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Terrestrial planets, which name is derived from a term for Earth, are usually defined as planets of sizeRp=0:51:7R
If Hydaelyn falls within that particular size, then there's a certain amount of velocity involved. A planet with extreme high velocity - In this case, 20 times faster if a Eorzean year were to match with an Earthen year - would potentially make it inhabitable as planets with extreme velocity have a much higher surface temperature than slower planets. As there are still tropical and winter-like climates on the Eorzean continent (and not on a global scale), you could conclude that the velocity of Hydaelyn is somewhat comparable with Earth. So that leaves us with just orbit length around the star and gravitational time dilation. Unless the orbit of Hydaelyn is 20 times longer around the star compared to Earth, the main cause for the massive time difference would be gravity. After all: 1 Eorzean year takes 7305 Eorzean days. Add in the fact that the average Eorzean adventurer can jump as high as their own height without effort, I don't believe it's far-fetched to conclude that gravity is significantly weaker on Hydaelyn compared to Earth.