Most planets have poles roughly aligned with the sun's, which we have labeled north and south. Not Uranus. For whatever reason, the seventh planet from the sun has always rolled on its side, throwing off all sorts of strange magnetic activity in the meantime. It's unlikely Uranus was tilted when it formed, and astronomers have struggled to understand the cause.
New research published today in suggests that Uranus got hit by a planet twice the size of Earth long ago. This collision could have radically changed the planet, resulting in its telltale tilt and making it relatively frigid compared to farther-out Neptune.
Uranus is about 14 times the mass of Earth and around four times larger in radius. Whatever hit Uranus is thought to have been between two or three Earth-masses. Such an object would have been sufficient mass and size to create a big collision, but small enough to strip Uranus of less than 10 percent of its atmosphere.
The object, according to the research, is likely closer to two Earth masses, as this would have kicked up more of the gas and dust that went on to form the inner Uranian moons. Uranus has 27 moons, but 13 of them are considered "inner" moons. Some of the mass from the planet would have been incorporated into Uranus, some went into the moons, and the rest possibly escaped the system entirely.
Planetary collisions in the early solar system weren't uncommon. The moon probably formed from another protoplanet smashing into the young Earth, and a runaway planet has long been the suspected culprit behind the odd tilt of Uranus. The new study bolsters that hypothesis and gives a good range for the mass of the object—which also explains where some of the objects between the size of Earth and Neptune disappeared to.
Now we may well know at least one of the reasons the ice giant is so weird—but plenty of other oddities are likely locked deep within Uranus.