NASA's Spitzer Space Telescope has found that exoplanet 55 Cancri e, which was first discovered in 2004, is less dense than previously reported; instead of a scorched, rocky world, water vapour and other gases likely steam from its molten surface.

The planet is the innermost of five known in the 55 Cancri system, which is just 40 light years from Earth, close enough and bright enough to be observed with the naked eye on a clear night. Orbiting at a distance 26 times closer to its star than Mercury does to our own Sun, and in less than 18 hours, 55 Cancri e claims the shortest time period known of any exoplanet.

The transiting nature of the exoplanet as seen from Spitzer's point of view enabled the new, detailed observations.

"The exquisite observations obtained by Spitzer provided a precise measurement of the planetary radius," lead author Brice-Olivier Demory of the Massachusetts Institute of Technology (MIT) tells Astronomy Now. The planet's radius was found to be just over twice that of Earth, and has a mass equivalent to 7.8 Earths. "The planetary mass has been known since its discovery, so our measurements provided a value for the density of 55 Cnc e, giving the opportunity to explore its composition. The Spitzer observations allowed us to firmly conclude a planet with a significant amount of volatiles, such as water vapor. This was unexpected for a planet orbiting so close to its star, where the high temperature should evaporate such atmosphere. 55 Cnc e therefore remains particularly enigmatic."

The planet's surface is estimated to reach searing temperatures of at least 1,760 degrees Celsius, with its close orbit similar to the inferno-worlds CoRoT-7b and Kepler-10b. Yet its low density is more in line with the cool super-Earth GJ1214b. Demory and his team suggest that around one-fifth of the planet's mass must therefore be light elements and compounds, like water, but that due to the high temperatures the water would exist in a super-critical state, that is, between a liquid and gas.

These curious characteristics imply an intriguing back story. Most likely, 55 Cancri e began life as a gaseous Neptune-like planet far from its sun. Gravitational interactions between the five known planets in the system caused migrations, setting the planet on course to eventually death-spiral into the star.

"Future observations will certainly help in computing an estimate [as to when this may occur]," says Demory. "We have new Spitzer observations scheduled next year that will observe the occultation of this super-Earth, which will help in constraining its orbital properties."

The paper describing the new results has been accepted for publication in Astronomy & Astrophysics.