# How does a planet-sized robot standing on a planet affect its gravity? [closed]

I originally asked this at the sci-fi exchange but was told it was off-topic as it was about real-world science.

Here is a picture of Unicron standing on top of the planet Cybertron, as written by Simon Furman and drawn by Geoff Senior in issue #75 of the Transformers comic.

Notice how Unicron is pretty much as tall of the planet's diameter. Assuming he was able to land there without causing the entire planet and himself to explode, wouldn't this have affected the planet's gravitational forces and possibly thrown it off its orbit?

• It would be a great nuisance to be a space faring robot with mass close to that of a planet. That massive, stuff would be gravitationally attracted to the robot and fall towards it. Just maintaining a non spherical physical shape would be difficult. Metal isn't strong enough to be a robot shape and a planet's mass. Unless the robot was largely hollow and as a result low density, it would be very problematic and probably impossible. Maybe this is better for world building? Nov 30 '20 at 3:46
• @userLTK If we replace the robot with an inert object that coalesces with the planet then it becomes an interesting orbital mechanics problem. All the time that it approaches it deflects the planet. But I guess the answer is (as always) yes, everything is always affecting everything gravitationally, and no, there's no specific orbit to be "thrown-off" of. So probably there's a good previous question to dupe this to.
– uhoh
Nov 30 '20 at 4:18
• Unicron (the robot in question) is able to transform into a spherical shape, so that would make it easier for him, I think. Nov 30 '20 at 14:17
• Voted to close. This has nothing to do with Astronomy. Worldbuilding.SE is the closest site, I think. Dec 30 '20 at 0:04
• I’m voting to close this question because planet sized robots are beyond the bounds of what is reasonable to manufacture. There can't be a reasonable answer to this that accepts the assumptions. and doens't invoke "magic" Dec 30 '20 at 0:23

Of course it would have had some effects! Gravity obeys the mathematical function:

$$F = {G\ m_1\ m_2\over D^2 }$$

where G is the constant of gravity (approx. $$6.67430 \times 10^{-11} N m^2 kg^{-2}$$), $$m_1$$ and $$m_2$$ are the masses of each body, and $$D$$ is the distance between each body (center-of-mass to center-of-mass).

Supposing the robot’s mass is about as much as the planet’s mass, it would definitely have thrown it off-course as it approached the planet—but, then again, the planet would also have affected the robot’s course.

It would not, however, have “affected the planet’s gravitational forces,” as the gravitational force exerted by a body depends only on its mass.

I would presume, though, that there are, on planet Cybertron (though we can’t see them on this image) bodies of water, or simply loose objects. These would have been affected by Unicron’s presence/approach, and would have been somewhat displaced by his presence/approach, as there was a force (basically) as strong as Cybertron’s gravity pulling them in the direction where Unicron was at that moment. Such objects on the opposite side of Cybertron may have been crushed (if they were not made very resistant), for example, and such objects on the side of Cybertron facing Unicron would have “lost their weight” (at least partly) and—no, not float—being easier to displace…

A lot is not know in the context of your question. Maybe reading the comix where this comes from would help know Unicron’s trajectory, etc., but I’ll leave that for others.

Be well!

• In the context of the comic, Cybertron does not have bodies of water, which is made clear by its inhabitants not knowing what water is. It definitely does have loose objects though, as there are plenty of vehicles and computers and things like that. It probably also has large liquid bodies but these are of some other liquid than water. Nov 29 '20 at 23:29
• So, as I said, these “mobile” things would mostly and definitely be affected. I also forgot to mention tidal forces. Parts of Unicron would obviously be closer to Cybertron than others — for example, on your image, is feet touch the planet, but his head is further. HE would be subjects to tidal forces, but so would Cybertron. This would have devastating effects on both. In other words, this is completely fictitious and wouldn’t be possible in real life. ;-) Nov 29 '20 at 23:32
• No reasonable assumption was made about the robot’s mass. Downvote. Dec 22 '20 at 17:45