A mysterious Mars-sized planet may be hiding at the edge of our solar system

By Sarah Kaplan, The Washington Post
A mysterious celestial body may be lurking in the frozen, far-flung reaches of the solar system, scientists say.
This is not the proposed “Planet Nine,” a ginormous body that Caltech scientists believe could be tugging at the orbits of the solar system’s most distant inhabitants. And it’s not Pluto. (Sorry Pluto, you still don’t count.)
Instead, University of Arizona astronomers Kat Volk and Renu Malhotra say it’s a Mars-si