Heliocentric, Tidal and Nebular Theory Help (page 4)

By — McGraw-Hill Professional
Updated on Sep 16, 2011


Certain questions remain difficult to answer—in particular the extreme tilts of Venus, Uranus, and the Pluto-Charon systems on their axes. You will remember that the axis of Venus is tilted nearly 180 degrees; another way of saying this is that its rotation is retrograde. The nebular theory does not specifically forbid this, although it suggests that most planets will end up rotating in the same sense as they orbit around the Sun. In the case of Uranus, some astronomers think that it was struck by an object so massive that its rotational axis was “knocked flat” by the encounter. In that scenario, both objects were nearly shattered; in the end, however, Uranus survived, and the other object did not. The same thing may have taken place with Venus and Pluto-Charon.

Are We Too Hope-driven?

The nebular theory explains why the planets orbit the Sun in a comparatively uniform manner. In addition, assuming that this theory is correct, we have good reason to believe that there are many such systems in our Milky Way galaxy, as well as in other galaxies, especially those of the spiral type with their abundant interstellar gas and dust.

Astronomers have found evidence of other planetary systems. Flat, circular clouds or rings, thought to be accretion disks, have been observed by the Hubble Space Telescope. If we actually are looking at stars with planets forming around them or in orbit around them, then it means that our Solar System is not a freak cosmic accident. If the Universe is teeming with planetary systems like ours, it is tempting to believe that there are many Earthlike planets too and that some of these planets have evolved intelligent life.

Critics of the nebular theory use the foregoing speculations against it. They say that hope drives the thinking of the proponents of the theory and that this emotion interferes with rational reasoning. If our Solar System is the only one of its kind in the whole Universe, they say, then so what? We are here to bear witness to the miracle of life on Earth simply because we are one of its products!

Decline And Death

Just as there are theories about how the Solar System was formed, there are notions concerning its long-term future. The ultimate fate of the Solar System depends on its parent star, the Sun. Most astronomers believe that the Sun eventually will swell into a red giant, burning up or vaporizing Mercury, Venus, Earth, and Mars and perhaps blowing the gas away from Jupiter, Saturn, Uranus, and Neptune. Then the Sun will shrink down and die out like an ember in a dying fire around which living beings were once encamped. Where those creatures, our distant descendants, will be by then is a question that no theory can answer.

Practice problems of this concept can be found at: Evolution of Solar System Practice Problems

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