The Hill sphere of a planet

Space is a vast, empty vacuum, but every planet manages to claim a tiny bit of yard space called a Hill sphere. Think of it as a celestial property line. Inside this bubble, the planet’s gravity acts as the landlord, holding onto moons like precious, overpriced furniture.

Step one inch outside this invisible boundary, and you’re evicted. The Sun, the ultimate high-rise developer, immediately snatches you up into its own massive portfolio.

Even a giant like Jupiter only owns a small fraction of the neighborhood. It’s a brutal market; most of the universe is just unrentable dead space.

So what actually determines the size of this celestial yard?

It all comes down to two things: how much "equity" the planet has—its mass—and how close it’s parked to the Sun’s corporate headquarters.

If you’re a heavy hitter like Jupiter, you can bully the Sun into giving you a bigger lot. But if you’re too close to the Sun, like Mercury, the landlord is constantly breathing down your neck, leaving you with a yard the size of a postage stamp.

Basically, the further you move into the suburbs of the solar system, the more space you can claim without the Sun foreclosing on your moons.

Wait, can a moon have its own tiny apartment inside that yard?

Technically, yes. A moon can offer a "sub-lease" to its own smaller satellite, creating a sub-moon. It’s like trying to fit a designer shoe box inside an already cramped studio apartment.

The problem is the zoning laws are a nightmare. Because the moon is already tucked inside the planet's yard, any sub-moon has to deal with two landlords—the planet and the Sun—constantly trying to evict it.

Most of these sub-leases end in a messy breakup. The gravitational tug-of-war usually results in the sub-moon being tossed into deep space or crashing into the floor, which is why "moon-moons" are incredibly rare finds on the market.

Has anyone actually managed to find one of these rare sub-leases?

In our local neighborhood, the vacancy rate for sub-moons is a staggering 100%. We’ve scouted the big estates like Jupiter and Saturn, but so far, no one has found a moon with a permanent live-in guest.

There were some hot tips about Rhea or Iapetus having tiny companions, but those deals fell through. In this cosmic market, the 'zoning laws' are so strict that any sub-lease is usually terminated by a collision or an eviction into deep space within a few million years.

Why is the 'eviction' notice so much faster for these tiny sub-moons?

It’s a matter of tidal friction—the cosmic version of a structural integrity failure. While a regular moon has a stable lease, a sub-moon is caught in a brutal tug-of-war between its parent moon and the planet next door.

This constant gravitational 'renovation' stretches the sub-moon's orbit into a wobbly shape. It’s like trying to live in a house where the walls are constantly shifting six inches every hour.

Eventually, the orbit gets so distorted that the sub-moon either slams into the moon or gets flung into the void. If you can't keep a steady circle, the universe cancels your lease.