Posted on March 14th, 2014 by george.
Q: Is it possible for humans to exist on a planet at the L1 Lagrange point of a gas giant orbiting a star?
A: It’s definitely possible, but it’s not likely to be a stable orbit over the lifetime of a solar system. In fact, no Lagrange point is inherently stable except in special circumstances. Most of the time they require station keeping to counteract gravitational perturbations from other planets. Orbits around L4 and L5 last the longest, which is why you see so many Trojan asteroids leading and trailing Jupiter. L4 and L5 can be stable for billions of years if the body in the Lagrange point is 25 times less massive than the primary. Earth is 318 times less massive than Jupiter, so Earth could be a Jupiter trojan long term without interference. But the L1 point is not as stable as L4 and L5. And since there’s no such thing as station keeping for a planet, your planet would only stay in the L1 point for something on the order of millions, not billions, of years. This may sound like a long time, but it’s the blink of an eye compared to the necessary timescales for evolution, geology, and solar system lifespan. Intelligent life would not have time to evolve on a planet in L1 before it drifted out to wander the solar system. So you’ll need to explain the presence of humans there. A primitive species could have been dropped off by an intelligent race. Otherwise life would have had to evolve in a different orbit (likely a stable one to set the stage for life), and then once humans existed, the planet somehow migrated to this L1.
The problem is it’s difficult to destabilize a planet in a circular orbit, get it to a Lagrange point, then make it stay there. You would need to conjure a very unlikely scenario. Say, for instance, that another star passed nearby the Oort cloud of this hypothetical solar system and perturbed several planet-sized frozen bodies hanging out in the boonies. One massive interloper comes zipping in from the Oort cloud and whizzes by your planet, kicking it out to where it starts interacting with the gas giant. But that Oort cloud planet wasn’t the only one kicked loose, and another one comes careening through the inner solar system at the precise speed and trajectory such that when your planet reaches the gas giant L1, it whizzes by your planet and robs it of the exact amount of energy to stabilize its orbit in L1. This is a one-in-a-quadrillion shot (I made up those odds), but it is strictly possible. It also avoids the need for an impact. Impacts are often what would cause a planet to move around, spin up, form moons, and stabilize in a different orbit. The only other origins I can think of are: 1) if your planet starts out as a binary planet, the binary system is perturbed to L1, and another perturbation kicks off the twin, leaving your planet alone and quasi-stable at L1 and 2) a hyper-advanced alien race has the ability to move planets in their orbits without disturbing them, and does this to your planet just for fun.
Long story short if you want your humans to evolve on this planet they won’t be able to survive an impact-based restructuring of their orbit to L1. Most of the time an impact large enough to change orbital parameters will result in partial to total destruction of the original planet. Even if a collision could move the planet to L1 without destroying it, the impact would likely turn the entire surface of the planet into molten lava. So you’ll need to explain the migration with momentum transfers. This may sound fanciful, but keep in mind we know this happens for capturing moons. Our Moon is the result of a giant impact, as is Pluto’s moon Charon. Phobos and Deimos may be asteroids captured into Martian orbit, or may be the result of a giant impact. There is evidence that many small moons of Jupiter and Saturn are captured asteroids or Kuiper Belt Objects, and we are all but certain that Neptune’s moon Triton is a captured KBO. Triton is in a retrograde orbit, meaning it spins the opposite way around Neptune from the direction Neptune is spinning and from the direction Neptune is moving around the Sun. There is no way to explain a retrograde moon except capture, even though capture is very difficult to piece together gravitationally. Scientists are still studying how Triton was stabilized in its current orbit, whether it ejected a moon of its own, suffered an impact near Neptune, grazed Neptune’s atmosphere, or circularized due to tidal dissipation.
So your planet is possible, though it will only remain at L1 for a few million years at best. Based on our discovery of super-Earths you could make it much bigger than Earth and have all your humans be super strong. Or you could leave it the same size and avoid having to answer questions about all the hydrogen in its super-thick atmosphere. If you give your planet a 24-hour rotation period it will have the same day/night cycle as Earth, and if you give it the same axial tilt it should enjoy the same seasons. If the planet evolved in one part of a habitable zone and migrated to this L1 in another part of the zone (we’ll leave alone the stability of such an orbit so close to the L1 of a gas giant), your humans will have had to weather unfathomable climate change, either from a frozen planet to a steamy jungle/dry desert or vice versa, with snowball periods if the migration doesn’t start and end in a single orbit. It’s probably easier to go from warm to cold since you don’t have to pass the gas giant in the process and risk ejection from the solar system. If you make it a super-Earth it may be difficult to explain away the radiation from the gas giant. You might try claiming that the magnetic field is much stronger that Earth’s because the molten iron core is larger (if this planet happened to enjoy a core donation similar to the one Theia gave Earth). The problem is that the higher gravity of a super-Earth may preclude the interior differentiation that powers Earth’s magnetic-field-generating dynamo. Or you could just invoke magnesium oxide and say, “Trust me, it has a magnetic field strong enough to withstand gas giant radiation.” If this were the case, you could get amazingly intense aurorae on this planet, perhaps visible at much lower latitudes than we see them on Earth.
The phase of the gas giant would never change; it would always appear full at night, would always reach its zenith at midnight, and would never be visible at the same time the sun was visible. If the planet ends up following a Lissajous orbit around L1, you could see some interesting effects: the shadow of your planet might fall on the gas giant from time to time and trace different paths across its face. You may be aligned so perfectly that you’re in perpetual transit, your shadow a permanent beauty mark on the face of the gas giant.
It’s a very interesting idea. It just takes some explaining and your planet won’t stay there very long. But we’ve evolved from hunter gatherers to spacewalkers in only 50,000 years, so your humans could definitely go through their formative stages and reach Type II status before the planet was kicked out of L1.
Comment on September 25th, 2014.
Thanks as always for making the mind blowing understandable for the rest of us…..and beautiful too
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