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View Full Version : How much can greenhouse effect extend "habitable zone"



CaptainToonces
2011-Apr-27, 05:47 AM
How much can the greenhouse effect extend the star-to-planet habitable zone for liquid water?

Like if you swapped the positions of Mars and Venus, would Venus become habitable?

Romanus
2011-Apr-27, 10:03 AM
I think it's impossible to say, as too many factors come into account. Water vapor is actually a more important greenhouse gas on Earth than CO2, but it would be more sensitive to changes in insolation than CO2, while CO2's abundance on Earth has been more or less balanced between sources and sinks for a very long time. Farther from the Sun you'd need a different mix of gases, such as methane and CO2, and probably just methane farther still; dependent on mass, activity, and initial composition, you have a lot of possibilities to play with.

As for Venus, if you moved it as far as Mars, it would not become habitable. No doubt most of that vast quantity of CO2 would condense and rain down onto the surface, albeit over a very long time period. Not all of it, though, which leads to the fascinating possibility of liquid CO2 seas under a still-dense atmosphere.

Ronald Brak
2011-Apr-27, 11:35 AM
Earth should be kind of habitable, for a while at least, if it were moved to venus's orbit. This thread touches on this:

http://www.bautforum.com/archive/index.php/t-40320.html

baric
2011-Apr-27, 03:19 PM
It can potentially be significant. I'm pretty sure that the effective temperature of Earth is slightly below freezing. This means that, without atmospheric warming, this planet could conceivably be a snowball.

However, keep in mind that the first effect of an atmosphere will not be to enlarge the habitable zone, but to shift it away from the star (since any atmosphere with water vapor will provide some warming).

After that, the degree of warming will depend upon the particulars of the atmosphere. A low-CO2/CH4 planet like Earth will not have as much warming as a high-CO2 planet.

A radically high-CO2 planet like Venus isn't really comparable since its high-CO2 levels are the result of the loss of water.

IsaacKuo
2011-Apr-27, 06:56 PM
How much can the greenhouse effect extend the star-to-planet habitable zone for liquid water?

In theory, it can extend it outward all the way to interstellar.

Githyanki
2011-Apr-27, 08:40 PM
A Europa-like world that orbits a Jupiter-like world that got ejected from their system could have liquid water.

My guess, is around the Ice-line (where Jupiter is).

Amber Robot
2011-Apr-27, 08:58 PM
In theory, it can extend it outward all the way to interstellar.

Really? How would the greenhouse effect keep a planet habitable in interstellar space?

baric
2011-Apr-27, 09:32 PM
Really? How would the greenhouse effect keep a planet habitable in interstellar space?

Maybe a geologically active super-Earth? It stretches plausibility, but you might be able to make a case for it.

The problem is that if your atmospheric pressures get too thick to compensate for a lack of stellar heating, you start running into issues with common compounds becoming super-critical (ie CO2 and Venus). This would be some kind of planet classification we haven't found yet -- a Terrestrial Soup of some sort.

IsaacKuo
2011-Apr-27, 10:03 PM
Really? How would the greenhouse effect keep a planet habitable in interstellar space?
A thick hydrogen atmosphere could do it: Possibility of Life-Sustaining Planets in Interstellar Space - David J. Stevenson (http://www.gps.caltech.edu/uploads/File/People/djs/interstellar_planets.pdf)

More recently, University of Chicago researchers have studied the possibility of more generic planets retaining liquid water underneath a thick layer of ice, but that's a different insulating effect than the greenhouse effect.

baric
2011-Apr-27, 11:43 PM
A thick hydrogen atmosphere could do it: Possibility of Life-Sustaining Planets in Interstellar Space - David J. Stevenson (http://www.gps.caltech.edu/uploads/File/People/djs/interstellar_planets.pdf)


This is exactly the kind of Terrestrial Soup planet I was talking about. H2 at that pressure would be a supercritical fluid. What are the atmospheric characteristics of something like that, and how would impact the ability to support life?

In addition, you have to plead for a new formation mechanism to create a planet massive enough to retain H2 while not progressing to a gas giant stage. The authors are leaning on some coincidentally timed nebula ejections to do this. Not impossible, but definitely contrived.

WayneFrancis
2011-Apr-28, 12:39 AM
A Europa-like world that orbits a Jupiter-like world that got ejected from their system could have liquid water.

My guess, is around the Ice-line (where Jupiter is).

In astronomical time frames it would not have liquid water for long.

eburacum45
2011-Apr-28, 08:57 AM
For a planet with a particular escape velocity the outer edge of the habitable zone will be defined by the maximum feasible greenhouse effect. Kasting et al have suggested that the distance at which CO2 starts to condense and form clouds in the cool upper atmosphere would place an outer limit on the habitable zone;
http://www3.geosc.psu.edu/~jfk4/PersonalPage/Pdf/Icarus_93.pdf
...the effect of water vapour and of any other, unknown greenhouse gases would extend this outer edge, but not indefinitely.

Planets that have habitable zones in deep space (as suggested by Stevenson) rely on internal sources of heat for energy rather than relying on weak sunlight from a distant star.