Who is the history’s greatest explorer? Marco Polo, Magellan, da Gamma and Cook are the main contenders, along—of course—with Christopher Columbus, whose star has fallen over the last few decades. Vancouver, Peary, Amundsen and Scott all have their partisans, as do Lewis and Clark. There are Cortez and de Soto, Stanley and Livingstone, and John Hanning Speke, who discovered the source of the Nile. A strong argument could be made for Jacques Cousteau, or some of the Apollo Astronauts. Let me toss another two new names into the discussion—men who are, improbably, in the prime of their careers: Mike Brown and his former graduate student Chad Trujillo.
Columbus discovered the Americas and Cook, Australia. Mike Brown and a cadre of collaborators, primarily Chad Trujillo and David Rabinowitz, have discovered dozens of planetary bodies beyond the orbit of Pluto, including all six “plutoid” dwarf planets: Quaoar, discovered by Brown and Trujillo, and Sedna, Orcus, Haumea, Makemake and Eris, discovered by Brown, Trujillo and Rabinowitz. Eris is the largest of the six—larger even than Pluto—and was for a time, in popular culture, thought of as the Solar System’s 10th planet.
Instead of producing a tenth planet, Brown, et al’s research turned into one of the catalysts for the demotion of Pluto, leaving us with just 8 planets and a passel of dwarfs. But if Brown’s latest work bears fruit and his latest predictions prove true, we’ll soon be back up to a full planetary compliment, with a new planet 9. A new planet 9 that will probably be about 10 times more massive than the Earth. (For scale, Uranus is 14.5 times the Earth’s mass.)
In 2014, Chad Trujillo and his colleague Scott Sheppard published a letter in the science journal Nature discussing a newly-discovered “Sedna-like body,” that had a peculiar, highly elliptical orbit, like the orbit of dwarf planet Sedna. The eight confirmed, full-sized planets all have more-or-less circular orbits. On the other hand, Sedna—which lives beyond Neptune—has an elliptical, stretched-out orbit. At its closest, it comes within 76 AU of the Sun—an AU being the average distance between the Earth and the Sun: about 93 million miles. At its furthest, Sedna is 936 AU from the sun. One complete orbit takes Sedna about 11,400 years. Trujillo and Sheppard noticed that both Sedna and this new body had elliptical orbits that took them away from the Sun in roughly the same direction, and speculated that this coincidence might be caused by the gravitational influence of a heretofore undiscovered planet.
Michael Brown and Konstantin Batygin looked at Trujillo and Sheppard’s data and took the idea further. They found four new objects gravitationally perturbed in the same way Sedna is—in the same direction—and used them to calculate a possible orbit for this new Planet 9, which they believed (as Trujillo and Sheppard believed) must be orbiting the sun opposite these 6 Sedna-like bodies, counterbalancing them.
Brown and Batygin made three specific predictions for future discoveries—predictions that they believed, if proven correct, would suggest that the Planet 9 hypothesis was correct: One—most Sedna-like objects discovered in the future would have orbits that skewed off in Sedna’s direction. Two—that these new objects would have orbital planes tilted in roughly the same direction as Sedna’s, and three—a small “population” of newly discovered objects would be skewed off opposite the majority, in the direction of the imagined Planet 9.
Since these predictions were published, four new Sedna-esque objects have been discovered. The first three objects match the first two of Brown and Batygin’s predictions. The fourth of those new objects, says Brown, “is my favorite. It is swept into an orbit exactly opposite of all the rest. This object was precisely the type predicted for the new population we had predicted [sic], and it was in exactly the right spot. How exciting was it to see this newly predicted population? Let’s just say I did a little dance in my office when I saw the orbit.”
So what does that mean for the plausibility of a 9th, giant, undiscovered planet? Says Brown, “we now have a score card! Originally there were six objects. Now there are ten. That’s a 66 percent increase, which is good work, mostly thanks to Sheppard & Trujillo’s efforts. And every single discovery fits a true prediction perfectly. By “true prediction” I mean an authentic prediction about something not yet seen, rather than an after-the-fact explanation. Those are hard. Those are the things that we give serious credence to, as a fun idea turns into a compelling hypothesis turns into a rigorous theory.
“Are we there yet? No. I would put us about halfway between compelling hypothesis and rigorous theory. There are still a few details about Planet Nine and its effect on the outer solar system that we can’t yet explain. But we’re close. When (or to be fair, I should say ‘if’) those details are nailed down, I will be happy to put Planet Nine into the category of rigorous theory. Of course, we might get lucky and actually find it first. Then is will simply be confirmed fact.”
According to the orbit Brown and Batygin have worked out for Planet 9, they predict it will be found near the constellation Taurus, which will rise in the autumn. So we may have something concrete to look at soon.
Does all this put Brown and Trujillo and their colleagues in the running for the title of history’s greatest explorers? If it does, you have to include Willian Herschel too, who discovered Uranus, and Urban le Verrier and John Couch Adams, who jointly discovered Neptune. There’s also the consideration that, like Moses gazing across the River Jordan, Brown and company won’t ever set foot on their discoveries. Meaning they can’t be said necessarily to have the boldness of a Cook or a Clark. Still, in terms of area explored and discoveries made, they stand alone. To have discovered literally dozens of new worlds hiding in the hinterlands of the solar system is an accomplishment for the ages.
(Of course, if we’re being serious, everyone knows that history’s greatest explorer is Christopher Columbus. But it’s bad manners to say so.)
