Terra's Big Neighbor

[Cray]

Harry Turtledove wrote the novel, "World of Difference," where Mars is replaced with Minerva, a habitable world slightly larger than Earth. That got me thinking about a somewhat different alternate history.

Would it be feasible to park a habitable planet, perhaps somewhat larger than Earth, in Venus's orbit? Or is Venus's orbit too close to Sol for any long-term habitability?

Also, would a somewhat larger planet in Venus's orbit (say, 2x Earth mass) be a problem for Earth's orbital stability?

[iancampbell]

Hmmm... It's distinctly arguable that whatever caused Venus to have such slow rotation is what doomed it to its present state. The reason is that slow rotation means no magnetic field, which means no protection from the solar wind, which means all the hydrogen (and therefore all the water) eventually being driven off. Which probably leads to no plate tectonics.

Even with no life on site, a tectonically active world will have its CO2 continuously removed from the atmosphere by weathering of newly created rocks and deposition of carbonates on the bottom of the ocean. We'll never know, but I think that it's quite likely a rapidly-spinning Venus would be much as we used to think it would look - a steaming, wet place. Maybe even a steaming wet jungle.

[radtech497]

Also, would a somewhat larger planet in Venus's orbit (say, 2x Earth mass) be a problem for Earth's orbital stability?

No, a planet that size in Venus' orbit (assuming eccentricity and semi-major axis remain the same) would only extend the planet's gravitational reach Solward by slightly more than 1 percent (1.013%, or 0.691 AU) and Earthward by slightly less than 1% (0.945%, or 0.755 AU). The planet would, all else but mass remaining the same, probably be a bit brighter in Earth's sky (its radius would be 34.891% larger) than Venus is now, though the cloud cover might be less.

Radtech497

[Cray]

Alright, so orbital stability isn't a major issue.

Next issue: temperature control for an Earth-like ecology. I suppose you'd want higher albedo and a lower greenhouse effect, which means...less water coverage? More clouds? Somewhat thinner atmosphere?

[stevebowers]

By increasing the mass, you make it less likely that the planet will lose all its water. You will probably end up with a hot water world - something like To'ul'h, or a hot ocean world. Make it too big, and you have a hydrogen/helium atmosphere, and it becomes a gas giant. It is difficult to get a large planet in Venus' location to support an Earth-like environment, but it might support some interesting lifeforms.

[radtech497]

Next issue: temperature control for an Earth-like ecology. I suppose you'd want higher albedo and a lower greenhouse effect, which means...less water coverage? More clouds? Somewhat thinner atmosphere?

An Earthlike albedo of 0.294 gives an equilibrium temperature of 300 Kelvin. An Earthlike atmosphere raises that to give a surface temperature of 334 Kelvin (warm but still able to support at least some forms of Terran life); this higher temperature results in a higher water vapor content (more clouds, which increases the global albedo, cooling the planet). Result? A warm, humid world, possibly covered in tropical vegetation.

[four]

Once you are moving planets, many Earthlike planets can orbit the Sun. A wide variety of factors affect a planet's habitability, like atmosphere density, geological activity, albedo, stellar radiation received, rotation speed, water content, magnetic fields, moons, atmospheric composition, continent placement, and many other factors. I wouldn't be surprised if a conventionally habitable planet can fit round the Sun anywhere from Mercury's orbit to Saturn's orbit, just by adjusting the various parameters, perhaps to unlikely but possible values.
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)
For your original question, a 2-Earth-Mass planet opposite Venus in its orbit could easily be habitable. Low levels of CO2, oceans, atmosphere with many water vapor clouds, ecosystem designed to be light colours to reflect sunlight, continents at poles to create icecaps, etc.

[kch49er]

If a Venus-analouge spun faster but still retrograde(perhaps whatever slowed its rotation was bigger and caused it to stop spinning and staart going the other way) Woul this have had any effect on Venusian life? Would it cause the magneitc north and south to flip? Possibly confusing migratory lifeforms that navigated via it?

[iancampbell]

If a Venus-analouge spun faster but still retrograde(perhaps whatever slowed its rotation was bigger and caused it to stop spinning and staart going the other way) Woul this have had any effect on Venusian life? Would it cause the magneitc north and south to flip? Possibly confusing migratory lifeforms that navigated via it?

Can't see it. Planetary magnetic fields periodically switch polarity anyway - at least that of Earth does. The reason is unknown, but probably connected with the chaotic motion of the molten iron of the core.

As a matter of fact, some people think we are at the start of a polarity switch now, with the field getting a good deal more complicated than a simple dipole.

It's probable that the polarity switch is usually slow enough for most species that use magnetic navigation to adapt. However, the average field across Earth in the middle of the switch would be close to zero; which leads to all manner of interesting consequences including aurorae at the Equator and an increased mutation rate. Probably weather changes too, as solar protons get into the atmosphere.

[stevebowers]

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.