Because Earth is not the only planet in the solar system. The gravity of the others would gradually affect such a planet, bringing it into view in no more than about 30 years. Also, it would have gravity of its own, and would have been discovered by the pull it exerted on the rest of the solar system, including Earth!
Not a planet of any significant size, which would be easy to see, but it is possible that some pieces of rock are in a similar orbit to the Earth at the five Lagrangian Points of the Earth's orbit.
Venus is closest to the Earths size and mass - often referred to as Earths sister or twin planet. Its also the planet that gets closest to Earth, the next one orbiting inside earths orbit.
Pluto
the earths orbit because the earths orbit is the size of earth + the size of the moon
It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation.
Venus
Neptune is the outermost planet, and is farthest from Earth's orbit.
Not a planet of any significant size, which would be easy to see, but it is possible that some pieces of rock are in a similar orbit to the Earth at the five Lagrangian Points of the Earth's orbit.
Venus is closest to the Earths size and mass - often referred to as Earths sister or twin planet. Its also the planet that gets closest to Earth, the next one orbiting inside earths orbit.
pluto
Pluto
Neptune, when Neptune is on the opposite side of its orbit from Jupiter.
No. A planet's mass does not determine the position of its orbit.
its orbit crosses with another plant
We call that path the "orbit" of the orbiting body. Note: You would not ever see a planet orbiting another planet. At least, if you did, you would not call them both planets.
the earths orbit because the earths orbit is the size of earth + the size of the moon
A moon is an object that orbits a planet. A planet cannot orbit another planet.