Gradually, the ebb and flow of gravity felt by the moon deforms it less and less, and the supply of frictional heat dwindles. The same gravitational forces that heat it up also mold its orbit into a circle. That water could come from chemical reactions with the carbon dioxide and hydrogen in the atmosphere, initiated by the impact of high-speed charged particles from space.īut such a moon won’t stay warm forever. A sufficiently thick, heat-trapping atmosphere, likely one dominated by carbon dioxide, might then keep the surface warm enough for water to remain liquid. In our own solar system, this process plays out on moons such as Saturn’s Enceladus and Jupiter’s Europa ( SN: 11/6/17 SN: 8/6/20).

Resulting friction inside the moon generates heat. Unless a moon’s orbit is a perfect circle, the gravitational pull of its planet continually deforms it.

Such orphaned planets might also have moons - and in 2021, researchers calculated that these moons need not be cold and barren places. Computer simulations suggest that there may be as many of these free-floating planets as there are stars in the galaxy. Astronomers have spotted about 100 starless planets, some possibly formed from gas and dust clouds the way stars form, others probably ejected from their home solar systems ( SN: 7/24/17). Habitability and stability don’t necessarily need to come from a nearby sun. “What we are looking for is places where these habitable conditions can be sustained for hundreds of millions, or billions, of years.” But life presumably also needs long-term stability. “There might be many places in the universe where habitable conditions can be present,” says Roccetti, of the European Southern Observatory in Garching, Germany.

A Moon Too Far by S.A. Garcia