Hello, How do you know how the weight of a very distant planet? How do they know this new plant would float on water. Is this science or just guessing?

Weighing planets is a tricky business. We can't even see the vast majority of extrasolar planets! For most of them, all we can do is watch how their gravity makes their star wobble. But astronomers can decipher that information and use it to estimate the orbit. The problem is, we can't tell whether the orbit of the planet is face-on or edge on in the sky, so there is always some uncertainty.

Every once in a while, though, we get lucky and discover a planet that passes in front of its star, so it blocks a little of the light. This is called a "transit", and tells us a lot more about the planet. For one, we know its orbit exactly, since we know that we are seeing it exactly edge on (otherwise we wouldn't see the planet go in front of the star). If we know the orbit exactly, then we can figure out what the planet's mass is compared to the star. We can get a good estimate of the star's mass based on its brightness and color, which lets us figure out the planet's mass too.

But the statement that a planet is "light enough to float on water" implies that we know the planet's density, not just the mass. To figure out the density, we need to know the size and the mass. Luckily, when a planet transits, we can watch how long it takes the star's light to dim, and figure out how big the planet is.

So, it's not guessing when you hear astronomers say that there is a planet out there that is the largest planet ever discovered, but only weighs half as much as Jupiter. The guessing comes in when we start trying to figure out why this planet is so huge. That is still up for debate...