Space/Science — HabitableZone

Space/Science » in reply to How do they know the radius?

Radius - depends on density which depends on compositionj which depends on....
n — (archived)

it isn't easy. In our own solar system the density of terrestrial planets and moons increases as they get larger, due to their larger gravfity and internal presure compressing minerals to denser forms. This effect is worth about 60% in density terms (Earth has an "uncompressed" density of 3.4 and an actual density of 5.9).

Different planets start from different baselines. Earth and Venus are very similar, but Mercury is denser and Mars lighter. If you think of it in terms of "cores", then Mercury has a very large metallic core whilst Mars has almost none.

A planet as light as 5 Earth masses would be unlikely to be a gas giant, in the habitable zone, because the lifetime of hydrogen and helium gas in its atmosphere would only be a few tens or hundreds of million years.

Of course, we have seen many reports of "Hot Jupiters" which are close to their sun and actively evaporating, but these systems are not stable in the long-term.

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