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We know that the conditions for life are rare in this solar system, and just the right atmosphere, water, and chemicals exist on earth in order for life to occur. However, if the earth was a bit closer to the sun, it would be too hot, and if it were a little farther away, it woud be too cold. So, our question is: how far is that "bit" of distance that would make life unsustainable?
Answer 1:

There are two main variables. One is distance from Sun; anything closer than about 0.7 AU and farther than about 2 AU presents a problem to LIFE AS WE KNOW IT.

The second MORE IMPORTANT FACTOR is the type of atmosphere a planet has. The Earth for example has an atmosphere dominated by N2 and O2 and neither of these gases are good greenhouse gases. That means that the Earths atmosphere is mostly TRANSPARENT. That means that sunlight is not absorbed by these gases so the atmosphere stays relatively cool. On the other hand, Venus has an atmosphere composed of 99% CO2...and CO2 absorbs sunlight very efficiently. That causes the atmosphere to heat up. This is why Venus has a surface temp of 600 C very HOT.

If the earth had a pure CO2 atmosphere it would be MUCH hotter than it is now. That is what global warming is all about? When we burn fossil fuel we enrich the atmosphere in CO2 and that causes global warming.

So in summary, defining the habitable zone around a star depends on the magnitude of the star, on the distance a planet is from that star as well as the composition of the atmosphere around the planet.

Answer 2:

That is a very good question and we don't really know the answer.

The reason why Venus is uninhabitable is because it has too much atmosphere. Now, the circumstances under which the Venusians atmosphere got to be as thick as it is may have something to do with its being close to the Sun, but we are not sure; it may have to do with the way plate tectonics work (or don't work, as the case may be) on Venus. It also may be due to Venus' very slow rotation; the presence of the Moon may stabilize the Earth's rotation such that the oceans continue to exist. It is unlikely that Venus ever hadoceans; the Earth has the carbon dioxide that makes the Venusians atmosphere, but it has almost all bound up with calcium in the oceans to make limestone, and thus isn't in our atmosphere. Of course, since Venus has been so hot, all of the water in its atmosphere has been broken up by ultraviolet light and the hydrogen has been lost into space, so even if the atmosphere were removed, Venus would still be uninhabitable. If you were to somehow transport Earth into an orbit around the Sun where Venus is now, the Earth would probably remain habitable for a while at least, because nearly all of the Earth's greenhouse gasses are trapped in its rocks.

Mars is uninhabitable because it doesn't have enough atmospheres. The surface of Mars at the equator is actually above freezing for significant parts of the year, but the atmospheric pressure there is so low that water would boil on its equatorial surface; even though temperatures are comparable to habitable parts of the Earth (Mars' poles of course are MUCH colder than anywhere on Earth). It is likely that a large part of the lack of a Martian atmosphere is due to the fact that Mars is smaller than the Earth, and thus possesses about one third of the Earth's gravity; it cannot hold the atmosphere on. Now, there is a growing body of evidence that Mars actually could have been habitable, had liquid water on its surface, and a thick enough atmosphere to sustain it, at times in its past. When and how it was that Mars got freeze-dried to be the way it is today is an unanswered question in planetary astronomy. It has been suggested that if Mars were instead the size of the Earth (and thus had Earth-like gravity), that it would be habitable. Of course, it isn't.

Venus is about 5 light minutes from the Sun. Earth is 8 light minutes, and Mars is 16. Light travels at 300,000 km per second. You do the math.

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