How do we know that Planet X is a gas giant?

6 relatively small objectes were observed in the Kuiper belt that had a clustering point in their orbits all at at their perihelion (the place where their orbits are closest to the sun). There is only a 1 out of 15000 chance that this clustering happened solely by chance. It's way more likely that a larger object corralled these objects. read more

First of all, we don't even know if there is a Planet X. Its existence is suggested by the orbits of other objects out in the far reaches of our solar system, but those orbits could be unusual just by chance. read more

The smallest known extrasolar planet that is likely a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope. A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. read more

It's way more likely that a larger object corralled these objects. And when you calculate the size of the object needed to corral these 6 objects it turns out it would have to be gas giant to make the math work. This is how we find the outer planets. Neptune was predicted to exist by close observation of Uranus. read more

Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are really a distinct class of giant planet, being composed mainly of heavier volatile substances (which are referred to as "ices"). read more