“Some prophecies are self-fulfilling
But I’ve had to work for all of mine
Better times will come to me, God willing
Cause I can’t leave this world behind” –Josh Ritter
You sure can’t leave this world behind. At least, not very easily. The reason for it, of course, is gravity.
Here on the surface of the Earth, the gravitational potential well is pretty large; large enough that there’s no easy way off. Sure, you can pour a huge amount of energy into a rocket to try and overcome this gravitational potential energy, but I think it’s more fun to think about what would allow us to leave for free.
In principle, bringing a large gravitational mass close to our surface might allow us to “jump ship” to the other world. Unfortunately, where we are, even our closest neighbor, the Moon, isn’t really all that close.
You might be used to seeing pictures like this, but these are false-perspective pictures, where the Earth and Moon are nearly lined up with the camera. If we illustrated the Earth-Moon system to scale, it looks far less impressive.
The Moon is practically useless for helping us escape the Earth’s gravity. It’s both small and low in mass, a combination that gives it a low surface gravity: just 17% of what we have here on Earth, and the Moon is quite far away, at least relative to the size of the Earth.
That might be the way it is here, in our Solar System, but as we’re just starting to learn, the Universe is an extremely diverse place, where every combination of planets and solar systems we can think of very likely exist.
For example, maybe there’s a solar system out there where it isn’t small, rocky planets that fill up the inner orbits, but a mix of Earth-like, rocky worlds and also gas giants, some of which are even closer to us than our closest planet, Venus, when it reaches perigee.
Wonder what that might look like?
From even a significantly greater distance than our Moon is, a gas giant planet, even a smallish one like Neptune, would appear huge and towering in the night sky. Our Moon, the biggest single object in the night sky (by angular size) at just over half a degree, would be dwarfed by the way a planet like this appears.
And a skyline would certainly have a different look to it.
Fortunately, we don’t have to speculate anymore! The star Kepler-36 was just announced to have two planets orbiting it: one is an Earth-like world just 50% larger than our planet, while the other is a Neptune-sized world, nearly four times our size and eight times as massive.
The truly amazing thing about these two planets is that they are so close together! While the Earth-like world orbits its star from a distance of 11 million miles, the Neptune-like world is just 1.2 million miles farther out, making these the two closest planets ever discovered in the galaxy!
Every 97 days, the inner, Earth-like world overtakes its next-door, Neptune-like world in its orbit. At that time, the planets are separated by a mere 1.2 million miles, or just 5 times the Earth-Moon distance. This is still too far to leap from one world to the next, but it’s on the right track.
You see, if you wanted another planet to come by, if its surface gravity was higher than Earth’s, we could literally get pulled off of our planet and get sucked into the gravitational well of the other world! If you brought the real Neptune close enough to the Earth, this is exactly what would happen.
Like Saturn and Jupiter (but not Uranus), Neptune has a higher surface gravity than Earth does. If the surface of Neptune came within about 1,000 miles (1,600 km) of Earth’s, the gravitational force from Neptune would exceed the gravitational force from the Earth, and objects on the side of the Earth facing the gas giant would be more attracted to the giant planet than to Earth, and would begin accelerating towards it.
Conversely, if you took a planet with lower surface gravity than the Earth and brought it close enough, someone on that world would find that our gravitational attraction was greater than their planet’s, and so they’d start accelerating towards our world!
Mars, for example, would only need to come within about 2,000 miles of Earth for someone on Mars to begin accelerating in our direction. There may even be alien worlds out there known as double planets (or binary planets) that are close enough for something like this to happen.
Of course, these are very very close distances for things as fragile as planet. Think about how small and far away the Moon is, and now think about how powerful the tides are on Earth. Now imagine replacing the Moon with something much more massive and about 100 times closer. Now, remember that gravity is an inverse square law, meaning that if it’s 100 times closer, the force is 10,000 times stronger.
In other words, it isn’t just you that will be accelerated towards an alien world; the world with the lower surface gravity itself will begin to be torn apart!
Earthquakes and volcanoes will certainly abound, but it’s more likely that even more severe changes would happen to such a world. It would be unlikely to be spherical, and — if they were comparable in mass — both worlds would likely be deformed by their mutual gravity.
It’s conceivable that even for the case of Kepler-36, the inner, Earth-sized planet has no atmosphere, because it’s been stolen, or siphoned off, by the close-by, larger-massed Neptune-sized world! Atmospheric thievery: in this Universe, it very likely happens!
It happens for binary stars, and there’s no reason to think there aren’t binary planets out there, too, for which the same thing happens.
So it might be hard to leave this world, but the more I think about it, the more I’m convinced that’s a very good thing!
(And if you think I got the idea to write about this from this week’s Futurama episode, I’ve just got one thing to say to you.)
All glory to the Hypnotoad!