class SpaceAge(object):
    def __init__(self, seconds):
        self.seconds = seconds

    @property
    def years(self):
        return self.seconds/31557600

    def on_earth(self):
        return round(self.years, 2)

    def on_mercury(self):
        return round(self.years/0.2408467, 2)

    def on_venus(self):
        return round(self.years/0.6151976, 2)

    def on_mars(self):
        return round(self.years/1.8808158, 2)

    def on_jupiter(self):
        return round(self.years/11.862615, 2)

    def on_saturn(self):
        return round(self.years/29.447498, 2)

    def on_uranus(self):
        return round(self.years/84.016846, 2)

    def on_neptune(self):
        return round(self.years/164.79132, 2)

class SpaceAge(object):
    YEARS = {"on_earth": 1,
             "on_mercury": 0.2408467,
             "on_venus": 0.61519726,
             "on_mars": 1.8808158,
             "on_jupiter": 11.862615,
             "on_saturn": 29.447498,
             "on_uranus": 84.016846,
             "on_neptune": 164.79132}

    def __init__(self, seconds):
        self.seconds = seconds

    @property
    def years(self):
        return self.seconds/31557600

    def __getattr__(self, on_planet):
        if on_planet in SpaceAge.YEARS:
            return lambda: round(self.years/SpaceAge.YEARS[on_planet], 2)
        else:
             raise AttributeError

Implement __getattr__ to reduce code