Do you miss Pluto? In 2006, the International Astronomical Union voted this bantam world off the island. It was no longer a planet — a peer of Mars, Jupiter and the rest. From then on, Pluto was to be known as a dwarf planet, a furtive designation that only an astronomer could love. And personally, I didn’t.
From now on Pluto was to be known as a dwarf planet, a furtive designation that only an astronomer could love. And personally, I didn’t.
Sure, the decision made astronomical sense, but the ninth planet was my sentimental favorite, enigmatic and beyond the capabilities of my small, home-built telescope. All the other planets had been mapped and photographed. They were familiar to anyone who could read. But not Pluto.
A few months before the astronomical union demoted Pluto, NASA launched the New Horizons spacecraft to map it — the only planet whose face we still hadn’t seen. How disappointing that long before New Horizons got there in 2015, Pluto had become an ex-planet.
But just as planets can be lost, they can also be found. And I’m hoping for a Pluto replacement. Astronomers are sifting the skies for a large, new world hanging out in the lonely hinterlands of the solar system, a world that no one could ever deny planethood.
What might be the payoff for finding a cold, dark and solitary world in the nether regions of the solar system? The principal benefit to science would be helping us understand how solar systems form. And you can be sure that NASA would soon make plans to send a probe its way.
But for me, such a hulking, far-off world has romantic appeal, more than Pluto ever did. I can imagine its future use as a way station for missions to the stars, either as a rest stop or simply as a booster for long-distance probes that could use it to slingshot themselves to the depths of space.
This isn’t a random search. For most of the 20th century, Pluto was assumed to mark the end of the planetary road – the last gas in the solar system. It was the only large body beyond Neptune. (Yes, there are billions of comets much farther out, but they’re small. Most would fit in New York City’s Central Park.)
But in the 1990s, astronomers began finding hundreds of objects farther than this supposed limit. Some were just comets, but there were also beefier bodies – balls of rock and ice hundreds of miles across. They were given weird names, like Makemake, Lempo, Haumea and Quaoar.
The astronomical union realized the problem this created. After all, if Pluto was a planet (which it was at that time), so were Makemake et al. They also orbited the sun, and were comparable in size to Pluto. But that would make for an awful lot of planets. Finally, feeling that the astronomical community had to deal with this situation, the group decided in 2006 to just throw all these icy rocks into another box and label it “dwarf planets.” Pluto was merely the first to be found.
But this story has a twist, making it about more than just semantics. Observations show that the farthest of the so-called trans-Neptunian objects have orbits that are roughly aligned. That’s suspicious. Imagine throwing a dozen pencils high in the air, after which you examine their orientation on the ground. You wouldn’t expect that nearly all of them pointed in the same direction. But that was true for the orbits of these distant objects.
In 2016, California Institute of Technology astronomers Mike Brown and Konstantin Batygin suggested an explanation. They argued that these objects had been slowly pulled into this improbable alignment by the gravitational tug of an unknown, heavy-duty planet.
Many of the planets we’ve detected around other stars were revealed by the slight dance their gravitational pull induces in their host stars.
That was certainly possible. In the 1840s, slight irregularities in the motion of Uranus aroused suspicion that an unseen planet, farther out in the solar system, was tugging on that world. This led to the discovery of Neptune. Many of the planets we’ve detected around other stars were revealed by the slight dance their gravitational pull induces in their host stars. Finding something by its influence on a neighbor is an old story in astronomy.
Brown and Batygin figured that they’d find this hypothesized planet within two years. But it didn’t happen. Recently, new discoveries have led them to slightly revise their prediction. It’s now believed that Planet 9, if it exists, will be about half the mass of Uranus. Maybe that doesn’t sound like much, but it’s more than the combined heft of Mercury, Venus, Earth and Mars. Formed in the outer solar system, this planet would likely be a cold ball of gas; like Jupiter and Saturn, although smaller, in a realm where the sun is no more than a bright dot in an inky sky. So, forget Planet 9 as a candidate for future colonization.
But why is it so difficult to find? Well, with a distance from the sun reckoned to be 10 times that of Pluto, Planet 9 would be dimmer than a cane toad. In addition, the predictions give only a general idea of where to look for this world. Finding Planet 9 won’t be easy, even assuming it’s there.
The Caltech astronomers are unfazed and are continuing the search. They’re combing through existing astronomical data and plan to introduce new telescopes to the search, such as the Vera Rubin Observatory.
I, for one, am rooting for them. As I see it, Planet 9 would someday be our descendants’ last port of call before they enter the dark seas of the final frontier.