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General

Read about using Erlang libraries in the official Getting Started guide, and in particular about formatting strings.

Use the provided module attribute with a list of letters representing planetary classes.
There is a built-in function for choosing an element from a list at random.

There is no Elixir function that would return a random float.
There is a built-in Erlang function that returns a random float x where 0.0 <= x < 1.0.
If x belongs to a range 0.0 <= x < 1.0, but you need a number from a different range a <= x < b, you can shift x's range by multiplying it by the range's width (b - a) and adding the range's start (a). That is: x * (b - a) + a.

There is no Elixir function that would be able to, in a single step, format a float as a string with a given precision.
There is a built-in Erlang function that takes a format string and a list of data, and returns a charlist.
There is a built-in function that changes a charlist to a string.
The format string of that function contains control sequences.
A control sequence starts with ~ and has the pattern ~F.P.PadModC, where F stands for the width of the output, P stands for the precision, Pad stands for the padding character, Mod stands for the control sequence modifier, and C is the type of the control sequence.
To format a float with a desired precision, a control sequence with the pattern ~.PC will suffice.
The control sequence type for floats is f.
The exact format string you need is ~.1f.