CHAPTER 10
Dwarf Planets and Solar System Debris
CHAPTER OUTLINE
The Discovery of Pluto
1. An analysis of the orbital data of Uranus indicated that 98% of its orbital variation
could be accounted for by the presence of Neptune; the remaining unexplained 2%
variation led to the search for Planet X.
2. In 1905 Lowell initiated a search for Planet X. Clyde Tombaugh finally discovered
Planet X—Pluto—in early 1930.
3. Tombaugh used a blink comparator to compare two photos of the sky taken a few days
apart. A moving object such as a planet will appear to jump from one spot to another as
the observer quickly changes views from the first photo to the second.
4. Pluto was discovered 6° from where Lowell had predicted it.
5. Pluto, however, is too small to cause the irregularities that had been seen in Uranus’s
orbit. Later it was shown that these irregularities were not caused by another planet but
were variations due to the limited accuracy of the data available.
6. Pluto’s discovery was an accident, though without Lowell’s predictions, the search
would not have been so vigorously pursued.
10-1 Pluto
Pluto as Seen from Earth
1. In 1989 Pluto was as close to the Earth as it has been for 248 years. (From 1979 to
1999 Pluto was inside Neptune’s orbit.)
2. Pluto’s average distance from the Sun is 40 AU, but its eccentric orbit causes it to vary
in distance from 30 AU to 50 AU.
3. Pluto’s orbit is tilted 17° to the ecliptic (no other planet is tilted more than 7°).
4. Stellar occultations indicate that Pluto has an atmosphere (nitrogen, methane, and
carbon monoxide). At aphelion it is probably too cold for the methane to remain gaseous.
The atmosphere is undergoing global cooling, while the planet’s surface is getting
slightly warmer.
5. On Pluto’s surface the temperature range from 235C to 210C.
Pluto and its Moons
1. In 1978, James Christy discovered that Pluto has a moon, named Charon.
2. Charon’s orbit is tilted at 61° to Pluto’s orbit around the Sun.
3. Charon orbits Pluto every 6.4 days, the same as Pluto’s and Charon’s rotational period.
Thus, these two objects always keep the same face toward each other.
4. Pluto’s atmosphere limits an accurate determination of its size, which probably ranges
from 2,300 km to 2,340 km. Charon’s diameter is about 1,200 km.
5. Pluto’s mass is about 12 times Charon’s, but only 1/5 of our Moon’s. Pluto’s density is
about 2 g/cm3.
6. Two additional moons were discovered in 2005 by the Hubble Space Telescope.
A Former Moon of Neptune?
1. Because Pluto is small and has an eccentric orbit, some theorize that it is a former
moon of Neptune that was somehow ejected.
2. The discovery of Charon made it seem less likely that Pluto was once Neptune’s moon.
3. It is likely that this system was formed when Pluto collided with a similar object,
leaving behind enough mass to form Charon and other moons.
10-2 Solar System Debris
1. Pluto and Charon are now classified as two of the largest examples of a group of
objects orbiting in a swathe of the outer solar system (the Kuiper belt).
2. Apart from the Sun (a large object) and the planets and moons (medium-sized objects),
most of the other objects in the solar system can be classified as debris—an accumulation
of rock fragments.
3. Solar system debris comes in a number of forms, including asteroids, meteoroids,
comets, and dust.
10-3 Asteroids
1. Almost 200 years after the first asteroid, Ceres, was discovered, the orbits of thousands
of asteroids have been accurately determined.
2. Astronomers estimate that hundreds of thousands of asteroids have been captured on
photographic surveys of the sky. Asteroids are also known as minor planets.
3. Ceres, at 950 km (590 mi) in diameter, is the largest asteroid and makes up about a
third of the mass of all asteroids.
4. Pallas and Vesta have diameters greater than 300 km. About 30 more asteroids have
diameters between 200 and 300 km. About 100 are between 100 and 200 km. All the rest
are under 100 km in diameter.
5. Asteroids are classified in a number of ways including by position (e.g., asteroid belt,
near-Earth, and Trojans) and by type (bright, metallic and reddish S-type, bright, metallic
and non-reddish M-type, and dark C-type).
5. Asteroids are irregular in shape, and thus reflect different amounts of light toward
Earth as they rotate.
6. Gaspra, an asteroid 19  12  11 km, was imaged by the Galileo spacecraft in 1990.
7. Some asteroids like Gaspra have a reddish tint and are fairly bright. Others are dark
like a lump of coal.
The Orbits of Asteroids
1. The asteroids revolve around the Sun in a counterclockwise direction like the planets.
2. Most asteroids orbit in or near the plane of the ecliptic.
3. Most asteroids orbit the Sun at distances from 2.2 to 3.3 AU (between Mars and
Jupiter) in what is called the asteroid belt.
4. Apollo asteroids are some 500 asteroids that have eccentric orbits that cross the
Earth’s orbit. About 70 of them are potentially hazardous to Earth because they are larger
than 1 km in diameter and get closer than 0.05 AU to Earth.
