R What do we learn from our own solar

We think that we now understand how our solar system formed, with its two types of planets in their concentric, near-circular orbits, ail travelling in much the same plane. But this model of formation is not echoed by what we have discovered about new worlds...

4.1 EARLY THEORIES: THE SEVENTEENTH CENTURY ONWARDS

Like neighbouring protoplanetary systems, the solar system formed within a disk

What is the Earth's place in the Universe? This question is intimately connected with that of the existence of other inhabited worlds, and is as old as the human race. The Greeks, following the teachings of Aristotle, placed the Earth at the centre of everything. For more than 1,000 years this was the prevalent theory, in spite of the work of early thinkers such as Aristarchus of Samos (who lived around 300 BC) and, many centuries later, Nicholas of Cusa (1401-1463), both of whom proposed a heliocentric (Sun-centred) model. It is to Nicolaus Copernicus (1473-1543) that we owe the accession of the heliocentric system, now known as the Copernican system. In 1543 (the year that Copernicus died) his radical book De Revolutionibus Orbium Coelestium was published.

Copernicus's theory, which sat ill alongside the teachings of the all-powerful Catholic church of the time, finally found acceptance thanks to the work of mathematicians and astronomers such as Tycho Brahe (1546-1601), Johannes Kepler (1571-1630), Galileo Galilei (1564-1642) and Isaac Newton (1642-1727), whose calculations and observations scientifically justified the Copernican model. Tycho's observations were profitably taken up by his pupil Kepler, who

The German philosopher Immanuel Kant (1724-1804) and the French scientist PierreSimon de Laplace (1 749—1827), who formulated the first theories concerning the origin of the solar system.

4.1 Early theories: the seventeenth century onwards 67

4.1 Early theories: the seventeenth century onwards 67

The Orion Nebula. This nebula is a complex of various nebulae - the brightest among them being M42 (the pink cloud at centre right). M43 is the smaller, round nebula (below centre), separated from M42 by a narrow blue band of gas. M42 and M43 appear pink due to the presence of hydrogen ionised by the young stars within them. To the left is the nebula NGC 1977, which, being non-ionised, reflects the light of nearby blue stars. The Orion Nebula is about 1,500 light-years from Earth.

The Orion Nebula. This nebula is a complex of various nebulae - the brightest among them being M42 (the pink cloud at centre right). M43 is the smaller, round nebula (below centre), separated from M42 by a narrow blue band of gas. M42 and M43 appear pink due to the presence of hydrogen ionised by the young stars within them. To the left is the nebula NGC 1977, which, being non-ionised, reflects the light of nearby blue stars. The Orion Nebula is about 1,500 light-years from Earth.

formulated empirically the famous 'Kepler's laws', describing the motions of the planets around the Sun. Galileo - an early promoter of the telescope for astronomical purposes - confirmed the Copernican theory in new ways, and Isaac Newton, in Philosophiae Naturalis Principia Mathematica, propounded the law of universal gravitation - mathematical proof of Copernicus's model.

With the Sun in its rightful place at the centre of its system, the question remained as to how it was made. The first hypotheses concerning the formation of the solar system were proposed by René Descartes (1596-1650). In his Théorie des Vortex Descartes imagined the solar system to have been formed from a 'whirlpool' of matter, but this model did not explain why the planets move in approximately the same plane (the ecliptic). Neither did it suggest why their orbits are almost circular, and why they all orbit in the same direction.

Later, Immanuel Kant (1724-1804) and Pierre-Simon de Laplace (1749-1827) introduced the nebular theory to explain these two fundamental characteristics of the solar system. The main aspects of this theory - which were intuitive at the time - are nowadays largely accepted. An initial, contracting gas cloud begins to turn with increasing speed, and finally collapses into a disk. Within this disk form planets, together with their satellites and ring systems (see p. 43).

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