Is there another planet in our solar system? Astronomers point to ‘Planet Y’

Astronomers have discovered new evidence that suggests the possibility of an unknown planet in our solar system's farthest reaches.

The study, published in the Monthly Notices of the Royal Astronomical Society: Letters, introduces what the researchers name Planet Y, a possible world based on the peculiar tilt of distant Kuiper Belt objects.

The Kuiper Belt, a huge ring of icy celestial bodies beyond Neptune, has long piqued the interest of astronomers looking for hidden planets. While Planet Y has yet to be actually discovered, the odd inclinations in the orbits of approximately 50 distant objects point to its presence.

“One explanation is the presence of an unseen planet, probably smaller than the Earth and probably bigger than Mercury, orbiting in the deep outer solar system,” said lead author Amir Siraj, a doctoral candidate at Princeton University, according to CNN.

“This paper is not a discovery of a planet, but it's certainly the discovery of a puzzle for which a planet is a likely solution.”

The notion of hidden planets beyond Neptune isn't new. Historical exploration for a so-called Planet X began in the early twentieth century, following Neptune's discovery in 1846.

Pluto was once considered a candidate for Planet X when it was found in 1930, but due to its small size, it was eventually reclassified as a dwarf planet instead.

Planet Y differs from the previously hypothesised Planet Nine, which is thought to have five to ten times the mass of Earth and orbits far farther away.

Siraj's team discovered that the tilt measured beyond around 80 times the Earth-Sun distance is approximately 15 degrees, which cannot be explained by a passing star or typical planetary formation models.

“It was quite a surprise to find that the solar system suddenly appears to be tilted by about 15 degrees, and this is what sparked the Planet Y hypothesis,” he said.

Computer simulations conducted by Siraj and his colleagues revealed that existing models, including Planet Nine, could not account for this tilt. They believe Planet Y is a Mercury- to Earth-mass planet orbiting at a distance of 100 to 200 times that of the Earth-Sun, with an orbital tilt of at least 10 degrees relative to the known planets.

Despite these observations, the existence of Planet Y remains unknown. “With these roughly 50 objects, the statistical significance is in the 96 per cent to 98 per cent range,” Siraj said.