5. Asteroids are not evenly distributed across the asteroid belt. At certain distances—2.5
and 3.28 AU—gaps appear and are related, respectively, to 1/3 and 1/2 of Jupiter’s
orbital period. These Kirkwood gaps are due to synchronous tugs from Jupiter.
6. Gaps in the asteroid belt also appear corresponding to 2/5 and 3/7 of Jupiter’s orbital
period.
The Origin of Asteroids
1. Astronomers originally thought that asteroids were the remains of the explosion of a
planet. However, there is no known mechanism for making a planet explode.
2. If all the asteroids were combined into one object, they would only form a body about
1,500 km in diameter, much smaller than our Moon.
3. It is most likely that asteroids are primordial material that never formed into a planet
because of Jupiter’s gravitational influence.
10-4 Comets
1. Comets are among the most spectacular sights available to the naked eye.
Cometary Orbits — Isaac Newton and Edmund Halley
1. Newton proposed that comets orbit the Sun according to his laws of motion and
universal gravity. He concluded that since comets were visible for only short periods of
time, their orbits were very eccentric, i.e., elongated.
2. Halley, a friend of Newton, used Newton’s methods, his own observations, and prior
comet descriptions to calculate orbits for a number of comets. He correctly surmised that
the comets of 1531, 1607, and 1682 were in fact the same comet and predicted its next
return in 1758. This comet was named in his honor his honor (Comet Halley) and is
probably the most famous periodic comet.
3. Comet Halley has a period of approximately 76 years. Because the comet is slightly
affected by the gravity of the planets during its return from deep space, its period varies
slightly one appearance to the next. Halley was the first to recognize this effect of other
bodies on comets’ orbits.
4. About 100 comets have periods of less than 200 years.
5. The planes of revolution of comets are not limited to the ecliptic but are randomly
oriented. Comets sweep past the Sun from all directions.
The Nature of Comets
1. Comets have three parts: the coma, the nucleus, and the tail. The coma and nucleus
comprise the comet’s head.
2. The coma may be as large as a million km in diameter. Some tails have been as long as
an astronomical unit.
3. Fred Whipple proposed in 1950 what is still the basic model of a comet: the nucleus is
essentially a dirty snowball made up of water ice, frozen carbon dioxide, and small solid
grains. We now include in the model a crusty layer on the surface of the nucleus.
4. The coma is made up of diffuse gas and dust. The nucleus is the solid chunk of a
comet. The tail is the gas and/or dust swept away from the comet’s head.
5. As a comet approaches the Sun, it becomes warmer and the ices inside melt and
vaporize.
6. The nucleus of Comet Hale-Bopp is about 25 miles across and spins once every 12
hours. As it spins, material is ejected from it in geysers and spirals away from it.
7. ESA’s Giotto spacecraft revealed that Halley’s coma is billions of times less dense
than the atmosphere of the Earth at sea level. It also confirmed that the comet is
composed of material as primitive as the original solar nebula.
8. The presence or absence of noble gases in a comet provides a tool for measuring the
thermal history of comets. Another tool is the structure of the dust that comets carry.
Comet Tails
1. Comets usually have two tails. The straight tail consists of charged atoms or molecules
(ions) that are being swept away from the comet by the solar wind. The curved tail is
caused by dust grains in the coma being pushed away by solar radiation pressure.
2. A comet’s tail always points away from the Sun (and thus does not always follow the
comet’s head). After passing the Sun, a comet’s tail actually leads the head.
3. Comet tails are typically 107 to 108 km long and may be 1 AU long.
4. Comets “die” through gradual evaporation of their entire nuclei, or through
evaporation of all their volatile materials, leaving chunks of rock, or by falling into the
Sun.
Missions to Comets
1. Deep Impact crashed on the nucleus of Comet Temple 1 in 2005; data showed the
presence of a mix of water, organic compounds, and silicates, similar to what is observed
in the long-period comets from the Oort cloud.
2. Stardust plunged through the coma of Comet Wild 2 in 2004, collected samples and
returned them to Earth; data showed the presence of olivine and high-temperature
minerals such as aluminum, calcium, and titanium.
3. Rosetta was launched in 2004 and will meet its target in 2014; its lander will drill into
the crust of the comet’s nucleus and collect and analyze material from the nucleus.
10-5 The Oort Cloud and the Kuiper Belt
1. In 1950 Jan Oort proposed that a comet cloud exists in a spherical shell between
10,000 and 100,000 AU from the Sun. Billions of comet nuclei are thought to exist in this
Oort cloud.
2. Long-period comets are believed to originate in the Oort cloud. Interactions between
comets in the cloud or between a comet and a star passing-by could deflect some comets
into the inner solar system.
3. In 1951 Gerard Kuiper proposed that a second, smaller band of comets must exist
inside the Oort cloud. The Kuiper belt is a disk-shaped region beyond Neptune’s orbit,
30–1000 AU from the Sun, and presumed to be the source of short-period comets.
4. The first object in the Kuiper belt was observed in 1992. Eris, currently the largest
dwarf planet, Pluto, Sedna and other trans-Neptunian objects are Kuiper-belt objects.
The Origin of Short-Period Comets
1. Long-period comets sometimes become short-period comets through the gravitational
influence of Jupiter and the Sun.
