Mysterious 'Planet X' detected beyond Pluto

This icy oddball may be the first of its kind from the far outer parts of our stellar neighbourhood.

By Nadia Drake
Published 4 Oct 2018, 09:19 BST
An illustration of Planet X, an as-yet unseen world that could be shaping the orbits of ...
An illustration of Planet X, an as-yet unseen world that could be shaping the orbits of smaller, extremely distant objects like the newly described 2015 TG387.
Photograph by Illustration by Carnegie Institution for Science, DTM, Roberto Molar Candanosa/Scott Sheppard

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 180 miles wide. And—like a handful of other distant solar system inhabitants—its orbital behaviour 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  in a notice distributed by the International Astronomical Union’s Minor Planet Centre.

“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.

Despite that relative proximity, you won’t be able to see it in the sky right now. 2015 TG387 is a 24th magnitude object, meaning that it’s about as bright as one of Pluto’s small moons—and you can’t even see Pluto itself without a decently sized home telescope and some practice. Sheppard estimates that it’s about 180 miles wide, although that calculation depends on how reflective its surface is.

Planetary Breadcrumbs

The far-off object has astronomers excited because 2015 TG387 joins several other recently discovered worlds that stay far away and trace exaggerated ellipses around our star, never coming closer to the sun than Neptune. These include Sedna, discovered in 2003, and 2012 VP113, which is nicknamed Biden.

An illustration shows the orbit of the new object 2015 TG387, nicknamed the Goblin, as compared to the rest of the solar system.
Photograph by Illustration by Carnegie Institution for Science, DTM, Roberto Molar Candanosa/Scott Sheppard

Its orbit also aligns with a cluster of other objects that together suggest that a large planet, perhaps several times more massive than Earth, is lurking in the inky darkness of the outer solar system.

“It suggests something is pushing these objects into similar types of orbits, that’s why we think there’s a large Planet X out there,” Sheppard says. “Something super-Earth to Neptune-size, shepherding these objects into these orbits, which is why they are stable.”

Sheppard and his colleagues are searching for this planet, and he says that 2015 TG387 somewhat helps narrow down the search area. For now, he suspects that Planet X is orbiting in opposition to 2015 TG387, creating a gravitational resonance that sweeps up and preserves those wacky, far-flung orbits.

But it’s also possible that astronomers are only seeing a small, biased subset of the Sedna-like inhabitants of the outer solar system, and that the rest of the population traces paths that don’t point to a planet.

For instance, Michele Bannister, an astronomer at Queen’s University Belfast, isn’t so convinced that a large mystery planet is out there and wants to see more tests done to make the case.

“The way you test these things is by knowing the abundances of different sorts of orbits and the shapes of orbits,” she says. “I’m looking forward to seeing one of these simulations in more detail with the addition of an extra planet.”

Ancient Relics

Still, Bannister is excited about this newly charted world because it could be the first emissary from a whole population of objects we don’t normally get to see.

“Every single one of these detections is the tip of the iceberg of a huge population, where we’re seeing the brightest one because it happens to be the closest to the sun or the larger member of its population, so it becomes detectable,” she says.

These motley collections of frozen fragments are key to understanding the full layout of our solar neighbourhood and its history, Bannister says. Until now, scientists have only been able to tap into several populations of these objects, including those with orbits that bring them within 50 astronomical units of the sun, those like Sedna that stop about 80 astronomical units away, and those that fling themselves straight into the heart of the inner solar system.

The last group, comets, are thought to come from the outer fringes of the Oort cloud, a distant cluster of icy fragments scattered between 2,000 and 200,000 times farther from the sun than Earth. 2015 TG387 is likely similar to a comet in composition in that it’s predominantly made of ice, but its orbit is not similar at all.

Instead, it likely originates well inside the inner fringe of the Oort cloud, a realm that we so far have not been able to probe very well.

Bannister says one of the outstanding mysteries swirling around these distant objects is the question of how, exactly, they came to be. It’s tricky building a population of objects that never venture closer to the sun than Neptune; there aren’t enough materials out there to craft these objects in place.

It’s similarly unclear how they might have been pushed so far away. Theories range from gentle gravitational nudges perturbing their orbits over time, to self-gravitating planetesimals, to close flybys of stars or rogue, starless planets.

“This population remains exciting because we don’t have a well-constrained explanation for what makes them,” Bannister says. “These could be fossilised planetesimals from the very dawn of our solar system, placed there by a yet-to-be detected mechanism.”

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