diff --git a/elixir/space-age/lib/space_age.ex b/elixir/space-age/lib/space_age.ex
index ce79dc4..48b3ca0 100644
--- a/elixir/space-age/lib/space_age.ex
+++ b/elixir/space-age/lib/space_age.ex
@@ -9,18 +9,21 @@ defmodule SpaceAge do
           | :uranus
           | :neptune
 
-  @planets ~w(mercury venus earth mars jupiter saturn uranus neptune)a
+  @type factors :: %{planet => float}
+  @factors %{
+    mercury: 0.2408467,
+    venus: 0.61519726,
+    earth: 1.0,
+    mars: 1.8808158,
+    jupiter: 11.862615,
+    saturn: 29.447498,
+    uranus: 84.016846,
+    neptune: 164.79132
+  }
 
-  defguardp is_planet(planet) when planet in @planets
+  @seconds_in_year 31_557_600
 
-  defp factor(:mercury), do: 0.2408467
-  defp factor(:venus), do: 0.61519726
-  defp factor(:earth), do: 1.0
-  defp factor(:mars), do: 1.8808158
-  defp factor(:jupiter), do: 11.862615
-  defp factor(:saturn), do: 29.447498
-  defp factor(:uranus), do: 84.016846
-  defp factor(:neptune), do: 164.79132
+  defguardp is_planet(planet) when is_map_key(@factors, planet)
 
   @doc """
   Return the number of years a person that has lived for 'seconds' seconds is
@@ -28,7 +31,7 @@ defmodule SpaceAge do
   """
   @spec age_on(planet, pos_integer) :: {:ok, float} | {:error, String.t()}
   def age_on(planet, seconds) when is_planet(planet),
-    do: {:ok, seconds / (factor(planet) * 31_557_600)}
+    do: {:ok, seconds / (@factors[planet] * @seconds_in_year)}
 
   def age_on(_planet, _seconds),
     do: {:error, "not a planet"}