Uranus Is Cold

Really cold.

You should probably get that checked out.

The amount of light a body receives plummets exponentially as the distance to a star increases (the formula is 1/distance squared). Uranus is just over 19 astronomical units away from the Sun, meaning it gets 0.27% of the sunlight we receive on Earth (or approximately 360 times less). It’s easy to surmise that Uranus does not need sunscreen.

Uranus is the coldest planet in the solar system. That’s a bit odd because Uranus is not the farthest planet from the Sun. One must travel a billion miles from Uranus to hit Neptune’s average distance from the center of the solar system. To date, the chilliest measurement on Uranus was -371.5 degrees Fahrenheit (49 K; -224.2 C), a full 10 degrees F below Neptune’s record.

Why is Uranus so frigid?

Something bizarre is happening deep inside Uranus.

Uranus is nearly the same size as Neptune, and the two share similar compositions. Neptune radiates 2.61 times as much energy into space as it receives from the Sun. Uranus, on the other hand, pumps out hardly any excess energy. Uranus emits 1.06 times the energy it receives from our star.

Scientists believe a rocky core inhabits the center of Uranus, just like other planets, which would suggest Uranus is capable of a toasty nucleus. For some reason, however, Uranus has a low thermal flux. In lay terms, heat generated at the core of Uranus does not transfer well.

Scientists do not currently understand why Uranus has a lower internal heat than other planets. Because we have only probed Uranus once, we know remarkably little about Uranus. Voyager 2 gave us a glimpse of Uranus, but everything else we have gleaned about Uranus has arrived thanks to telescopes. Two basic scenarios exist: either Uranus internally produces heat and it does not transfer well to the rest of the planet or the core of Uranus is as cold as Cole Palmer.

Perhaps future probes of Uranus will yield a definitive answer, but, for now, we do not know why Uranus is so cold.

We can, however, speculate about the causes of the frigidity of Uranus.

You might recall that Uranus is tilted. Unlike the other planets, Uranus is askew relative to the ecliptic of the Sun. One hypothesis for this oddity stems from a potential collision with a body about Earth’s size in the solar system’s early days. A clash with a mass of these proportions could have sent Uranus into a slanted tizzy. Some scientists wonder if such a crash could also have prompted Uranus to expel its primordial heat, a shock from which Uranus perhaps never recovered.

Others speculate that the core of Uranus produces ample heat but some mechanism or barrier keeps the core’s heat from circulating to the atmosphere. Certain modes of convection can hamper heat transport. In 2021, researchers recreated the elemental conditions of Uranus and Neptune in miniature to see if they could replicate the difference in thermal flux. The scientists discovered that if they introduced magnesium to the soup, they could create an insulation layer. So, if Uranus somehow has more magnesium than Neptune, it could explain why Uranus is so cold.

Or Uranus could be frosty for a completely different reason. We simply have not plumbed Uranus enough to know. Missions to Uranus have been prioritized by NASA and, potentially, by the Chinese Space Agency. With further inquisition into Uranus, the mysteries of Uranus might reveal themselves.

Until then, however, Uranus will hold onto its cold secrets.