If there was another small planet 92m miles from the Sun at 180° to Earth's position with a 365.25 day year...

Question

... how would we have known that planet existed at all?

Ineed, any planetary body that was ALWAYS on the other side of the Sun from us at all times?

I know it's an unlikely coincidence, but no more unlikely than the Moon being closer than the Sun to the Earth by almost exactly the same factor as it is smaller than the Sun (enabling perfect Solar eclipses to occur)... surely?

Thanks for all answers.

Answer 1

Not only would that be an unlikely coincidence, but such a planet would not stay there. The position exactly 180 degrees around the Sun from Earth is the Lagrange point L3, which is not stable. Over millions of years, small chaotic interactions with the gravity of the other planets, including the Earth itself, would cause it to drift to one of the stable Lagrange points, L4 or L5, 60 degrees ahead of or behind the Earth in its orbit.

Answer 2

The orbits of all the planets are not completely stable, they wobble and move around a bit over the years due to gravitational influences from the other planets (mostly Jupiter).
Even if there was a planet orbiting exactly opposite to the Earth, at some point in the 4 billion years of the solar system's existence, the orbit of one or the other of the two worlds would have changed a bit.
And with elliptical orbits, one small change can multiply over time (especially if the influence that caused the change is cyclical, meaning happens more than just once).
So it is not possible for another planet to be in our orbit but on the opposite side of the sun.

The moon size is purely coincidental, and simply a matter of timing.
When the moon was formed, it was closer to the Earth and would have appeared much larger to us than the sun would have.
But the moon is moving away from the Earth at about 3 cm per year, so in a few hundred thousand years or so it will appear smaller to us than the sun and we won't have total solar eclipses anymore.

Answer 3

The fanciful idea of there being a Counter-Terra is the creation of Science Fiction writers.

The key point is that we now live in an age of satellites and rocketry probes launched from earth that transmit photographs back to Earth. We can look round corners in effect. Nothing has been noticed lurking 180 degrees from the earth on the opposite side of the Sun by rockets that have travelled to the Sun. So there is nothing there and the whole idea is just so much pie in the sky.

The idea would only be possible if the counter-terra was tiny compared to the earth. The 5 Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects (such as a satellite with respect to the Earth and Moon).

The Lagrangian points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to rotate with them. They are analogous to geosynchronous orbits in that they allow an object to be in a "fixed" position in space rather than an orbit in which its relative position changes continuously.

However, Wikipedia says of the L3 Lagrangian Point:

"The L3 point lies on the line defined by the two large masses, beyond the larger of the two.

Example: L3 in the Sun–Earth system exists on the opposite side of the Sun, a little outside the Earth's orbit but slightly closer to the Sun than the Earth is.

Here, the combined pull of the Earth and Sun again causes the object to orbit with the same period as the Earth. The Sun–Earth L3 point was a popular place to put a "Counter-Earth" in pulp science fiction and comic books – though of course, once space based observation was possible via satellites and probes, it was shown to hold no such object.

In actual fact, Sun–Earth L3 is highly unstable, because the gravitational forces of the other planets outweigh that of the Earth (Venus, for example, comes within 0.3 AU of L3 every 20 months)."

Answer 4

For one, we would know it exists due to its gravitational pull on the nearest planets to it (Mercury and Venus) as well as the gravitational wobble the Sun would put exceed for a planet being that close.

I think it is unlikely there would ever be such a planet that near, as being only 92 million miles from any parent star would literally fry it to a charred crisp before destroying the planet outright from the high temperatures and strain it would undergo.

Such was the thought for a planet once conceived of as "Vulcan" around 100 years ago, but soon dismissed for those reasons.

Answer 5

We would notice its impact on other planets.

And that is just incorrect about the moon/size analogy. The degree of coincidence for a planet to be in just the right position and have just the right period to have not moved more than the sun's width in recorded history is far, far greater.

Answer 6

There was a movie about exactly this called "Journey to the Far Side of the Sun"(1969). It's available from Amazon.com.....Bascally, they shot an astronaut there and he returned but everything was reversed.....because at the same time, the other "twin" planet also shot an astronaut over from the other side of the sun.......cool movie