As is the case with the other outer planets, we see Neptune best when it is at or near opposition (Fig. 7-8), although its absolute brightness does not vary much as the Earth revolves around the Sun. Neptune cannot be seen with the unaided eye; powerful binoculars or, better yet, a good telescope is necessary to observe it. You need to know exactly where to look; the Weather Underground or Celestron CD-ROM maps can be used to locate it. Even when viewed through a large amateur telescope, Neptune will only look like a blue star.
THE YEAR AND THE DAY
Neptune is tilted on its axis by 291Ⲑ2degrees. This is to say, the equatorial plane of Neptune intersects its orbital plane at an angle of 291Ⲑ2 degrees.
This compares with 231Ⲑ2degrees for the Earth. The seasonal variations in
the Sun’s path across the sky on Neptune would be somewhat familiar to Earthlings, except for one fact: No human would ever live long enough to see all four Neptunian seasons go by. As a matter of fact, no human likely would be able to survive long enough beneath the sapphire-blue haze and snow-white clouds of Neptune to eat a decent supper, let alone carry out a lifetime’s research.
Neptune takes 165 Earth years to make one complete journey around the Sun. Its orbit is almost a perfect circle, so any seasonal effects on Neptune’s climate must be caused entirely by the tilt of its axis and not by variations in the amount of sunlight it receives.
COMPOSITION, ATMOSPHERE, AND WEATHER
In composition, Neptune is thought to be similar to Uranus, but it is more dense. Neptune is about 49,500 kilometers (30,800 miles) in diameter; this is a little less than four times the diameter of the Earth (Fig. 7-9). Neptune generates more internal heat than Uranus; in this respect it more nearly resembles Jupiter and Saturn. Neptune is more blue in color than Uranus, and astronomers are not quite certain what is responsible for this vivid sapphire hue.
Orbit of Earth
Opposition Neptune
Orbit of Neptune Sun (at center of Earth’s orbit)
Figure 7-8. The orbits of Earth and Neptune, to scale. As with the other outer planets, the best viewing is at opposition.
When Voyager passed by Neptune in 1989, it proved to have a more interesting atmosphere, at least visually, than its aquamarine cousin. There were dark spots and bright clouds; one of the clouds raced around Neptune independently of other weather phenomena and was named Scooterfor this reason. There was an oval-shaped spot of deep indigo, similar in shape to but about half the diameter of Jupiter’s Great Red Spot. This system had winds that blow faster than those on any other planet in the Solar System, approximately three times the speed of the wind in a maxitornado on Earth.
The Great Dark Spot, which was as large in diameter as the Earth, disap- peared after a few years. Apparently the self-sustaining forces that keep storms alive on Jupiter are not as effective on Neptune.
1989N1R
Neptune has a ring system similar to those of Uranus and Jupiter but fainter. These rings were revealed by the Voyager probe. The rings of Neptune are unique in the Solar System because the outermost one, called 1989N1R, is nonuniform. This name derives from the year of discovery (1989), the planet (Neptune), the ring number (1 means “outermost”), and R (which stands for “ring”).
Astronomers are not sure why 1989N1R is “clumped.” One theory holds that it was created only a little while ago, on a cosmic scale, possibly with- in the last few centuries. If an asteroid or comet ventured too close to the planet, it would break up because of gravitation. Eventually the particles would spread all the way around Neptune and form a uniform ring, but this process would require some time. Maybe it hasn’t had time to do this yet.
Another theory for the clumping of 1989N1R involves gravitational interaction between the ring particles and a tiny moon,Galatea. It is possi- ble that certain gravitational resonances could cause the clumping.
Earth
Neptune
Figure 7-9. The equatorial diameter of Neptune is about the same as that of Uranus, four times the diameter of Earth.
Orbit of Earth
Sun (at center of Earth’s orbit) Orbit of
Neptune
Orbit of Pluto
Figure 7-10. The orbits of Earth, Neptune, and Pluto, to scale. The orbits of Pluto and Neptune overlap, but orbital resonances ensure that the two planets will never collide.
The particles that make up the rings around Neptune are in general less than an inch in diameter. This was revealed by analyzing radio waves pass- ing through or reflected from the particles. Large chunks of rock affect radio waves at medium and long wavelengths, and as the rocks become smaller, they affect radio waves at shorter wavelengths. Neptune’s rings were not detected when observations were made at radio wavelengths.
