def generate_missing_link(star=None):
    """Give exoplanet.eu link if link is missing.

    See https://github.com/DanielAndreasen/SWEETer-Cat/issues/135.

    Parameters
    ----------
    Star: str Optional
        Star name from Sweet-cat.

    Returns
    -------
    link: str
        Link to exoplanet.eu.
    """
    if star is None:
         return "http://exoplanet.eu/catalog/"
    else:
        star_formatted = star.lower().replace(" ", "_")
        return "http://exoplanet.eu/catalog/{}_b/".format(star_formatted)