Planet 9 Might Be Lurking in the Darkness

Evidence is mounting for a ninth planet hiding in the outer solar system.

Babylonian priests first documented the 5 planets observable with the naked eye in the 8th century BCE. Priests like Nabu-rimanni, the first known Babylonian astronomer, created new mathematics to calculate the movement of Mercury, Venus, Mars, Jupiter, and Saturn. It was not until a millennia later, thanks to the invention of the telescope, that Neptune and Uranus were found. William Herschel observed Uranus and Neptune in 1781, though it would take another century to confirm Neptune was a planet. Pluto was discovered and confirmed in 1930 and then sadly demoted to a dwarf planet in 2006.

No other planets have been discovered beyond Neptune’s orbit. In fact, solar system models suggest that the existence of a distant, massive body outside Neptune’s orbit is highly improbable. And then Sedna was discovered in 2003.

It’s All About the ETNOs

Dwarf planet Sedna is a particularly interesting trans-Neptunian object (TNO), bodies whose orbits extend beyond Neptune’s. While there are a multitude of TNOs, such as the millions of icy objects in the Kuiper Belt, nearly all of these follow a regular, predictable orbit between 30 and 50 AU. The astronomical unit (AU) is defined as the average distance between the Earth and the Sun.

Sedna’s orbit is extremely eccentric and tilted about 15 degrees off the elliptical plane. Its eccentricity has been measured at 0.84961,000. For reference, Earth’s orbital eccentricity is a measly 0.017. Sedna’s aphelion (its further distance from the Sun) is 937 AU, while its perihelion (its closest distance to the Sun) is a little over 76 AU. This is odd compared to the orbits of the planets and other bodies in the solar system.

Then astronomers found VP113 in 2012, Leleākūhonua (TG387) in 2015, and RR205 in 2021. Their orbits are similarly strange. Leleākūhonua’s orbit varies between 2000 and 65 AU every 32,000 years. Astronomers classify these 4 bizarre objects as Sednoids.

So far, astronomers know of 29 ETNOs. While some of their extremely eccentric orbits can be explained by interactions with Neptune, the majority of them cannot. There’s something missing.

The Hidden Planet Hypothesis

In the wake of Neptune being confirmed as a planet in 1846, Percival Lowell predicted the existence of what he called Planet X. By looking at unexplained anomalies in Neptune’s and Uranus’ orbits, he calculated that there must be a massive, influential body even further out. Geoge Forbes calculated that there could be 2 planets beyond Neptune in 1890. However, when Voyager 2 flew by Neptune in 1989, NASA scientists took more accurate measurements of the gas giant’s mass, and these anomalies disappeared. Hopes for a new planet were dashed.

After the discovery of Sedna and other ETNOs, though, hopes for Planet 9 were renewed. And new research is pointing to Planet 9 as the only explanation for such bizarre orbits.

In 2021, Brown and Batygin created 121 computer simulations. In each, they varied Planet 9’s mass, average distance from the Sun, and eccentricity. They found that a planet with a mass of 6.2 Earth masses and an average distance of 380 AU would explain ETNOs’ orbits. They believe “a massive planet on a distant inclined eccentric orbit remains the simplest hypothesis.”

Batygin, Morbidelli, Brown, and Nesvorny wrote a paper in April 2024 in which they ruled out competing explanations. These include the galactic tide, the influence of other large planets, and passing stars. They included these in their models, and they were not sufficient to explain the orbits of ETNOs. It’s only with the addition of Planet 9 do their models fit observation. They said that “our results reveal that the orbital architecture of this group of objects aligns closely with the predictions of the P9-inclusive model.”

Though models suggest Planet 9 exists, no one has actually seen it.

Finding Planet 9

If it exists, Planet 9 would be 600 times less luminous than Pluto. Though it is likely a large body, Planet 9 would reflect very little sunlight and emit a tiny amount of infrared radiation. Planet 9 is thought to be near its maximum distance from the Sun, making detection even more difficult.

Despite this, Batygin and Brown have poured through historical data in search of Planet 9. They looked at data from the Wide-field Infrared Survey Explorer, the Catalina Sky Survey, and the Zwicky Transient Facility and came up empty-handed. They said, “This survey rules out 56% of predicted Planet Nine phase space.” Fortunately, though, this data does not include areas in the direction of the aphelion or those near the Milky Way’s plane. So it could still be out there.

Some have suggested that Planet 9 could be a primordial black hole. Its gravitational influence would still be present, though detecting it directly would be exceedingly more complicated.

Fortunately, new tools will be coming online soon. Perhaps the most promising is the Vera C. Rubin Observatory in Chile. It’ll be functional in 2025 and produce about 20 terabytes of data every day. Its camera will be the biggest ever made, big enough to hopefully find Planet 9.