Planetrise: an artist's impression of the 'hot neptune', Kepler 36c, lurking in the skies above the volcanic surface of Kepler 36b. Image: David A Aguilar (CfA).

Two planets that are such close neighbours they loom in one another's skies at less than five times the average Earth-Moon distance threaten to challenge what we know about planet formation and migration.

The two worlds, Kepler 36b and 36c, were discovered 1,200 light years away by NASA's planet-hunting Kepler Space Telescope, which looks for the miniscule dimming of a star's light as a planet passes in front of it. The extent of the dimming can tell us the diameter of the eclipsing planet while its periodicity tells us the planet's orbit.

In the case of Kepler 36, found in the constellation of Cygnus the Swan, the transit signals of two planets were found. One, designated 36b, is a so-called 'super-earth', a rocky planet one and a half times bigger than Earth and 4.5 times the mass of our planet. The second, designated 36c, is a more exotic 'hot neptune', which is 3.7 times the diameter of Earth and eight times as massive. What's particularly special about this pair is that while planet b orbits its star every 14 days at a distance of 17.7 million kilometres, planet c orbits every 16 days at a distance of 19.3 million kilometres. When the planets are in conjunction, every 97 days, there is just 1.6 million kilometres between them. For comparison, the Moon's average distance from Earth is 384,400 kilometres while the distance to the nearest planet, Venus, at conjunction is 41 million kilometres.

According to research published in the 21 June issue of the journal 'Science Express', at closest approach planet c will grow as large as the Moon appears in our sky. Forget about the spring and neap tides on Earth; the gravitational tides these two planets will experience are far greater. Kepler 36b's innards will be pulled and squeezed, similar to how Jupiter's gravity kneads the interior of its moon Io to produce tremendous amounts of volcanic activity. Kepler 36b may well be a volcanic world.

"These are the closest two planets to one another that have ever been found," says Professor Eric Agol of the University of Washington, who was co-lead on the discovery. "The bigger planet is pushing the smaller planet around more, so the smaller planet was hard to find."

The greatest puzzle surrounding this tight-knit pair is, of course, why they have ended up so close to one another. Agol and his team of 45 other scientists from the USA, Europe and Australia point out that in our Solar System small rocky planets and large gas or ice giants are separated, suggesting that they formed in different regions of the Solar System and perhaps even in different ways. In the case of Kepler 36, however, there's a small terrestrial world right on the doorstep of a giant planet. Planetary formation models describe how planets can migrate in-system from colder reaches farther out and this may have occurred with planet c. Either way, the two planets have a larger density contrast than any other pair of adjacent planets known to exist, with rocky planet b eight times denser than planet c, akin to a golf ball next to a beach ball. Indeed, the science team estimate that planet b has an iron core that constitutes 30 percent of its mass, surrounded by an atmosphere of less than one percent hydrogen and helium and no more than 15 percent water. Planet c may also have a rocky core but it is surrounded by a much thicker atmosphere of hydrogen and helium. Unfortunately, it is unlikely that either world has denizens to appreciate the view in the sky as the two plants near – both worlds are too hot for life as we know it.