Researching the Protoplanet That Smashed Into Uranus

A new paper suggests that Uranus might still contain evidence of the catastrophic impact that knocked it on to its side and shaped much of its evolutionary history. If it’s accurate, there are some incredibly exciting discoveries lurking within the ice giant’s mantle.

If we held a competition for weirdest planet in the solar system, Uranus would easily win. Not only does it orbit in retrograde (a characteristic it shares with Venus), its orbital inclination is 98 degrees. This means it orbits essentially on its side, with its north and south poles approximately where its equator should be. It also radiates very little heat. Neptune radiates 26.1x more energy than it receives from the Sun. Uranus, in contrast, only emits ~1.06x. Its heat flux is lower than Earth’s, and it ranks as the coldest planet in the solar system — colder, in fact, than either Neptune or Pluto, despite being closer to the Sun than both.

Uranus’ magnetic field is particularly strange. Earth’s magnetic field tilts roughly 11 degrees from its spin axis. Uranus’ magnetic field is tilted a whopping 59 degrees from the axis of rotation. Earth’s magnetic field passes through the center of the planet, while Uranus’ very much does not. The magnetic dipole is shifted from the center of the planet towards the south rotational pole by roughly one third of the planet’s radius. This makes the field highly asymmetrical, with variations ranging from 0.1 gauss to 1.1 gauss. Earth’s magnetic field, in contrast, varies from 0.3 gauss to 0.6 gauss.

The traditional explanation for how Uranus wound up in this configuration is that at least one protoplanet slammed into the ice giant billions of years ago, knocking it on its side. And according to the scientists behind this paper, there might still be traces of that impact remaining.

The team ran multiple simulations of various scenarios. One of their findings is that, depending on the angular momentum and composition of the impacting protoplanet, a large amount of additional ice could have been deposited deep within the planet. At low mass and lower impact velocity, much or all of the ice winds up sitting on top of Uranus’ icy mantle (Uranus is believed to consist of three zones — an atmosphere of hydrogen, helium, and methane, an icy mantle comprised of ammonia and water ice, and then a solid rock core). Under this theory, the additional ice of the impactor body may have effectively sealed Uranus’ heat under the icy mantle or created a thermal inversion layer. The off-angle magnetic field could have been shaped by the impactor — multiple simulation runs show that the rock from the protoplanet doesn’t always wind up evenly distributed and could have changed the overall composition of the core (again, depending on impact speed and angle). A close analysis of Uranus’ material composition could tell us a great deal about the events that left the planet in its current orbital inclination.

Of course, no model can hold a candle to the value of actually getting a probe on-site to examine Uranus and its moons, but for now models will have to suffice. There are no plans for a Uranus mission that would echo the exploration of Jupiter or Saturn by the various probes we’ve sent to both. The sideways-spinning ice ball of the solar system will remain a mystery for a while longer.

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