Terra's Big Neighbor

[iancampbell]

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?

This is for use in a fictional work, right? Personally, I think that a planet a large proportion of which is uninhabitable makes for some interesting story possibilities.

Yes, of course there would be a strong greenhouse effect at sea level. As a more severe version of this, it is thought that the conditions at an altitude of about 40km in Venus's atmosphere are relatively benign. (As far as temperature and pressure are concerned, anyway.)

Incidentally, an atmosphere with a high proportion of N2 and a partial pressure of oxygen similar to Earth's would probably be useless to humans. NItrogen narcosis sets in at a relatively low pressure.