Some planets are unlike any planet in our solar system.

The exoplanet Kepler 138d is a water world. The giant ocean and water vapor cover the entire planet. And there is the large solid nucleus and that planet's mass is 1,17 times Earth's. The size of that exoplanet is larger than Earth's. 

Its classification is "super earth".  But in the case of Kepler 138 d that classification comes from its size. Not from its gravitation. The mass of Kepler 138 d is, depending on sources 0,6-1,17 Earth. 

A planet's mass determines its gravitational field and whether it is larger than Earth. But if the mass of the planet is the same or less, it means that there are fewer heavy elements than on Earth. If a planet has a greater mass than Earth but is smaller in size.

This tells us that there are more heavy elements than there are on Earth. In other words, a planet may be smaller in size than Earth, but its gravitational field may be stronger than Earth's.

So because the mass of the Kepler 138d is almost similar to Earth that makes this planet interesting. There is some conflict between the sources that determine the mass of Kepler 138d. Wikipedia tells. That the mass of Kepler 138d is 1,17 Earth mass(1). 

But NASA's database tells. That the mass of Kepler 138d is 0,64 Earths. (2)If the last of those numbers is true. That means there are not so many heavy elements on that planet as on Earth. 

(1) https://en.wikipedia.org/wiki/Kepler-138

(2)https://exoplanets.nasa.gov/exoplanet-catalog/1557/kepler-138-d/

But its size is bigger which means the solid core of that planet is not formed of at least very heavy elements. There are fewer heavy elements than on Earth. And that makes Kepler 138d very interesting. The mass of the exoplanets is the thing that makes them similar to Earth, not the size. 

So that means the planet is very unlike Earth. The Kepler 136 B orbits the star Kepler 138 in 23,1 days. And the star Kepler 138 is an M-type star. Which means its surface temperature is about 2400–3700 Kelvin. So that star is very cold. And that means the Kepler 138 d has a level in its ocean where fish-type habitats could exist and live. 

The thing that makes Kepler 136B interesting is that this very hot exoplanet might have some kind of life forms. The extremely high temperature doesn't mean that the water can vaporize on that planet because there is so much water. There is high-pressure water inside the giant ocean. And clouds are cooling the surface of that strange world. 

https://exoplanets.nasa.gov/exoplanet-catalog/1557/kepler-138-d/

https://scitechdaily.com/water-worlds-astronomers-discover-that-two-exoplanets-are-unlike-any-planets-in-our-solar-system/

https://en.wikipedia.org/wiki/Stellar_classification