 INTERLUDE 11-1 Almost a Star? |
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| Jupiter has a starlike composition — predominantly hydrogen and helium, with a trace of heavier elements. Did Jupiter ever come close to becoming a star itself? Might the solar system have formed as a double-star system? Probably not. Unlike a star, Jupiter is cold. Its central temperature is far too low to ignite the nuclear fires that power our Sun. Jupiter's mass would have to increase 80-fold before its central temperature would rise to the point where nuclear reactions could begin, converting Jupiter into a small, dim star. Even so, it is interesting to note that although Jupiter's present-day energy output is very small (by solar standards, at least), it must have been much greater in the distant past while the planet was still contracting rapidly toward its present size. For a brief period of time—a few hundred million years—Jupiter might actually have been as bright as a faint star, although its brightness never came within a | factor of 100 of the Sun's. Seen from Earth at that time, Jupiter would have been about 100 times brighter than the Moon!
What might have happened had our solar system formed as a double-star system? Conceivably, had Jupiter been massive enough, its radiation might have produced severe temperature fluctuations on all the planets, perhaps to the point of making life on Earth impossible. Even if Jupiter's brightness were too low to cause us any problems, its gravitational field (which would be 1/12 that of the Sun if its mass were 80 times greater) might have made the establishment of stable, roughly circular planetary orbits a fairly improbable event. The size of the "Jupiter," or second largest body, in a newborn planetary system may be a very important factor in determining the likelihood of the appearance of life. |