The physics of an Earthlike planet with four large moons
The moons would likely end up in periodic orbits. Each moon might take twice as long to orbit as the previous one. For example, Io takes 1.8 days to orbit Jupiter. Europa takes 3.6 days. Ganymede takes 7.2 days. Each is about 1.5 times as far as the previous one.
If each moon was about 2000 km wide, together they would probably still weigh less than Earth's moon. If they happen to be lined up in the sky, the farthest would appear a quarter as wide as the nearest, and 1/16th as bright.
The nearest moon might be only 25,000 kilometers from the surface, passing overhead about once a day, in the opposite direction as the sun. It could have an elongated or triaxial "American football" shape (Dedekind ellipsoid), somewhat like the mystery planet "2003 EL61" Haumea (a Jacobi ellipsoid), but would look round when overhead, roughly half as wide as a fist on an extended arm. It would appear elongated on the horizon. Tides would be a hundred times stronger. There would be more earthquakes, vulcanism, and regularly spaced hurricanes. Surface gravity would not be affected.
Asteroid spins and sunlight
Small asteroids are irregular. When they rotate they absorb, reflect, and re-emit sunlight in a repeating but unstable pattern.
This, and the fact that they're less massive, makes them act like crude solar sails. Reflected sunlight over many millennia (and infrared emissions as they cool during the brief night) could speed up their rotations until they break apart.
There should be a cut-off at a few hundred meters: below that size, there are fewer asteroids than expected. There's also more loose rubble orbiting the sun than expected. Some gets pushed into interstellar space.
A few small asteroids have very slow rotations. This allows them to accumulate dust, and become almost spherical. Some small worlds like Methone have very strange or almost organic shapes.
Giant Earthlike planets
Most detectable extrasolar planets are many times larger than Earth. A few have solid surfaces, with surface gravities of 2G or 3G, almost comparable to Jupiter's.
Even when located in their stars' habitable zones, they're likely to have dense atmospheres with a high percentage of hydrogen, and ultra-deep oceans. There would be no free oxygen, except perhaps in complex compounds, maybe something like hemoglobin foam.
Huge planets would have huge quakes, 'hot spots', unstable canyons and mountain ranges. Mountains and volcanoes would be lower but wider.
Instead of plate tectonics there could be interlocking geological 'islands', with central volcanoes that disperse internal heat by slowly turning inside-out, or even flipping over. Ocean waves would roll in and break much faster than on Earth.
Imploding planets
A gas-giant planet could collapse in a 'micro-nova' when the radioactive decay-powered heat source at its core cools enough.
At that point, the shrinking giant would initiate an era of lithium fusion. It would have to be more than fifty times heavier than Jupiter. The higher core density would also boost various forms of fission.
The local system would gain a small, temporary star, too dim for most Earth telescopes to see. In a hundred million years, the planet will reemerge as a blazing, storm-swirled globe, less than half its original volume.
Completely unverifiable speculations
Height of the tallest waterfall in the observable universe: 50 kilometers.
At the bottom is a solid white splash funnel rising several kilometers, hidden by a permanent rainstorm. The roar is loud enough to kill anything within a few kilometers.
We are not alone!
Distance to the nearest self-replicating molecules: 50 lightyears
Distance to the nearest living cells: 1000 lightyears
Distance to the nearest intelligent aliens: five billion lightyears (they're no longer organic, but are made of pure information now, and are approaching us at the speed of light).
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Five billion years ago, when enough elements were already available for complex organisms to evolve, no intelligent life existed within four billion lightyears of Earth. If it had, it would have eventually invented lightspeed transportation, and have arrived by now.
That implies that for every advanced civilization, there must be quintillions of planets with no or non-intelligent life, in most cases barely detectable semi-biotic molecules.
Some non-sentient lifeforms may become advanced in ways no one can imagine, or form planet-wide microbe societies.
The ultimate fate of the universe
Space/Time is expanding at an accelerating rate. Given endless eons, every particle (electron, positron, photon, neutrino, etc.) will eventually be googols of meters from its nearest neighbor.
No further communication will then be possible. By necessity, each particle will become the core of its own quantum universe, filling up the space around it with increasingly derivative potential swarms of virtual particles. These must eventually become as complex as the random universe that started it all, unless the laws of physics can somehow be brought under our control before then.
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