Terra's Big Neighbor

[Drashner1]

In fact, with Solsys being popular among modosophonts for being the birthplace of terragenkind, perhaps a transapient would do exactly that, e.g. create a Counter-Earth and Counter-Luna opposite Earth in the same orbit, to create the most faithful Earth replica possible. (Barring simulations of course)

Based on some existing writings on the site, GAIA seems to limit the level of development in the Sol system to avoid potential disruptions to Earth's biosphere. So She would probably object to any attempt to build additional planets there. Possibly with godtech weapons.

Other systems may be a different matter however. The Ecotopia system has hundreds of planets in its Lab.

http://www.orionsarm.com/eg-article/48fca9d97f6c5

Todd

[Cray]

An Earthlike albedo of 0.294 gives an equilibrium temperature of 300 Kelvin.

How does albedo react to percentage cloud coverage?

An Earthlike atmosphere raises that to give a surface temperature of 334 Kelvin

What's the impact of atmospheric pressure on greenhouse gas? If the atmosphere was thinned to between 0.9 to 0.5 atmospheres, how much does the planet cool off?

[radtech497]

How does albedo react to percentage cloud coverage?

The albedo of a cloudless world is lower than that of one whose disk is obscured by clouds; the amount of change is related both to the reflectance of the clouds (clouds composed of nitrogen oxides reflect less incoming sunlight than do clouds made of water vapor or ammonia) and to the degree of cloud cover.

What's the impact of atmospheric pressure on greenhouse gas? If the atmosphere was thinned to between 0.9 to 0.5 atmospheres, how much does the planet cool off?

The three gases usually considered in calculations of radiative forcing, or the greenhouse effect, are carbon dioxide, water vapor, and methane (there are, of course, other gases, but their concentrations are usually to small to have much influence on the results). These calculations require three values be known about each gas: 1) the gas' fraction of the atmosphere by volume (f), the partial pressure (p) of the gas (which requires the mean surface pressure be known), and the optical thickness factor (k) of the gas (kCO2 = 0.029, kH2O = 0.087, and kCH4 = 0.609).

As one of the required factors, changes in the partial pressures (and thus the mean atmospheric pressure) change the amount of temperature caused by the greenhouse gases. If the pressure is increased, the influence of these gases increases, while for decreases in pressure the opposite is true.

As for the reduced pressures (0.9 and 0.5 atmospheres) requested, the temperature increases caused by the greenhouse effect are 40 K and 30 K, respectively, for an Earth-like atmosphere; thinning the atmosphere does result in lower mean surface temperatures.

Radtech497

[iancampbell]

There is very little correlation between magnetic pole changes and extinctions, so whatever happens during a change it doesn't seem to affect the biosphere much.

Indeed. As I said, slow enough to adapt. A similar thing can be said about the effect of the ice age cycle (which is approximately 30k years) on the survival of various plant species. Trees can move between geographic areas, if given enough time; what's actually happening, of course, is that areas that wouldn't support a particular species before become marginal and then ideal, as the climate zones change.

One of the potential problems with AGW is that the changes would be too fast for tree species to cope.

[Cray]

Alright, so let's narrow this down and get a habitable planet in or about Venus's location. I'm not actually after a retro-scifi swampy Venus. I'd prefer it to have a diverse range of terrain like Earth. And the "bigger than Earth" part can go if it's a problem.

I've been looking into "Habitable Zone Limits for Dry Plants" by Abe et al and "Towards the Minimum Inner Edge Distance of the Habitable Zone" by Zsom et al. Venus's 0.723AU is a bit close, especially for very Earth-like planets. So, how does the following combination of planetary characteristics work for Earth-like habitability, not to mention bothering Earth's orbit?

1) 0.8AU orbit. That cuts illumination to 82% of Venus. (Feel free to move it a bit to avoid a resonance with Earth. The further out, the better.)
2) Near-0 tilt. That should give larger ice caps and higher albedo, and help cool temperate latitudes.
3) Equatorial desert continents help raise albedo (see "...Dry Planets"), though you don't want to block ocean circulation.
4) Higher temperatures should generate more cloud cover.
5) 0.8-bar atmospheric pressure should lower the greenhouse effect.
6) Fast rotator like Earth. Call it 28 hours for giggles.
7) It'll probably need a moon to keep a magnetic field and maybe stabilize its tilt.

Could that bring average temperatures below 320K?

[radtech497]

Near-zero obliquity (axial tilt) would reduce the size of the polar regions (and the temperature gradients due to seasons). The obliquity sets the extent of the Arctic and Antarctic Circles.

