14-10 Trans-Neptunian Objects and Pluto’s Reclassification

For many years astronomers attempted to find other worlds beyond Neptune using the same technique used to discover Pluto (see Figure 14-17). The first to succeed were David Jewitt and Jane Luu, who in 1992 found an object with a semimajor axis of 42 AU and a diameter estimated to be only 240 km. This object, named 1992 QB1, has a reddish color like Pluto, possibly because frozen methane has been degraded by eons of radiation exposure. As of 2013, more than 1200 trans-Neptunian objects—icy worlds whose orbits have semimajor axes larger than that of Neptune—have been discovered. Most of these are relatively small, like 1992 QB1, but a number are larger than Charon and one is larger than Pluto itself (Figure 14-20). In light of these recent discoveries, Pluto is best understood as a particularly large (but not the largest) trans-Neptunian object, rather than a planet.

Figure 14-20: The Largest Trans-Neptunian Objects This artist’s impression depicts Earth and the largest objects known beyond Neptune (as of 2012) to the same scale. The largest of these, Eris, has a diameter between 2300 km and 2400 km. There is less uncertainty about Pluto’s diameter, which is close to 2300 km. Note the differences in color among these objects and that five of them have satellites of their own. Table 7-4 in Section 7-5 lists the properties of seven of these objects.

(NASA; ESA; and A. Field [STScI])

Reclassifying Pluto as both a dwarf planet and a trans-Neptunian object (both new terms) instead of a planet, was controversial. However, Pluto was already known to be somewhat of an oddball planet, as it is much smaller than the true planets and has an orbit inclined from the ecliptic plane significantly more than the planets. Furthermore, compared to the planets, Pluto also has a significantly lower density, suggesting it has a much larger proportion of water-ice. However, once large trans-Neptunian objects were discovered—one even larger than Pluto—a new class of objects came into view, of which Pluto was only a member. The main decision then facing astronomers was whether to add several more oddball objects like Pluto to the list of planets, or to consider Pluto and these new objects members of a different category. The solution adopted by the International Astronomical Union in 2006 was to create a category called trans-Neptunian objects, and in addition, to determine that a larger object like Pluto anywhere in the solar system could also be called a dwarf planet if it met certain requirements (see Section 14-9).

With improvements in the sensitivity of telescopes, new objects beyond Neptune are being discovered at a rapid pace. Based on these observations, it is thought that there could be 35,000 or more objects beyond Neptune with diameters greater than 100 km. One of the larger objects could have collided with Pluto in the past, giving rise to Pluto’s retinue of satellites (see Section 14-9). At least 40 trans-Neptunian objects have satellites, which suggests that such collisions have taken place many times.

The Kuiper Belt and Beyond

Most of the trans-Neptunian objects lie within the Kuiper belt, which extends from about 30 to 50 AU from the Sun and is relatively close to the ecliptic. When the solar system was young, a large number of icy planetesimals formed in the region beyond Jupiter. Over time, the gravitational forces of the massive Jovian planets deflected most of these planetesimals beyond Neptune’s orbit, concentrating them into a belt centered on the plane of the ecliptic. Most of the trans-Neptunian objects within the belt are in orbits that are only slightly inclined to the ecliptic; these objects are thought to have formed beyond Neptune and to be in roughly their original orbits. Other objects such as Makemake and Haumea (see Figure 14-20 and Table 7-4) are in orbits that are inclined by about 30° from the ecliptic. These objects are thought to have been pushed into their steeply inclined orbits by gravitational interactions with Neptune.

Figure 14-21: RIVUXG
A “Kuiper Belt” Around Another Star The star HD 139664 is only slightly more massive than the Sun but is thought to be just 300 million years old. This Hubble Space Telescope image shows a ring of material around HD 139664 that is similar in size to the Kuiper belt in our solar system. HD 139664 is 57 light-years (17 parsecs) from Earth in the constellation Lupus (the Wolf).

