Monday, November 30, 2009

Uranus and Neptune from Earth

It is possible for the amateur astronomer to see both Uranus and Neptune. In fact, if you know where to look, Uranus is visible, albeit very faintly, even to the naked eye, provided that the night is very dark, very clear, and you are far from sources of light pollution. To view Neptune, which is much fainter than Uranus, requires an advanced amateur telescope. You don’t have to spend years sweeping the skies to find these dim and distant worlds.
But what can you expect to see? Uranus will appear as a greenish disk, probably featureless—though it is not impossible, given a very good telescope and superb atmospheric conditions, to see atmospheric features and bright spots. It is even possible to see Titania and Oberon, the largest of the planet’s five moons. Even many advanced amateur astronomers have not seen Neptune. Blue in color, it is aptly named for the Roman god of the sea. If you locate the planet at all, it will be a featureless disk.

Understanding Uranus and Neptune

Since ancient times, the inventory of the solar system was clear and seemingly complete: a sun and, in addition to Earth, five planets, Mercury, Venus, Mars, Jupiter, and Saturn. Then came along one of those scientific busybodies that the eighteenth century produced in abundance. Johann Daniel Titius, or Tietz (1729–1796), a Prussian born in what is now Poland, poked his curious nose into everything.
He was a physicist, biologist, and astronomer who taught at the University of Wittenberg.
It occurred to him, in 1766, that the spacing of the planetary orbits from the sun followed a fairly regular mathematical sequence. He doubled a sequence of numbers beginning with 0 and 3, like this: 0, 3, 6, 12, 24, 48, and so on.
He added 4 to each number in the sequence, then divided each result by 10. Of the first seven answers Titius derived—0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0—six very closely approximated the relative distances from the sun, expressed in astronomical units (remember, an A.U. is the mean distance between the earth and the sun), of the six known planets.
No one paid much attention to Titius’s mathematical curiosity until another Prussian astronomer, Johann Bode (1747–1826), popularized the sequence in 1772. Neat as it was, the sequence, which became known as the Titius-Bode Law or simply Bode’s Law, is now generally thought to be nothing more than numerology. For one thing, there is no planet at 2.8 A.U. This gap would be filled later by the discovery of the asteroid belt at this location. While the rule gives a number that is close to Uranus, it breaks down for the positions of Neptune and Pluto. Since those planets had yet to be discovered, no one saw it as a problem. But what about the numbers beyond 10.0 A.U.? Did the Titius-Bode law predict other, as yet unknown, planets?
The people of our planet did not have to wait long for an answer. On March 13, 1781, the great British astronomer William Herschel, tirelessly mapping the skies with his sister Caroline, took note of what he believed to be a comet in the region of a star called H Geminorum. On August 31 of the same year, a mathematician named Lexell pegged the orbit of this “comet” at 16 A.U.: precisely the next vacant slot the Titius-Bode Law had predicted.
Herschel, with the aid of a telescope, had discovered the first new planet since ancient times.
Once the planet had been found, a number of astronomers began plotting its orbit. But something was wrong. Repeatedly, over the next half century, the planet’s observed positions did not totally coincide with its mathematically predicted positions. By the early nineteenth century, a number of astronomers began speculating that the new planet’s apparent violation of Newton’s laws of motion had to be caused by the influence of some as yet undiscovered celestial body—that is, yet another planet. For the first time, Isaac Newton’s work was used to identify the irregularity in a planet’s orbit and to predict where another planet should be. All good scientific theories are able to make testable predictions, and here was a golden opportunity for Newton’s theory of gravity.
On July 3, 1841, John Couch Adams (1819–1892), a Cambridge University student, wrote in his diary:
“Formed a design in the beginning of this week of investigating, as soon as possible after taking my degree, the irregularities in the motion of Uranus … in order to find out whether they may be attributed to the action of an undiscovered planet beyond it ….”
True to his word, in 1845, he sent to James Challis, director of the Cambridge Observatory, his calculations on where the new planet, as yet undiscovered, could be found. Challis passed the information to another astronomer, George Airy, who didn’t get around to doing anything with the figures for a year. By that time, working with calculations supplied by another astronomer (a Frenchman named Jean Joseph Leverrier), Johann Galle, of the Berlin Observatory, found the planet that would be called Neptune. The date was September 23, 1846.

