Comparatively little is known about the formation and early history of the solar system, even though our knowledge of the sun and its planetary system is steadily growing. Various theories exist that explain how the planet earth and its companions were formed about 4.5 billion years ago, but the answer may lie not in the observation of our immediate neighborhood, but of other stars in our galaxy.
The Titius-Bode law
It has long been known that the distances of the planets from the sun can be fitted to certain mathematical sequences, the most famous of which is the Titius-Bode law. This predictive sequence was first determined by the German mathematician Johann Titius in 1766, and publicized by Johann Bode in 1772. Titius discovered that if the numbers 0,3, 6,12, 24, and so on in that sequence, with every number after 3 being double its predecessor, have the value 4 added to each one, and if the distance of the
earth from the sun is taken as 10, the end values correspond with the known planetary distances from the sun.
The Titius-Bode sequence was regarded with skepticism by those who thought that any able mathematician could invent a sequence and suit it to the problem. When Uranus was discovered in 1781, however, and was found to conform to the law (with the predicted distance being 191.8 and the actual distance being 196), some astronomers banded together to search for the “missing” planet that would fill the gap at the value of 28. The discovery of Ceres, on January 1,1801, by Giuseppe Piazzi from his home in Palermo, Sicily, was thought to solve the problem of the gap. But Ceres, at a distance from the sun of 27.7, turned out to be just one in a belt of many thousands of asteroids orbiting the sun between Mars and Jupiter.
Neither Neptune nor Pluto corresponds to its predicted positions within the Titius-Bode sequence, but the search for, and the calculated positions of, Neptune narrowed the range of possibilities and ultimately aided in its discovery. In fact, Pluto occupies the position that Neptune should hold if it were to follow from Uranus, at an actual distance of 393.0. In spite of this discrepancy, it is evident that the planets are spread out in a fairly distinct arrangement, which could be a clue to the formation of the solar system.
The rotation of planets
It is accepted that most of the planets in the solar system have, or have had, rotation periods of about 10 hours. Both Jupiter and Saturn maintain this average, whereas it is evident that other bodies have had their periods altered by gravitational interference, such as Earth. The effects of the moon’s gravity have contributed greatly toward the lengthening of our day from the 10 hours that it was originally, to the present 24.
The sun’s axial rotation period also differs from what it should be. The sun contains almost all the mass present in the solar system, and to match this it should have a proportionate amount of the angular momentum of the system. Consequently, it should rotate in only a few hours; but in fact, the sun’s rotational period is nearly a month.
The origin of the solar system
Virtually all the different theories on the formation of the solar system agree on two facts. One is the age of the system—around 4.5 billion years. This figure has been arrived at from the examination of certain types of meteorites. Particular elements (such as uranium and thorium) decay into lead overtime, and scientists can deduce the age of rock particles from meteorites by comparing the respective amounts of these elements and the lead present within the samples.
The other point of agreement is that the
planets were formed from gas and dust in the vicinity of the sun. Opinions vary on the origin of this material, ranging from gaseous rings thrown off by the sun to matter pulled from a passing star by the sun’s gravity. The gas and dust seem to have condensed into tiny bodies, or planetesimals, which eventually built up the planetary system we know today. This formative sequence would explain the presence of asteroids, which are planetesimals that never accreted into a planet, possibly because of the gravitational influence of the giant planet Jupiter.
Toward the end of the eighteenth century, the French mathematician Pierre Simon de Laplace proposed that the planets were formed from rings of gas thrown off by the young sun, but this theory would not explain the sun’s disproportionately low momentum. If the solar system were formed in the way he suggested, then the sun would have almost the correct amount of momentum, which is not the case. According to the Australian astronomer A. J. R. Prentice, however, Laplace’s idea would work if the solar core was assumed to have collapsed at a sufficiently rapid rate to push most of the angular momentum outward into the surrounding gas. Prentice’s modification of Laplace’s original idea has gained some popularity.
Other planetary systems
Looking to other regions of the galaxy for answers to their questions, astronomers have studied groups of stars, and in particular T Tauri variable stars. T Tauri is the prototype of a particular kind of star that varies irregularly in light intensity. This star, and others of its kind, are young stars in the early stages of evolution. They are throwing off mass, which takes the form of nebulosity surrounding the central star. The density of this matter is not uniform, and the star is seen to vary in brightness as veils of gas and dust pass between the star and ourselves. The ejection of matter from the star in this way would tend to decrease its angular momentum. If our sun had gone through a similar phase, this would explain the distribution of momentum and rotation that exists in our solar system. The planets themselves could have condensed from the gas and dust around the young sun, building up from local condensations, through the planetesimal stage, and finally into the planets that we observe today.
If T Tauri variables are an indication of possible early processes within our own solar system, then they could also indicate other similar embryonic planetary systems. The idea of a planetary group forming from a nebula surrounding a young star is more credible than that of an accidental creation with, for example, matter being dragged from a passing star. The former notion indicates that the process of planetary formation happens on a fairly regular basis, that other systems may be commonplace, and that they are the standard byproduct of a young star.
On the other hand, the chances of any particular star passing close enough to another to effect a matter transfer on such a scale are very remote. This would mean that our solar system is almost unique. But stellar collisions, or near-collisions, are not the only way that a planetary system could form accidentally—a star could pass through a nebula and draw off matter from the nebula while doing so. This matter could accumulate and condense into planets, although this, too, would be unlikely.
Theories may be proposed unendingly, but without actual observational proof, the idea of extra-solar planetary systems remains just an idea. Even the world’s largest telescopes are unable to detect planets orbiting other suns because the light from any nonluminous companions would be obscured in the glare from the parent star. Such objects may, however, be detected from the observation of gravitational effects within other systems.
Two gravitationally bound bodies orbit each other around their mass center—their common center of gravity—the position of which is determined by the relative masses of two objects. For example, because the earth has 81 times the mass of the moon, the center of gravity of the earth-moon system lies 81 times closer to the earth’s center than the moon’s. This point, the barycenter, lies beneath the earth’s surface. As a result of the moon’s gravitational effect on the earth, our planet spins around the barycenter, each spin lasting 27.32 days—the length of time it takes the moon to make one complete orbit This gravitational effect causes waves in the earth’s path as it travels around the sun, each wave lasting 27.32 days.
Under favorable conditions, it should be possible to detect similar deflections in the motions of stars—particularly nearby and small stars—with planets in orbit around them. Some astronomers claim to have detected such wobbles in the paths of a few close stars. Peter van de Kamp, a Dutch-born American astronomer, announced in 1975 that he had evidence of wobbles in four stars. In particular, he had observed a tiny deflection of only a few hundredths of an arc second in the path of Barnard’s Star, an object only six light-years away and the third nearest star (including the sun). The deviations apparently indicated that there are two large planets in orbit around Barnard’s Star, one of which has a mass slightly larger than that of Jupiter, and the other with a mass half that of Jupiter. Van de Kamp also claimed that the star Epsilon Eridani has a planetary companion with a mass six times that of Jupiter.
Other investigators, however, have been unable to duplicate van de Kamp’s findings satisfactorily, even when using techniques that were more sensitive than those he used. Most astronomers are coming to believe that van de Kamp’s “star wobbles” were caused by miniscule changes in his telescope—for example, when the main lens was cleaned and replaced. The verdict on the existence of other planetary systems remains open for the moment.