## two planets

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Quantum Jack
Posts: 13
Joined: Wed Nov 26, 2008 5:11 am UTC

### two planets

There are two planets in two solar systems in two galaxies.

On planet 1:

A civilization arises. They determine to explore space. They begin with the usual rockets, space stations, etc. Eventually they decide they should go all-out. They begin with a large space station in orbit, piecing on more and more bits, until it has so much gravity that, 1: launching rockets becomes easier, and, 2: the orbit is better described as a binary planet orbit.

Eventually, the station gets so big, that it houses the entire populace, and all that is left of the original planet is inert, unusable material. So they set sail to explore the galaxy.

On planet 2:

Civilization arises, they decide to explore space. Instead of building a space station, they reorganize the planet, move things around, dig caves/mines. build shields, rockets/other propulsive means. When their planet is good and ready, they take off.

The same, intergalactic space ship meets both of these. how do they decide which one is driving a planet, and which is driving a spaceship?

Lopsidation
Posts: 183
Joined: Tue Oct 27, 2009 11:29 pm UTC

### Re: two planets

Cool puzzle. Here's an answer that may or may not work.
Spoiler:
Think about the order the spaceships/planets were built in. The space station will have more advanced technology towards the outside, the planet will (probably) have more advanced technology near the center.

Who
Posts: 108
Joined: Wed Oct 02, 2013 2:53 am UTC

### Re: two planets

Spoiler:
Station 1 hasn't achieved hydrostatic equilibrium and is thus not a planet.

Station/planet 2 sounds as if it achieved hydrostatic equlibrium but depending on just how much they re-organized stuff, that may have changed its classification.

Also, they both need to be orbiting a star and they need to not have other shit in their orbit to even be possibly considered a planet.

Assuming they're both orbiting a star and neither has other stuff in its orbit, whichever one is a planet is the one which is a sphere due to its gravity rather than due to being built that way.

marzis
Posts: 29
Joined: Fri Oct 17, 2014 5:19 pm UTC

### Re: two planets

Who wrote:
Spoiler:
Station 1 hasn't achieved hydrostatic equilibrium and is thus not a planet.

Station/planet 2 sounds as if it achieved hydrostatic equlibrium but depending on just how much they re-organized stuff, that may have changed its classification.

Also, they both need to be orbiting a star and they need to not have other shit in their orbit to even be possibly considered a planet.

Assuming they're both orbiting a star and neither has other stuff in its orbit, whichever one is a planet is the one which is a sphere due to its gravity rather than due to being built that way.

I think that Quantum Jack is really asking if the third civilization can tell the difference between a planet-turned-spaceship vs. a spaceship of equivalent size, based on the process/exposure/organization/etc. In either of these situations, if condition A is present on the planet-turned-spaceship, it is either 1) indicative of it being a former planet, or 2) reasonable to assume it is present on the spaceship of equivalent size.

As to hydrostatic equilibrium, I believe both civilizations will have bypassed this through 'advanced technology', if for no other reason than I'm pretty sure if you calculate the force necessary to actually take a planet (or planet sized spaceship) and propel it anywhere, you're going to end up destroying the planet through all sorts of awful sheering forces or other such things, unless you have thoroughly covered it in 'advanced technology' which will solve that problem.

Quantum Jack
Posts: 13
Joined: Wed Nov 26, 2008 5:11 am UTC

### Re: two planets

I like Lopsidation's idea. Higher technology in areas built later. I didn't have a preset "solution" just thought it was a neat thought puzzle. I do like the idea of a space construction project with enough gravity to make transport between planet and space station. Even though it would cause many other problems.

ekim
Posts: 109
Joined: Mon Dec 18, 2006 12:40 pm UTC
Location: Seattle

### Re: two planets

Quantum Jack wrote:I do like the idea of a space construction project with enough gravity to make transport between planet and space station. Even though it would cause many other problems.
In What-If #58, XKCD wrote:This leads us to the central problem of getting into orbit: Reaching orbital speed takes much more fuel than reaching orbital height.

I'm curious whether a big space station like this (i.e. non-insignificant percentage of Earth's mass) would make it more or less difficult to ferry things between the two. If most of the fuel will be spent matching the station's speed, not escaping the Earth's gravity, the fuel you bring along to land on the station (to fight the station's gravity) is a big problem since that's extra mass you're bringing through the hard part. But:

• The Earth would lose mass to the construction of the station, so its gravity well would be smaller and a shuttle would need less fuel at launch
• The Station's mass would have its own gravity well requiring fuel for slow-down, increasing total shuttle mass, greatly increasing initial launch fuel
• The lower Earth mass would mean orbital speed (for a given height) was lower, so less fuel would be needed for the shuttle to match the station's velocity
• Wait, orbits are weird. I don't know what this does to our relative speeds/fuel requirements. Since we're building everything, maybe we can pick an orbit to minimize this?

I would cross-post this question to Science, but don't know how / if that's possible.

jestingrabbit
Factoids are just Datas that haven't grown up yet
Posts: 5967
Joined: Tue Nov 28, 2006 9:50 pm UTC
Location: Sydney

### Re: two planets

Yeah, this is maybe not so much a logic puzzle. To Fictional Science with you!
ameretrifle wrote:Magic space feudalism is therefore a viable idea.