Planet X. This name was given by astronomer Percival Lowell to an undiscovered planet that he suspected was orbiting beyond Neptune. His search for Planet X began in 1905 at his observatory in Flagstaff, Arizona. Although Lowell died before finding Planet X, the search he began continued. Finally, in 1930, at Lowell’s observatory, Clyde Tombaugh discovered the planet Pluto. Planet X did exist!
Astronomers immediately began to wonder, ‘Could another Planet X be found?’ Six decades of intense hunting followed, and in later years of the search, even spacecraft were used. Although thousands of asteroids, stars, galaxies, and nebulas were discovered, no new planets were identified.
Yet, the search did not cease. Scientists began to use new technologies and more powerful telescopes to detect orbiting objects that are millions of times fainter than can be seen by the naked eye. Their efforts finally paid off. Amazingly, dozens of minor planets beyond Pluto have now been sighted!
Where are those small planets? How many more may yet be found? Are they the most distant objects in our solar system?
The Farthest Objects
The solar system consists of nine planets orbiting the sun. In addition, thousands of rocky asteroids hurtle through space, mostly in a belt between Mars and Jupiter. Nearly a thousand comets have been spotted as well.
Which of these objects travel the greatest distance from the sun? Actually, the comets do, by far.
The word “comet” originates from the Greek word ko·meʹtes, meaning “long haired”—referring to the long, sweeping tails trailing behind their bright heads. Comets have been the source of much superstition and hysteria. Observers still refer to comet visits as apparitions. This stems from early beliefs that they were ghostly objects. Why have they been greatly feared? One reason is that their appearances have sometimes coincided with tragic events.
Comets still evoke fanaticism. In March 1997, in California, U.S.A., 39 members of the Heaven’s Gate cult committed mass suicide as the comet Hale-Bopp approached the sun. Why? Because they expected that an alien spacecraft, supposedly hiding behind the comet, was coming to pick them up.
Not everyone has viewed comets irrationally. In the fourth century B.C.E., Aristotle proposed that comets were clouds of luminous gas high up in the sky. A few centuries later, the Roman philosopher Seneca astutely suggested that comets were orbiting, heavenly bodies.
With the advent of the telescope and the discovery of Newton’s law of gravitation, the study of comets became a more precise science. By 1705, Edmond Halley had determined that comets orbit the sun on long, elliptic paths. Furthermore, he noted that comets appearing in the years 1531, 1607, and 1682 had similar trajectories and were separated by regular intervals of about 75 years. Halley correctly suggested that each of these sightings was of the same orbiting comet, later named Halley’s Comet.
Researchers now know that comets have a solid nucleus, generally between 1 and 12 miles [1 and 20 kilometers] across. The nucleus can best be described as a dark, dirty iceberg made up mostly of water ice mixed with dust. Close-up images of Halley’s Comet taken by the Giotto spacecraft in 1986 show jets of gas and dust emanating from the comet. These emissions make up the bright comet head and tail seen from the earth.
Two families of comets orbit the sun. A comet’s classification is based on its orbital periods, or the amount of time it takes to complete one revolution around the sun. Short-period, or periodic, comets—like Halley’s Comet—take fewer than 200 years to make each trip around the sun. Their orbits follow paths close to the ecliptic, the celestial plane in which the earth and other planets orbit the sun. There may be a billion periodic comets, most of which are orbiting beyond the outermost planets Neptune and Pluto, billions of miles from the sun. Occasionally, some of these, such as Encke’s Comet, have their orbits pulled in nearer to the sun by close encounters with planets.
What about the orbits of long-period comets? Unlike their short-period counterparts, long-period comets circle the sun from all directions. They include the comets Hyakutake and Hale-Bopp, which put on spectacular displays during their recent appearances. However, they are not expected back for thousands of years!
A huge collection of long-period comets move about in the outermost parts of the solar system. This swarm has been named the Oort cloud, after the Dutch astronomer who, in 1950, first suggested its existence. How many comets make up this cloud? Astronomers estimate more than a trillion! Some of these comets travel to distances of a light-year or more from the sun. With those distances, a single orbit may take well over ten million years!
Myriads of Small Planets
The newly identified minor planets mentioned at the outset share their realm beyond Pluto with short-period comets. Since 1992, astronomers have discovered about 80 of these small, planetlike bodies. There may be tens of thousands of them larger than 60 miles [100 kilometers] across. These miniplanets make up the Kuiper belt, named for a scientist who nearly 50 years ago suspected its existence. The Kuiper-belt objects are probably made up of a combination of rock and ice.
Have the recent discoveries of these small planets changed the way the inner solar system is viewed? Indeed! Pluto, its moon Charon, Neptune’s satellite Triton, and some other icy objects in the inner solar system are now thought to be objects that came from the Kuiper belt. Some astronomers even think that Pluto no longer qualifies as a major planet!
Where Did They Come From?
How did comets and minor planets come to be in abundance in the Kuiper belt? Astronomers suggest that these objects grew from an early cloud of dust particles and condensing ice, which stuck together to form larger objects. However, these objects were too thinly spread out to continue to grow into large planets.
Long-period comets also represent a substantial part of the solar system. Altogether, these comets have a mass some 40 times as great as earth’s. Most are thought to have formed early in the solar system’s history in the region of the outer gas-giant planets.
What propelled these comets out to their present orbits so far from the sun? Apparently, large planets, such as Jupiter, acted as powerful gravitational slingshots on any comets that came near them.
Comets consist of some of the most primitive material in the solar system. How can these fascinating objects be explored further? Occasional visits to the inner solar system by some comets enable them to be studied up close. Various space agencies plan to send a number of spacecraft to explore comets over the next several years.
Who knows what may yet be found in our solar system? The new discoveries and understanding of distant objects orbiting the sun add force to the Bible passage recorded at Isaiah 40:26: “Raise your eyes high up and see. Who has created these things? It is the One who is bringing forth the army of them even by number, all of whom he calls even by name.”