«Astronomers had a problem: Something was wrong with the orbit of Uranus, the seventh planet from the Sun. Then came the discovery of Neptune, the ...»
Astronomers had a problem: Something was wrong with the orbit of Uranus, the seventh planet from the
Sun. Then came the discovery of Neptune, the eighth planet. But something was still wrong with the orbit of
Uranus. Could yest another planet lurk unseen in the distant reaches of the Solar System, and could such a
planet be affecting the orbit of Uranus?
The first part of the question was answered in 1930, when Clyde Tombaugh, an Illinois farmboy with a high
school education and a burning interest in Astronomy, discovered a tiny planet after examining hundreds of thousands of heavenly objects on photographic plates.
Named Pluto, the planet Tombaugh discovered has revealed itself with great reluctance. It took fifty years for astronomers to measure Pluto's diameter with some degree of accuracy, yet even today no two figures are quite the same. It took as long for Pluto's moon Charon to be discovered. Yet some astronomers questioned whether Charon is even a moon, believing it instead to be a double planet system with Pluto.
The second part of the question asked above has not been answered to astronomer's satisfaction. Pluto, it turns out, does not influence the orbit of Uranus the way it should, so some astronomers are once again looking for another planet, a tenth planet far, far away in the distant reaches of the solar system.
What will turn up is, at this point, anybody's guess. There is a good chance however, that anything that does turn up may be unexpected, like Charon or Pluto were. And of course, the unexpected may not turn up for years.
Or you never know. It could turn up tomorrow.
1. Uranus and Neptune THE PLANET URANUS is the seventh planet out from the Sun. It is about 1,784 million miles from the Sun, or about nineteen times as far from the Sun as the Earth is. It takes 84 years for Uranus to make one journey around the Sun.
Uranus was discovered in 1781 and, after that, was very closely studied by astronomers. They expected it to move about the Sun in a certain way, according to the law of gravitation first worked out by the English scientist Isaac Newton (1642-1727) in 1687. According to this law, the Sun ought to exert a strong gravitational pull on Uranus, a pull governed by the sizes of the Sun and Uranus and the distance between them.
Jupiter and Saturn, which are the largest planets and also the two closest to Uranus, ought to exert small gravitational pulls of their own.
If the pulls of the Sun, Jupiter, and Saturn were all taken into account, Uranus ought to move around the Sun in a certain elliptical orbit. In moving so it would, as seen from Earth, move among the stars in a certain path from night to night and astronomers should be able to tell exactly where it would be every night, The trouble was that this turned out not to be so. With time, Uranus slowly moved out of the calculated position. The error wouldn’t seem much to ordinary people, but to astronomers it was a horrifying situation. It might have meant that Newton’s law of gravitation was wrong. And if that were the case astronomy might find itself in a very confused situation.
Astronomers decided that the trouble was that they weren’t considering all the different gravitational pulls. Suppose there were another planet beyond Uranus that had not yet been discovered. It would exert a small pull on Uranus that in turn might cause those errors in its position that were troubling astronomers.
2. Percival Lowell In 1877, an Italian astronomer, Giovanni Virginio Schiaparelli (1835-1910), had studied Mars closely and made a map of the markings he could see on it. He thought the dark markings might represent water, and the light markings, land. He noticed that some of the dark markings were long and narrow, and he called them canali, which is Italian for “channel.” A channel is any long, narrow body of water connecting two larger bodies. The English Channel between England and France is the best-known example on Earth of a body of water known by that name.
The word, however, was translated into English as canals. This was unfortunate, because a canal is an artificial waterway dug out by humans. As soon as English-speaking people heard that there were “canals” on Mars, they believed there were intelligent beings on Mars. They also thought that Mars, being smaller than the Earth and having only two-fifths its gravitational pull, was not able to hold water over long periods. For that reason, Mars was drying out, and the Martians must have dug the canals to conduct water from the planet’s polar ice caps to the warmer regions near its equator, where they could grow food.
