Keys to access vector fields - enum class or enum in namespace?

Let's say I have an alias for vector:

typedef std::vector<double> PlanetData;

And I want it's fields to be accessible via some keys:

double x = planet_data[PlanetDataKeys::PosX]; //planet_data is of type PlanetData

How can I achieve it?

I can define an enum inside a namespace:

namespace PlanetDataKeys {
  enum {
    PosX = 0,
    PosY = 1
  };
}

But enum class is safer:

enum class PlanetDataKeys {
  PosX = 0,
  PosY = 1
};

However, as implicit casting of enum class to int type is disabled, this would require to write:

double x =  planet_data[static_cast<int>(PlanetDataKeys::PosX)];

which is a bit awkward .

Which approach is better in this case and why?