The observable universe shrunk to the size of Earth still leaves the solar system vanishingly small. Under that compression, a single atom in the human body would remain far too large to stand in for the entire planetary system.
The comparison rests on orders of magnitude that stretch intuition. The observable universe spans tens of billions of trillions of kilometers, while Earth is only a few thousand kilometers across. Apply a uniform scale factor so that this cosmic horizon fits inside a globe, and the solar system contracts by the same ratio. Yet the solar system is already mostly vacuum: planets and star occupy a tiny fraction of its volume, defined by orbital mechanics and gravitational potential rather than solid matter.
An atom, by contrast, is set by quantum mechanics and the Bohr radius, with electrons described by probability clouds rather than miniature planets. Even so, an atom is enormous compared with the scaled solar system in this thought experiment. The diameter of an atom relative to its nucleus is large, but the gap between the solar system and the observable universe in linear size, and even more in volume, is vastly larger. The result is a hierarchy where planets shrink past dust, past molecules, and past atoms, slipping into a regime where familiar physical analogies fail.
