New Object Beyond Pluto Hints at Mysterious ‘Planet X’

October 3, 2018

The solar system’s retinue of known, faraway worlds has gained another member: a small, icy body that takes 40,000 years to plod once around the sun, traveling farther away from our home star than all known solar system objects except for comets. The last time 2015 TG387 was anywhere within whispering distance of the sun, mammoths and cave bears trampled Eurasian grasses, and modern humans were crafting tools from stone.

Called 2015 TG387 (and nicknamed the Goblin), the world is likely spherical and about as wide as the state of Massachusetts. And—like a handful of other distant solar system inhabitants—its orbital behavior might signal the presence of an unseen Planet X lurking in the distant outer dominions of the solar system.

“Every small object we find that is isolated like this will bring us closer to finding the planet,” says Scott Sheppard of the Carnegie Institution for Science, who reported the finding today in a notice distributed by the International Astronomical Union’s Minor Planet Center.

“Or, you never know, if we find more of these, maybe they’ll stop pointing toward the planet.”

Something Fishy

As its ungainly name implies, 2015 TG387 first caught astronomers’ attention in 2015. For years, Sheppard and his colleagues have been using some of the sharpest telescopes on Earth to peer deep into the outer solar system and uniformly search the sky for some of the farthest-flung worlds still gravitationally tethered to the sun.

Doing this type of survey requires a substantial amount of time and patience, because merely seeing a small pinprick of light doesn’t tell you much. Instead, astronomers must painstakingly track objects like 2015 TG387 as they inch across a star-drenched background.

“It took three years of observations to actually determine its orbit to a precision we’re comfortable with,” Sheppard says. “We’ve found several more objects that are similar distances to this one, but it will take another year or two to look at their orbits and see if they’re actually interesting.”

Right now, 2015 TG387 is in the northern sky near the constellation Pisces. It’s about 80 astronomical units away, meaning it’s 80 times farther from the sun than Earth, or about twice as far as Pluto. It’s currently moving inward, and at its closest approach, the tiny iceball will still be 65 astronomical units away. At its most distant point, it’ll be nearly 2,300 times that distance.

In 1930, Clyde Tombaugh discovered Pluto when he compared this image (with Pluto circled) with another taken six days earlier and noticed the bright speck had moved.

In 1994, this was the best view of Pluto and its moon Charon (right) that the world had ever seen. Taken by the Hubble Space Telescope’s Faint Object Camera, the image showed both objects clearly, but little else.

In 1996, the world got its first look at the surface of Pluto through the Hubble Space Telescope. Taken with the European Space Agency’s Faint Object Camera, the image was 100 pixels across and showed intriguing hints of lighter and darker areas.

In 2006, Hubble added two small moons to Pluto’s lineup: Nix and Hydra (far right). Pluto now has five known moons, including its large companion Charon (right of Pluto), and New Horizons has been looking for more.

In 2010, an analysis of Hubble images revealed a mottled world of orange, white, and black. The center held a mysterious bright spot, prompting NASA to time the New Horizons mission for a better view of the area, now seen as a heart shape.

On April 9, 2015, the New Horizons spacecraft took this photograph of Pluto and Charon from a distance of about 71 million miles (115 million kilometers). It was the first color photo of the Pluto system made by an approaching spacecraft.

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