For centuries, the planetary giants— Jupiter with its distinctive red spot and Saturn with its massive rings— have been the most glamorous of the planets, while the outer orbs— Uranus, Neptune, and Pluto— have been viewed, when they've been viewable at all, as dim, remote, drab, and mysterious stepsisters. Then 11 years ago Voyager 2, its mission to Jupiter and Saturn completed, was allowed to fly by and snap a few images of Neptune. What it eventually sent back to Earth changed everything. Neptune was literally seen in a new light, with images of a gorgeous blue orb with swirling white clouds that seemed, well, Earth-like. Three years later, the discovery of the Kuiper belt, a field of remote icy objects that includes Pluto, redefined Neptune as a major force in the arrangement of the entire solar system. Astronomers now believe Neptune's gravity not only skewed the orbits of the small, distant bodies in the Kuiper belt but also affected the position of the giant gas planets.
"No one planet can tell us everything about the universe," says Heidi Hammel, a senior scientist with the Space Science Institute in Boulder, Colorado, "but Neptune seems to hold more than its share of information about the formation of our own solar system— as well as the solar systems beyond."
Voyager 2's flyby in 1989 provided the first close-up of Neptune— and the first full view of its wispy rings. previous images had revealed only INCOMPLETE arcs. The bright spots in the background are stars.
Photo by NASA/FPG
For decades astronomers have thought that planets remained stationary after the initial formation of the solar nebula. But something about Neptune's strange small neighbor, Pluto, tickled their curiosity. While the other planets travel in circular orbits, Pluto's orbit is a peculiarly tilted ellipse. Astronomers now know that Pluto's orbit is strange only for a planet. It is perfectly normal for some 300 known objects in the Kuiper belt. To Renu Malhotra, a staff scientist at Houston's Lunar and Planetary Institute, these elliptical orbits have explanatory power: They suggest that Neptune— a planet with the mass of more than 17 Earths— influenced the orbits of all its diminutive neighbors.
How could this happen? The answer lies in the interactions that occurred during the solar system's early history. After the planets formed, their gravitational pull interfered with the orbits of leftover building blocks called planetesimals. "What evolved was a sort of planetary game of handball, involving Neptune, Uranus, Saturn, and Jupiter," says Malhotra. By the rules of this interaction, Neptune literally tugged planetesimals from their orbits and "handed them down" to the giant gas planets closer to the sun. As the planetesimals moved into smaller orbits, they lost orbital energy and angular momentum— and that energy was instead absorbed by the planets. "The greater the orbital energy, the bigger the orbit," explains Malhotra. Neptune, Uranus, and Saturn began to move outward— with Neptune taking a 30 percent leap that moved it to its present location of roughly 2.8 billion miles from the sun. Jupiter, which lay on the receiving end of the planetesimals, either absorbed or ejected them. As a result, Jupiter lost orbital energy, and its circuit around the sun shrank by about 2 percent.
Malhotra had theorized that Pluto's orbit had been shaped by Neptune, but it took the discovery of the Kuiper belt in 1992 to prove it. As Neptune glided outward, only those objects beyond a certain distance would have been able to escape being captured and pushed inward. Pluto, for instance, circles the sun two times for every three orbits of Neptune. Thus Pluto is said to be in a 2:3 resonance orbit with Neptune, says Malhotra, and other Kuiper belt objects have similar orbital relationships with Neptune. Any object orbiting closer than that, she believes, would have long been ripped from its circuit by the great planet. Pluto and the other objects in the Kuiper belt are just far enough away to avoid this fate.
Color-enhanced images from the Hubble Space Telescope Reveal Neptune's stormy weather. Winds approach 1,000 miles an hour at the equator, and storms the size of Earth itself can sweep across the planet. Methane in the uppermost atmosphere absorbs light, making the planet appear blue. The patches of white and yellow indicate areas of very high clouds.Photos by NASA
Such planetary rearrangements appear to be common in other solar systems. The giant gas planet that orbits star 51 Pegasi, for example, baffled astronomers when it was discovered in 1995. The planet was about the same size as Jupiter, but it orbited eight times closer to its star than Mercury does around our sun. "No gas planet could have formed that close to a star," says Malhotra, "so it had to have moved there." The theory by which Neptune and the outer planets may have moved gives one possible explanation for this— although there will inevitably be others, especially since the giant Pegasi planet must have moved inward, rather than outward like Neptune, Uranus, and Saturn.
The planetary movement of Neptune, strangely enough, may be relevant to Earth's development. One somewhat speculative theory holds that water came to Earth by way of a distant icy body— such as those found in abundance in the Kuiper belt and the more remote Oort cloud, an icy, comet-filled field. The outward movement of Neptune may have perturbed the motion of that ice-bearing body, sending it on a course that eventually caused it to crash-land on Earth.
