On a related note, why didn't Venus remain habitable for longer than it did? Is the current hypothesis as to the time when Venus's atmosphere became hostile and what event triggered it, proven true?
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4$\begingroup$ This question doesn't make any sense. The planets have all 'survived' but there are no planets that are alive - there is no usual sense. Can you clarify what you mean. $\endgroup$– Rory AlsopNov 22, 2014 at 11:48
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$\begingroup$ I apologise for that and have edited it. @Superdavo's answer answered the original question, but the edit was made after, so I'm keeping this open for now. $\endgroup$– pratik_mNov 23, 2014 at 15:22
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$\begingroup$ Re the second part of your question, if it was proven true then it wouldn't have been called a hypothesis (assumed solution to a scientific problem that requires further investigation to confirm or dismiss it), would it? $\endgroup$– TildalWaveNov 25, 2014 at 11:49
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$\begingroup$ What is the "current" hypothesis you are talking about? $\endgroup$– JonathanNov 28, 2014 at 21:31
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1 Answer
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By survived, do you mean remained habitable?
If so, then there are several reasons:
For Venus, the problem was that it was just barely on the edge of the habitable zone when the Sun was younger and less bright. As it aged and brightened towards the values we see today, any oceans began to evaporate. The resultant water vapour is a very powerful greenhouse gas, and so would have trapped yet more heat, causing a runaway greenhouse effect, until it is the baked, hellish world we see today.
Mars, on the other hand, was believed to be too small. It (probably, assuming modern theories are correct) started off as another habitable world, but since it was smaller, it would've had less residual heat left over from its birth, and would've cooled quicker. This would've caused the core to solidify, removing the magnetic field. This meant that the atmosphere was vulnerable to the solar wind, which would've gradually blown it - and any warming greenhouse gases - away, and with the lower pressure, the water would evaporate away as well, rendering Mars uninhabitable.
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$\begingroup$ That quite answers the original question. Could you also answer the part about Venus? $\endgroup$– pratik_mNov 23, 2014 at 15:24