I planned to make a space game so I need a knowledge about 3d planet making so anyone help me to make it possible

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I've done some studying on this as well and the best way to accomplish this is by transitioning using shaders.

it all depends on what your trying to accomplish really, so if your making a space exploration game where you can land on planets and explore the world and leave the planet and go to another one, shader transitioning would be the way to go.

while your in space you would have the 3D spherical object (planet) and once your player is within a certain distance of the planet you would actually transition the world into a flat terrain. Your terrain would need to be separated into at least 9 sections, like a tile map, and programmically place the tiles of the terrain to simulate like the player is exploring the whole world like they are on a spherical planet.

you could use actual spherical objects to land on and ect. But the actual computational and programmical challenges are very complex. One would be texture mashing as the poles of a sphere. The way the Geomtry of the sphere is, the triangles become mashed at the poles making creating a seamless transition at the poles hard. Then there's also the precision problem, since most game engines are based on floats, using spheres as a terrain your always cutting off values beyond the precision of a float. So eventually, you may experience odd behavior because the values have been stretched past the point of precision.

so best practice would be to use 3D spheres in space and transition to flat terrains when landing. You can make the transition smooth by hiding the transition while your flying through clouds ect.