KELT-11b was considered to be one of the largest objects known, and it’s about 40 percent wider than Jupiter. A team of astronomers find an exomoon. It was the first of its kind to be discovered. Exomoons are satellites orbiting a planet around another star. What makes this find interesting is the scale of the planet-moon system, although “moon” is quite a loose term as the object is reputedly around the size of Neptune, and the planet it is orbiting is ten times the mass of Jupiter or 3,000 times that of Earth.
This changes the way we categorize systems and invites questions on how big can planets really get. There are two ways to look at it. First is in terms of mass. Planets do not create their own energy out of nuclear fusion. A planet 13 times the mass of Jupiter can create pressure and heat that could trigger fusion reactions of deuterium, which is a heavy isotope of hydrogen. By then, the object is a brown dwarf and not a planet. Larger masses, on the other hand, are able to create their own energy and are no longer planets.
If one wanted to take this literally, there is a funny answer. Jupiter is 11 times bigger than Earth’s diameter, and it is as far as a planet can be, any more matter on Jupiter will cause it to be crushed by the gravity and become tightly packed. Still, there is only a small gap between Jupiter-like planets and brown dwarfs in terms of size. One example of this principle is the star Trappist-1A that had Earth-sized planets orbiting it. The red dwarf is about 1/2000th brighter than the Earth’s Sun and is powered by nuclear reactions that last for trillions of years. It is actually 80 times as large as Jupiter but is 10 percent larger than the planet in diameter. One can note that smaller stars like this are dense and dim and has a low rate in the outpour of energy.