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frudi_x · 09-02-2004, 10:45 AM

there are actually some good reasons to assume only Earth-like planets could sustain life.

for one, carbon is the only element that can produce the great abundance of different complex molecules that seem to be necessary for complex life to evolve. iirc known organic compounds outnumber known unorganic compounds by about 10:1. the only other element, that could theoreticly produce a similar multitude of compounds is silicon (which has the same number of valence electrons as carbon, so they are chemically quite similar), however long chains of silicon atoms are typicly a lot more unstable than carbon chains. we can therefore reasonably assume evolution will preffer carbon based organisms to theoretical silicon based ones. luckily carbon is one of the most abundant elements in the universe, so there's enough of it around in most solar systems.

the problem is however, that most organic molecules break apart at higher temperatures, higher in this case meaning 50+°C, so this sets the limit on the surface temperature of a planet that we can expect to support carbon based life. temperatures also can't be too low, or complex organic compounds cannot form, though i don't really remember what 'too low' is in this case. there is however another sensible lower limit for temperature - the freezing point of water. running water is considered to be one of the basic requirements for life, as it's an excelent solvent for most unorganic and many organic compounds, catalysing chemical reactions between them. it is also one of the rare compunds that has it's liquid phase in the temperature range that is suitable for formation and existance of organic compounds and of those it is by far the most common (as oxygen and particularly hydrogen are both very abundant in the universe) and uniquely useful.

so, what else do we need besides carbon, water and a temperature in the 0-50°C range? well, for one we need the temperature to be in that range for enough time for complex life to develop. our only estimate for that is the time it took here on earth, so let's use a rough figure of a few billion years. that eliminates the more massive stars as the more massive the star is the faster it uses up it's nuclear fuel and dies. stars are ranked into spectral classes based on their surface temperature (which depends on mass) - O B A F G K M - ranging from the hottest and most massive O class to the coldest and smallest M class. as only F class stars and smaller live for more than a billion years it's unlikely that life could develop on O B and A class stars (which by the way, are very rare anyway).
but, a star also can't be too small as then for the planet to be sufficienlty warm it would have to be so near the star that it's rotation would be slowed down because of tidal forces. this happened to Mercury, which only turns around it's axis one and a half times in the time it takes for it to go around the Sun once. the result are extremly high temperatures on the day side and extreme cold on the night side. this eliminates M and some K class stars as candidates for life bearing planets. that's a shame, as most stars in the universe are M class.

so, we want a planet with enough carbon and water, orbiting an F, G or K class star at a right distance (so the surface temperature is in the 0-50° range). there are also further possible requirements, like the presence of a Jupiter-like gass giant that will clean up all the debris left over from the formation of the solar system (debris that would otherwise regularly hit the planet, reseting the evolutionary clock to zero), no companion stars or migrating giant planets that would perturb the orbits of the planets...

finding these planets is the ultimate goal of the planet-hunting astronomers, but our current instruments are not precise enough to find them yet. it's even doubtful that the current method for finding planets can ever be made precise enough to find other Earths. however, if NASA's and ESA's plans for the following 20 years become realised, we will have space based systems of telescopes capable of directly detecting the light from these worlds.

09-02-2004, 10:45 AM	#10
frudi_x Drow Warrior Join Date: June 29, 2001 Location: the pale blue dot... Age: 45 Posts: 294	there are actually some good reasons to assume only Earth-like planets could sustain life. for one, carbon is the only element that can produce the great abundance of different complex molecules that seem to be necessary for complex life to evolve. iirc known organic compounds outnumber known unorganic compounds by about 10:1. the only other element, that could theoreticly produce a similar multitude of compounds is silicon (which has the same number of valence electrons as carbon, so they are chemically quite similar), however long chains of silicon atoms are typicly a lot more unstable than carbon chains. we can therefore reasonably assume evolution will preffer carbon based organisms to theoretical silicon based ones. luckily carbon is one of the most abundant elements in the universe, so there's enough of it around in most solar systems. the problem is however, that most organic molecules break apart at higher temperatures, higher in this case meaning 50+°C, so this sets the limit on the surface temperature of a planet that we can expect to support carbon based life. temperatures also can't be too low, or complex organic compounds cannot form, though i don't really remember what 'too low' is in this case. there is however another sensible lower limit for temperature - the freezing point of water. running water is considered to be one of the basic requirements for life, as it's an excelent solvent for most unorganic and many organic compounds, catalysing chemical reactions between them. it is also one of the rare compunds that has it's liquid phase in the temperature range that is suitable for formation and existance of organic compounds and of those it is by far the most common (as oxygen and particularly hydrogen are both very abundant in the universe) and uniquely useful. so, what else do we need besides carbon, water and a temperature in the 0-50°C range? well, for one we need the temperature to be in that range for enough time for complex life to develop. our only estimate for that is the time it took here on earth, so let's use a rough figure of a few billion years. that eliminates the more massive stars as the more massive the star is the faster it uses up it's nuclear fuel and dies. stars are ranked into spectral classes based on their surface temperature (which depends on mass) - O B A F G K M - ranging from the hottest and most massive O class to the coldest and smallest M class. as only F class stars and smaller live for more than a billion years it's unlikely that life could develop on O B and A class stars (which by the way, are very rare anyway). but, a star also can't be too small as then for the planet to be sufficienlty warm it would have to be so near the star that it's rotation would be slowed down because of tidal forces. this happened to Mercury, which only turns around it's axis one and a half times in the time it takes for it to go around the Sun once. the result are extremly high temperatures on the day side and extreme cold on the night side. this eliminates M and some K class stars as candidates for life bearing planets. that's a shame, as most stars in the universe are M class. so, we want a planet with enough carbon and water, orbiting an F, G or K class star at a right distance (so the surface temperature is in the 0-50° range). there are also further possible requirements, like the presence of a Jupiter-like gass giant that will clean up all the debris left over from the formation of the solar system (debris that would otherwise regularly hit the planet, reseting the evolutionary clock to zero), no companion stars or migrating giant planets that would perturb the orbits of the planets... finding these planets is the ultimate goal of the planet-hunting astronomers, but our current instruments are not precise enough to find them yet. it's even doubtful that the current method for finding planets can ever be made precise enough to find other Earths. however, if NASA's and ESA's plans for the following 20 years become realised, we will have space based systems of telescopes capable of directly detecting the light from these worlds.