What scientists know about the way planets form suggests that if a planet is much larger than our own Earth, it it probably becomes a "gas giant." This means that it is made of a variety of gases. The pressure at the center may have helped squash the gas into something more like a liquid or a gooey ice, but there won't be any solids like we find on Earth, Mars, Venus or other smaller planets, and whatever solidifying the gas does happens only at extreme temperatures and pressures.
Until they got a look at a place called Kepler 10-C. Orbiting the sunlike star Kepler 10 in the constellation Draco, it's a rocky planet 17 times the mass of the Earth -- well past the limit at which astronomers thought the only kind of planets around were balls of gas. Neptune in our own solar system is 17 times the Earth's mass.
Although Kepler 10-C probably orbits its sun too close for life to exist, its presence shakes up a couple of commonly-held theories, one being the above-mentioned size limit for rocky worlds. The other is its age -- the system may have formed less than 3 billion years after the Big Bang, compared with our own solar system that didn't show up until about 7 billion years later. Rocky worlds were also thought to be rare in the earlier universe, with higher temperatures and energy levels making the formation of heavier elements more difficult.
Seems the universe isn't in any hurry to be completely figured out any time soon.