The giants of the solar system flank the Moon tonight. Jupiter, the largest planet, rises to the upper right of the Moon late this evening. It looks like a brilliant star. And the second-ranked planet, Saturn, follows them in the wee hours of the morning, well to the lower left of the Moon. It’s not nearly as bright as Jupiter, but there aren’t any other bright stars or planets around it, so it should stand out.

Although both planets are big and heavy, they’re a bit puny compared to some planets found in other star systems. Some of those are many times as massive as Jupiter. Yet none of them is much bigger than Jupiter. It seems that Jupiter is near the “sweet spot” for the sizes of giant planets.

That’s thanks to gravity. A more-massive planet has more stuff, which you’d expect to make it bigger as well. But the extra material also adds to the planet’s gravitational pull, which squeezes it more tightly. That makes it shrink.

And as it shrinks, the planet’s interior gets hotter. At about a dozen times the mass of Jupiter, it gets hot enough that the object is classified as a brown dwarf — an intermediate step between planets and stars. And at about 80 times Jupiter’s mass, the object’s core gets hot enough to ignite nuclear fusion — and shine as a star. Even then, though, it’s not much bigger than Jupiter itself — it’s at the “sweet spot” for big planets, brown dwarfs, and small stars.

More about the Moon and Saturn tomorrow.

Script by Damond Benningfield