# The Naked Scientists Forum

### Author Topic: Could a planet be orbited by another planet instead of a moon?  (Read 5159 times)

#### JakubTyl

• First timers
• Posts: 9
##### Could a planet be orbited by another planet instead of a moon?
« on: 02/12/2013 14:06:43 »
Would it be possible for Earth like planet to have Larger planet instead of moon? As we know moon is important for life on the Earth but I try to find way how to compensate moon by something else. The scenario would be like this:
There would be no moon. There would be Large planet behind the Earth so the Earth would be between that Large planet and between Sun. This Large planet would orbit in the same way as Earth so they would always be in straight line (Sun-Earth-Large planet). Sun would have gravity pull on the Earth from one side and Larger planet would have gravity pull from the other side. That way there would be tide exchange as it is with moon. and the pull of sun and Large planet would stabilize Earth axis so it would not go crazy. The Larger planet would be placed in proper distance from earth to not consume it but just pull it in the right way.
Is that concept theoretically possible? Or how could you modify it to make it possible. In way that the planet could hold similar environment as Earth does today.
And would it be possible for life to exist if Earth would have 0 axial tilt as Mercury (but spinning), or it would have 90 axial tilt as Uranus (but spinning towards the sun so the planet would change day and night), or it would have 45 degree axis tilt? thank you
« Last Edit: 02/12/2013 23:43:58 by chris »

#### evan_au

• Neilep Level Member
• Posts: 4113
• Thanked: 245 times
##### Re: Large planet instead of moon?
« Reply #1 on: 02/12/2013 20:13:41 »
It is theoretically possible for an Earth-sized moon (with an atmosphere) to be in orbit around a Jupiter-sized planet - such an arrangement is often used in movies like Star Wars.  It is possible for the Earth-sized Moon to be habitable, if the orbit were close enough to the star, and the radiation levels were safe.

We haven't been able to observe different planetary systems with enough detail to know how common different arrangements are. But you can try building your own planetary systems inside a computer, and see how stable they are.

When you have more than 2 bodies in a gravitational system, the orbits become chaotic, and the results are often unpredictable, with one of the bodies plunging into the star, or being flung off into space.

The mathematician Lagrange identified 5 configurations of 3 bodies which give a stable configuration. They are labelled L1 to L5.

The one you describe with Earth between Sun and large planet is called L1. Unfortunately, the L1 point is unstable, and any small disturbance (like other planets in the system) will cause it to move out of this orbit. The L4 and L5 points are a better location, as they are stable against moderate disturbances.

It is important that the Earth rotates relative to the star, or one side would be baked, and the other side frozen. If the Earth were too close to the larger planet, Gravitational forces would tend to tidally lock the smaller body to the larger one.

#### CliffordK

• Neilep Level Member
• Posts: 6321
• Thanked: 3 times
• Site Moderator
##### Re: Large planet instead of moon?
« Reply #2 on: 02/12/2013 21:08:04 »
Our moon is one of the largest moons with respect to planet sizes, and the Earth/Moon is sometimes referred to as a binary system.  However, Pluto & Charon may be closer in mass to each other.

If a second planet the size of Earth was locked in orbit the same distance as our moon, assuming both were spinning, the tides would be huge.  If they were spin-locked (so, a revolution a month), then the tides would not be an issue, but the day/night cycle would be longer.  Or, one might just consider tides as only being an issue for coastal communities.

There would be a distance where a binary system would experience the same gravitational pull as Earth experiences from the moon, but as Evan mentioned, the orbits may not be stable with respect to other planets and the sun.

Tides would be lower if there was less water in the oceans, although tectonic activity might still be a problem with two earth-sized planets locked in a close binary orbit.

#### RD

• Neilep Level Member
• Posts: 8126
• Thanked: 53 times
##### Re: Large planet instead of moon?
« Reply #3 on: 02/12/2013 21:49:11 »
There would be no moon. There would be Large planet behind the Earth so the Earth would be between that Large planet and between Sun. This Large planet would orbit in the same way as Earth so they would always be in straight line (Sun-Earth-Large planet)

I don't think constant planetary alignment is possible, even if you restricted this solar system to 2 planets and a sun  ... https://en.wikipedia.org/wiki/Syzygy_%28astronomy%29

... the pull of sun and Large planet would stabilize Earth axis so it would not go crazy.

Crazy chaotic motion not inevitable ... https://en.wikipedia.org/wiki/Double_planet

#### JakubTyl

• First timers
• Posts: 9
##### Re: Large planet instead of moon?
« Reply #4 on: 02/12/2013 23:59:21 »

The one you describe with Earth between Sun and large planet is called L1. Unfortunately, the L1 point is unstable, and any small disturbance (like other planets in the system) will cause it to move out of this orbit. The L4 and L5 points are a better location, as they are stable against moderate disturbances.

Interesting, I was reading that wikipedia article.. But still I have question. What if that Earth like planet with moon (just same as Earth with same environment, people etc..) would be on the same orbit with Huge planet (size like Jupiter or so) and it would be located in L4 position to this huge planet. Would that anyhow affect that human like planet? Or it would be only nice view at some point of the day for the beings on Earth? Thank you

#### CliffordK

• Neilep Level Member
• Posts: 6321
• Thanked: 3 times
• Site Moderator
##### Re: Could a planet be orbited by another planet instead of a moon?
« Reply #5 on: 03/12/2013 00:35:09 »
Jupiter, of course, has Trojans and Greeks.  So, putting a large planet such as Earth into L4 or L5 would likely open the planet up for numerous asteroid collisions.  However, it is quite possible that a Trojan or Greek planet would mop up many of the other smaller asteroids, so by this time in evolution, most of the old asteroids would be gone.

