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uranus and neptune

6 march 2023

I once saw Uranus through binoculars … no, I really did; the hilarity of such remarks is something that you just have to power through when you are reading a book about the outer planets of the Solar System. Anyway, I saw Uranus while taking a lab astronomy course in college. It is not hard to locate, or wasn't, 45 years ago when the night skies were darker out on the vast fields south of my Midwestern agricultural college. Uranus is also not that interesting in binoculars, a tiny greenish-blue dot; definitely not a star but definitely no competitor to Mars, Jupiter, or Saturn for visual interest.

250 years ago, says Carolyn Kennett in her new book Uranus and Neptune, Uranus was so hard to locate that a bunch of Europe's top astronomers recorded sightings of it without knowing what they were looking at. The planet can just barely be seen with the naked eye under perfect conditions; through 18th-century telescopes the seeing wasn't a whole lot better. Pierre-Charles Le Monnier recorded a dozen sightings of Uranus between 1750 and 1771 and chalked it up as just another star.

Le Monnier was mortified, a decade later, when William Herschel observed Uranus and figured out what he was seeing. Herschel's discovery was epochal. Five other inner planets were known in prehistory. The 17th and 18th centuries had seen a proliferation of fictions about unknown other worlds. Now, of a sudden, there was a real-life unknown planet to speculate about.

Neptune would follow in 1846, either another lucky stab or the result of careful hypotheses; opinions still differ, says Kennett. But astronomers theorized at least the basic idea of Neptune by observing oddities in the orbit of Uranus. Neptune didn't account for all those oddities; and it is a lot further, dimmer, and harder to see than Uranus. But into the ledger it went, and the list of planets was complete (later to be supplemented by Pluto and then reduced again after Pluto was decommissioned).

I imagined, growing up well before Voyager, that scientists knew everything about places like Uranus and Neptune, because … scientists just did. I knew they hadn't been there, but I figured they had their ways of getting information. Kennett makes clear that almost everything known about the two "ice giant" planets comes from the Voyager 2 mission, which visited Uranus in 1986 and Neptune in 1989. Space telescopes have since elaborated some of the spacecraft's findings, but what we know about these distant neighbors, and especially what we know about their weird moons, is almost all due to a single unmanned spaceship.

Both Uranus and Neptune are big fuzzy globes, but are otherwise dissimilar in big ways. Uranus seems to have no internal heat source; Neptune purrs along on the heat generated, most likely, by its own gravitational collapse. Uranus is knocked so that its poles lie in the plane of its orbit; Neptune is more like Earth and other planets in having a modest tilt.

Their moons, as Kennett discusses them, are highly varied and hold most of the interest of these faraway systems. Some are "dead" and heavily cratered. Titania and Ariel, moons of Uranus, have wrinkly surface features produced by geological (or, I suppose, titanialogical and arielogical) processes. Triton, a moon of Neptune, is distinguished by its volcanos that spew liquid-methane lava, and by its "cantaloupe regions" that … look like cantaloupes. One imagines that back in Herschel's day, they'd have been dubbed with some mythological moniker; but late-20th century American astronomers' breadth of cultural reference seems to have come from the produce section of supermarkets.

Every once in a while you see a story about a contemplated probe of Uranus, but funding is parlous. Even Voyager, now agreed to be one of the most inspiring feats in all of human exploration, was a patchwork effort, extended as far as it got by the ingenuity of team members who outwitted budget-cutters.

Of course, as I've noted before, planetary knowledge isn't very generalizable, and Uranus and Neptune in particular are so remote that we wouldn't know they were there if it weren't for the Discovery Channel. Still, Kennett says, if we're looking for planets around distant stars, we should learn more about Neptune, at least. Neptune is a template for many exoplanets, and the more we can learn about the one nearest us, the more we may learn about planets – and life – in the rest of the galaxy

Kennett, Carolyn. Uranus and Neptune. London: Reaktion, 2022.