why are natural satellites always smaller than planets ?

Is it gravity alone ??
Any exceptions to this ??

Are you asking why all natural satellites in the solar system are smaller than any of the planets? If so, that’s not true, since Ganymede and Titan are bigger than Mercury. (In size, anyway…they both have less mass).

Or are you asking why all satellites are smaller than the planet they orbit? I think the obvious answer is that if the satellite were bigger, it would be the planet.

If the satellite was bigger, it would be the planet, and the planet, being smaller, would be the satellite. The larger body is going to have more momentum (m*v) at the same speed, so it is going to take the trajectory closer to the perfectly eliptical trajectory of the center of mass of the two body system (barycenter). Remember, the center of mass of the two bodies is going to form that eliptical trajectory, and if the more massive body is big enough relative to its satelite, the barycenter will be inside of the larger body. See here for some neat animations.

First, it’s not always true… Mercury (a planet) is smaller than Ganymede (a natural satellite) is in terms of radius/surface area/volume, although it is denser and therefore more massive.

A lot of it is strictly in terms of definitions - if there are a bunch of natural objects orbiting each other in close proximity, we pick the biggest one and say, “That’s a planet, and all the smaller ones are its satellites”

Also, many of the objects in our solar system that don’t orbit larger bodies and are smaller than a certain size have been redefined as ‘minor planets’, such as Pluto. That limit is below the size of the largest natural satellites we know of.

I’ll stop rambling at this point, since 2 other people have answered already.

Both Ganymede (D = 5268km) and Titan (D = 5152km) are larger than Mercury (D = 4879).

Has anyone mentioend that Ganymede and Titan are larger than Mercury yet? :smiley:

I also keep hearing a rumor that two moons of the first (closest to the sun) planet in the Solar System are larger than the planet they orbit. They apparently have a lower density.

Thanx for all replies. ignorance fought !
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Are you asking why all natural satellites in the solar system are smaller than any of the planets? If so, that’s not true, since Ganymede and Titan are bigger than Mercury. (In size, anyway…they both have less mass).*

yes , that is what I meant …

Just to avoid any possible misunderstandings: Ganymede and Titan aren’t satellites of Mercury, but rather of Jupiter and Saturn, respectively. Mercury doesn’t have any moons.

This is approximated for laymen as “the lighter body becomes the satellite and orbits the heaver body, which is the planet orbiting the Sun.” As Santo shows, like Newtonian physics that’s only an approximation of the truth: both bodies orbit a common center of mass, with the lighter body moving much further than the heavier one – in fact, the center of mass is generally within the heavier body. If the center of gravity of the Mars-Phobos-Deimos system is as much as a centimeter offet from the center of Mars, I’ll be very surprised. And Jupiter is so much larger and heavier than its satellites (in fact, than anything else in the Solar System other than the Sun) that, ceteris paribus, the same is true for the Jupiter+satellites system.

However, when the two bodies are closer in relative mass, the distinction between planetary center and barycenter becomes significant. In terms of astrodynamics, the Moon does not orbit the Earth; it orbits the Sun in an orbit dynamically lockstepped to that of Earth (52% Solar pull-48% Earthly pull). The barycenter of the Earth-Moon system is about 1,000 miles from the center of the Earth in the direction of the Moon. And if one were to obervse the Earth in fine detail from a point outside the Earth-Moon system, one would note the Moon making large rosettes around the Sun with Earth always at the midpoint of the arcs, while Earth makes tiny monthlong shimmies in its orbit around the Sun as it revolves around the E-M barycenter deep within its core.

The situation with Pluto and Charon, complicated by the two smaller moons, is even more extreme. Pluto is theorized to essentially lurch in its orbit while Charon orbits the Sun in lockstep with it – the same as Earth-Moon but even more so.

I’d say that if the barycenter is under the surface of the larger body we have a “planet/moon” system, if the barycenter runs above the surface then it’s a true double-planet. But it’s something astronomers will have to define someday.

They also need to define a size cut-off for a “moon”. Unless I am outdated, currently a softball sized rock would qualify for a “moon”. Thus, Jupiter, Saturn and Neptune (at least) likely have hundreds or thousands of “moons”. And Mercury could even have a few.:dubious:

Yeah, I was kinda wondering why they even entered into the discussion, since they have nothing to do with Mercury, but it was more fun to just go with the joke.

Mercury vs. Ganymede or Titan were cited as examples that it was not absolute size but nature of the primary orbited that defines “planet” vs. “moon”/“satellite” – As may have been mentioned here already, both Ganymede and Titan (satellites of giant planets) are larger than the planet Mercury. (Cue “Rio” by Duran Duran. :p)

:smiley: yes , I already checked a NASAlink.

I was just going to mention that, myself. If you ask me (which the IAU never does, for some reason), Luna has every reason to be called a planet, rather than a satellite.

Far more than that-- There are trillions of objects making up the rings of Saturn.

And if anyone’s worried about the point that Mercury is still more massive than any satellite in the Solar System, then you might be interested to know that Pluto, Eris, and their kin are all less massive than several of the larger satellites.

For those saying the Earth/Moon combo should be a double planet, and that the moon orbits the sun and not the earth, BadAstronomer tackled this one not too long ago.

Basically, the moon is contained within the earth’s Hill’s sphere, meaning that the moon is considered to orbit earth. If we we at the same distance from the sun as Mercury (aside from it beaing really friggin’ hot) the sun’s gravity would be enough to strip the moon away and send it off into its own orbit. But since we’re out here and not way in there, the moon orbits the earth.

And since I’m not an astronomer and bad at explaining stuff like that, go check out BA’s article in which he makes a great deal more sense than I just did.

Yeah, but they’re not as massive, so I’ve heard.