On this day in 2005, Eris was discovered. It is the largest dwarf planet known to exist in our solar system. But who knows; with objects so small, so far away, it is hard to say that we won’t find something larger. As it was, it took just under 75 years to discover a dwarf planet larger than Pluto. Of course, “larger” is a vague word. Pluto is very slightly more voluminous than Eris; but it is more than 25% more massive than Pluto.
Now I don’t want to start a fight. If you want to call Pluto a planet, fine. And everyone I talk to who wants to do so is fine with calling Eris a planet too. But let’s be clear. If we hadn’t all been raised with Pluto being called a planet, no one would be running around saying that Eris ought to be called a planet. I mean, it only has a quarter the mass of our Moon. It has a volume of one-third. It would rightly be called some misbehaving moon. And it may well be just that: a moon that got dislodged from some planet. It certainly does have an amazingly strange orbit that is more befitting a comet than a planet.
Will we visit Eris? Frankly, I don’t much care. I think we are going to find that it is a lot like Pluto. But calculations have been made. If we launch in 2032 or 2044, it would take a bit less than 25 years to reach the dwarf planet. It would be roughly 90 AU (distance from Earth to Sun) at that point. I’m not sure if there are any plans to do it, but I’ll be dead by then anyway.