With its 18-hour year, this bizarre lava-world underlines the incredible diversity of planets in our galaxy. And it’s a bit like Star Wars planet Mustafar

First they thought it was a water world, a planet larger than Earth covered in nothing but ocean. Then they thought it might be a diamond world, covered in mountains of graphite and diamond. Now, researchers think that near-by 55 Cancri e has an entire hemisphere engulfed in lava.

The planet orbits a sun-like star located just 40 light years away. It orbits its parent star about 100 times closer than Earth to the sun, completing a circuit in just 17.68 hours.

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So close to its parent star, the planet is locked by gravity to show only one face to the star rather like the moon shows only one face to Earth. This means that one hemisphere of the planet is permanently sunlit, while the other is in perpetual darkness.



The planet has attracted a lot of interest since 2011, when it was discovered to cross the face of its star and block out some of its light. This allowed the planet’s atmosphere to be analysed. No water vapour was found, putting paid to the idea of it being a water world.



An analysis of the parent star, however, showed a higher than usual concentration of carbon-bearing elements. This led researchers to suggest next that 55 Cancri e could be a diamond planet with a landscape composed of graphite and diamond mountains.

The latest work involves observations of the planet with the Spitzer space telescope, Nasa’s orbiting infrared observatory. It shows that the temperature of the sunward facing hemisphere soars to 2500°C, while the permanently dark hemisphere reaches around 1100°C.

At these temperatures the hot side must be completely molten. At the terminator, the name for the boundary between the light and dark side (sorry, another Star Wars reference), their must be some form of lava shoreline as the molten rock solidify into landforms. In the twilight of the terminator region, the lava will be glowing red hot casting a hellish appearance across the alien landscape.

Dr Brice-Olivier Demory of the University of Cambridge’s Cavendish Laboratory is the lead author of the paper announcing the new results. Although the work answers some questions about the nature of the planet, it raises others.

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For example, despite the proximity of 55 Cancri e to its star and the tremendous amount of blinding sunlight it receives as a result, the temperature calculated from the infrared observations is higher than expected. So there must be another source of heat in the planet.

At eight times the mass of the Earth, it seems certain that the planet will contain a lot more radioactive elements than our world. As these decay, they would heat the interior, perhaps providing the extra heating.

One thing is certain, 55 Cancri e must now be a top target for the James Webb Space Telescope. This Nasa-built spacecraft is the successor to the Hubble Space Telescope and will be launched in 2018 by the European Space Agency. Its mirror will be more than seven times larger than the Spitzer’s. Although it works at somewhat different infrared wavelengths it will be able to study nearby planets such as 55 Cancri e in unprecedented detail.

But perhaps the best thing about the announcement of this discovery is that none of the astronomers felt duty bound to reference Mustafar, the lava planet on which Obi-Wan Kenobi and Anakin Skywalker fought their climatic light sabre battle in Star Wars: Revenge of the Sith. That’s what journalists are for. Enjoy:

Facebook Twitter Pinterest Hayden Christensen and Ewan McGregor in a scene from Star Wars Episode III Revenge of the Sith. Photograph: Ho/Reuters

Stuart Clark is the author of The Unknown Universe (Head of Zeus), and co-host of the podcast The Stuniverse (Bingo Productions).