Another Earth

A standard element of science fiction stories and movies is colonies on other worlds. In reality, of course, no planet in the Solar System is habitable and planets in other star systems that seem like they might be habitable are way too far to get to using current technology.

But what if there were an Earth-like planet in our Solar System? Imagine, for instance, another planet of about the same size and composition and with a breathable atmosphere in the same orbit. What if it’s exactly opposite us, so that if you picture Earth’s orbit as a clock, when Earth is at twelve, the other planet is at six. That would put the planet at about 186 million miles away.

Do we have the technology today to colonize it on a large scale? If not now, how soon could we do so?

And if this planet was directly opposite Earth, it would never be directly visible, since it would always be behind the Sun. So when, if ever, would we discover it? Would its impact on the orbits of the other planets lead to its assumed existence? Would it be visible in photos taken by an interplanetary probe (like the famous Pale Blue Dot photo taken by Voyager 1)?

It’s been done. Saw it at the Liberty Theater when I was a kid.

How does that broken link answer the questions posed in the OP?

The Earth II would affect the orbits of other planets. Unfortunately for Earth II, the other planets would also affect it, making its orbit unstable. Think of it this way - if we’re approaching Jupiter as we go around the Sun, Jupiter’s gravity is going to pull us more quickly around in our orbit. But Earth II would be past Jupiter, so Jupiter would be pulling it backwards, slowing it down in its orbit. Eventually these types of interactions would make Earth II visible, and at some point Earth II would end up colliding into Earth I, which would be very bad for both us and the intelligent apes that rule Earth II (according to some documentary I’ve seen starring Charlton Heston).

If you assume that Earth II is held in place 180 degrees away from us by Magic™, then we would have detected its presence from its gravitational interactions with the other planets, and its presence would have thrown some of our probes to Mars and Venus off course. And since that didn’t happen, we know Earth II isn’t there. There are also a few probes that we have sent out that would have seen it if it were there.

If Earth II happened to be significantly smaller than Earth I (in which case it wouldn’t be much of an Earth II), there are two stable points where it could hang out and not eventually crash into us, but these points are 60 degrees ahead of us and behind us in orbit, which would make mini-Earth II visible. Jupiter actually has a small collection of captured asteroids at these two points, which are called Legrange points if you want to google them for more info.

As for getting there, we certainly could, if we wanted to devote the resources to it. We have the technology now. You just need some heavy-lifting rockets to get the stuff into space. It’s probably easiest to do the final assembly of your spaceship in space, like how the ISS was assembled.

We have had the technology to do this since shortly after the moon landings. The only difference between the early 1970s and now is that we could engineer something that is quite a bit safer than what they had back then. It’s still not without risks. Getting there isn’t going to be quick, and that’s a long time to keep a bunch of people alive in a fragile little tin can floating around in space.

Apparently it’s been done at least twice!
Another Earth (2011)

I know; the thread title was not a coincidence.

Emphasis on the “least.”

There is at least one known Earth Trojan (and for every planet other than Mercury.)

I believe you are referring to the planet known as Gor.

Yes, I will admit that I read some of them when I was a horny teen.

You would have to take something like a Saturn V with you and land it on Earth II so that you can return.

To make this more interesting, lets say that the Moon is the other Earth. But to make it appear the size of the Moon, it’s going to have to be farther away, and both planets orbit each other around a common center. Tides will have the same magnitude, but months are going to be a lot longer, since the orbit is going to be slower.

We could land people on Earth II, the big problem is getting them home again. The Apollo landers could return because to escape the Moon’s gravity you need only a small fraction of the rocket propellant you need to escape Earth’s gravity.

For Earth II landers, as carnivorousplant says, if you want to get home you’d need to bring another Saturn V with you, or build another one when you get there. Good luck with that.

I imagine there’d be a few Elon Musk types who’d spend a billion dollars for a one way trip to Earth II, and I’m sure their adventures there would be the biggest media spectacle ever, until they died.

Thank you for your thoughtful reply. One question, though; you mentioned a (hypothetical) smaller planet at one of the Lagrange points. Does it have to be smaller? It is possible for a planet the same size as Earth to have a stable orbit at one of these points?

Since the Earth’s orbit is elliptical and not circular, it’s not possible for another planet in the same orbit to always be exactly 180 degrees from the Earth, even over the course of one year. When the Earth is at perihelion it will be moving faster than the counter-Earth which at the same time is at aphelion. I’m not sure if the deviation from the ideal path would be large enough (equal to the Sun’s radius as seen from Earth) to actually reveal the counter-Earth or if it would still remain hidden behind the Sun despite this.

An additional planet with Earth mass in an Earth-like orbit will render orbits dynamically unstable and would have long tossed the entire inner solar system into chaos, leading to one or more planetary collisions over the lifetime of the solar system. A large Earth-like planet in Earth’s current Lagrangian point would have so much gravitational influence that it would shift Earth’s current Lagrangian points. There would be a new set of Lagrangian points for the Earth - Earth II system. So, no, you cannot have another Earth at our Earth’s Lagrangian points and hope for a stable orbit. Any object that would like to reside in a planet’s Lagrangian for the long term would need to have significantly smaller mass than the primary. How much less is a question for the experts. Not a scientist, JMHO.

Why not borrow one of their Saturn A rockets for the return?

Also, why does everyone assume the other Earth would have evolved exactly like ours?

An excellent idea!

Well, of course it wouldn’t. But it would be fascinating to see what would have happened on Earth without the humans to muck up everything. Like those giant herds of bison that were all over the Great Plains in our world. Would they be there in the absence of humans? Or would the alpha predators that were in North America before humans hunted them to extinction have kept the bison population down?

Wait, so is Earth II so much like Earth I that it has North America II, and bison II? And if you took a bison II from Earth II and mated it with a bison I from Earth I, would it be interfertile?

Dude, that’s fine, but there better be buxom cave girls in fur bikinis on Earth II who are easily impressed by bespectacled middle-aged explorers from Earth I, or no sale.

As for using Lagrange points, you could do it with a klemperer rosette, but for that you’d want six Earthlike bodies in a hexagon around the sun, each in the lagrange point of 2 other bodies. The only problem is that this isn’t going to be 100% stable over millions of years. But if you’ve got the astroengineering chops to build one of these in the first place, a little station keeping every few hundred thousand years isn’t too much to ask.

The polarity is reversed! Didn’t you ever see the science documentary Doppelgänger?

Maybe there already was an Earth II. But since L3 is not a stable point, it eventually drifted around and collided with Earth I. But that’s pure science fiction, of course.