But that's just one of Neptune's mysteries. When Voyager 2 passed by the planet in 1989, it picked up images of a giant swirling storm the size of Earth that bobbed and weaved on the surface of the planet. Until then, nobody expected to find capricious storms on Neptune. Unlike the Great Red Spot on Jupiter, a turbulent region that has been observed since the days of Galileo, this storm proved unusually volatile: When the restored Hubble Space Telescope first took a look at Neptune in 1994, the dark spot had vanished. Changeable weather is no surprise to Earthlings because the sun heats up our planet's surface and sends water vapor and air rising to create clouds and wind. Farther out in the solar system, however, the sun's energy is so minimal, its effect is supposed to be reduced by the law of inverse squares. "Since Uranus is twice as distant as Saturn, it receives four times less solar energy— and should have four times less climactic activity," says Carolyn Porco, a planetary scientist at the University of Arizona.
That law certainly seemed to be heeded by Uranus, which was so devoid of clouds it looked like a great cue ball hovering in space when Voyager 2 flew by. Most astronomers predicted an equally boring Neptune— which lies 30 times beyond the distance of the Earth from the sun. Boy, were they wrong.
This image from Voyager 2 shows Neptune's Great Dark Spot, a storm system that resembles Jupiter's Great Red Spot. The key difference is that Neptune's Spot comes and goes.
Photo by NASA
First, Neptune proved to be a more brilliant blue than expected. Methane in its upper atmosphere absorbs red light, creating the blue effect. And frozen methane high in its atmosphere produces wispy clouds. Within Neptune's interior, a spherical shell of water sloshes around a solid core, throwing the planet's magnetic field off 47 degrees from its axis of rotation. None of this was as surprising as the discovery of storm systems. Near-1,000-mile-per-hour winds and ever-changing clouds "went against all the models of what had been expected, and we're still trying to understand the reasons why," says Hammel. "Since the energy is not coming from the sun, that leads to the question: Where is it coming from?"
Neptune itself must harbor the answer. Planetary scientists can calculate a planet's radiation balance— the amount of energy it emits compared to the amount of sunlight it receives— and Neptune's ratio is intriguing. "You've got great heat from the inside and very little heat from the outside, so you're not really in a stable situation at all," says Hammel.
Unlike Earth, which has only a very thin layer of atmosphere, a thick swath of gas surrounds Neptune's small solid core. In the outer layers of Neptune's atmosphere, the pressure would be similar to that on Earth, but with temperatures of –350 degrees Fahrenheit. Deeper into the planet, however, heat and pressure probably increase, until they rise so sharply that the atmosphere— much of it hydrogen— turns into a liquid state, "probably the consistency of pudding," says Hammel. Under these conditions, electrons become stripped from molecules, and the nuclei pack together. That is not a bizarre chemical configuration— it is the definition of a metal. "But it's unusual to think about hydrogen, which we know as a gas, being found in a metallic state," Hammel says.
Last year, a team at the University of California at Berkeley came up with a hypothesis about what might be occurring in the planet's atmosphere to explain Neptune's extraordinary internal energy. Robin Benedetti, a graduate student in physics, put some methane in a pressurized chamber and used a laser beam to create "pressure and heat conditions that you might find about a third of the way toward the planet's center." Methane is composed of four hydrogen atoms that surround a carbon atom. Benedetti found that under extreme pressure the bonds holding the hydrogen atoms onto the carbon dissolved, and the carbon atoms began binding to one another. Under different conditions, carbon atoms would form a coal-like substance, but under this extreme pressure, they formed diamond dust. "We're not sure how big diamond crystals might form on Neptune," she says, but they could conceivably make the Krupp diamond on Elizabeth Taylor's finger seem like a chip. If those conditions exist on Neptune, it's possible that diamonds are literally raining down through the atmosphere toward the planet's center, releasing heat as a result of friction. "This could be a really huge amount of energy," says Benedetti, and it may explain in part why Neptune radiates 2.6 times as much heat as it absorbs from the sun.
Photo by NASA
The actual answers to this and other new questions raised by Neptune will have to wait until the day an orbiter can be launched to collect more information. "We've come a long way in our understanding," says Hammel. "But until we get the sort of data that only an orbiter can provide, the best we can hope for is a blurry— but tantalizing— view of this amazing planet."
For an on-line peek at Neptune, visit NASA's gallery of planetary images: photojournal.jpl.nasa.gov.For astronomy buffs, the Web site space.com offers a searchable archive.Views of the Solar System, a wonderful on-line resource, includes a thorough introduction to Neptune: www.solarviews.com/eng/neptune.htm.David Jewitt, a University of Hawaii astronomer who helped discover the Kuiper Belt, discusses his research and keeps tabs on the denizens of the outer solar system at www.ifa.hawaii.edu/faculty/jewitt/kb.html.
