Some astronomers have a knack for making it seem like humans have truly become masters of the universe. They built the Gaia space observatory, currently scrutinizing a billion stars to create the definitive map of the Milky Way. They detected ripples in space and time caused by black holes crashing into each other in a distant galaxy. They have analyzed the afterglow of the Big Bang — the very beginning of existence! — to measure the precise amount of matter and energy in the entire universe.
And then there is astronomer Mike Brown of Caltech, always ready to deliver a dose of humility. His specialty is exploring the outer reaches of our solar system, a mere 1/1,000th the distance to the sun’s nearest star, and finding it full of shadowy unknowns.
Brown is most famous as the guy who discovered that Pluto is surrounded by a whole population of related objects. It was a revelation that prompted the International Astronomical Union to reclassify the former ninth planet as a “dwarf planet” and Brown to christen himself “@plutokiller” on Twitter. Recently, Brown and his colleague Konstantin Batygin made headlines again, this time by reporting convincing evidence of a true Planet 9 — a world some 5,000 times the mass of Pluto, orbiting even farther from the sun. Perhaps the most striking thing about Planet 9 is not that it (probably) exists, but that nobody has found it yet: a giant body, circling right in our celestial backyard, sight unseen.
Masters of the universe? Ha. We don’t even know how many planets are in our own solar system.
Brown likens his fumbling investigations of the realm beyond Pluto to the journeys of 16th-century European navigators. “Think of it the way you would if you were jumping on a ship going across the ocean and didn’t know what was going to be around the next corner,” he says. “I have some ideas of what might be there, but right now it’s really still a ‘there be dragons’ world that we don’t know anything about.”
Slowly, Into the Dark
Exploring the outer solar system has always been a halting, painstaking process. Uranus, the first planet discovered in the modern era, was probably spotted by Greek astronomer Hipparcos in 128 B.C. but not recognized for what it is until William Herschel recorded it in 1781. Galileo apparently observed Neptune in 1613 but mistook it for an ordinary star; the planet wasn’t identified for real until 1846. Despite an intensive search for a hypothetical Planet X, pursued with fanaticism by wealthy eccentric Percival Lowell, it took another 84 years after Neptune’s discovery before Clyde Tombaugh discovered Pluto. Then 75 more years passed before Brown located the distant dwarf planet Eris and showed that Pluto is not alone.
As Brown explains, there is a simple reason for the snail’s pace of discovery: “It’s a big solar system, and things get faint fast!” Solar illumination gets dimmer with the square of distance from the sun, and then any light reflected off a distant body likewise dims with the square of its distance from Earth. Put the two effects together, and the consequences are daunting. If you moved Pluto twice as far from the sun, its apparent brightness would decrease by 2 to the 4th power — a factor of 16.
Brown and a handful of colleagues have now pretty well scanned the zone around Pluto, out to around 5 billion miles from the sun, for large objects. A frozen ball known as V774104, currently the most distant solar system object known, is twice that far. Go just a little deeper, though, and all kinds of things could be circling about, invisible to even the best telescopes.
For now, the evidence for Planet 9 comes solely from its gravity, not from its light. Starting about a decade ago, astronomers noticed odd patterns in the distant solar system. Brown and Batygin were especially struck by the orbits of six of the most extreme objects in the Kuiper Belt (the population of outer objects that includes both Pluto and Eris), all of which cluster on one side of the sky.
They deduced that a large planet, roughly 10 times the mass of Earth, was lurking on the other side of the sun, its gravitational pull sweeping any smaller stuff out of the way. The pattern of clustering indicated a likely orbit for Planet 9. The fact that nobody had noticed it yet offered another clue: The planet must be on the darkest, farthest part of its looping path, possibly 100 billion miles away.
That still leaves a lot of sky to examine. To visually track down his putative Planet 9, Brown is requesting 20 nights of observing time on the Subaru telescope in Hawaii, the only large instrument with a wide enough field of view to practically pull off such a search. Even if he gets his observing time (there’s a lot of competition), the project will take at least a year and a half.
Brown has learned to be philosophical about these things. “There’s always a chance that the six times you’re looking for Planet 9, it happens to align with a star and you miss it,” he says, “but eventually we’ll find it, and then we’ll study it to death.”
The Great Beyond
An important lesson from the discovery of Eris and the rest of the Kuiper Belt is that Planet 9 is certainly not alone. Astronomers now realize that the formation of the solar system was a messy, chaotic process that ejected all kinds of bodies out to the far fringes.
Today, there are four giant outer planets. Planet 9, if it exists, is most likely a stillborn fifth giant planet, a smaller version of Neptune that got sent off on a very different evolutionary track. As Brown points out, it’s perfectly possible that six, seven or even more giant planets started to form before some of them were ejected. It is even more likely that smaller bodies — Mars size? Moon size? — are floating around on the fringes.
“There’s a ton of stuff out there,” Brown says. And could Subaru spot a moon-size thing at a Planet 9-like distance? “Nope, nope, nope. Not a chance.”
One way to extend human vision is to look for heat rather than light. Truly giant planets would retain a lot of thermal energy from the time of their formation, and therefore could show up in telescopes tuned to infrared rays or even colder millimeter waves. The WISE space telescope did a sky survey and found nothing, ruling out distant versions of Jupiter or Saturn. But Planet 9 would be too small and cool to show up in WISE’s detectors.
Brown and the other outer explorers are much more jazzed about the Large Synoptic Survey Telescope (LSST), currently under construction atop Cerro Pachón in Chile. Starting in 2023, the LSST will perform extremely sensitive, repeated scans of the full night sky. It should eventually spot hundreds or even thousands of bodies out to the distance of Planet 9. Perhaps that bounty will include colder versions of Pluto, or a weird deep-freeze analog of Mars. The orbits of all those objects, in turn, could indirectly reveal the presence of even more distant worlds. “LSST will be great,” Brown gushes. “And I guarantee you, we’ll see patterns we didn’t anticipate. Then we’ll have to start looking to see what’s causing those other patterns.”
Such bootstrapping will quickly hit a wall, however. There is no greater survey telescope planned after LSST — “a scary thing to think about,” Brown says — but we already know that the solar system keeps going far beyond that instrument’s limits. The Oort Cloud, a loosely bound flock of dormant comets, extends to at least 100 times Planet 9’s distance from the sun. There could be planet-size objects lurking out there as well. We have no way to see them, no way to detect them, not even any ideas on the drawing board about how to find them.
When it comes to our celestial backyard, mystery — not mastery — is the name of the game. “It’s a big unknown,” Brown says. “I would love to just get in a spaceship and drive around the outer solar system, taking a look at this object and this object and seeing what’s really going on out there.”
