The six planets you see above are all considerably larger than Earth, and yet five of them orbit their star at a closer distance than Mercury. It's the most ridiculously packed solar system we've ever seen, and our current theories about how planets form are completely unable to explain how they all got there. It's a real exoplanet mystery, and the biggest discovery since we found exoplanets in the first place.
That's the opinion of lead author Jack Lissauer, part of NASA's Ames Research Center. The Kepler 11 solar system has crushed ten times the mass of Earth inside the orbit of Mercury, with another large planet out around where Venus orbits our Sun. This wouldn't be quite so remarkable if it was, say, one huge gas giant, which previous models show could have formed further out and then moved inwards. But five different planets? That's much harder to explain.
The researchers say that forming all five planets further away from the Kepler 11 star might better explain how they exist at all - it's hugely unlikely enough solid particles would be found right around the star for that many planets to form - but it can't speak to how the planets actually moved into orbit. The planetary migration model can explain one or maybe two planets in close proximity to a star, but it's hugely unlikely five different planets could make the move without smashing into each other. Indeed, we know they must have had almost no interactions with each other, because they're all on an incredibly regular, flat orbital plane - otherwise we wouldn't be able to see them at all.
Lissauer explains how fundamental a problem this is, and that there's a lot of theoretical work ahead of them to explain how this planetary system could possibly exist:
"Large planets very close in orbit around a single star were just totally unexpected. We think this is the biggest thing in exoplanets since the discovery of 51 Pegasi - the first exoplanet - in 1995. I come from a planet formation theory perspective, and this has sent me back to the drawing board."