1. Member
Join Date
Nov 2006
Posts
25

## Earth sized moons?

Hi everyone,

I was wondering the other day; is it possible that a body the size and mass of Earth could orbit a planet the size of Jupiter? I know Jupiter is 318 times the mass of Earth; so I would think it's possible. However, is there something I might be missing? I have searched for this, but could not find anything. Thanks.

2. Over what period of time, and in what configuration?

If you mean, just Jupiter and Earth, nothing else, then they would of course orbit each other. Or do you mean, would an Earth-sized planet be able to fit in amongst the current Jovian population, and exist (along with Saturn et al.) for billions of years with no problems?

3. Originally Posted by orochi
Hi everyone,

I was wondering the other day; is it possible that a body the size and mass of Earth could orbit a planet the size of Jupiter?
An Earth-sized object could even orbit another Earth-sized object [in which case they´d share a common CG]*

*Assuming equivalent masses.

4. Looking at the Earth-Moon system, the Earth is 81 times the mass of the Moon, and is 4 times the diameter. That's a much smaller ratio than a Jupiter-Earth system. In fact, the ratio of the Earth-Moon system is smaller than any other planet in our solar system. In other words, our Moon is larger than any other moon in comparison to its planet.

Edit: Actually, the above is wrong if you still count Pluto as a planet. Pluto's moon Charon is 1/8th of Pluto's mass.

Here's an interesting article on Planet-Moon systems that I dug up.
Most satellites in our solar system are too small, compared to the planet they orbit, to put the balance point very far from the center of the primary body in the system. But the mass ratio of Pluto to Charon is just 8:1 (compared to typical planet:satellite mass ratios of 10,000:1), and the balance point of Pluto-Charon lies a few thousand kilometers above Pluto, toward Charon.

Although Pluto-Charon is the only recognized binary planet in the solar system, there are binary asteroids and binary Kuiper Belt Objects. Of course, there are also many binary stars in the galaxy as well. New Horizons is expected to be the first mission to visit any kind of binary object.
http://pluto.jhuapl.edu/science/ever...ry_planet.html

5. Established Member
Join Date
Aug 2006
Posts
432
Titan is as big as Mercury and happily orbits Saturn. Saturn is less than a third of the mass of Jupiter.

6. Originally Posted by dvb
Edit: Actually, the above is wrong if you still count Pluto as a planet.
<Dr. Zoidberg voice>
It is NOT!
</Dr. Zoidberg voice>

7. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
There is theory to suggest that gas giant moons should total just 1:10000th of the mass of the parent planet. That would imply that the parent would have to exceed 30 jupiter masses to have an Earth-sized moon: putting it in the brown dwarf range.

Grant Hutchison

8. For that math to work (the planet needing to be 30 jovian masses in order to have 1 terran mass in orbit), the total mass of the moons and rings around Jupiter right now would need to add up to only a thirtieth of the Earth's mass or less. But more than one of Jupiter's observed moons already beat that fraction individually.

9. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
Originally Posted by Delvo
For that math to work (the planet needing to be 30 jovian masses in order to have 1 terran mass in orbit), the total mass of the moons and rings around Jupiter right now would need to add up to only a thirtieth of the Earth's mass or less. But more than one of Jupiter's observed moons already beat that fraction individually.
It's an order-of-magnitude thing: a factor of two or three (which is what's involved for Jupiter) is not considered a significant deviation from theory.

Grant Hutchison

10. Order of Kilopi
Join Date
Jun 2006
Posts
7,156
I assume a jupiter sized object could capture an earth sized object and so have an earth sized moon.

11. Originally Posted by grant hutchison
It's an order-of-magnitude thing: a factor of two or three (which is what's involved for Jupiter) is not considered a significant deviation from theory.
I think Delvo's point is that--according to that weblink--a Jupiter size planet might entertain a Mars size moon, so an Earth size moon would only need ten Jupiter masses, more or less sub-brown dwarf. Why that makes a difference, I'm not sure.

12. Established Member
Join Date
May 2006
Posts
1,075
Presumably a Mars- or Earth-sized moon around a planet like Jupiter would be tidally locked.

Is it possible for a tidally locked moon to maintain an atmosphere?

(please excuse any visions of Ewoks this evokes)

13. Titan is tidally locked with Saturn. Yes, no problem there.
Unlike a tidally locked planet, a moon will still be rotating with relation to the sun, like ours!

14. Originally Posted by Ronald Brak
I assume a jupiter sized object could capture an earth sized object and so have an earth sized moon.
With the caveat that a third body would have to be involved, since a "two-body" capture is not possible!

15. Originally Posted by grant hutchison
There is theory to suggest that gas giant moons should total just 1:10000th of the mass of the parent planet. That would imply that the parent would have to exceed 30 jupiter masses to have an Earth-sized moon: putting it in the brown dwarf range.
Not exactly a theory, but a model. Our current level of data is far to small to come to this conclusion. Coincidence perhaps? I'm not in a position to draw any conclusions myself.

16. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
Originally Posted by dvb
Not exactly a theory, but a model. Our current level of data is far to small to come to this conclusion. Coincidence perhaps? I'm not in a position to draw any conclusions myself.
Well, the model had quite extensive theoretical underpinnings, if I recall the paper correctly. But I take your point about the small dataset that's currently available for us to judge accuracy or otherwise.

Best wishes to Thunder Bay, BTW: I worked there in the winter of '79-'80, and spent a lot of time skiing out at Big Thunder (which I guess may or may not still exist, after 26 years ...)

Grant Hutchison

17. Originally Posted by grant hutchison
Best wishes to Thunder Bay, BTW: I worked there in the winter of '79-'80, and spent a lot of time skiing out at Big Thunder (which I guess may or may not still exist, after 26 years ...)

Grant Hutchison
Thanks Grant, and have a happy new year.

OT: Big Thunder went bankrupt after the Nordic Winter Games came to town in 95 I think it was. We still have the other two ski hills though.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•