* We haven't explored the whole galaxy: Our knowledge of the Milky Way is limited. We can only see and study a small fraction of the billions of stars and planets it contains.
* New discoveries are constantly being made: Astronomers are always finding new and larger objects in the galaxy. What we consider the biggest today might be surpassed tomorrow.
However, we can talk about some of the largest objects we know of:
Biggest known star: The current record holder for the largest star is UY Scuti. It's a red supergiant estimated to be about 1,700 times the size of our Sun. If it were placed where our Sun is, its surface would extend past the orbit of Jupiter!
Biggest known planet: The title of "biggest planet" is more complicated. Some planets have larger diameters, while others have larger masses. Currently, the planet with the largest diameter is HD 100546 b. It's about 6 times the size of Jupiter. However, the planet with the largest mass is DENIS 0255-4700 b, which is about 28 times the mass of Jupiter.
It's important to remember that these are just the largest objects we've discovered so far. There could be even bigger stars and planets out there that we haven't seen yet.
