We all know that life on Earth, in the grand scheme of the universe, is a pretty lucky deal. Our planet is just the right distance from the sun, with just the right atmospheric conditions, to support life. That's so rare, we obviously haven't found another planet with similar conditions. If we had, our sci-fi depictions of alien races would probably be a lot different.
For as unique as Earth is, though, scientists have typically considered our solar system to be, structurally, unremarkable. As NPR writes, we thought our solar system was normal. A star sits in the center. Small(ish) rocky planets, formed from dust melted by the star, orbit fairly close to that star. Beyond a certain point, called the frost line, larger, gaseous planets like Saturn and Jupiter hold much wider orbits. Their formative dust never got close enough to the sun to melt and condense into a rocky planet.
That was the theory for years. It makes sense. But it's wrong. By peering at other solar systems, astronomers have discovered systems with gigantic, Jupiter-size gas planets nestled closely to their stars, with orbits of only a few Earth days. Mercury, the closest planet to our sun, takes 88 Earth days to make its orbit. Many systems even have twin gas planets in close orbit to their stars.
Given the proximity, these planets are obviously hot, not like the cold gaseous planets we expected. Other systems have rocky planets close, like ours--except not like ours, because they're also orbiting extremely close to their stars.
"As of this month, we've discovered 884 planets, 692 planetary systems, 132 of them with more than one planet and, strange to tell, almost none of them look like us...Though it will take a while to discover smaller planets, right now there's only one planetary system that looks a lot like our own" writes NPR's Robert Krulwich. According to an astronomer from the University of California, Santa Cruz, these solar systems are changing how we view the universe. That systems with very tightly clustered planets are more common than not.
All this makes our own solar system even more unique, but it raises questions, too. How did gaseous planets form so close to stars? One theory is that they form beyond the frost line, like our own gaseous planets, but then move closer to the star. Will that happen in our solar system? DId we have other planets in our system at one point that have been absorbed by the sun?
We don't have the answers, yet, but the questions are interesting all by themselves.