars, so you can admire it at your leisure.

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Some comets appear to originate from locations far beyond the Kuiper belt. The source of these is thought to be a swarm of comets that forms a spherical “halo” around the solar system called the Oort cloud. This hypothesized “halo” extends to 50,000 AU from the Sun (about one-fifth of the way to the nearest other star). Because the Oort cloud is so distant, it has not yet been possible to detect objects in the Oort cloud directly.

Question

ConceptCheck 4-10: If a comet is observed to have an orbit that is perpendicular to Earth’s orbit about the Sun, did it most likely originate from the Kuiper belt or the Oort cloud?