"When Mars is at its closest point to Earth, could its gravity have an effect on us, or is it still too far away?"
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Question
In terms of habitability, Mars and Venus seem like near-misses and so our own solar system might easily have had more than one habitable planet. Do you think that in other solar systems where there is life, a significant number will have more than one habitable planet?
The question of the width of the habitable zone in our solar system is complex. Both Venus and Mars have been described as near the edge of the habitable zone, as you note. The situation is even more complicated, however, since the habitable zone has shifted outward over the past 4 billion years as the brightness of the Sun has gradually increased. Yet another complication arises when we consider that the temperature at the surface of a planet also depends on the composition of the atmosphere (and thus how strong is the greenhouse warming), as well as the brightness of the Sun and the distance of the planet. Thus we really don't know the answer to your question. In terms of the Drake equation, it is common to guess that there are either 1 or 2 habitable planets in a typical planetary system -- but this is really just a guess. After the Kepler mission, we will have a much better idea of the distribution and sizes of planets in the habitable zones of other planetary systems.
David Morrison
NAI Senior Scientist
September 15, 2008
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