Gliese 581e is an extrasolar planet orbiting the star Gliese 581, approximately 20.4 light years from Earth in the Libra constellation. It is the third planet discovered in the system (or fourth if the refuted planet candidate Gliese 581d is included) and the first in order from the star.
The planet was discovered by an Observatory of Geneva team led by Michel Mayor, using the HARPS instrument on the European Southern Observatory 3.6 m (140 in) telescope in La Silla, Chile. The discovery was announced on 21 April 2009. Mayor's team employed the radial velocity technique, in which the orbit size and mass of a planet are determined based on the small perturbations it induces in its parent star's orbit via gravity.
At a minimum mass of approximately 1.7 Earth masses, it is one of the least massive extrasolar planets that has been discovered near a normal star, and relatively close in mass to Earth. It is also the exoplanet with the smallest accurate true mass known. It orbits its star at a distance of 0.03 AU (4,500,000 km), and orbits too closely to be in the habitable zone. It is unlikely to possess an atmosphere due to its high temperature and strong radiation from the star, although scientists believe that it likely has a rocky body like Earth, making it a terrestrial planet. It is also likely to experience intense tidal heating similar to (and likely more intense than) that affecting Jupiter's moon Io. Gliese 581e completes an orbit around its parent star in approximately 3.15 days.