'Planet Y' theory hints at hidden Earth-size world lurking in the solar system — and it could be much closer to us than 'Planet Nine'

An artists intepretation of Planet Nine shows a large dark planet with the sun in the distance.

Astronomers believe they have uncovered evidence of a previously unknown ninth planet lurking in the not-too-distant reaches of the solar system. This hypothetical world, dubbed "Planet Y," could be around the same size as Earth and may exist alongside another popular Planet Nine candidate — potentially bringing our cosmic neighborhood's planetary tally to 10, if both can be confirmed.

However, despite researchers' claims that this newly theorized world could be found within three years, there is no proof of its existence, and some experts are skeptical of the new findings.