55 Cancri e (abbreviated 55 Cnc e) is an extrasolar planet orbiting the Sun-like star55 Cancri A. Its mass is about 7.8 Earth masses and its diameter is about twice that of Earth's, thus classifying it as the first Super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It takes less than 18 hours to complete an orbit and is the innermost known planet in its planetary system. 55 Cancri e was discovered on August 30, 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star.In October 2012, it was announced that 55 Cancri e could be a carbon planet

 

Discovery

Like the majority of known extrasolar planets (found previous to the Kepler mission), 55 Cancri e was discovered by detecting variations in its star's radial velocity. This was achieved by making sensitive measurements of the Doppler shift of the spectrum of 55 Cancri A. At the time of its discovery, three other planets were known orbiting the star. After accounting for these planets, a signal at around 2.8 days remained, which could be explained by a planet of at least 14.2 Earth masses in a very close orbit. The same measurements were used to confirm the existence of the uncertain planet 55 Cancri c.

 

55 Cancri e was one of the first extrasolar planets with a mass comparable to that of Neptune to be discovered. It was announced at the same time as another "hot Neptune" orbiting the red dwarf star Gliese 436 named Gliese 436 b.