Pluto and Charon
Pluto, named after the god of the underworld, follows an eccentric orbit that ranges from slightly inside that of Neptune (29.7 AU) to about 50 AU at aphelion. Pluto and its moon,Charon, named after the ferry boatman who took dead souls to Pluto for judgment, receive 1/900 as much sunlight per unit area as Earth when the two are at perihelion. At aphelion, the sys- tem receives only 1/2500 as much sunlight per unit area as Earth.
Figure 7-10 illustrates the orbits of Pluto-Charon, Neptune, and Earth to scale. You might wonder if either Pluto or Charon will ever crash into Neptune. The answer is no because of a phenomenon called
orbital resonance. The Pluto-Charon system makes exactly two solar orbits for every three orbits of Neptune; as a result, the two systems can never get any closer than 17 AU to each other. Unless some other celes- tial object intervenes and gravitationally upsets the orbit of Neptune or the orbit of Pluto-Charon, a cosmic collision will never take place.
THE YEAR AND THE DAY
The equatorial plane of Pluto intersects its orbital plane at an angle of 58 degrees, but the rotation of Pluto is retrograde. There are pronounced sea- sonal changes in the path of the Sun across the sky at any particular loca- tion; the Sun always appears to rise in the west and set in the east.
Pluto takes about 6 days and 91Ⲑ2Earth hours to rotate once on its axis.
Charon follows a prograde orbit (in the same direction as Pluto rotates) over Pluto’s equator and completes one orbit every Plutonian day, so Charon always stays over the same spot on Pluto. An observer on Pluto would see Charon hanging almost perfectly still in the sky. In addition, Charon, like most planetary moons, keeps the same side toward Pluto constantly.
The Pluto-Charon system takes 248 Earth years to make one complete journey around the Sun. Its orbit is a pronounced ellipse. Thus the varia- tions in this system’s distance from the Sun, as well as its extreme axial tilt, affect the seasons. The maximum-to-minimum ratio of solar irradiation is about 2.8:1.
COMPOSITION
Pluto is approximately 2400 kilometers (1500 miles) in diameter; this is smaller than the Earth’s moon. Charon is about half the diameter of Pluto.
The centers of the two objects are about 20,000 kilometers (12,500 miles) apart. If the Pluto-Charon system could be brought close to Earth for size comparison, the result would look like Fig. 7-11.
Both Pluto and Charon have about twice the density of water. This implies that they consist of a combination of ices and rocky materials. They may in fact be huge “dirty snowballs,” consisting of primordial matter that never accreted into an object large enough to properly be called a planet.
Controversy has arisen here; an outspoken group of astronomers has expressed their belief that Pluto would not be called a planet if it were dis- covered today.
ATMOSPHERE
Pluto has a thin atmosphere consisting largely of nitrogen. However, astronomers think that this atmosphere, which is on the order of one-millionth the density of Earth’s atmosphere at the surface, exists only when Pluto is near perihelion. When the system moves farther from the Sun, the atmosphere is believed to freeze onto the surface. Because Pluto’s gravitation is weak, the atmosphere extends to considerable distances from the planet, enveloping Charon. The atmosphere might be blown into a teardrop shape by the solar wind, in much the same way as a comet’s tail is blown away from the Sun.
Although no probe has yet flown near Pluto, images of the planet have been obtained through the Hubble Space Telescope. The surface is pinkish red; this is thought to be caused by the presence of methane ice. There are bright and dark regions, with the south polar region being especially reflec- tive. High-resolution images of Pluto and Charon will be obtained when and if a close flyby is made. Some astronomers believe that when this hap- pens, Pluto and Charon might be reclassified as a double comet.
What Makes a Planet?
Astronomers have been searching for a large planet beyond Neptune ever since Neptune itself was discovered. Pluto is not massive enough to account for observed aberrations in the orbits of Uranus and Neptune. Perhaps such
Charon
Pluto
Earth
Figure 7-11. The Pluto-Charon system, to scale as it would appear next to Earth.
a “Planet X” does not exist, and the so-called perturbations in the orbits of Uranus and Neptune are caused by some unseen (or unseeable) object or effect. Maybe “Planet X” is a large, massive object with albedo (reflectivi- ty) so low that we cannot see it even with the largest Earth-based telescopes.
Astronomers are almost certain that there are thousands or millions of asteroids and dormant comets in solar orbits beyond the orbit of Neptune.
