Oh, okay! So if gravity is a centrifugal force, and the moon is devoid of this, how come all those probes N.A.S.A. sent up there haven't fallen off....and Neil Armstrong wasn't sucked immediately into space instead of taking that first step (giant leap for mankind) on its surface
Actually the moon is one sixth the size of earth, has a solid rocky core, rotates about the same as earth does, hence the phases of the moon and it is this rotation the creates a centrifugal force that is one sixth the force earth generates hence...the moon has one sixth the gravity of earth.. Atmospheric pressures, or barometric pressure is caused by the amount of moisture contained in the air which lends it density. That the surface of our planet floats, as it were, atop a molten core...the rotational energy causes friction thus preventing the core from solidifying. This in turn prevents the tectonic plates from fusing into one solid mass. Volcanic activity is due to the instability that friction generates. As the continents move toward, away or from side to side one another the stress of that movement opens magma chambers, weakening the strata above and releasing the pressure built up within.
Planets which have been observed to consist of a single crust, and thus are devoid of tectonic plates, oceans, rivers and vulcanic activity, are said to have such dense atmospheric pressures and gravitational forces that they are incapable of sustaining life as we know it.
The moon has no atmosphere as the gravity well generated by it's curvature isn't strong enough to hold one. Planets Jupiter, Saturn, Uranus and Neptune are gas giants. Much larger and denser than earth, their mass alone creates a curvature in local space orders of magnitude greater than ours. The amount of mass and it's corresponding density determines how much space is curved around it.
A star two to three times the mass of ours will, at the end of it's life span, swell to what is called a red giant stage in which it begins to burn helium rather than hydrogen in the envelope surrounding it. This phase will continue for several thousand years, depending on it's mass. It will then implode, collapse upon itself, and wind up in one of three states, again, depending on it's mass. A brown dwarf, which our sun will become after four billion years or so, a neutron star if the mass is greater than three solar masses (three times our sun's mass), whose density is so great that a teaspoon full would weigh in at around a ton or thereabouts or, if the star is ten or more solar masses heavier could result in the creation of a black hole whose mass and density is powerful enough to slow light and, therefore time, to a standstill.
On a lighter note there is Drakes Equation......in our galaxy there 250 billion stars.
Ten percent of that could G-Type like ours, 25 billion. Ten percent of that, 2.5 billion could have planets. Ten percent of that #, 250 million could have planets capable of sustaining life. Ten percent of that could have sentient life(intelligent, self aware).
Ten percent of that, 2.5 million could have post industrial civilizations (like ours). Another tern percent or 250 thousand could be capable of space travel, even on a limited basis (like us). So whose to say. I think the possibility of at least a thousand ET cultures isn't so far fetched. As far their visiting here is concerned...only the powers that be have the answer to that.
