Right, but the context was a moon of a gas giant, and gas giants all have huge magnetic fields.
We just had a thread on this, but such an arrangement is unstable. The two planets wouldn’t stay directly opposite each other.
First of all, “just kicking out of the Earth’s gravitational pull” is far from trivial. Second, even if you did, you’d still be in an orbit around the Sun very similar to Earth’s orbit; you wouldn’t just be waiting around at a point in space. Third, even if you could somehow just wait around at a point in space (you can’t), you’d have to wait 6 months for the other one to catch up, not 12 hours.
It wouldn’t stay in that exact orbit for long, largely due to the gravity of the other planets but also due to the Earth’s gravity.
I can identify at least four possible misconceptions in this sentence. 1) Earth’s gravitational influence stops at distance near the Earth (it doesn’t); 2) if you could somehow “get rid of” the gravitational force of the Earth, you wouldn’t be moving along in its orbit any more (you’d still be moving relative to the Sun at the Earth’s speed, which you would need to cancel out somehow); 3) if you could cancel out the Earth’s speed, you could just hover in space (you’d fall towards the Sun instead); 4) the Earth goes around its orbit in 24 hours (it goes around in one year, so even if you could do all this you’d need to wait six months for the counter-earth to arrive.)
ETA: And it seems that Chronos said much the same thing.
You might be thinking of Jupiter. It has massive and complicated magnetic fields that produce very dangerous radiation belts, so the danger would be passing through those.
Conditions are pleasant around Saturn, Uranus and Neptune, radiation-wise, and some of Saturn’s moons are good life candidates as well as potential places to visit someday.
Although the situation where two planets are directly opposite in the same orbit is unstable, there are other possibilities. For instance the two moons of Saturn, Janus and Epimetheus, are in almost the same orbit, and periodically exchange enough momentum to swap orbital characteristics.
Another possibility is a 1:1 resonant relationship with a gas giant, which itself might have a habitable moon (or moons). An Earth-like planet could possibly remain in such a 1:1 resonance for many billions of years.
By a 1:1 resonance, you mean a Trojan point? A gas giant with a habitable planet each in its leading and trailing Trojan points should work just fine. Any of the other Lagrange points would be unstable, though.
As I understand it, what is I think termed a “Klemperer rosette” is quite possible: six Earthlike planets at the vertices of a regular hexagon in the same orbit, so that each is in the Trojan point of two others. Such an arrangement would not form naturally without a third body, e.g. a Jovian or Superjovian in the next orbit out and in resonance with the rosette, nudging the individual planets into the Trojan points, but once formed it would be stable.
Re post #2, it’s my understanding that Mars is just within the "Cinderella zone:, but that its rela6tively small size (and hence lower escape speed), lack of strong magnetosphere, and minimal internal heat work together to keep it from being Earth-habitable; that terraforming Mars is *relatively]/i] simple (granted that any terraforming project on a whole planet is a barely-imaginably-huge project)
IIRC Trojan points only work when one of the bodies is significantly larger than the other; so you might have a habitable planet following a gas giant, but two habitable planets are not dynamically stable.
Just two habitable planets might not be, but two habitable planets, one each leading and trailing a gas giant, would be.
Sleeping Beauty zone…
I thought the problem with Mars was its small size which prevents (or caused it to lose) a substantial atmosphere, and the problem with Venus was its extremely high surface temperatures caused by greenhouse gases.
In other words, we’d have three habitable planets right here in our own solar system if Mars and Venus had different atmospheres. So, theoretically at least, our solar system proves multiple habitable planets are possible.
Several different 1:1 resonances are examined here
The typical L4/5 Lagrange orbits are the ‘tadpole’ orbits, I think; horseshoe orbits and the circular/eccentric pairs described in that paper are a little more exotic.
L4/5 is a degenerate case of a tadpole orbit, where the tadpole is shrunk to zero size. A horseshoe orbit is what you get when the leading and trailing tadpoles grow big enough for their tails to come together at L3. I don’t know anything about the eccentric ones.
The Straight Dope is the only reliable provider of extraterrestrial weather forecasts.
<Checks> It’s not actually stable; the arrangement lasts fine if there’s no perturbations, but anything that does perturb the orbit of one of the planets causes the rosette to undergo increasing instability.
Not much of problem for an artificial version as seen in Ringworld; presumably if you can move planets like that in the first place, you can probably correct minor perturbations in their orbits as well. But it does mean you won’t see a natural version.