How compact could a solar system get?

A solar system, named Trappist-1, has been discovered about 39 light-years away from us, with planet orbits so close together that from the surface of one of them we would be able to see all the others in the sky in the same way that we see the moon on Earth.

The system is made up of seven rocky planets orbiting a red dwarf slightly larger than Jupiter, at a distance less than that separating the Sun from Mercury.

All of them could potentially have water on their surfaces, on the planet’s twilight zones, due to them being all tidally locked in synchronous rotation around their star.

This system, thus made, resembles much more that of gas giants such as Jupiter or Saturn and their moons, rather than the Sun and its planets. It would be, for all intents and purposes, a miniature solar system.

A study also reveals that the theoretical number of possible stable planets is much higher than one would imagine. In fact, it would be possible to have as many as 252 planets identical to Earth, arranged evenly on six orbits.