Our simplistic nine-planet view of the solar system was shattered years ago when scientists learned Pluto was not unique in the outer solar system. We have since discovered more “dwarf planets,” and an international team of astronomers has just spotted the most distant such planetoid yet. The object known as “Farout” is 120 times farther from the sun than Earth, putting it far beyond the orbit of Pluto. There is some hope that learning more about Farout could point the way to a true ninth planet.
Astronomer Scott Sheppard from the Carnegie Institution for Science imaged Farout for the first time on Nov. 10th with the help of researchers from the University of Hawaii and Northern Arizona University. They used the Subaru 8-meter telescope on Mauna Kea in Hawaii for that initial observation, which the Las Campanas Observatory in Chile confirmed earlier this month. Based on the data we have so far, the team has made educated guesses for Farout’s size, brightness, color, and approximate location.
Farout is between 500 and 600 kilometers (310-370 miles) in diameter, so its mass should be sufficient to make it spherical. So, scientists consider it a dwarf planet and not a very large comet. Farout has a pinkish hue, indicating the surface is icy — no surprise at that distance. Ice tends to turn pink or red after long-term exposure to solar radiation.
At 120 AU (an AU is the distance from Earth to the sun), Farout is the most distant dwarf planet so far discovered in the solar system. Previously, the dwarf planet Eris held that distinction at 96 AU. Pluto is a mere 34 AU from the sun. Farout, which is officially known as 2018 VG18, is so far away that it takes light almost 17 hours to reach it.
We can’t be sure of Farout’s orbit just yet. Most trans-Neptunian objects have eccentric orbits, and we don’t know which way Farout is going to swing. It may take as long as 1,000 Earth years for it to complete a single orbit, so we will need several years of observation to estimate its path. 120 AU might be as far away as it gets from the sun, or it could travel even farther out before drifting closer.
Astronomers hope that Farout can help pin down the location of the theorized ninth planet, sometimes called Planet X or Planet Nine. The orbit of trans-Neptunian objects like Farout suggest there’s something massive out there — at least several times heavier than Earth. Farout is so far out that it may fill in a lot of gaps. There are two options, the more interesting of which is that Farout was dragged into its distant orbit by Planet X. If future observations suggest it swings in close to the orbit of Neptune, it’s also possible an encounter with the eighth planet flung it out there instead. It’ll take time to figure out which it is.