eli5 How do we know there is not an alternate planet in the same orbit as the earth?


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?

In: 0

* The Earth’s orbit is elliptical. This means the other planet wouldn’t always be precisely behind the sun from our perspective.
* We have sent a number of probes into the outer solar system, some of which have taken composite panarama photographs of the entire system, and they have seen nothing.
* Each of the planets has a measurable gravitational effect on the other planets as well as on asteroids and comets. A planet on the other side of Earth’s orbit would be affecting Venus, Mars, and comets/asteroids in ways we could detect. None of these bodies behave in ways we can’t account for.
* None of the other planets in our solar system have “twins” in their orbits, and no planets we have found so far in other star systems have either. Why would Earth be unique in this regard?

The orbits of planets and other bodies in our solar system are carefully studied and modeled. We need to be very good at this to for example launch something from Earth and hit a specific point on Mars with a probe.

Everything with mass has its own gravity and pulls on everything else. If there was a planet in the L3 point then it would be pulling on everything else in the solar system and those effects would be easily spotted. It would introduce an unexplained element in the calculations and someone would very quickly figure out what must be causing it. We have also launched probes which could directly observe the area as well.

So unless this planet is invisible and massless we can be pretty sure it doesn’t exist.

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.

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.