def _get_constants(planet='earth'):
    """Retrieve planetary constants for calculations.

    Parameters
    ----------
    planet : str
        The name of the planet of interest.
        (default='earth')

    Returns
    -------
    radius : float (km)
        The average radius to the surface of a planet.
    mass : float (kg)
        The average mass of the planet.
    gravity : float (m**3 kg / s**2)
        Newton's gravitational constant

    """

    radius = {'earth': 6371.2}
    mass = {'earth': 5.9722e24}
    gravity = 6.6743e-11

    if planet not in radius.keys():
        raise KeyError('{:} is not yet a supported planet!'.format(planet))

    return radius[planet], mass[planet], gravity