Lets just say both planets travel at the same speed to avoid collision and the other planet is on the opposite side of the sun?
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Two planets sharing a single orbit is not a stable configuration. Even if they did start off on perfectly opposite sides of the Sun, the slightest gravitational nudge (say, from a third planet in the solar system) will upset the balance and the two will gravitate towards each other, falling out of 180° opposition and eventually colliding. The leading hypothesis for how the Moon was formed is that the Earth collided with a smaller planet which shared its orbit very early in the history of our solar system, within a few tens of millions of years. We’re past four *billion* years without any collision now.
Also, we’ve sent numerous robotic spacecraft across the solar system that would have seen such a planet if it were there. The famous “Pale Blue Dot” photo of the Earth as a single blue pixel was part of a wider overview of the Solar System in which a counter-Earth would have been plainly visible. From Mars, Earth shines brightly in the evening skies like Venus does from Earth; any martian rover would have spotted a counter-Earth very quickly.
We would notice its gravitational effects on other objects. We’ve also sent plenty of space probes that would have directly observed such a planet, but we wouldn’t have needed to to detect it. [EDIT: and as /u/antithesys correctly notes, Earth’s orbit is elliptical and varies in speed, so the two objects would wobble around that 180-degree alignment – although I think Earth’s orbit is actually close enough to circular that the other body would never be visible.]
But it turns out we don’t even need *that*, because the orbital configuration you’re describing isn’t stable. Any unevenness away from exactly 180 degrees apart will tend to grow, and the objects will end up taking [horseshoe orbits](https://en.wikipedia.org/wiki/Horseshoe_orbit) (from the perspective of the other object) that would render them visible at times.
Such a configuration exists today for [two of Saturn’s moons](https://en.wikipedia.org/wiki/Epimetheus_(moon)#Orbit), which share an orbit but trade-off for which moon is “ahead” and which is “behind” in the orbit; the two moons come quite close to one another but never collide.
In fact, it’s believed that this *did* happen for Earth in the early Solar System. Earth is believed to have shared its orbit with a Mars-sized body called [Theia](https://en.wikipedia.org/wiki/Theia_(planet)). But unlike the Saturn system, perturbations from the gravity of the other planets made Theia’s orbit unstable with respect to the Earth, and eventually led to a collision that was probably the most violent event in the history of our planet. The resulting debris formed Earth’s Moon.
Two planets sharing a single orbit is not a stable configuration. Even if they did start off on perfectly opposite sides of the Sun, the slightest gravitational nudge (say, from a third planet in the solar system) will upset the balance and the two will gravitate towards each other, falling out of 180° opposition and eventually colliding. The leading hypothesis for how the Moon was formed is that the Earth collided with a smaller planet which shared its orbit very early in the history of our solar system, within a few tens of millions of years. We’re past four *billion* years without any collision now.
Also, we’ve sent numerous robotic spacecraft across the solar system that would have seen such a planet if it were there. The famous “Pale Blue Dot” photo of the Earth as a single blue pixel was part of a wider overview of the Solar System in which a counter-Earth would have been plainly visible. From Mars, Earth shines brightly in the evening skies like Venus does from Earth; any martian rover would have spotted a counter-Earth very quickly.
Modern ephemerides (orbital predictions) are based on extremely precise numerical simulations that model the pull of the planets on each other. The forces involved are much smaller than that caused by the sun but are still large enough to cause detectable effects in the planets’ positions. An anti-earth, apart from being an impossibly unlikely coincidence, couldn’t exist without us seeing effects which we’re not seeing. The only out would be for the body to be very small and then it wouldn’t count as a planet.
Two planets sharing a single orbit is not a stable configuration. Even if they did start off on perfectly opposite sides of the Sun, the slightest gravitational nudge (say, from a third planet in the solar system) will upset the balance and the two will gravitate towards each other, falling out of 180° opposition and eventually colliding. The leading hypothesis for how the Moon was formed is that the Earth collided with a smaller planet which shared its orbit very early in the history of our solar system, within a few tens of millions of years. We’re past four *billion* years without any collision now.
Also, we’ve sent numerous robotic spacecraft across the solar system that would have seen such a planet if it were there. The famous “Pale Blue Dot” photo of the Earth as a single blue pixel was part of a wider overview of the Solar System in which a counter-Earth would have been plainly visible. From Mars, Earth shines brightly in the evening skies like Venus does from Earth; any martian rover would have spotted a counter-Earth very quickly.
We would notice the gravitational effects. Like an asteroid going around near this other planet we can’t see, once we find the asteroid again we would notice it got deflected onto a new path.
Also not to mention our own orbit isn’t stable. Everything with mass effects everything else with mass gravitationally. Jupiter, Earth, the Sun, Pluto, they all change how things move throughout the solar system. In the 4.5 billion years our solar system has existed the planets have flung each other around the solar system and where we are now is just what’s survived, but there’s no guarantee it will last forever. If there were another planet opposite the sun, it probably wouldn’t stay there, and while there is a Lagrange point there, it’s an unstable one, so if the planet were nudged slightly out of place (a very likely thing to happen) it would then continue moving away from that point. If something were there, it wouldn’t stay there for long.
We would notice the gravitational effects. Like an asteroid going around near this other planet we can’t see, once we find the asteroid again we would notice it got deflected onto a new path.
Also not to mention our own orbit isn’t stable. Everything with mass effects everything else with mass gravitationally. Jupiter, Earth, the Sun, Pluto, they all change how things move throughout the solar system. In the 4.5 billion years our solar system has existed the planets have flung each other around the solar system and where we are now is just what’s survived, but there’s no guarantee it will last forever. If there were another planet opposite the sun, it probably wouldn’t stay there, and while there is a Lagrange point there, it’s an unstable one, so if the planet were nudged slightly out of place (a very likely thing to happen) it would then continue moving away from that point. If something were there, it wouldn’t stay there for long.
We would notice the gravitational effects. Like an asteroid going around near this other planet we can’t see, once we find the asteroid again we would notice it got deflected onto a new path.
Also not to mention our own orbit isn’t stable. Everything with mass effects everything else with mass gravitationally. Jupiter, Earth, the Sun, Pluto, they all change how things move throughout the solar system. In the 4.5 billion years our solar system has existed the planets have flung each other around the solar system and where we are now is just what’s survived, but there’s no guarantee it will last forever. If there were another planet opposite the sun, it probably wouldn’t stay there, and while there is a Lagrange point there, it’s an unstable one, so if the planet were nudged slightly out of place (a very likely thing to happen) it would then continue moving away from that point. If something were there, it wouldn’t stay there for long.
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