Dwarf planet is the most distant solar system object we’ve ever observed

Astronomers have spotted a new object that’s the most distant ever discovered in our solar system— a dwarf planet that’s roughly four times farther away from the Sun than Pluto. The discovery was announced on Monday by the International Astronomical Union’s Minor Planet Center.

When Scott Sheppard, who helped detect the dwarf planet, first saw how slowly the distant object moved across the sky, he had one thought, murmured quietly to himself: “far out.” He was amazed, because to astronomers like Sheppard, a slow object is a very distant object in our solar system. And this one was extraordinary. “It was the slowest-moving object we’ve ever seen,” Sheppard, an astronomer at the Carnegie Institution for Science says.

By measuring how slowly the dwarf planet moved across the sky, Sheppard and his colleagues calculated that the object, now known as 2018 VG18 (and nicknamed ‘Farout’), was at least 120 astronomical units (AU) away from Earth. For perspective, one AU is the distance between the Earth and the sun, or 93 million miles away.

An artist’s illustration of ‘Farout,’ a recently-discovered dwarf planet in the outer solar system.
Roberto Molar Candanosa/Carnegie Institution for Science

That puts Farout at around the same distance from Earth as Voyager 2, the space probe that recently crossed into interstellar space. Another spacecraft, Voyager 1, is even farther away at 145 AU, and is getting farther all the time.

These extreme distances mean that figuring out what’s going on with the Voyager spacecrafts is hard enough, but discovering what’s happening with other, natural objects in these outer reaches of the solar system is far more challenging. It took an international team of astronomers to confirm the existence of Farout, scanning through data from extremely high-powered telescopes to look for small signs of movement that might indicate the presence of a planet.

Two images of Farout taken by telescopes. The dwarf planet is the only thing in the images that moves.
Scott S. Sheppard/David Tholen

Sheppard and the team responsible for this discovery detected another dwarf planet that was announced earlier this year, nicknamed the Goblin. Both discoveries, and others, are thanks to a massive search of the sky that’s been going on for the past six years. “We’ve been doing the largest, deepest survey for solar system objects,” Sheppard says.

One of the goals of this survey is to look for a large planet-sized object (also called Planet 9 or Planet X) that might exist in the outer reaches of the Solar System. Since 2012, the team has covered about 20 percent of the night sky, and while they’ve found a lot of dwarf planets, they still haven’t found Planet X.

Even with objects that the team has located, like Farout, researchers don’t have a whole lot of information. “All that we currently know about 2018 VG18 is its extreme distance from the Sun, its approximate diameter, and its color,” David Tholen, an astronomer at the University of Hawaii and a co-discoverer of the object said in a statement.

They can tell that it’s roughly 500-600 kilometers (310-372 miles) around, large enough to qualify as a dwarf planet. It’s also a distinctive reddish-pink color, which is common for that area of the solar system.

Sheppard says the color indicates that it is probably an icy object. “When ice is bombarded by radiation over the age of the Solar System it turns kind of reddish in color,” Sheppard says.

And in this area of our cosmic neighborhood, objects are constantly bombarded with radiation, just as they’ve been for hundreds of millions of years, if not longer. Out there, they are at the edge of the protective bubble formed by plasma streaming off the Sun. When they cross that boundary, called the heliopause, the levels of cosmic radiation go up, and the interaction changes icy objects, giving them that characteristic tinge of color.

“There are hundred of thousands of worlds of various sizes from the size of islands to the size of continents in this realm of the solar system and almost all of them are going in and out of the heliopause as they travel in their orbit around the Sun,” Michele Bannister, an astrophysicist at Queen’s University Belfast, says. “If it wasn’t pinkish, I would be astounded.”

While researchers can detect a blush of pink through the telescope, other details about 2018 VG18 (Farout) remains stubbornly out of focus.

Astronomers can tell that it probably takes a thousand years or more to make one full circuit around the sun, but they still aren’t sure what the shape of its orbit is, whether it’s moving away from the Sun, or toward it. They still have no idea how far into the Solar System it might come, and whether or not it is affected by the gravity of the giant planets, like Neptune, Saturn, or Jupiter. All of that information will take at least a year, if not more, to tease out from data gathered by telescopes around the world.

“That this object is particularly distant is in many ways not the most interesting thing about it,” Bannister says. Bannister, who was not involved in the survey, points out that the discovery of objects such as this one, and future analyses of its orbit will help astronomers get a better picture of how the Solar System formed and developed. For her, the fact that we managed to see this faint object at such a great distance is more of a bonus than a feature. “This is what comes out of doing patient, careful searching,“ Bannister says. “That we’re seeing it at this distance is purely an accident of cosmic time.”


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