G

 The differences most likely arose from small initial differences in the initial temperatures and atmospheric concentrations of carbon dioxide and oxygen. Studies have shown that several billion years ago the atmosphere of Venus was very similar to that of Earth today, and that there was likely a large amount of liquid water on its surface. Venus appears to have suffered a 'runaway greenhouse effect' where the carbon dioxide in the atmosphere lead to an increased planetary temperature, which freed trapped carbon from the oceans, increasing the amount of carbon dioxide in the atmosphere, which in turn increased the temperature, etc. The carbon dioxide in Earth's early atmosphere got dissolved into the oceans and trapped into solid compounds in the surface minerals, so that nearly all of it is gone from the present atmosphere.

h. During the formation of the Solar System, the inner solar system, being close to the Sun, was too warm for water and methane to condense, so the planets that formed there were largely composed of compounds with high melting points. Further out from the Sun, lighter compounds could remain solid, and so the gaseous planets were able to form from these materials. Gaseous elements were more common in the cloud which formed the Solar System, so the outer planets were able to gather more material and are therefore larger.

i. Uranus was first observed in 1690, but was not discovered to be a planet until 1781. It was initially thought by William Herschel to be a star, and then a comet, but then it was eventually realized that its orbit was nearly circular and he concluded that this must be a planet. The discovery of Neptune was quite different and far less accidental. In the 1820s it was realized that the orbit of Uranus acted as if it were being 'tugged' by another planet further from the Sun. In 1843 John Couch Adams calculated the orbit of the eighth planet which would account for these observations. Simultaneously, Urbain Le Verrier produced his own calculations. Both men observed the new planet a day apart from each other, very close in the sky to where they had predicted it would be found.

j. Comets are like leftover building blocks from the Solar System formation process and can offer clues to the mixture from which the planets formed by their chemical composition. The asteroids tell us a great deal about the effects that Jupiter must have had on the early objects which formed in the Solar System, since a planet would undoubtedly have formed from the asteroids if not for Jupiter's influence.