Building a Planet From Scratch (Part Two)

[Kitti]

Atmosphere​

No atmosphere
It is possible to have a planet essentially without atmosphere, as is the case with Mercury. Due to its small size and resulting little gravity, it's believed that Mercury's atmosphere was blown away by solar winds. There are some remnants remaining, traces of water vapor and potassium among a few others, but it is unlikely such a planet could support life. An atmosphere is important in stabilizing water and temperature and in protecting organisms on a planet from radiation.

Runaway Greenhouse Planets
A planet whose atmosphere is too heavily composed of carbon dioxide - such as Venus's is - will often become victim to the runaway greenhouse effect. The atmosphere of Venus is approximately 96.5% carbon dioxide, which trapped heat from the sun on the planet and continually raised the planet's temperature until almost all the water that was on Venus had long since boiled away. This means that Venus's surface temperature, without any ability to regulate itself with the water having boiled away and the heat unable to escape, is 894 °F (480 °C). Not exactly welcoming!

Balanced Atmospheres
In order to support life in such a form as we know it, an atmosphere must be present, it must have the correct thickness (Venus's atmosphere is much too thick to support life, resulting in problems with greenhouse gases and atmospheric pressure). Earth's atmosphere is estimated to be about 1000km in thickness. The atmosphere also needs to have certain elements present (carbon, nitrogen, hydrogen, and oxygen) to support life, at least the life as we know it. For life forms utilizing alternate forms of respiration or whose primary structure is different from life we know, there may be other requirements instead.

Jovian Planets
These are the "gas giants", one whose primary element is not a solid material such as rock. It is difficult with such planets to determine where the atmosphere begins. In the ordinary gas giants (Jupiter and Saturn), their atmosphere and much of the planet itself is composed of hydrogen and helium. The "special" gas giants (Neptune and Uranus) are often referred to as ice giants for their atmospheric differences from the other gas giants. Water, ammonia, and methane are the contributors which make them "ice" giants and methane absorbs red lightwaves, giving them their tranquil blue color. As these are not mainly made of a stable substance, life on these planets is unlikely.

Spoiler

Lunine, Jonathan I. (September 1993). "The Atmospheres of Uranus and Neptune". Annual Review of Astronomy and Astrophysics31: 217–263.

Ingersoll, A. P.; Dowling, T. E.; Gierasch, P. J.; Orton, G. S.; Read, P. L.; Sanchez-Lavega, A.; Showman, A. P.; Simon-Miller, A. A.; Vasavada, A. R. "Dynamics of Jupiter's Atmosphere"
Ward, Peter D. and Brownlee, Donald, Rare Earth, Copernicus Books, 2000.

 

[Sakura]

You seem like you have a lot of astronomical knowledge, so this is something I want to ask!

How do surfaces exist on Jovian/gas giants? Is there a solidified form of helium or gas that allows for that, or is it possible to pass through such planets?

(Sorry if this is a silly question, I don't know much about planets! )

 

[Minibit]

Where could one find information on planet's atmospheres in relation to the kind of sun they are orbiting? I imagine their proximity to the star and the kind of star have an effect on what kind of atmosphere the planet has

Please tell me these will continue, I find it quite insightful and interesting!

 

[Kitti]

Oops, sorry it's been so long, I didn't see your questions.
[mention=13]Sakura[/mention]: The "surface" doesn't exist. The closest approximation is the point where pressure compresses gas to a state whose density is similar to a liquid.
Mini: First of all, this article made me giddy and you might also enjoy it. The star, when it comes to the planets, doesn't have as much an effect on the atmosphere as the elements that make up the atmosphere for the planet. It depends on the elements present and the sun exerts an effect on these, which is why our atmosphere can support life and Neptune's "hydrocarbons" cause it to appear blue, despite orbiting the same star.