Discovery of the Solar System

The Copernican System (Courtesy of Rice University)
The discovery of the solar system belongs to the period called the "Renaissance", when philosophers decided to admit nothing but observation and logic in building the science enterprise, and to reject tradition. Perhaps the best-known exponent of this new (and courageous) approach is the French mathematician, philosopher and scientist René Descartes (1596-1650). His statement "cogito ergo sum" (I think therefore I am) is symbolic for the Renaissance attitude. (Some have pointed out that you can't just think without thinking of something, so that it is impossible to start science with pure logic. Anyway, Descartes tried it and came up with lots of interesting results, and some pretty strange notions, too.)

General acceptance of the sun-centered system of celestial motions took several hundred years. The main idea of the solar system was proposed by the Polish astronomer Nicolaus Copernicus (1473-1543) who said that "the Sun is the center of the Universe" and made the planets move around it in perfect circles (in his book entitled, "On the Revolution of the Celestial Spheres", written in Latin and published in 1543). He thereby revived an ancient idea going back to the Greek philosopher Aristarchos (flourished ~270 B.C.) who suggested that the Sun is much bigger than the Earth and that it, and not the Earth, is at the center of the universe. (Nobody paid much attention to this, since it is plain that the Sun is moving around a lot: once a day across the sky, once a year across the stars at different elevations above the horizon.)

Kepler’s model of the solar system (Source: Rice University)
The German astronomer Johannes Kepler (1571-1630) supported the Copernican concept that the Sun is at the center, but gave to the planets elliptical orbits, with the Sun in one of the foci of each ellipse, to describe their complicated motions more correctly. The direct observations by Galileo Galilei (1564-1642), who showed that Venus has phases like the moon (using the telescope he invented and built) clinched the matter for the heliocentric system. (The Church disagreed. It put Copernicus' book on the index of forbidden works in 1616, and left it there till 1835. Also, Galileo had to recant and was forbidden to teach and to leave his home.)

Galileo was one of the great minds of all time. As the French philospher Yves Bonnefoy has said: "With Galileo, the Moon ceased to be an object of adoration and became an object for scientific study." Another Italian astronomer, Giovanni Domenico Cassini (1625-1721) (whose name is associated with the large gap between the inner and outer rings of Saturn) determined the size of Earth's orbit. His value was only 7 percent short of the modern one (150 million km). He also established the size of the solar system. (Aristarchus had been off by a factor of 20 in estimating the distance to the Sun.)

Galileo's telescopes. (Source: Rice University)
Then came Isaac Newton (1642-1727) who brought the laws of physics to the solar system. Isaac Newton explained why the planets move the way they do, by applying his laws of motion, and the force of gravitation between any two bodies, letting the force decrease with the square of the distance between the two bodies. (Besides formulating the laws of motion, Newton invented the concept of gravitational force and a new kind of math to calculate planetary motions. The math is now called calculus. It was invented independently, and published earlier, by the German mathematician Gottfried Wilhelm Leibniz (1646-1716) whose notation is still used in textbooks today. Newton also built the first reflecting telescope to survey the sky with.)

Knowing that the Sun is in the center of the system, and the rotating planets move around it in their proper orbits following Newton's laws became the basis for further exploration of "celestial mechanics". Many details of the motions remained to be worked out. Major contributions came from the French mathematicians and astronomers Pierre Simon de Pierre Simon Laplace (1749-1827), Joseph Louis Lagrange (1736-1813), and Urbain Leverrier (1811-1877). (Even now celestial mechanics is an active field of study, because of the amount of computation it takes to work out planetary positions for millions of years.)

Comet Halley (Source: NASA)
The English astronomer Edmond Halley (1656-1742) realized that comets are part of the system (at least one of them, Halley's Comet, kept on coming back every 75 years). Neptune, a major planet, was only discovered in 1846 (by the German astronomer Johann Galle, following instructions from Leverrier); Pluto was found in 1930. Many astronomers now think Pluto is really just an unusually big comet-type body (or rather two bodies) in the "Kuiper Belt", with hundreds or thousands such bodies. The Kuiper Belt takes its name from the Dutch-American astronomer Gerard Kuiper (1905-1973) who made many contributions to the knowledge of satellites of the outer planets.

In addition, there is the "Oort cloud", asteroids and comets that surround the solar system like a swarm of mosquitoes surrounds a group of people on a picnic in summer, in Minnesota. The "Oort Cloud", named after the Dutch astronomer Jan Hendrik Oort (1900-1992) who proposed its existence (in 1950), is an enormous assemblage of comets at great distance from the inner solar system. It serves as a seemingly inexhaustible reservoir for comets coming close to the Sun (and therefore becoming short-lived). Comets that cross the path of the inner planets do so, it is thought, because their orbits were disturbed by the gravitational influence from passing stars.

The Sun as viewed in X-rays. (Courtesy of SOHO/EIT Consortium)
Two fundamental discoveries about our world are among the greatest contributions to human thought: The age of the solar system and the energy-source of the Sun. The two are related, because only the energy of nuclear fusion (whereby mass is made into energy) can fuel the Sun for as long as it has warmed our home planet.

The discovery of the energy-source of the Sun — fusion of hydrogen nuclei (i.e., protons) to make helium nuclei (i.e., alpha particles) — owes much to the discovery of radioactivity (by Antoine Becquerel and the wife-and-husband team Marie and Pierre Curie); to the discovery of the proton and of the mutability of atomic nuclei (by Ernest Rutherford); and to the realization that mass itself is a manifestation of energy (by Albert Einstein). Together, these discoveries are less than a century old. The concept of the Sun as a nuclear furnace has only been reasonably well understood since the 1930s.