Searching for an invisible Ninth

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One more in that vein? Our solar system had nine planets, with even this excellent mnemonic to remind us of their names: My Very Educated Mother Just Showed Us Nine Planets. All in orbits around the Sun, that’s Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

Yes, Pluto. Remember that one? We of a certain vintage were taught that our solar system’s ninth and outermost planet was Pluto. That number and that name, as also the names of the other planets, were like fundamental truths of life. Which is why it came as a definite shock to us of a certain vintage when Pluto was “reclassifed” in 2006—as a “dwarf planet”. It still goes around the Sun, but it no longer qualified as a full-fledged planet, like the other eight.

The story here goes back to what led to the discovery of Pluto in the first place – and in fact, all the way back to how Uranus and Neptune were discovered.

Leaving aside our Earth, the first five planets outwards from the Sun – out till Saturn, that is – have long been known to humans, and known to be distinct from stars. They were discovered in prehistoric times, millennia before telescopes came along. That is itself a tribute to the observational skills of our ancestors, as also to the glorious night skies they were lucky enough to live under, unlike our light-polluted modern times.

While Uranus is difficult to see with the naked eye, it had indeed been observed in ancient times. Only, the ancients thought of it as just another star. It was in 1781 that it was finally identified as a planet, using a telescope.

Neptune, though, is not visible at all to the naked eye. How, then, was it discovered? Now that’s the story that leads us to Pluto and, well, beyond.

After Uranus was discovered, the French astronomer Alexis Bouvard compiled astronomical tables about its orbit and those of Jupiter and Saturn. His data for the two earlier-known planets nicely matched his observations. But he noted that with Uranus, there were serious discrepancies. Its orbit would change unexpectedly, not following the path his table predicted. Bouvard hypothesized that there must be another planet whose gravity was responsible for these irregularities. He recorded his observations, but was unable to find such a planet. After he died in 1843, other astronomers used his data about these perturbations to predict the position of an eighth planet. In 1846, Johann Gottfried Galle pointed a telescope at that position and found an object very close to it. Galle knew exactly what he was looking at: a “new” planet, the one we now know as Neptune. As with Uranus, astronomers realized it had been seen before, though through telescopes. Galileo, for example, recorded seeing it in 1612. Though of course, nobody realized it was a planet till Galle did.

The point here is that Neptune was the first planet in the Solar System that wasn’t identified as one by direct observation. Instead, it was found using what we knew about another planet, and mathematical techniques to interpret those observations. That’s already, it seems to me, a triumph of science. But there was more to come.

Through the rest of the 19th Century, astronomers came to believe that Neptune’s presence did not fully explain the disturbances in the orbit of Uranus. In 1903, Percival Lowell published a book laying out the case for yet another planet, the ninth. It had to be beyond the orbit of Neptune, and large enough to influence the behaviour of Uranus. The early years of the 20th Century saw an intense search for what Lowell called “Planet X”. In 1930, Clyde Tombaugh found an object that seemed to fit the requirements Lowell laid out.

That was Pluto. For about the next 70 years, students like me learned about the Solar System with Pluto as its ninth, and farthest-flung, planet.

But it was always an outlier, and not just for being far-flung. In particular, it is much smaller than the mathematical calculations predicted. In fact, it is less than a fifth the size of our Moon. Thus, and again, its presence doesn’t fully explain what Uranus is up to.

This is what led to its 2006 “reclassification”. That year, the International Astronomical Union officially defined planets as celestial bodies with three characteristics. One of those is that such an object should have “cleared” its neighbourhood—meaning that it is so gravitationally dominant that there are no similar sized objects nearby. Far from doing such clearing, Pluto actually weighs less than a tenth of the combined mass of the other objects in or near its orbit. Thus it can’t be a planet. While its reclassification has been contested, the consensus today seems to be that Pluto is just one of a collection of similar-sized objects in what’s known as the Kuiper Belt.

So yes, we’re a Solar System with only eight planets. Or eight known planets. For we still have to account for the eccentric orbit of Uranus, as well of other Kupier Belt objects. That means that the search for a ninth planet continues. One team of astronomers used a telescope in Hawaii to examine areas of the sky where calculations had shown Planet Nine might turn up (“A Pan-STARRS1 Search for Planet Nine”, https://arxiv.org/pdf/2401.17977.pdf, 1 February 2024). As they write: “Planet Nine [P9] is now seen to be capable of accounting for a range of additional otherwise unexplained phenomena in the solar system.”

They didn’t find the planet, but they did rule out about 78% of the possible locations. Intriguingly, they also reported some “predictions for P9 parameters”. If it exists, it will likely be about 6.6 times as massive as the Earth; it is in an elliptical orbit whose maximum diameter is about 500 AU; and it is currently about 550 AU (astronomical unit: the distance from the Earth to the Sun) from us.

Compare to Neptune, the eighth and most remote known planet, whose distance from us varies between 18 and 30 AU. Then give some thought to the idea of predicting the presence of, even the possible location of, these otherwise unknown planets.

Mathematics. All over again, it’s mathematics.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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Published: 08 Mar 2024, 01:14 PM IST

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