The Pluto Files: The Rise and Fall of America's Favorite Planet (4 page)

In a research paper submitted to the Royal Society the following month, Herschel proposed “star-like” as a descriptor, which in Greek becomes the more familiar “aster-oid.”

At 600 miles in diameter, Ceres is dwarfed by Mercury, the reigning smallest planet. But that won’t concern us just yet. By 1807, three more of these diminutive planets had been discovered: Pallas, Juno, and Vesta. By 1851, 11 more had been logged, and the solar system’s planet count reached 18—duly recorded this way in textbooks of the times. The spate of new planets were all small and traveled in orbits similar in size and location to that of Ceres. By 1853, it was clear that a new class of objects had been identified:
the asteroids
. These bodies occupied a new swath of real estate in the solar system:
the asteroid belt
. Practically overnight, the planet count dropped back to seven: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus (Figure 2.4).

Figure 2.4.
The number of planets over time. From the era of ancient Greece until 1543, the number of planets remained unchanged at seven. After Copernicus advanced the Sun-centered system, the number dropped to six, reached a peak of 23 with the discovery of a bunch of asteroids, dropped back to eight when the asteroids became a category of their own, went back up to nine with the discovery of Pluto in 1930, and fell back to eight in August 2006. (Adapted from Steven Soter, “What Is a Planet?”
Scientific American
, January 2007.)

Ceres was discovered first because it’s the brightest and largest of its class. At twice the mass of all other asteroids combined, of which there are hundreds of thousands known, Ceres swiftly went from being the smallest in the class of planet to being the largest in the class of asteroid.

From the time of ancient Greece up to the publication of Nicolas Copernicus’s 1543 magnum opus
De Revolutionibus
, the planet count for the known universe was seven. With gods drawn from Roman and Norse mythology, we derive names for the seven days of the week from these objects. The Greeks determined that of all the celestial bodies, only seven move against the background sky. With paths not fully understood, they called this elite group the “wanderers,” which in Greek translates to “planets.” The planet inventory was clear, clean, and classic: Mercury, Venus, Mars, Jupiter, Saturn, Sun, and Moon.

The Earth-centered view of the universe collapsed after Copernicus, who placed the Sun in the middle, the Moon in motion around Earth, and the Earth-Moon system in motion around the Sun. Thus was born the “solar” system that today we take for granted. But what then becomes of the magnificent seven? The Moon and Sun were pulled from their planet status while Earth joined the list, in commensurate rank with the other revolving objects. This commonsense reassessment of the word
planet
dropped the number to six and did not require a resolution or a formal, agreed-upon definition. It seemed unambiguous enough. Or so we thought.

P
LUTO CONTAINS ABOUT
70
PERCENT ROCK AND
30
percent ice if you measure things by mass. But rock is denser than ice, leaving the rocky parts to occupy only 45 percent of Pluto’s volume. If volume is what matters to you, then you can rightly declare that Pluto is mostly ice. This fact sits within a long list of properties that are not shared with any other planet in the solar system. While each planet is unique in one way or another, Pluto’s list just might be as long as all the other planets’ combined.

Pluto is by far the least massive planet, with less than 5 percent the mass of Mercury, the solar system’s next smallest planet.

Pluto’s orbit is so eccentric—so flattened from a perfect circle—that flit crosses the orbit of Neptune, its nearest planet (Figure 3.1). In fact, Pluto spends 20 years out of its 248-year orbit closer than Neptune to the Sun.

Pluto’s orbit is not only oblong. It tips more than 17 degrees from the plane of the solar system, a full 10 degrees more than Mercury’s orbit, the next most tipped in the set (Figure 3.2).

Pluto’s largest moon, Charon, named for the Greek ferryboat driver who would carry your unfortunate soul across the River Acheron to the underworld, was discovered in photographs obtained from the Kaj Strand 61-inch telescope at the U.S. Naval Observatory’s Flagstaff station in Arizona. It was June 1978 when USNO’s James Christy spotted a suspicious bump—an odd elongation to Pluto’s grainy image.
¹²
Richard Binzel (then a graduate student at the University of Texas at Austin) and his collaborators confirmed Charon’s existence in February 1985, when the moon eclipsed Pluto, visibly reducing the total light emitted by the system.
¹³
Pluto had finally reached a place in its orbit where, from Earth’s line of site, Charon would pass directly between us and Pluto.

Charon is so large compared with Pluto—equivalently, Pluto is so small compared with Charon—that they orbit a spot that sits not within Pluto itself, but in free space. In other words, unlike the moons of every other planet in the solar system, where their center of motion falls within the physical body of the host planet, the Pluto-Charon system orbits a point in space outside of the physical extent of Pluto itself. Binzel’s measurements enabled accurate estimates of Charon’s period of revolution around Pluto, which perfectly matched the time it took Pluto to turn once on its axis.

Figure 3.1.
Orbits of the outer planets. When seen from “above,” Pluto’s orbit is so elongated that it’s the only planet to cross the orbit of another planet, in this case Neptune.

Figure 3.2.
Orbits of the outer planets, in perspective. From this view, the excessive tip of Pluto’s orbit from the plane of the solar system is evident.

