Kepler-36b and its companion planet, Kepler-36c, are far too close to their sun to be in the habitable zone, but they are bizarrely close to each other. On their closest approach, they come within about 1.2 million miles (1.9 million kilometers) of each other.

Despite being in a similar orbital distance from their star, the pair are very different.

Kepler-36b, is a super-Earth that orbits a G-type star 1,533 light-years from Earth in 13.9 days. Its mass is about four times greater than Earth's.

Its companion planet, Kepler-36c is Neptune-sized with a mass more than seven times Earth's; it takes 16.2 days to complete one orbit of the star, which is older and hotter than our Sun. The two planets have repeated close encounters, experiencing a conjunction every 97 days on average. At that time, they are separated by less than five Earth-moon distances. Such close approaches stir up tremendous gravitational tides that squeeze and stretch both planets, which may promote active volcanism on Kepler-36b.

"If I could withstand the high temperature, I would love to stand on the inner planet as it passes its outer companion. The close proximity and large size would cause Kepler-36c to loom large in the sky, several times wider than our Moon appears from Earth. That would be a sight to behold!" said Eric Agol, a professor of astronomy at the University of Washington and one of the system's discoverers.