Planet Nine isn’t hiding because it would be small — if it exists, it may be several times Earth’s mass and more than twice as wide. But hundreds of times farther from the Sun than Earth, it receives hundreds of thousands of times less sunlight, and the tiny fracti - Space Daily

Planet Nine, if it exists, would not be a pebble overlooked in the dark. The current hypothesis describes a world several times Earth’s mass, probably built more like a cold sub-Neptune than a rocky …