(NASA; ESA; and P. Kalas [University of California, Berkeley])

The processes that gave rise to the Kuiper belt in our solar system also appear to have taken place around other stars. Figure 14-21 shows a disk of material surrounding the young star HD 139664. This disk has dimensions comparable to our Kuiper belt. A number of other young stars have been found with disks of this same type.

There are relatively few members of the Kuiper belt between the orbits of Neptune and Pluto. Remarkably, there are about 100 objects that have nearly the same semimajor axis as Pluto. These so-called plutinos, which include Pluto itself, have the property that they complete nearly two orbits around the Sun in the same time that Neptune completes three orbits. The plutinos thus experience rhythmic gravitational pulls from Neptune, and these pulls keep them in their orbits. (In Section 13-1 we saw a similar relationship among the orbital periods of Jupiter’s satellites Io, Europa, and Ganymede, though the ratio of their orbital periods is 1:2:4 rather than the 2:3 ratio for Neptune and the plutinos.) Most Kuiper belt objects orbit at distances beyond the plutinos but within about 50 AU from the Sun, at which distance an object completes one orbit for every two orbits of Neptune. At this distance the rhythmic gravitational forces of Neptune appear to pull small objects inward, giving the Kuiper belt a relatively sharp outer edge.

Two examples of trans-Neptunian objects that are not members of the Kuiper belt are shown in Figure 14-20. Eris, the largest known trans-Neptunian object, has a semimajor axis of more than 68 AU and an orbital eccentricity of 0.442. This orbit takes Eris from inside the orbit of Pluto to far beyond the Kuiper belt. Eris is also the most reflective body in the solar system—more reflective than ice or snow, and even more reflective than a mirror! Eris reflects about 96% of the light striking its surface, whereas a mirror reflects about 90–95%. It is thought that this unusual reflectivity comes from a very thin layer (less than a millimeter) of nitrogen-rich ice mixed with methane frost. Due to Eris’s highly elongated orbit, as it moves farther from the Sun, its atmosphere can freeze onto the surface to produce this reflective layer.

Figure 14-20 also shows an even more extreme case with Sedna: Its orbital semimajor axis is 518 AU, giving it an orbital period of more than 10,000 years, and the orbital eccentricity has the remarkably high value of 0.85. It is not well understood how Sedna could have been moved into such an immense and elongated orbit.

New Horizons

Excitement about the worlds beyond Neptune has motivated the development of a spacecraft called New Horizons. Launched in 2006, New Horizons swung by Jupiter in 2007 and will make the first-ever flyby of Pluto and Charon in 2015. To make the journey in nine years, New Horizons was launched fast enough to pass our Moon nine hours after launch, while it took more than three days for the Apollo 11 mission; New Horizons had the greatest launch speed of any man-made object.

One of many surveys New Horizons will attempt is to observe methane frost gathering on the nighttime side of Pluto. But isn’t the nighttime side dark? No. Just as moonlight can illuminate portions of Earth during nighttime, Pluto’s moon Charon can do the same for Pluto. Even though sunlight is weaker at Pluto compared to Earth, Charon is so close to Pluto that Charon-moonlight is twice as bright as the light we experience from our Moon. Thus, an astronaut could read a book by the bright Charon moonlight!

Ironically, the nine years it takes to get to Pluto might save the New Horizons mission by allowing time to solve a potential problem. Some recent estimates suggest that small but frequent impacts in the Kuiper belt could endow Pluto with a large cloud of dangerous dust; even colliding with dust a few millimeters in size could destroy the spacecraft. We are searching for this dangerous dust cloud from Earth, and one “thread the needle” idea is to send New Horizons through such a cloud on a path that has already been cleared of dust by Charon. The alternative is to veer farther away from Pluto, but that would lower the resolution of features imaged on Pluto’s surface.

After surveying Pluto and its moons, New Horizons will go on to explore other bodies in the Kuiper belt. The high-resolution images and other data to be returned by New Horizons promise to revolutionize our understanding of these most remote members of the solar system.