Astronomers still haven’t found a ninth planet in our solar system. To their credit, they haven’t stopped trying since Neptune, the eighth planet, was first spotted by Johann Gottfried Galle in 1846.
Astronomers initially suspected the ninth planet because of peculiarities in Neptune’s orbit that can be explained only by the gravitational tug of a planet beyond it.
When Clyde Tombaugh discovered Pluto in 1930, he was expecting a massive gas giant in the mold of Uranus or Jupiter. He was among the first to complain that Pluto, a tiny hunk of practically nothing, was too insignificant, gravitationally speaking, to explain the weirdness in Neptune’s orbit.
The search for Planet Nine has a long and checkered history. Johann Galle first spotted Neptune because of its gravitational effect on the orbit of Uranus, the next planet in from Neptune. Urbain LeVerrier, a French astronomer, had correctly calculated the position of Neptune based entirely on its effects on Uranus.
LeVerrier was not content with his discovery. He studied the orbit of Mercury and concluded that a planet inside its orbit must be causing it to be slightly off course.
Seeing that planet would be challenging. Mercury itself is hard to observe because of its proximity to the sun. The sun’s glare would drown out an even closer planet.
The only hope seemed to be to look for the unseen planet as it passed across the sun’s disk. This event, called a transit, happens when Mercury is directly between Earth and the sun.
The race was on. In December 1859, LeVerrier received a letter from Edmond Lescarbault, a rural French physician, who claimed to have seen a tiny black speck traveling across the sun on March 29, 1859.
LeVerrier was skeptical, and he had a right to be. Lescarbault was from the little town of Orgeres. He timed the transit using a clock without a second hand.
He made due by measuring the seconds with a makeshift pendulum consisting of a ball on a string. The pendulum hung from a nail in the wall of his observatory.
Paper was scarce in Orgeres, so Lescarbault recorded his observations on a wooden plank. (He supplemented his income from country doctoring by doing some carpentry on the side.) When he used up the plank, he planed off the surface and started over.
LeVerrier visited Orgeres and, despite his original reservations, came away totally convinced of Doc Lescarbault’s discovery. He even went to the trouble of naming the planet Vulcan after the ancient blacksmith of the gods.
The name is apt. LeVerrier calculated Vulcan’s distance from the sun at a staggeringly close 13 million miles. Mercury is, on average, 33 million miles away from old Sol.
He plotted the planet’s orbit with great precision. He calculated that it took precisely 19 days and 17 hours to orbit the sun. Vulcan zipped around the sun in less than an Earth day, which provides plenty of opportunities to look for it.
There was only one problem with Lescarbault’s discovery and LeVerrier’s calculations. Vulcan doesn’t exist.
Astronomers have made numerous attempts to see it. It should be easy.
Thanks to LeVerrier, we have an exact accounting of where Vulcan should be at any given time. LeVerrier went to his grave defending Vulcan, but the danged planet just isn’t there.
As it turned out, it took an Einstein to explain the oddities in Mercury’s orbit. In 1915 Albert Einstein proposed looking at the orbit of Mercury to validate his General Theory of Relativity. Because Mercury is so close to the sun, the sun’s gravity warps spacetime in a more pronounced way than it does farther out where we live.
Mercury is precisely in the correct location if we factor in the effects of Einsteinian gravity. The reason that LeVerrier suspected an intra-Mercurial planet has simply disappeared.
The same can be said for the supposed ninth planet. Careful analysis of Neptune’s orbit proves that the planet is precisely in the correct orbital location.
If Planet Nine exists, it must be far, far away from Neptune’s influence — in the outer reaches of a ring of material out past Neptune called the Edgeworth-Kuiper Belt (EKB) and beyond.
Since Planet Nine has no gravitational effect on Neptune, LeVerrier’s methods won’t work. More indirect methods are required.
The distance to such a planet makes the use of measures like miles or kilometers practically meaningless. Instead, astronomers refer to extreme solar-system expanses in “astronomical units” (au).
Astronomers define one au as Earth’s distance from the sun, approximately 93 million miles. Neptune averages 30 au, almost 2.8 billion miles, from the sun.
