[thedoc (OP)]

Jonathan asked the Naked Scientists:
   Is it technically possible for two planets to share the same orbit, as in exact polar opposites to each other travelling at the same speed (wouldn't want a collision!) or would this additional gravitational field somehow have an effect on other planets in a solar system?

I ask because i was wondering if it would be possible many years from now to move a moon or planet (yeah i know that would require a LOT of power!) into the same orbit as earth directly opposite on the same orbit of 365.25 days. I think Europa would be a good candidate, but i may be getting a teensy bit ahead of myself!
What do you think?

[chiralSPO]

I think you are allowed to get a little bit ahead of yourself in questions such as these.

I don't think there would be any obvious immediate danger to the balance of the solar system if Europa happened to be adjusted to an orbit precisely opposite ours. There would certainly be some minor changes for the rest of the Jovian moons.

However, the Europans would view this change in orbit as apocalyptic.

I doubt it would be very good for us Earthlings either. Europa is somewhat smaller than our moon, and has between 2x and 3x as much water as the content of the Earth's oceans. Since our moon doesn't have enough gravity to hold on to its water given the amount of sunlight it receives from the Sun, Europa certainly couldn't either if it were at the same distance from the Sun. As its water boiled away, it would presumably leave a trail of water vapor and ice in its wake, which the Earth would slowly capture. I think eventually the Earth would flood completely, and would be entirely covered in deep ocean!!!

I don't have any clue how long this would take, but it certainly would mean some pretty dramatic changes down here...

[evan_au]

Is it technically possible for two planets to share the same orbit?

Officially, you can't have two similar-sized planets in the same orbit, because since 2006, the definition of a planet now includes the requirement that it has "cleared the neighborhood" around its orbit.

exact polar opposites to each other travelling at the same speed?

At one time there was a theory that another planet could be "hiding" on the other side of the Sun from the Earth, as you suggest.

But astronomers were able to dispel this theory, because the tugs of the other planets would have sometimes pulled it far enough away from "exactly opposite" that it would have become visible in the morning or evening sky.

A polar opposite planet is fine in the short term (centuries), but is a real problem in the long term (millions of years), because these two large bodies are continually changing their orbits slightly, due to the tugs of the other planets.

Sooner of later, it is likely to approach the Earth closely. Although a direct collision is unlikely on a first approach, it is likely that they will both be disturbed into a fairly elliptical and chaotic orbits, which would be disastrous for life on these planets. It may end up with a collision with each other, or with Mars or Venus.

if it would be possible many years from now to move a moon...

There is another way that a small body can share an orbit with the Earth, and that is at one of the five Lagrangian points, labeled L1 to L5. Favorites are L4 & L5 points,  60 degrees ahead or behind the Earth in its orbit around the Sun. Because the other object is much smaller than the Earth, it won't disturb the Earth's orbit, and so there would be less disastrous consequences for Earth.

The L4/L5 orbit is still disturbed a bit by the other planets, but presumably if we had the resources to move a moon there, we would have the resources to keep counteracting the disturbing forces from Jupiter, Mars and Venus.

In fact there are a group of "Trojan Asteroids" that share an orbit with Jupiter.

There are currently some satellites orbiting the L1 point between Earth and Sun - very useful for Sun observation. These continually burn small quantities of fuel to keep themselves on station.

Since Lagrange did his work, several other semi-stable orbits have been discovered, including Cruithne, an asteroid that roughly shares Earth's orbit, continually approaching Earth from one side, then, hundreds of years later, approaching from the other side.