This Kuiper Belt is a disk-shaped swarm of primordial rocks and “dirty snowballs”; the Oort Cloud is a larger, spherical congregation of such objects that encloses the Solar System like a bubble. Every once in a while, an object from one of these swarms undergoes a gravitational interaction or collision with another object and is hurled into the main part of the Solar System. If the object passes near Neptune or Uranus, the gravitation of the large planet can send it diving toward the Sun. A few decades later, we on Earth discover a new asteroid or comet.
We might say that in order to be a planet, a celestial object must be spherical, must orbit the Sun (and not some other planet), and must be larg- er than a certain diameter (say, 500 kilometers) or have more than a certain amount of gravitation (say, 5 percent that of the Earth). However, no offi- cial standard yet exists. Depending on the set of criteria adopted, assuming scientists ever agree on one, Pluto-Charon may be “demoted” to the status of a double comet or else hundreds, maybe thousands, of objects now con- sidered primordial matter will be reclassified as planets.
Quiz
Refer to the text if necessary. A good score is 8 correct. Answers are in the back of the book.
1. Which of the following planets generates the least amount of internal heat?
(a) Jupiter (b) Saturn (c) Uranus (d) Neptune
2. The Pluto-Charon system is unique in that
(a) they always keep the same sides facing each other.
(b) they are the smallest of the gas giants.
(c) they have atmospheres consisting entirely of helium.
(d) they actually orbit Neptune, not the Sun.
3. The dark side of Uranus is much colder than the sunlit side because (a) the axis of Uranus is so greatly tilted.
(b) the atmosphere is so thin.
(c) there are no winds on Uranus.
(d) No! The dark side of Uranus is just as warm as the sunlit side.
4. Most astronomers believe that the surfaces of the gas giant planets (a) are liquid water.
(b) are liquid methane.
(c) do not exist as definable boundaries.
(d) are peppered with craters.
5. The tops of the highest clouds on Jupiter (a) reflect sunlight very well.
(b) are red or brown.
(c) spin counterclockwise because they are high-pressure systems.
(d) are actually smoke from volcanic eruptions.
6. An Earthly analog of Jupiter’s Great Red Spot might be (a) a tornado.
(b) a hurricane.
(c) a high-pressure system.
(d) a volcano.
7. Saturn appears in its crescent phase, as seen from Earth, when it is at (a) conjunction.
(b) quadrature.
(c) opposition.
(d) No! Saturn never appears as a crescent to Earth-bound observers.
8. The magnetosphere of Jupiter is distorted by (a) the solar wind.
(b) Jupiter’s gravitation.
(c) Jupiter’s rings.
(d) Jupiter’s moons.
9. The most oblate planet is (a) Jupiter.
(b) Saturn.
(c) Uranus.
(d) Neptune.
10. An astronomical unit is
(a) the mean distance of Earth from the Sun.
(b) 299,792 kilometers (one light-second).
(c) the mean distance of the Moon from Earth.
(d) the radius of the Solar System.
CHAPTER 8
An Extraterrestrial Visitor’s Analysis of Earth
Suppose that you were an extraterrestrial being visiting Earth for the first time. What would you see? How would you interpret your observations?
Imagine yourself in the role of the explorer/reporter assigned the task of visiting Earth and writing a report about the planet for a magazine article back home. This chapter is written from the point of view of an imaginary explorer/reporter from the fictitious planet called Epsilon Eridani 2.
Until now in this course, measurement units such as kilometers, miles, and degrees usually have been written out in full. However, starting with this chapter, I will use more abbreviated symbology. This is the way scien- tists usually write such expressions, so you should get used to it too.
This chapter is written as a fictitious story, but all the scientific infor- mation is based on well-documented knowledge. While the explorers from Epsilon Eridani 2 are make-believe characters, the things they see are real, although viewed from perspectives we don’t normally consider. It has been said that it is difficult to see a big picture when you are inside the frame.
Let’s step outside the frame for awhile. Here is the log of the first officer of the fictitious Epsilon Eridanian exploration vessel, the Dragon.
Copyright 2003 by The McGraw-Hill Companies, Inc. Click Here for Terms of Use.
The Blue and White Planet
As our ship approaches the planet Sol 3, the third planet in orbit around the star that we call Sol, we are struck by the amazing blue and white colors.
Previous probes have shown us that the white regions are clouds of water vapor and ice ranging in altitude from zero (at the surface) up to about 16 kilometers (km) or 10 miles (mi). The surface is well-defined, and more than half of it is liquid water. Some of the surface is frozen water; other regions show an amazing variety of features, the details of which it is part of this mission to catalogue.