As of Go 1.8+ you can now use sort.Slice to sort a slice:

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sort.Slice(planets, func(i, j int) bool {
  return planets[i].Axis < planets[j].Axis
})

There is normally no reason to use an array instead of a slice, but in your example you are using an array, so you have to overlay it with a slice (add [:]) to make it work with sort.Slice:

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sort.Slice(planets[:], func(i, j int) bool {
  return planets[i].Axis < planets[j].Axis
})

The sorting changes the array, so if you really want you can continue to use the array instead of the slice after the sorting.

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type Planet struct {
    Name       string  `json:"name"`
    Axis       int64   `json:"Axis"`
}

planets := [...]Planet{{"a", 12}, {"b", 9}, {"c", 20}}

Slices.Sort

Go: 
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package main

import (
    "fmt"
    "slices"
)

func main() {

    strs := []string{"c", "a", "b"}
    slices.Sort(strs)
    fmt.Println("Strings:", strs)

    ints := []int{7, 2, 4}
    slices.Sort(ints)
    fmt.Println("Ints:   ", ints)

    s := slices.IsSorted(ints)
    fmt.Println("Sorted: ", s)
}