Placement of deserts depends on atmospheric circulation and topography (mountain ranges forcing moisture in prevailing air currents to condense on the windward side before reaching the leeward side). Cold oceanic currents that inhibit the moisture content of the overlying atmosphere can produce deserts as well (e.g. the Atacama, Namib, and Kalahari Deserts). One way to produce widespread equatorial deserts is to ensure that the equatorial regions are covered by land that is distant from an ocean (where the oceans cross the equator, tall mountain ranges near the western coasts to force condensation into narrow coastal regions should suffice).

Radtech497

[Cray]

Regarding obliquity and planetary temperatures, some models indicate that global average temperatures drop with lower tilt. For example, in the following link:
http://physics.bd.psu.edu/faculty/willia...ja2003.pdf

Table 1 gives a range of different scenarios. Run "PRES0" with 0 tilt, present geography, present CO2 and levels lower average temperatures by about 3K. Shifting to a Sturtian glaciation-era geography drops temperatures a bit more.

Supporting discussion is on pages 10-12 and Figure 15, where it shows the ice caps - or glacial ice sheets - grow significantly at low tilts compared to Earth's actual obliquity.

Much cruder modeling also shows the same growth of ice caps and tundra for low tilts:
http://www.bbc.co.uk/blogs/legacy/23degr..._tilt.html

The issue seems to be that the low tilt leaves larger areas of the planet with permanently cool temperatures. Not necessarily freezing (e.g., Britain stuck at 7C year-round), but not something that encourages summer thawing because, y'know, there is no summer per se. Just permanently small air conditioning bills.

Are those two models incorrect in the growth of ice caps due to near-0 obliquity? If so, do you have any other, crunchy references showing the impact of low obliquity on planetary temperatures?

[four]

Question: Does the planet have to have an Earthlike climate throughout? Or just in parts of it?If the latter, how about a planet with two polar continents (covering the poles, extending to 50-70 degrees north and south), with an unbroken equatorial ocean. Planet about 2-3 earth mass, and at Venus' distance from the Sun. Planet has Earthlike atmosphere composition, thinner atmosphere creating more temp. variation, about 0.75 atm, avg. temp. 80-90 celsius at equator, 5-40 celsius on coasts of polar continents depending on latitude, interior of polar continents variable but often below freezing. There is a mountain range circling the south pole around 75 degrees south.
The oceans at the equator regularly evaporate, giving the Equator heavy cloud cover reflecting sunlight, and almost permanent rain, and these clouds and muggy air travel toward the poles and deposit their load in cooler areas.
Thus this planet features: Many Earthlike climates on the coasts of the continents, varying based on latitude and local topography, at the south pole little water gets through giving either an ice cap or Scandinavian climate, not sure which, and at any islands on the equator there will be very exotic forms of life. Perhaps To'ul'hian tweaks even. So a combination of Earthlike and alien climates.
Disclaimer: I am not a scientist, and this model could be entirely wrong. Just giving some ideas for you to work with if wanted.

[iancampbell]

Four - Another way of getting a planet some of which is habitable and some not is a planet with extensive high-altitude upland regions. Something about the size and height of the Tibetan plateau, on a planet otherwise generally like Earth except that sea level is about 2-3 bar of pressure and 60C or so, would probably be quite comfortable. (I'm not professional in this area, so the details might be wrong and probably are.)

In this context, I think it's worth mentioning that Venus has at least one area rather like that, at least in terms of height.

[Cray]

Question: Does the planet have to have an Earthlike climate throughout?

Well, it's obviously going to be warmer regardless. But the more Earthlike the merrier.

Or just in parts of it?If the latter, how about a planet with two polar continents (covering the poles, extending to 50-70 degrees north and south), with an unbroken equatorial ocean.

Equatorial oceans put low albedo terrain at the highest insolation area of the planet. The references I provided indicate you probably want desert terrain around the equator. You can put the cloud-generating oceans at middle latitudes. The poles, if the low tilt is cooperating, will be large ice caps.

Planet about 2-3 earth mass, and at Venus' distance from the Sun. Planet has Earthlike atmosphere composition, thinner atmosphere creating more temp. variation, about 0.75 atm, avg. temp. 80-90 celsius at equator, 5-40 celsius on coasts of polar continents depending on latitude, interior of polar continents variable but often below freezing.

Why not push for as Earthlike as possible before modeling data pushes back?

Four - Another way of getting a planet some of which is habitable and some not is a planet with extensive high-altitude upland regions. Something about the size and height of the Tibetan plateau, on a planet otherwise generally like Earth except that sea level is about 2-3 bar of pressure and 60C or so, would probably be quite comfortable. (I'm not professional in this area, so the details might be wrong and probably are.)

Putting high pressure at sea level is begging for a severe greenhouse problem. Why not make the whole planet more habitable with lower pressure?