No, there isn't another fully formed planet within Earth. However, compelling evidence suggests remnants of another protoplanet, Theia, may exist deep within Earth's mantle.
Theia and the Moon-Forming Impact
The prevailing theory for the Moon's formation is that a Mars-sized protoplanet named Theia collided with early Earth. This impact ejected vast amounts of material into space, which eventually coalesced to form the Moon.
The Presence of Theia Remnants in Earth's Mantle
Seismic studies have revealed two large, dense structures deep within Earth's mantle, beneath Africa and the Pacific Ocean. These structures, known as Large Low-Shear-Velocity Provinces (LLSVPs), have puzzled scientists for years. One hypothesis suggests that these LLSVPs could be remnants of Theia's mantle that sank into Earth's mantle after the impact. These remnants likely have different chemical compositions and densities than the surrounding mantle material, which is why they can be detected seismically.
What the Remnants Aren't
It's important to clarify that these potential Theia remnants are not separate, fully formed planets. They are large, distinct regions within Earth's mantle that haven't fully mixed with the surrounding material. They may retain some of Theia's original characteristics but are now integrated into Earth's structure.
In Summary
While there isn't another planet orbiting inside the Earth, there is increasing evidence that a large portion of Theia, the planet that collided with Earth to form the moon, may still exist as distinct structures within the Earth’s mantle. These structures are not planets themselves, but rather integrated remnants of a protoplanetary collision.
