Jupiter
SPECIAL NOTE: The ideas in the article have come under question due to recent computer models of what effect Jupiter's gravity plays in the orbits of small bodies. However, until more evidence is gathered, I'll keep it here.
You might be asking yourself what role a big ball of hydrogen some 300 million miles away could play in your everyday life. Well, don't ask an astrologer, because they're full of crap. Instead, consider the following.
Back in the early days of our solar system, there were a lot more little rocks and balls of ice swinging wildly out there in bizarre orbits around the Sun. If life were to evolve here, something would have to clean the majority of those pesky little cosmic rodents up before they could crash here.
That's what Jupiter did. It's huge gravitational influence on the rest of the solar system did several things. It attracted copious amounts of these asteroids to itself over the ages, making impacts on other planets - like the Earth - far less frequent. It's gravitational field altered the orbits of other bolides, and flung them out of the inner solar system into the far reaches of stellar space, and possibly beyond. It also helped shape the orbit of the rocks in the asteroid belt, helping keep them safely in the region of space between Mars and Jupiter, where we don't have to deal with them.
This is important because asteroids can, and have, caused major devastation to our planet. And I'm not just talking about the millions of brain cells that committed suicide when you forced them to process the sheer stupidity of Michael Bay's Armageddon.
I'm talking the dinosaurs. I'm talking about one ball of rock, a few kilometers in diameter, that managed to wipe out the majority of species on our planet. Of course, it wasn't just the impact, it was also the aftermath. Besides the flaming shockwave that incinerated just about anything in it's path, the survivors had to deal with other problems.
The impact sent vast quantities of flaming rock into the upper atmosphere and possibly beyond. When these came crashing down, all over the globe, huge fires sprang out. The brunt force of the impact put tremendous strain on our planet's crust, triggering massive earthquakes and volcanoes. And if that wasn't enough, the area that the bolide hit - the Yucatan peninsula of what we now call Mexico - is rich in sulfur. All of this sulfur released into the atmosphere caused massive amounts of sulfuric acid - aka acid raid - to fall. And for any creature fortunate enough to survive all of that, the ash left in the atmosphere blocked a good portion of the sunlight reaching the surface, causing global temperatures to fall drastically.
So limiting the amount of big rocks that could possibly hit Earth is important. Jupiter, according to theory, did just that. Of course, it didn't eliminate all impacts, as evidenced by the surfaces of Mars, the Moon, Venus, Mercury, and even our own - but that's not necessarily a bad thing.
For instance, in the early days of the Earth, our planet had no water; it had all been boiled away by the sun and impacts before the surface had cooled enough to hold liquid water. So where did the water come from? Comets. And not just the long period comets from the Oort Cloud and Kuiper belt, but from short period comets.
What sent those comets on a trajectory to Earth? Jupiter.
Jupiter formed further from the Sun than it's current location[1]. Constant collisions from dust, ice, and gaseous debris from the evolution of our solar system, as well as the exchange of orbital momentum from near collisions with thousands of relatively large bodies, caused it to lose speed and angular momentum; it then migrated about 40-45 million miles closer to the sun.
Jupiter's migration, combined with it's massive gravity, sent what scientists refer to as "a metric crap-load" of these Jupiter Family[1] short period comets into the inner solar system, where they collided with the smaller, rocky planets, like our own. And with a cooler surface and a thicker atmosphere, the water wasn't boiled away.
Which is good for us, since water lets us shower, making water important to a healthy social life, as well as life in general.
1. - http://en.wikipedia.org/wiki/Planetary_migration#In_the_Solar_system
2. - These are short period comets who's orbit around the sun last less than 20 years. Their orbits generally don't venture much further than Jupiter.