|Would you want to tell him there's something wrong with a planet being a dwarf*? Image courtesy of Seraph777|
So where are they?
Lets start at the beginning: For millions of years the inner solar system was filled with hundreds of planetary embryo's, colliding and merging. These gradually grew from planetesimals to Cere's sized worlds, then to Moon size.
Above: An artists impression of the growth of a protoplanetary disk. Courtesy of NASA
At the same time there was a cosmic demolition derby going on: Protoworlds were tugging on each other with gravity, destabilising each others orbits.
Eventually the inner solar system found some sort of stability. But, many other solar systems have worlds closer to their Suns than Mercury, and our solar system doesn't. In fact, the space between Mercury and the Sun is devoid of any kind of world, even asteroids - and that's also strange, since the missing matter makes our solar system lightweight compared to most.
Now Rebecca Martin and Mario Livio from the University of Nevada have found an explanation: There was a planet, bigger than Earth, inwards of Mercury, which absorbed all the asteroids and dwarf planets near its orbit.... then, in the outer solar system, Jupiter's orbit shifted inwards. All the inner planets were forced to move inwards too.... and the innermost world fell into the Sun.
|Above: The Sun - a giver of life, but also a destroyer of worlds....|
Why did Jupiter's orbit shift so disastrously?
Researchers trying to replicate the growth of the giant planets in computer simulations have found that it's hard to do so and end up with the four giants we have today. But making a solar system with five giant worlds is much more likely.
So we're now missing two extra worlds - and it turns out they may have been victims of the same cosmic disaster: According to a team form the University of Toronto, the fifth giant planet was an ice giant in the outer region of the solar system. It wandered too close to Saturn, setting off a chain reaction: It was pulled inwards, simultaneously moving Saturn further out. Then it had a similar encounter with Jupiter, and the king of the giants threw the smaller giant entirely out of our solar system. That same encounter moved Jupiter closer to the Sun, dooming the young super-Earth. It was a planetary demolition derby - and it sent one world into fire, and doomed another to an icy exile.
What would these worlds have been like? We can look at other solar systems for clues. The planet Janssen is a super-Earth orbiting the star 55 Cancri, at 1/25th the distance of Mercury, and it's an incredibly harsh place: Its surface temperature is 2000 kelvin, it's atmosphere is made of hydrogen, helium and hydrogen cyanide with lots of carbon. The surface is very black, and temperature variations may point to active volcanoes.
|Above: An artists impression of Janssen (55 Canrcri e), a volcanic super earth closer to its Sun than Mercury is to ours.|
If recent evidence of a ninth outer planet pans out then that means our solar system once had eleven planets - making the ancient solar system an even stranger place than we'd already thought.
*Don't go looking for that fanfic on the internet, nothing good ever came from such a googling.