2. Most comet orbits are either elliptical or parabolic (but not hyperbolic). Understanding
how comets are distributed and what they are made of will put important constraints on
models describing the formation and early evolution of our solar system.
3. The average distance between comets in the Oort cloud is greater than 16 AU! The
Oort cloud and the Kuiper belt are far from crowded.
10-6 Meteors and Meteor Showers
1. A meteor is the phenomenon of a streak in the sky caused partially by the burning of a
rock or dust particle as it falls into our atmosphere.
2. A meteoroid is an interplanetary chunk of matter smaller than an asteroid.
3. A meteorite is an interplanetary chunk of matter after it has hit a planet or moon.
4. The first confirmation of a fall of rocks from an exploding meteor occurred in France
in 1803.
Meteors
1. A fireball is an extremely bright meteor.
2. A meteoroid’s typical speed is 50 km/s (100,000 mph), so when it hits the Earth’s
atmosphere, it heats up and begins to vaporize.
3. A typical meteor begins to glow at a height of 130 km (80 mi) and burns out at about
80 km (50 mi).
Meteoroids
1. Most meteors are produced by meteoroids with masses ranging from a few milligrams
(grain of sand) to a few grams (marble-size rock).
2. Since a meteor can be seen only if it is within 150–200 km of the viewer, it is
calculated that over the entire Earth there must be about 25 million meteors a day visible
to the naked eye.
3. It is estimated that 1,000 tons of meteoritic material hit the Earth every day.
4. It is estimated that only 1 in 1 million meteoroids that hit the atmosphere survives to
reach the surface.
5. Unlike most asteroids, meteoroids may orbit the Sun in any orientation.
6. It is thought that many small meteoroids are debris from asteroid collisions.
7. Many meteors come from material evaporated from a comet’s nucleus.
Meteor Showers
1. A meteor shower is the phenomenon of a large group of meteors seeming to come
from a particular area of the celestial sphere. It is actually caused by the Earth passing
through a swarm of small meteoroids.
2. Meteor showers are named after the constellation from which they seem to originate.
3. The radiant of a meteor shower is the point in the sky from which the meteors of a
shower appear to radiate.
4. Most of the major meteor showers are associated with comets.
5. Some showers change in intensity from year to year because the particles that cause the
shower clump together in one region of the comet’s orbit.
6. The best time to observe a meteor shower is in the early morning hours. This results
from the Earth’s rotation and its motion through the swarm of particles left behind by the
comet.
10-7 Meteorites and Craters
1. Meteorites are classified into 3 categories:
(a) Irons—iron meteorites that are made up of 80%–90% iron (with some nickel).
(b) Stones—stony meteorites that can contain flakes of iron and nickel.
(c) Stony irons—meteorites that are half stone and half iron.
2. About 95% of all meteorites are stones but if you find one it will likely be an iron.
3. Chondrites comprise an important class of stony meteorites; they make up 91% of the
about 24,000 known meteorites. They formed early in the history of the solar system and
they are thought to be the building blocks of the planets. They consist mostly of
chondrules, small rock spheres in a mix of other mineral or metal grains.
4. Carbonaceous chondrites are the most important subclass of chondrites. They contain
high levels of water, organic compounds and minerals.
5. The Hoba meteorite in Namibia weighs 65 tons and is the largest meteorite ever found.
6. The second largest (34 tons) is on display in New York City at the American Museum
of Natural History.
7. One of the most prominent impact craters on Earth is Meteor Crater near Winslow,
Arizona. It is nearly a mile across, 180 m (600 ft) deep and has a rim rising 45 m (150 ft)
above the surrounding desert.
8. The meteorite that formed Meteor Crater is estimated to have had a total mass of 300
million kg (300,000 tons) and to have been about 45 m across. It struck about 25,000
years ago at a speed of about 11 km/s (25,000 mph), releasing an energy equivalent to a
25-megaton bomb.
9. It is estimated that a meteorite larger than 1 km in diameter strikes the Earth on
average once every few hundred thousand years.
10. A hit by a 1-km meteorite would produce a crater 10-km in diameter and be
equivalent to a 5000-megaton bomb.
11. There is compelling evidence that an asteroid some 10 km in diameter struck the
Earth (near the Yucatan peninsula) 65 million years ago and led to the subsequent
extinction of the dinosaurs.
10-8 The Importance of the Solar System Debris
1. We believe that life on earth started about 3.8 billion years ago, at the end of the heavy
bombardment period. There is evidence for biological activity at the end of this period
and known fossils on Earth date as far back as 3.5 billion years ago.
2. The building blocks of life could have been delivered by asteroid and comet impacts.
3. Understanding the chemical makeup of comets helps in understanding the composition
and conditions of Earth’s formation 4.6 billion years ago.
4. Collisions supplied Earth with water, volatiles and carbon-based molecules.
5. Understanding the structure, composition and orbits of space debris will help protect
against catastrophic collisions.
6. Asteroids and comets may prove to be a rich supply of water, life-sustaining carbonbased molecules and structural raw materials to supply space exploration and
construction.