Gas Planets Statistics

The most immediately striking difference between the terrestrial and jovian worlds are in size and density. It is useful to recall our rough scale: If the earth is a golf ball 0.2 miles from the sun, then Jupiter is a basketball 1 mile away from the sun, and Pluto is a chickpea 8 miles away. At this scale, the sun’s diameter would be as big as the height of a typical ceiling (almost 10 feet). While the jovian planets dwarf the terrestrials, they are much less dense. Let’s sum up the jovians, compared to the earth:
We have given a gravitational force and a temperaturenat the surface of the jovian planets, but as we’ll see, they do not really have a surface in the sense that the terrestrial planets do. These numbers are the values for the outer radius of their swirling atmosphere. One surprise might be that the surface gravity of Saturn, Uranus, and Neptune is very close to what we have at the surface the earth.
Gravitational force depends on two factors, you’ll recall, mass and radius. Although these outer planets are much more massive, their radii are so large that the force of gravity at their “surfaces” is close to that of the smaller, less massive Earth. Of the jovians, Jupiter and Saturn have the most in common with one another. Both are huge, with their bulk mainly hydrogen and helium. If you recall from our description of the early, developing solar system, the outer solar system (farther from the sun) contained more water and organic materials, and the huge mass and cooler temperatures of the outer planets meant that they were able to gravitationally hold on to the hydrogen and helium in their atmospheres.
The terrestrials consist mostly of rocky and metallic materials, and the jovian planets primarily of lighter elements. The density of a planet is determined by dividing its mass by its volume. While the outer planets are clearly much more massive (which, one might think, would make them more dense), they are much larger in radius, and so encompass a far greater volume. For that reason, the outer planets have (on average) a much lower density than the inner planets, But what of Uranus and Neptune—distant, faint, and unknown to ancient astronomers? While they are both much larger than the earth, they are less than half the diameter of Jupiter and Saturn; in our scale model, they would be about the size of a cantaloupe.
Uranus and Neptune, though less massive than Saturn, are significantly more dense. Neptune is more dense than Jupiter as well, and Uranus approaches Jupiter in density. Take a look at the following “Astronomer’s Notebook” sidebar to understand
why this is so.
Consider Neptune. Remember, density is equal to the mass of an object divided by its volume. While the mass of Neptune is about 19 times smaller than that of Jupiter, its volume is 24 times smaller. Thus, we expect its density to be about 24⁄19 or 1.3 times greater.
While we cannot yet peer beneath the atmospheric surface of Uranus and Neptune, the higher densities of these two planets provide a valuable clue to what’s inside.
Reflecting their genesis, all of the jovian planets have thick atmospheres of hydrogen and helium covering a core slightly larger than Earth or Venus. The rocky cores of all four of the jovian planets are believed to have similar radii, on the order of 4,300 to 6,200 miles (7,000 to 10,000 km); but this core represents a much smaller fraction of the full radius of Jupiter and Saturn than do the cores of smaller Uranus and Neptune— thus the higher average density of the latter two planets.
The atmospheres of the jovians are ancient, probably little changed from what they were early in the creation of the solar system. With their strong gravitational fields and great mass, these planets have held onto their primordial atmospheric hydrogen and helium, whereas most of these elements long ago escaped from the less massive terrestrial planets, which have much weaker gravitational pull. But here’s where it gets really strange. On the earth, we have the sky (and atmosphere) above, and the solid ground below. In the case of the jovians, the gaseous atmosphere never really ends. It just becomes denser with depth, as layer upon layer of it presses down.
There is no “normal” solid surface to these planets! As the gases become more dense, they become liquid, which is presumably what lies at the core of the jovian worlds. When astronomers speak of the “rocky” cores of these planets, they are talking about chemical composition rather than physical state. Even on the earth, rock may be heated and pressed sufficiently to liquefy it (think of volcanic lava). Thus it is on the jovians: gas giants, whose atmospheres become increasingly dense, but never solid, surrounding a liquid core. In the case of Jupiter and Saturn, the pressures are so great that even the element hydrogen takes on a liquid metallic form.