Do we know...

[JarodRussell]

... what's on the other side of the sun, directly opposite to Earth? And since when do we know it (which probe took a look at it)?

Would be funny if there's another planet or thing hiding in the same orbit around the sun. We would never be able to detect it from here since the sun blocks the view.

 

[RobertVA]

Orbital mechanics aren't that simple. Another planet forming in an orbit 180° out of phase with Earth would be extremely unlikely. Although gravity from planets like Mars, Venus and Jupiter has very faint influence over Earth's orbit, the differences between their influence over Earth and your hypothetical alternate planet would over a few centuries move them out of their hiding places. After a few million years there would be a risk the two planets would even collide.

 

[diankra]

The 'mirror panlet' is an old SF idea that is no longer credible. But I do like the film Dollepganger/Journey to the Far Side of the Sun.

 

[scotpens]

I don’t know from orbital mechanics, but is it even theoretically possible for more than one planet to share the same orbit?

Journey to the Far Side of the Sun, aka Doppelganger, had some gorgeous visuals and impressively realistic miniature work by Gerry Anderson veteran Derek Meddings. (And props, vehicles and costumes that were recycled in the UFO TV series.) But as a science-fiction story, it’s preposterous rubbish. It might have worked as an alternate-universe thing, like Star Trek’s MU. But a mirror-image Earth in the same orbit, with everything duplicated, including every person on the planet, only in reverse? Balderdash and bollocks!

 

[Shatnertage]

Yeah, we found out in 1986. It's Mondas.

 

[Christopher]

Another planet existing opposite Earth in its orbit is impossible for multiple reasons. For one thing, if there were something there, we'd be able to see its gravitational effect on the other bodies of the Solar System. We could've proven it was there centuries before we launched space probes to look at it directly, once we understood Newton's and Kepler's laws. Remember, you don't have to see a planet directly to identify its presence. Neptune's existence was predicted by its gravitational effect on Uranus's orbit. Most of the extrasolar planets we've discovered have not been detected visually, but are known only because of the "wobble" their gravity induces in the stars they orbit.

More importantly, the reason planets form in the first place is because things that share an orbit will eventually run into each other. Their precise orbits are constantly being tweaked by the gravitational effects of other bodies, so they speed up or slow down relative to one another and sooner or later they converge and either collide or get drawn together.

I don’t know from orbital mechanics, but is it even theoretically possible for more than one planet to share the same orbit?

There are two ways that can happen. One is if it's a pair of companion planets orbiting a common center of mass that in turn orbits the star -- like the Earth and Moon or Pluto and Charon. There has been some debate over whether Charon should be considered a companion dwarf planet to Pluto, and some have even argued that Luna is a companion planet to Earth (because it's actually twice as strongly affected by the Sun's gravity as it is by Earth's).

The other is if a smaller planet occupies the L4 or L5 Trojan point of a larger planet. The Trojan points are zones of gravitational stability formed by the interaction of other gravity fields; the competing pulls of the planet and the star balance out so that the L4 and L5 points function as "dips" in the gravitational landscape; anything at either point will tend to be pulled back toward it if it drifts away. There are plenty of asteroids at Jupiter's Trojan points as well as Neptune's, and probably smaller amounts in other planets' Trojan points. Theoretically it's possible that a planet-sized body could settle at a Jovian's Trojan point, though it's rather unlikely. The problem is that the early phases of planetary formation are extremely turbulent and it's unlikely that a Trojan planet would avoid being kicked out of the Trojan point. But it's not impossible.

 

[Silvercrest]

Yeah, we found out in 1986. It's Mondas.

I thought it was Gor!

 

[backstept]

Yeah, we found out in 1986. It's Mondas.

I thought it was Gor!

I thought it was Fire and Water!

 

[Skellington]

Haven't probes looked back at the Sun anyway? One would think that they'd have spotted a honking big planet where none should be.

 

[JustAFriend]

2009 - STEREO spacecraft returned images of the far side of the Sun.

http://www.space.com/scienceastronomy/090123-sun-far-side.html

BTW: Saw Journey to the Far Side of the Sun, aka Doppelganger when it first came out...

 

[diankra]

Just occurred to me... the Occam's Razor approach to this is why should Earth had an 'opposite' planet, and not Mars, Venus, etc? But if they did, we would be able to spot themir duplicates very easily, so we know they don't exist, and hence...

 

[diankra]

Just occurred to me... the Occam's Razor approach to this is why should Earth had an 'opposite' planet, and not Mars, Venus, etc? But if they did, we would be able to spot themir duplicates very easily, so we know they don't exist, and hence...

 

[Christopher]

^Well, it doesn't follow that just because the other planets don't have something, Earth can't have one either. After all, Earth is the only non-dwarf planet to have a satellite within an order of magnitude of its own size. So if it were physically possible to have a counter-orbiting planet at all, the lack of such objects in the other planets' orbits wouldn't rule out the possibility of one in our own. The real answer is that it's not physically possible for the reasons I spelled out above. It's just not a formation that would be gravitationally stable. Any two large bodies sharing the same orbit would inevitably either crash together or go into orbit of each other, if one or both of them didn't get jettisoned from that orbit first. The only exception would be an L4 or L5 planet, which would be only 60 degrees away rather than 180 and thus be easy to detect.