Lowell was very interested in the Martian canals, and he made up his mind to study them with great care. He used his fortune to establish a private observatory in Flagstaff, Arizona, where the altitude, the desert air, and the remoteness from city lights made the night sky particularly clear. The Lowell Observatory opened in 1894.
For fifteen years, Lowell studied Mars as carefully as he could, taking thousands of photographs. He was sure that he could make out the canals. In fact, he saw far more than Schiaparelli ever did, and he drew detailed pictures that eventually included over five hundred canals. These followed straight lines that crossed one another. At the crossings, the dark areas seemed to broaden, and Lowell called these oases.
The canals seemed to become double at times.
There were changes with the Martian seasons.
Lowell lectured on the subject, wrote popular books, and was completely covinced that there was intelligent life on Mars. As a result, the British writer Herbert George (H. G.) Wells (1866-1946) wrote a book in 1898 called The War of the Worlds in which he described a Martian invasion of Earth. This made the notion of intelligent (and dangerous) life on Mars even more popular.
Few other astronomers managed to see the canals the way that Lowell did, but Lowell wasn’t upset by that. He simply pointed out that he had better eyes, a better telescope, and a better observatory.
Yet, as it turned out, Lowell was wrong. We now know that there are no canals on Mars. We have sent unmanned spacecraft to Mars since the 1960s, and they have mapped the whole planet in detail. They found no canals and no signs of any intelligent life.
Apparently, Lowell, trying to see things he could just barely make out, was fooled by optical illusions. Little patches of irregular dark markings seem to form straight lines when the eyes strain to see them.
Nevertheless, all this showed that Lowell was not afraid to take up difficult tasks and to deal with subjects that other astronomers avoided.
Beginning in 1902, Lowell became interested in the possible existence of a planet beyond Neptune. In 1905, he began a search for the planet, keeping that search a secret so that other astronomers wouldn’t take up the task and perhaps beat him to the discovery.
In 1908, he began to call the unknown distant world Planet X.
Lowell’s secrecy was of no use, however. Another aristocratic Boston astronomer, William Henry Pickering (1858-1938), was also interested in the possible existence of a planet beyond Neptune.
Pickering had already made some discoveries about the outer planets. In 1898, for instance, he had detected a ninth satellite of Saturn, one that was farther from the planet than any of the others. He called it Phoebe.
Pickering used the tiny errors in Uranus’s motion to venture an estimate of the location of a planet beyond Neptune (a planet which he called Planet O). He believed that the planet beyond Neptune would probably be about 4,800 million miles from the Sun, or about one and three-quarters times as far from the Sun as Neptune is. It would take 373 years to move once around the Sun, or two and one-quarter times as long as it takes Neptune to make its own circuit.
Pickering also believed that the new planet would be about twice the mass of Earth. In addition, he believed that its magnitude would be between 11 and 13, which meant it would be surrounded by millions of stars of the same brightness.
Pickering announced his figures in 1908. When Lowell heard this, he was upset and decided to do some figuring of his own. His results predicted that the distant planet was about 4,400 million miles from the Sun, a little nearer than Pickering thought, and that it would go around the Sun in 327 years, again less than Pickering’s figure. He also thought it would be about six or seven times the mass of the Earth, or almost half the size of Uranus or Neptune.
Pickering, however, did not follow up his figures by actually trying to find the planet in the sky. But Lowell was more determined.
He began what was an enormous task. He made photographs of sections of the sky under conditions that would pick up stars as dim as magnitude 13. Such a photograph might contain hundreds of thousands of stars. He would then take another photograph of the same part of the sky a few days later. All the dim stars on it would remain in place, but if one of the stars was actually a new planet, that “star” would have changed its position slightly.
Lowell would then search the two photographs with a magnifying glass, looking at each star and trying to see if he could detect a change. It was the kind of work that led to one disappointment alter another, and by 1912, Lowell suffered a nervous collapse. He later recovered, however, and went right back to the search.
Lowell died of a stroke in 1916, and at the time of his death, he had still not found the planet. He was only 61 when he died, and his life may well have been shortened by his continuous searching.
