Instead of silicate rocks, such a world would boast an incredibly heat-resistant ceramic crust of silicon and titanium carbides. The atmosphere would be rich in carbon monoxide and methane, which would react in the sunlight to produce a thick smog, hiding oceans of oily hydrocarbons. Most bizarrely, high pressures deep underground would forge the carbon into a complete shell of diamond.
Description: A terrestrial planet completely smothered in deep oceans. Formation: This could develop in two ways. Either a planet would form in the cold outer solar system from ice-rich material and then migrate inwards – the ice thawing into a globe-spanning ocean. Alternatively, the discs of planet-forming material around other young stars could become mixed-up, perhaps by the shifting orbit of a gas giant world, and so build Earth-like worlds out of wetter stuff from further out.
This kind of world could contain up to a hundred times more water than the Earth, and so its rocky crust would be completely swamped in a liquid shell hundreds of kilometres deep. Not even the highest mountains would be tall enough to pierce the surface, so it would form a single unbroken global ocean.
Detectability: Waterworlds could be pretty common in the galaxy, but even detecting a lot of water vapour in the atmosphere doesn’t indicate whether the whole surface is smothered with ocean. Again, a trip out there would be required.
Description: A hellish world pockmarked with volcanoes and oceans of magma.
Formation: A rocky planet orbiting too closely to its sun would not only experience searing heat, but also powerful tidal effects from the star’s gravity. These tidal forces would constantly flex and knead the planet’s crust, causing intense internal heating. Such a tormented world would be violently volcanically active and sport churning oceans of bubbling magma.
‘Chthonian planets’, named after the Greek gods of the infernal underworld, could be formed when a gas giant like Jupiter spirals too close to its star and its atmosphere becomes blown away into space. All that remains would be the rocky core feeling the pummelling tidal effects of the overbearing star.
Detectability: The newly discovered planet CoRoT-7b orbits its star so closely it could be the first example of a Chthonian world.
Description: A terrestrial planet pitched onto its side.
Formation: A planet could become knocked over so that it spins on its side, like a marble rolling around the Sun, either by a large impact (as happened with Uranus) or periodically tipped by the gravitational interaction of gas giant planets in the system. Such a topsy-turvy world would have a bizarre climate and seasons, with the equator receiving less sunlight than the poles and each hemisphere experiencing constant night-time for half of each year.
Detectability: A large space telescope array in the future could watch the spin of a world.
Description: A planet almost completely composed of the element iron.
Formation: Our Earth consists of a core of molten iron surrounded by a mantle and crust of silicate rocks. The planet Mercury, however, is exceptionally dense, and is thought to have had most of its silicate layer blasted off in a colossal impact. If this happened to a larger proto-world, the iron core left behind would be an Earth-sized planet, which could form oceans and atmosphere and a very odd chemistry on its iron-rich surface.
Detectability: A cannonball world would have a very high density – something we could spot. But ultimately, we’d have to go there to know its composition for sure.