Figure 3.3.
Discovery photo (shown in negative) of Pluto’s moon Charon. In June 1978, U.S. Naval Observatory astronomer James Christy spotted, in the grainy photo of Pluto on the left, a suspicious elongation to the image. An earlier image of Pluto on the right showed no such distortion, hinting at an orbiting companion to Pluto, which was later confirmed to be a moon.

Pluto and Charon are in a rare double tidal lock, always showing the same face to each other. Tidal locks themselves are common. Jupiter and Saturn have tidally locked their closest satellites. And Earth has tidally locked the Moon, creating a genuine “near side” and “far side.” Although the Moon is trying to tidally lock the Earth, forcing our days to get longer and longer until they equal the lunar month, it will not succeed within the Sun’s life expectancy. This leaves Pluto and Charon as the only double-tidally locked system among the planets.

Although Pluto’s orbit crosses that of Neptune, they will never collide because Neptune has Pluto in another kind of lock, called an orbital resonance. For every three trips around the Sun that Neptune takes, Pluto takes exactly two, leaving them in an eternal three-to-two resonance with each other. No other pair of planets behaves this way.

Figure 3.4.
James Christy (seated), the discoverer of Pluto’s moon Charon, shown in 1978 with his colleague Robert S. Harrington in front of some computers that were surely considered fast in their day. Harrington would calculate that a series of eclipses between Pluto and Charon might occur in the 1980s.

Pluto is further distinguished by the presence of other small icy objects that share its orbital space. If you add up all the debris—leftovers from the formation of the solar system—that can collide with Pluto, it rivals the mass of Pluto itself. All other planets handily dominate their zones. While they (Earth included) continue to be hit by wayward comets and asteroids, the total mass of these impactors remains small compared with the planet itself.
¹⁴
Italian astronomer and asteroid hunter Vincenzo Zappalá could not resist creating a cartoon about it, with Pluto steeped in anthropocentric household trash as the rest of the planets look on in disapproval.

Figure 3.5.
Cartoon by Italian astronomer and asteroid hunter Vincenzo Zappalá, poking fun at Pluto and its debris-filled orbit around the Sun.

Alas, both Pluto and Charon share an important physical property with the rest of the planets. They’re both round. Apart from crystals and broken rocks, not much else in the cosmos naturally comes with sharp angles. The list of round things is practically endless and ranges from simple soap bubbles to the entire observable universe. Spheres tend to take shape from the combined action of simple physical laws. You can show, using freshman-level calculus, that the one and only shape that has the smallest surface area for an enclosed volume is a perfect sphere. In fact, billions of dollars could be saved annually on packaging materials if all shipping boxes and all packages of food in the supermarket were spheres. For example, the contents of a big box of Cheerios would fit easily into a spherical carton that had a 4-inch radius. But practical matters prevail. Spheres don’t pack or stack well and nobody wants to chase packaged goods down the aisle after it rolls off the shelves. We already do this for apples and oranges.

For cosmic objects larger than a particular threshold, energy and gravity conspire to turn them into spheres. Gravity is the force that serves to collapse matter in all directions, filling in the low places with material from high places. But gravity does not always win; the chemical bonds of solid objects are strong. The Himalayan range in Tibet continues to grow against the force of Earth’s gravity. But before you get excited about Earth’s mighty mountains, you should know that the spread in height from the deepest undersea trenches to the tallest peaks in the world is about a dozen miles, but Earth’s diameter is nearly 8,000 miles. Contrary to what it looks like to teeny humans crawling on its surface, Earth, as a cosmic object, is remarkably smooth; if you had a gigantic finger and you rubbed it across Earth’s surface (oceans and all), Earth would feel as smooth as a cue ball. Expensive globes that portray raised portions of Earth’s landmasses to indicate mountain ranges depict a grossly exaggerated reality.

If a solid object has a small enough surface gravity, the chemical bonds in its rocks will easily resist the force of their own weight. When this happens, almost any shape is possible. Two famous celestial nonspheres are Phobos and Deimos, the Idaho-potato-shaped moons of Mars. On 13-mile-long Phobos, the bigger of the two moons, a 150-pound person would weigh only about 4 ounces. All but the largest of the asteroids and comets are too small for their gravity to force them in the shape of a sphere—hence the classic view of craggy chunks of tumbling rock and ice that we carry for these populations.

Our earlier concept of Pluto as a round object, commensurate in other ways with the eight major planets in the solar system, led to the illustration of the nine-planet solar system, etched on a gold plaque, that accompanied
Pioneer 10
and
Pioneer 11
to the outer solar system. Launched by NASA in the early 1970s, when Pluto’s family status was rarely questioned, the
Pioneer
spacecraft were the first objects to ever achieve escape velocity for the solar system. They would never come back. Not ever. And so the schematic view (Figure 3.6) was intended to alert curious aliens from other star systems to the basic form of our solar system and to offer our return address, with a line clearly emanating from Earth, the third planet from the Sun. What the etching did not convey was that given the size of Jupiter and Saturn as illustrated, the size of Pluto would be a pinprick. The plaque further does not show the seven moons of the solar system that are bigger than Pluto. And unlike what is illustrated, with Saturn’s ring clearly drawn, the rest of the gas giant planets (Jupiter, Neptune, Uranus) bear rings as well. So spacefaring aliens would indeed be confused by our illustration and would surely pass us by in search of an actual star system that looks like what we drew.