When the Imperials went looking for a quiet spot to host Emperor Palpatine’s personal retreat, the Forest Moon of Endor must have seemed a natural choice. It’s 46,000 light-years from the office, populated by cuddly locals, and features verdant woods which are completely dissimilar to the redwood forests of upstate California. No wonder this treechanger’s paradise also goes by another name – the Sanctuary Moon.
Okay, so it wasn’t all forest walks and fishing trips. The Imperials turned Endor into a serious construction site, displacing the local Ewoks, felling trees, causing widespread seismic disturbances, and generally exhibiting a flagrant disregard for local planning laws. Everything pretty much went down like an episode of Grand Designs, except in this case the grand design involved turning Luke to the dark side and wiping out the Rebel fleet. Also, no-one got pregnant (although…)
But what about the Forest Moon itself? What do we know about it? Could such a promising spot for a couples therapy retreat really exist in our galaxy?
Building a Planet, Step One: Obeying the Lore
What do we know about the Forest Moon of Endor? A surprising amount, actually:
- The Forest Moon is one of nine moons of the gas giant Endor, which in turn orbits two stars, Endor I and Endor II, with an orbital period of 402 Earth days.
- At least one of its local stars has the the same surface temperature and apparent magnitude as Sol (Earth’s local star).
- It has an approximately Earth-like day-night cycle.
- It has a diameter of 4,900 km (or a radius of 2,450 km). For comparison, that’s about two fifths of Earth’s.
- Its surface gravity is strong enough to make rocks dropped from a glider an effective weapon for toppling intergalactic space empires, but weak enough to keep C3PO from breaking when he falls out of a tree.
- It is mostly forest, swamp, mountain, and savannah. 8% of the surface is covered in water (Earth has about 70%). It has blue sky and water clouds and an atmosphere breathable by Harrison Ford.
- It has three sentient species. In addition to the Ewoks, there are also Duloks (swamp-dwelling carnivores) and Yuzzum (horrific CGI jazz singers).
- Ewok jerky is a favoured snack throughout the Outer Rim.
That’s quite a bit to go on. Unfortunately, that makes filling in the gaps all the more difficult.
Building a Planet, Step Two: Obeying the Law(s of physics)
There are three oddities about the Forest Moon that we need to explain:
- It has a high surface gravity (approximately Earth-like) despite the planet as a whole being rather small.
- It seems a little on the large side for a moon, but nonetheless persists in a stable orbit around a gas giant in a binary star system.
- It has a breathable atmosphere, a temperate climate, and sufficient rainfall to sustain all those forests, despite having no oceans.
These three features set the Forest Moon apart from any planet that we know of in our own solar system. Such observations imply some very idiosyncratic features for the Ewok home planet. Let’s look through them one at a time.
Endor’s been in the wars
It seems that the Death Star was not the first interloper to threaten the Forest Moon.
For the Forest Moon to maintain a relatively high surface gravity, it needs to be at least as dense as Earth. But small planets exert less pressure on their interiors, and tend to be a bit puffier as a result. This means the Forest Moon must be made of heavier stuff than the Earth – a lot more iron, a lot less silicon and oxygen. How probable is that?
It turns out we have a great example close to home – Mercury. Mercury is about the size of the Forest Moon while maintaining a density close to Earth’s. Its iron core makes up almost three quarters of its volume – way more than ours. Scientists reckon Mercury may have been involved in a cosmic biffo at some point and lost most of its mantle to space.
If the Forest Moon had the same experience as Mercury, it could plausibly wind up with as much as half Earth gravity. That’s plenty enough to keep AT-STs stuck to the ground and Ewok combat gliders soaring through the canopy. It could even go some way to explaining how the heroes manage to fall fifteen feet from an animal trap and leap from speeder bikes without injury. Low gravity would also encourage the growth of the tall forests seen all over Endor and justify the flimsy construction of Ewok villages. On the other hand, it makes Luke’s levitating god stunt a lot less impressive – at least half as impressive, to be precise.
Big moons may be the rule – not the exception
The field of planetology packs in twists with all the restraint of an M. Night Shyamalan film.
Earlier this year, NASA announced the discovery of a weird new star system: Trappist-1. It’s an ultra-cool dwarf star not much bigger than your average gas giant hosting a family of Earth-sized rocky planets with orbital periods (‘years’) shorter than February. When it was announced a lot of press focussed on the possibility of life, but the real breakthrough was the discovery of just how astonishingly variable planetary systems can be.
If I had been writing this piece at the beginning of this year (not possible – I was in too much grief) I would have suggested that the Forest Moon of Endor was implausible – too big to orbit anything much smaller than our Sun. Now it looks like giant ‘moons’ are a dime a dozen.
Why does it always rain on
Earth has a good mix of biomes: a little desert, some forest, ice caps, and a whole lot of water. The Forest Moon, by contrast, is just what it says on the tin – forested all over. Is this simply a case of George Lucas dozing off in 8th grade geography, or could there be a scientific reason?
It turns out that forests are rain-making machines. Dry places like my home inferno of Australia rely on the ocean to deliver rain. But the Amazon rainforest keeps the faucet running all year round and couldn’t care less what global weather patterns have to say. In fact, when the Andes first popped up, the Amazon basin was cut off from the sea for millions of years. It didn’t bother the rainforest much, although it may have bothered the Pacific marine species that got stuck there while the river searched around for a new exit.
As for sunlight, you actually don’t need much of it to sustain a forest. In fact, most of Earth’s forests are in the high latitudes – think Canada and Siberia. In ye olde times there were even forests in Antarctica, as depicted in my favourite episode of Walking with Dinosaurs. So long as an environment has a good starter kit of water and carbon dioxide and at least a little bit of sunlight, forests will tend to happen eventually. And anyway, if the Forest Moon has two local stars and a funny orbit around its host planet, it could easily get a much more even spread of sunlight than we do here on Earth.
Building a Planet, Step Three: The Estimate
It looks like the Forest Moon is definitely plausible. But is it probable? Let’s have a crack at an order of magnitude estimate:
- Number of stars in our galaxy = 250 billion
- Fraction of stars which are in binary systems = 85%
- Fraction of stars that are G-type = 20%
- Fraction of binary systems that host planets = 50%
- Planets per star = 10
- Fraction of planets that are gas giants = 20%
- Moons per gas giant = 20
- Fraction of moons that are very large = 1 in 20
- Fraction of large gas giant moons that are rocky = 1 in 5
- Chance of moon getting whacked (but not too much) = 1 in 20
- Chance of acquiring the right amount of volatiles = 80%
- Chance of not being blown up by a peevish galactic emperor = 100%
- Standard ‘things I forgot’ factor = 1%
- Gut instinct factor = 50%
- Number of Forest Moons in our galaxy ~ 2 million
Distance to the nearest one:
- Volume of galaxy = 3 E+61 cubic metres
- Volume surrounding each Forest Moon = 3 E+61 / 2 E+6 = 1.5 E+55 cubic metres
- Distance between each Forest Moon = cubed root of volume = 250 light-years
- Average distance to the nearest Forest Moon ~ 125 light-years
Unless I dropped a few orders of magnitude somewhere, that’s pretty damn close.
The bottom line
All up, the Forest Moon of Endor has got to be one of the more plausible science-fictional planets out there. In fact there’s a good chance a real-life planet fitting the bill exists only a few hundred light-years from here. Sure, that’s a long way to schlepp for a lame camping trip. But you could at least stock up on some of that delicious Ewok jerky while you’re there.