Origin of the solar systemAs the amount ofdataon the planets, moons, comets, and asteroids hasgrown, so too have the problems faced by astronomers in forming theoriesof the origin of the solar system. In the ancient world, theories of the originofEarthand the objects seen in the sky were certainly much lessconstrained by fact. Indeed, a scientific approach to the origin of the solarsystem became possible only after the publication of IsaacNewton’s laws ofmotionandgravitationin 1687. Even after this breakthrough, many yearselapsed while scientists struggled with applications of Newton’s laws toexplain the apparent motions of planets, moons, comets, and asteroids. In1734 Swedish philosopherEmanuel Swedenborgproposed a model for thesolar system’s origin in which a shell of material around theSunbroke intosmall pieces that formed the planets. This idea of the solar system formingout of an original nebula was extended by the GermanphilosopherImmanuel Kantin 1755.Early scientific theoriesTheKant-Laplace nebular hypothesisKant’s central idea was that the solar system began as a cloud of dispersedparticles. He assumed that the mutual gravitational attractions of theparticles caused them to start moving and colliding, at which point chemicalforces kept them bonded together. As some of theseaggregatesbecamelarger than others, they grew still more rapidly, ultimately forming theplanets. Because Kant was highly versed in neitherphysicsnormathematics, he did not recognize theintrinsiclimitations of his approach.His model does not account for planets moving around the Sun in the samedirection and in the same plane, as they are observed to do, nor does itexplain the revolution of planetary satellites.
A significant step forward was made byPierre-Simon Laplaceof Francesome 40 years later. A brilliant mathematician, Laplace was particularlysuccessful in the field ofcelestial mechanics. Besides publishing amonumentaltreatiseon the subject, Laplace wrote a popular bookonastronomy, with an appendix in which he made some suggestions aboutthe origin of the solar system.Laplace’s model begins with the Sun already formed and rotating anditsatmosphereextending beyond the distance at which thefarthestplanetwould be created. Knowing nothing about the sourceofenergyin stars, Laplace assumed that the Sun would start to cool as itradiated away its heat. In response to this cooling, as thepressureexertedby its gases declined, the Sun would contract. Accordingto the law ofconservation of angular momentum, the decrease in size wouldbe accompanied by an increase in the Sun’s rotational velocity.Centrifugalaccelerationwould push the material in the atmosphere outward, whilegravitational attraction would pull it toward the central mass; when these