Mysterious Planet X May Really Lurk Undiscovered in Our Solar System

Planet X" might actually exist — and so might "Planet Y."

At least two planets larger than Earth likely lurk in the dark depths of space far beyond Pluto, just waiting to be discovered, a new analysis of the orbits of "extreme trans-Neptunian objects" (ETNOs) suggests.

Researchers studied 13 ETNOs — frigid bodies such as the dwarf planet Sedna that cruise around the sun at great distances in elliptical paths. [Meet Our Solar System's Dwarf Planets]

Theory predicts a certain set of details for ETNO orbits, study team members said. For example, they should have a semi-major axis, or average distance from the sun, of about 150 astronomical units (AU). (1 AU is the distance from Earth to the sun — roughly 93 million miles, or 150 million kilometers.) These orbits should also have an inclination, relative to the plane of the solar system, of almost 0 degrees, among other characteristics.

But the actual orbits of the 13 ETNOs are quite different, with semi-major axes ranging from 150 to 525 AU and average inclinations of about 20 degrees.

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