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Message: <blockquote data-quote="radferth" data-source="post: 1604771" data-attributes="member: 5791">two unrelated thoughts1) A planets orbit would need to be quite eliptical to have much affect on temperature.  Earth's seasons are caused by the angle of sunlight changing due to the axial tilt.  The northern hemisphere is acutally a bit further in the summer.  Albeit the Earth's orbit only varies by about 1% as to how far we are from the sun.2)  If you want humans to be able to survive on a planet without too much equipment, you want to have the partial pressure of oxygen about equal to Earth's.  If we round off and assume Earth has 1 atm of pressure and 20% oxygen, then we have .2 atm partial pressure of oxygen.  So if you want a planet with .5atm, you could breath alright with 40% oxygen.  At 2 atm you would need a atmosphere around 10% oxygen.  I don't know how low you can go in overall pressure before other physical problems occur, but I do know (from watching old Jasque Cousteau) that at high pressures the toxicity of other stuff in the air becomes a problem at high pressures.  When they would go down very deep, they would use an oxygen/helium mixture, since helium is quite inert and caused no problems.  If you want a high pressure planet, I would suggest having Argon as the main component, as it is inert like helium but has a similar density to oxygen and doesn't make your voice so funny.Danny -former science teacher, lifetime science geek</blockquote>