Toward the end, however, he had found a better way of looking for the planet. This was through the use of a blink comparator. Carl Otto Lampland (1873-1951), then the assistant director of the Lowell Observatory, had urged Lowell to get this device, and finally he did. This is how it worked.
Two photographic plates were taken of a particular sector of sky a few days apart. These two plates were placed in the blink comparator, which shone a light through one of the plates and projected it onto a screen. Then it shone a light through the other negative and projected it onto the same screen. The blink comparator switched from one negative to the other, back and forth, back and forth, and very quickly. If the plates didn’t fall on exactly the same part of the screen, the stars would appear first in one place, then in the other, shuttling back and forth rapidly. The plates would be adjusted till both projections were aligned on exactly the same part of the screen. Then, as the light beam switched back and forth, all the stars showed up motionless.
If one of those “stars” on the screen were a planet, however, it would have moved during the time between which the two plates were taken, and it would jump back and forth with the rapid switch between plates. If the move was a large one, the object was probably an asteroid, which would be a comparatively close object. In order for it to be a far distant planet, it would have to blink back and forth only a small amount.
The blink comparator was a great invention, because it was far easier to look at a photographic plate and watch for a single blink among many thousands of stationary stars, than to inspect each star with a magnifying glass and try to detect a small movement with the human eye alone.
Yet, even with the help of a blink comparator, Lowell’s Planet X was not located in his lifetime.
3. The Discovery of Pluto
From this reflected light, astronomers decided by 1987 that the surface of Pluto was rich in methane, a substance which on Earth is a major part of the natural gas we use as fuel. Methane freezes at a very low temperature, so that even in Pluto’s unbelievable cold, some of it would still be a gas. Pluto has a methane atmosphere about 1/900 as dense as Earth’s and one-tenth as dense as that of Mars. Pluto seems to be lighter at its poles, where more of the methane freezes than at its equator.
Pluto’s surface is slick with ice-like solid methane, so it reflects more light than most small worlds close to the Sun do. If it were a rocky world, it would reflect considerably less light and would be even dimmer than it is. It would have been much harder to discover.
Charon’s reflected light is quite different from Pluto’s. Because Charon is smaller than Pluto, it has a smaller gravitational pull. It can’t hold on to the molecules of gaseous methane very well, so that any it may have once had escaped long, long ago. What is left is frozen water, which doesn’t vaporize at Charon’s frigid temperatures and therefore isn’t lost.
Consequently, where Pluto has mostly a frozen-methane surface, Charon has a frozen-water surface. Charon has no atmosphere of its own, but Pluto’s methane atmosphere seems to stretch out so far from the little planet that its very thin outermost fringe of atmospheric gas extends beyond Charon. Charon thus circles Pluto inside wisps of Pluto’s atmosphere.
6. Beyond Pluto ONCE PLUTO WAS discovered, astronomers concluded from its dimness that its discovery was just a lucky coincidence.
Pluto was clearly too small for its tiny gravitational pull to have any noticeable effect on Uranus.
Pluto was found almost exactly where Lowell had said the distant planet would be, but Pluto was not the object Lowell was looking for. It just happened to be in the right spot.
Well, then, if the tiny errors in Uranus’s motion were to be explained, there must be still another planet, a tenth planet, which must lie beyond Pluto. It would be larger than Pluto in order to produce the effect on Uranus, and the farther beyond Pluto it was, the larger it would have to be.
On the whole, then, even if it were farther than Pluto, its larger size would make it brighter than dim little Pluto, and therefore it would be easier to find.
But where is it?
Tombaugh, who had discovered Pluto and then realized it couldn’t be Lowell’s Planet X, exactly, continued to use his blink comparator for years afterward, and by 1943 he had examined 45 million stars. In the process, he found all sorts of astronomical objects far outside the solar system. Inside the solar system, he discovered a new comet and no fewer than 775 asteroids that hadn’t been seen before. But he found no new planet.