You are a...
Look to space to see planets eight,
They are eight-legged creatures.
You'd probably waste where's I'd just wait.
They are incredibly resilient, able to survive in places where other creatures would waste away and die.
Multiply Fifty and Three One Hundred Million times,
Tis when evolution made me,
Living in pound scum,
What a great time whoopee.
Exist in fossils dating back ~530 million years, which seems off by an order of magnitude from the above number?
Seventeen 73 was important to me,
Twas when a month discovered me,
Discovered in 1773 by Johann August Ephraim Goeze.
Please take note for i only live Three,
But take my water and I'll out live even thee.
Average lifespan is a few months, but they're capable of living much longer (100+ years?) in a dehydrated state.
The title is...
An anagram for "Johann August", as frabjrew also mentioned in a comment.