Schematic view (not to scale) with our solar system on the upper level and a habitable planetary system with a "hot Jupiter" on the lower level. Our solar system has four terrestrial planets: Mercury, Venus, Earth and Mars. Earth is in the habitable zone, where water can exist on its surface. The lower planetary system contains a hypothetical "hot Earth" a few times larger than the Earth but very close to the star, a theorized ocean-covered planet in the habitable zone, and several icy planets in the outer system. Credit: Sean Raymond/NASA