The Edgeworth-Kuiper Belt (EKB) is a ring of icy bodies encircling the sun along the solar system’s plane. The EKB’s primary disc extends from 30 au, the orbit of Neptune, to about 50 au. After that, a “scattered disc” spreads out to nearly 1,000 au.
Farther out, the Oort Cloud, a sphere of icy comets, starts at about 2,000 au and extends to 200,000 au, about 3/4s of the way to Proxima Centauri, the nearest star to the sun.
Collectively, such objects are lumped together as “trans-Neptunian objects” (TNOs). Anything in the scattered disk and the Oort cloud is called an “extreme trans-Neptunian object” (ETNO).
Astronomers have discovered thousands of TNOs 60 or more miles in diameter. Some of them are large enough to be classified as dwarf planets. They include Pluto at 1,447 miles wide, Eris at 1,445 miles wide, and Sedna at 618 miles wide.
The discovery of Sedna in 2004 led astronomers to suspect the existence of a planet-sized object as far as 500 au from the sun.
Astronomers were impressed by Sedna’s unusual orbit. Sedna is currently 84 au from the sun, which puts it in the scattered disk. At its closest point to the sun, the dwarf planet will be 79 au from the sun. At its farthest point, Sedna reaches an astounding 937 au, a distance that carries it to the inner fringes of the Oort Cloud.
It will take a while to get there. Sedna orbits the sun once every 11,408 years. By comparison, Neptune, the farthest known planet, completes a full solar orbit in “only” 165 years.
Sedna’s highly stretched-out orbit has led some astronomers to conclude that a gravitational encounter with the proposed Planet Nine could have dragged Sedna out of its original, more circular orbit.
In 2016, CalTech astronomers Konstantin Batygin and Mike Brown used computer simulations and mathematical modeling to uncover a group of six ETNOs, called sednoids, that have orbits similar to Sedna’s.
Brown and Batygin hypothesized that a planet-sized object at an extreme distance from the sun might be gravitationally shepherding the sednoids into “clustered” orbits.
The proposed Planet Nine has a mass of 5-10 times that of Earth. At 16,000-32,000 miles wide, it might be as much as four times the diameter of Earth, about the size of Neptune or Uranus.
It orbits the sun at an average distance of perhaps 450 au, 15 times farther from the sun than Neptune.
Enormous distances make for long orbits. Planet Nine could take as many as 20,000 of our Earth years to complete one orbit around the sun.
How could Planet Nine have taken its distant position in the solar system?
It might have coalesced in the usual planetary way as smaller objects accreted together to form a more massive planet. However, such a planet would have to have been within the orbit of Neptune. Not enough planetary material exists 450 au from the sun.
Astronomers now think that the planets migrated inward and outward during the solar system’s early days. Planet Nine might have been gravitationally expelled into the solar system’s outer darkness by another planet that passed nearby.
Alternatively, Planet Nine could have been stripped from the orbit of a passing star.
Or it could be a “rogue” planet ejected from another star system billions of years ago. It would have wandered alone into the space between the stars for a long time before it was captured into orbit by the sun’s gravity.
Or it may not exist at all. Alternative explanations abound.
For example, a nearby (but yet-unseen) primordial black hole could have altered the sednoids’ orbital paths.
Some astronomers claim that “observational bias” skews the evidence for Planet Nine. They argue that insufficient data exists to prove —or disprove — the existence of Brown and Batygin’s planet.
Others maintain that the gravitational effects of so many TNOs, many of which remain undiscovered, are impossible to calculate. The orbital similarities of the sednoids might be temporary or coincidental.
Conclusive evidence will come with the first observation of Planet Nine — if it exists at all. That observation will be extraordinarily challenging because of the proposed planet’s relatively small size and extreme distance. It simply wouldn’t reflect much sunlight into Earth-based telescopes.
As Batygin himself put it, “Until Planet Nine is caught on camera, it does not count as being real. All we have now is an echo.”
Tom Burns is the former director of the Perkins Observatory in Delaware.