Celestia Forums

I'd been quasi working on a little setting material and had been advised, given my wish for an earth-ish like planet with an inordinately large 'moon' that would otherwise qualify as its own planet to have the following.

One Earth-Massed Body.

One body roughly the mass of mars.

One sitting at three times the distance the earth's moon is from the earth from the other.

What I'm wondering is the basic plausability of this, and if the tides on the larger body would in any way resemble those on earth, or if they'd be of the 'daily flooding' sort.

There is nothing implausible about your configuration, just unlikely. Its possible to have two earth like planets or larger orbiting a common berry center, and orbiting a star or even a binary star system. Obviously the more planets that you have and the closer they are together the more likely it is to be unstable.

Just off the top of my head, Mars is 8.7 times more massive then the Moon, so placing it three times further makes it about 8 times weaker. As for the tieds that is a vastly more complex question that depends on rotation of the planet, axis tilt and inclination of the "Moon" However it should all average out to about the same as earth, if in the identical configuration.

Hill sphere radius is given by [tex]r_H \approx a \left( \frac{m}{3M} \right)^{\frac{1}{3}}[/tex] where a is Earth's orbital radius, m is mass of Earth and M is mass of the Sun, which gives a Hill sphere radius of 1.5 million km. Prograde orbits are stable out to about a third of the Hill sphere radius or about 500,000 km.

Moon's orbital radius is 384,400 km which means your satellite at three times the moon's orbital distance is going to be unstable if they planet+moon system is located at 1 AU from a 1 solar mass star.

Its bene a long time since i've dropped by this thread. However since the answer still eludes me, anyone know a stable configuration for an earth-mars binary system that would remain reletivly stable (at least so far as the possibilities of life evolving on the 'earth' of the system is concerned anyway)?

Feel free to tinker with the companion's mass or the distainces. However the assumptions I'm making ofr the bit of fiction I'm writing are as follows:

A longer orbital period for the binary pair, currently working off one of the martian calanders for a 'year' timeframe. However since I don't know what the atmospheric composition would have to be for habitability by known complex life (I'm working under the presumption that the average temperatures are comperable to ice-age europe).

The 'mars' of the system is geologically dead, or at least its processes and movements are so slow that nothing has yet been observed. Also my only requirement that it be extraordinarily large, prefferably, if possible, the size of merceury (I forget how dense that planet is, but given its closeness to the sun I'm fairly sure it's dense for its size considering the heat there).

The 'earth' of the pair has two continents. One at the southern pole where antartica is now. And the other that's roughly the size of the american, african, and asian continents further up, straddling the equater. I'm unsure on what effect this has on local climate, and am willingto take smal llibrities with a plot device I plan on using. However the end result shouldn't be that without handwaving the place would be a gigantic iceball, or an atmosphere required to keep thigns warmed up enough for complex life being somehow lethal (I don't know what breathing a high carbon dioxide mix would do to lungs, even with a more tha nadiquit oxygen supply. I think that around nine or ten percent carbon dioxide the human brain starts goingwierd, but I could easily be wrong there).

The sun this pair is orbiting around is reasonably like our own, at least with a habitable zone large enough that this planet could orbit around, but without having to endure unreasonable amounts of UV/xray radiation.

As for the rest of the system. That i'm somewhat unsure of on specifics. However I had considered the idea of two planets orbiting closer than this binary pair, another terrestrial behind it, a 'jupiter', probably far more massive than ours (well, twice to five times jupiter's mass), and debris after that point. Play with the configuration if what I'm suggesting would produce something either unstable, or unsuitable to life forming in the target pair. The ability to not only currently sustain life via terraforming/introducing life but also for a naitive biosphere to have florished before being wiped out (current idea is a gamma ray burst goingoff close enough to have destroyed the ozone layer, and the parent's uv/other radiation doing the rest of the job).

I appologize if this seems more than a bit unreasonable and appreciate any constructive feedback.

Even though an Earth-mass / Mars-mass binary is unlikely, it is possible, through the same mechanism that created the Pluto-Charon binary. This type of binary could be created by a collision.

The hypothetical binary would be stable if the binary orbited an early F star. F stars are still considered to be 'solar-type'. let's look at at an example case of the binary orbiting an F2V star. An F2V star would have 1.4 times the mass of the sun, 4.5 times the sun's luminosity, and the habitable zone would be centered at 2.46 au. An Earth-mass planet in the central HZ of an F2V star would have a hill sphere of 3.68 million km, and prograde orbits would be stable out to 1.21 million km. For the hypothetical binary, the tides experienced by the earth-mass planet would be identical to Earth's if the two worlds are about 770,000 km apart, and the configuration would be stable.

I was reading a thread on Yahoo Answers about the impossibilities of terraforming Mars and mainly due to its inert magnetosphere. It would require pumping immense energy into Mars' core to bootstrap its convective engine.

Then I said to myself, "hmmmmmmmmmmmmmmmm", and I googled Earth mars binary and found this thread.

Great minds think alike, and I'd like to think that my mind is great... don't mind me taking credit for this, it's just a habit. Ok fine, it was your idea and your mind is great. But mine is too.

Now, I'd just like you to know that even though an Earth-Mars binary system is very creative and imaginative, if you make so much as a tiny glitch in your calculations, then everybody and everything is DEAD! What might be fun is to think of the ways it could happen:

• Binary orbit causes quite a massive shift in weather, greater temperature differentials for longer periods of time. If this tweaks the planet albedo too much, then Earth could fall into an ice age doomed to become a solid white golf-ball for millions of years. But this is a boring death, what else can we come-up with...
• Longer nights due to the weird binary orbit causes everyone to mutate into creatures like Tolkien's Gollum (without all the drama)
• Binary orbit becomes unstable and sends Earth hurtling too close to the sun and we're about to get fried, but in a desperate attempt to buy time, physicists manage to stop the Earth from spinning and for months it is eternal darkness for those still alive, while on the other side of the planet its so hot that everything is either dead, molten, or boiling.
• Tidal volcanism run a-muck on one side of the planet, too much volcanic activity, Earth becomes lopsided (like Mars is) and the magnetosphere collapses, northern lights become extremely frequent and vivid, atmosphere gradually thins more and more.