Expert: Jayendra Upadhye - 5/4/2008

Question
Hypothetically, is it possible, given the influence of a larger body on a smaller body, that a planet and its moon would share enough characteristics that both would be habitable by the same people?  

Answer
Hi,
The universe is very very large.
That in itself makes all sorts of things possible.

In the case of the earth, and the solar system, in its early stages of evolution, the late bombardment phase occured, when debri from the outer solar system came careening into the inner solar system, the resultant heating of the the surfaces of the inner planets blew off  the atmospheres of smaller planetary bodies, as their gravities proved unequal to the task of retaining them.

That is how the moon lost its atmosphere. Mars too slowly lost its atmosphere to attenuation by solar radiation, after its core cooled completely and it lost its magnetic shield.

However, elsewhere, it is easy to contemplate a world where the satellite is earth sized, with oceans of liquid water, and Oxygen in its atmosphere, and the main planet say 4 earth masses, and also having liquid water and oxygen, and an axial tilt to bring about seasons, and each with a strong magnetic field supported by a liquid convecting core.

Such a world is indeed possible. Interesting possibilities emerge.
1 - One world develops humanoids first, and is considered a place of pilgrimage where the "gods once resided".

2 - Two cultures evolve simultanously, with one colonising the other, and eventually the colony frees itself, and they have an interplanetary UNO!!

3 - The ion tail of the satellite and the ion tail of the main planet lash each other periodically once a month, triggering massive displays of corona borealis even at the tropics! And triggers many popular religious myths and gods!

4 - One planet evolves bi-symmetrical organisms like this earth, whereas the other has a hexa-symmetrical arachnid species, and both are involved in a perpetual titanic struggle.

5 - As it becomes difficult for artificial satellites to go around just one planet, they are forced to put satellites that either go around both, or do a figure 8 around them!!

6 - To compound matters, there is a 3rd planet going around the first two, at an angle with the solar-system plane! (a captured rogue that itself is also habitable)! And which is slowly alligning its orbital plane with the rest of the system.

Think..
The possibilities are endless!

Variety is indeed the spice of life.

regards.
Do rate the answer if you find it meaningful.
Jayen