The planet would likely not occupy a single point in L3/L4, so it would likely follow an orbital path that takes it closer and farther from the host planet.   Distances would still be quite long, but perhaps there would be significant tides on the closest approach to the large planet, and very small tides when farthest from the planet.

#### JakubTyl

• First timers
• Posts: 9
##### Re: Could a planet be orbited by another planet instead of a moon?
« Reply #6 on: 03/12/2013 00:42:52 »
Jupiter, of course, has Trojans and Greeks.  So, putting a large planet such as Earth into L4 or L5 would likely open the planet up for numerous asteroid collisions.  However, it is quite possible that a Trojan or Greek planet would mop up many of the other smaller asteroids, so by this time in evolution, most of the old asteroids would be gone.

The planet would likely not occupy a single point in L3/L4, so it would likely follow an orbital path that takes it closer and farther from the host planet.   Distances would still be quite long, but perhaps there would be significant tides on the closest approach to the large planet, and very small tides when farthest from the planet.

Yeah I meant like just the size of that host planet would be as Jupiter not that it would have also the Greeks and Trojans. It would be clear of that, just instead of Trojans and Greeks there would be Earth in L4. So it would not stay L4? It would shift on that orbit of that Jupiter size planet? That is confusing..  I thought that object in L4 the Earth like planet with moon would just follow the orbit of that Jupiter size planet constantly in L4 position, if the orbit is perfectly circular.

#### CliffordK

• Neilep Level Member
• Posts: 6321
• Thanked: 3 times
• Site Moderator
##### Re: Could a planet be orbited by another planet instead of a moon?
« Reply #7 on: 03/12/2013 01:24:08 »
When you look at the L4 L5 equi-potential diagrams:

You see the L4 & L5 describe essentially a loop path, which is also why they are relatively stable.  There may be a single point in the middle that wouldn't move, but many objects move somewhat.

Consider Asteroid 2010 TK7 sharing an L4 co-orbit with Earth.  Both it, and Earth share an orbit around the sun, but when the orbit is plotted with respect to Earth, the orbit looks like this:

So, in the first half of the year, it is closer to the Sun than the Earth, and thus orbits faster than the Earth.  In the later half of the year, it is farther from the sun than the Earth.

It should be in an eliptical orbit.

Hmmm....  is there something wrong with NASA's website which seems to show 2010 TK7 both in front of Earth, and behind Earth?

Ahh, the orbit for 2010 SO16, in L5 looks better.

#### JakubTyl

• First timers
• Posts: 9
##### Re: Could a planet be orbited by another planet instead of a moon?
« Reply #8 on: 03/12/2013 01:32:20 »
You see the L4 & L5 describe essentially a loop path, which is also why they are relatively stable.  There may be a single point in the middle that wouldn't move, but many objects move somewhat.

Oh I see now.. thank you.. and so the sizes of those bodies matter? I read somewhere that it may be unstable if the one body is too large as I was giving the example of Jupiter size planet and Earth planet. You think that it could work in with Earth like planet and Jupiter size planet or is there any size balance maximum of those two bodies on the same orbit to be stable? thank you

#### evan_au

• Neilep Level Member
• Posts: 4113
• Thanked: 245 times
##### Re: Could a planet be orbited by another planet instead of a moon?
« Reply #9 on: 03/12/2013 09:40:26 »
Let's assume that Jupiter is in a circular orbit around the Sun, at a distance of 150 Million km=100 Million miles = 1 AU = Earth's current orbit. (This would not be unusual - astronomers have now found many Jupiter-sized planets at 1 AU or closer to their star.)

You could place Earth at Jupiter's L4 or L5 point, where Earth would also follow a roughly circular orbit with radius 1 AU, which would make it a very comfortable average temperature for us.

The Earth would also be 1 AU from Jupiter, so Jupiter would exert a very weak gravitational braking effect on Earth's rotation, much less than the Sun or Moon does today. So Earth could maintain a 24-hour rotation, keeping the temperature range quite comfortable.

Jupiter would be a bright point in the sky, easily visible before sunrise or after sunset (depending on whether Earth was at L4 or L5).

However, if you placed the Earth at the orbit of Io, Jupiter would be a giant sky-filling orb (I calculate that Jupiter would be 19 degrees across as seen from Io, hundreds of times bigger than the Moon as seen from Earth). If Earth were tidally locked to Jupiter, the day would be a rather tiring 42 hours - but with a natural siesta every noon.

#### JakubTyl

• First timers
• Posts: 9
##### Re: Could a planet be orbited by another planet instead of a moon?
« Reply #10 on: 03/12/2013 10:38:09 »
Jupiter would be a bright point in the sky, easily visible before sunrise or after sunset (depending on whether Earth was at L4 or L5).

However, if you placed the Earth at the newbielink:https://en.wikipedia.org/wiki/Jupiter_moons#Table [nonactive], Jupiter would be a giant sky-filling orb (I calculate that Jupiter would be 19 degrees across as seen from Io, hundreds of times bigger than the Moon as seen from Earth). If Earth were tidally locked to Jupiter, the day would be a rather tiring 42 hours - but with a natural siesta every noon.
Thank you, Kind of what I wanted to hear :)
I am interested in that Earth being Giants moon idea. Often there are imagined Earth like planets as being moon of some Gas Giant. To have night/day cycle on both sides of that moon it has to orbit the way that it does not get behind Giant at any time or the whole planet would be in complete darkness for some time and one part would never see the sun. I was trying to find out 3d image of Ios path around Jupiter but did not found. It has to orbit in very special way kind of like our moon that it can take sun on both sides during the cycle of day. Or how that work in order to have day and night on both sides?

#### The Naked Scientists Forum

##### Re: Could a planet be orbited by another planet instead of a moon?
« Reply #10 on: 03/12/2013 10:38:09 »