Yesterday, Michael Brown—the very man who demoted Pluto—and Konstantin Batygin announced that a new planet might once again be swimming into our ken. In the debris fields of the outer solar system, at 20 times Neptune’s distance to the Sun, something dark and unseen is stirring chunks of rock and ice into strange orbits. Something massive.
Batygin and Brown say that only a planet could make these disturbances. A planet that was, until yesterday, conspicuously absent. When astronomers zoom in on other stars in our galaxy, they often find planets that are roughly 10 times the size of Earth—and yet, no planets of this size had ever been found locally. Now it appears that one has. Batygin and Brown have given it the unimaginative name, “Planet Nine.” If it’s soon spied by a telescope, as is their hope, it will likely be renamed for one of the gods the Romans stole from Homer’s “realms of gold,” just like the solar system’s other planets. Perhaps it will be Nox, the seldom-seen Roman goddess of night and shadow.
Upon hearing about Planet Nine, I reached out to Freeman Dyson, the 92-year-old physicist from the Institute for Advanced Studies at Princeton. Dyson is widely regarded as one of the singular geniuses of 20th century physics, which is saying something given how many great physicists that century produced. In the decades following World War II, Dyson did pioneering work in quantum electrodynamics, but he also devoted years of his life to Project Orion, a famous NASA-funded study into whether it might be possible to propel a starship with nuclear explosions. I vaguely remembered him writing something about how objects in the outer solar system might serve as stepping stones in an interstellar voyage, places to resupply and refuel before heading out into the great unknown.
“Would Planet Nine make an ideal waystation?” I asked him. “No,” he replied, quickly. “The galaxy is like the Pacific Ocean, scattered with small islands that are abundant and easy to visit.” But these islands are comets, not planets. “Planets are rare, hard to land on, and harder to take off from,” he said. “Comets are far more abundant and more friendly to visitors.”
What a killjoy, that Freeman Dyson. I wrote him right back: “But would there be any benefit to having another potential gravity assist way out there?” After all, we often send space probes toward Jupiter, in order to use its gravity like a slingshot, to accelerate to the speeds you need to reach Pluto or the interstellar void. I wondered if it might help to have another planet—another potential slingshot—in the outer reaches of the solar system, for when we send a probe or a crewed ship to Alpha Centauri, or one of the other nearby stars.
I waited an hour. Dyson didn’t reply. I felt bad for wasting his time. I tried Steinn Sigurdsson, an expert in orbital mechanics from Penn State University. “Suppose you used a gravity assist from Jupiter and Planet Nine,” I wrote. “Would that help you escape the Sun’s gravity with less fuel?”