GJ 876 d was, when it was found in 2005, by far the lightest planet known around a normal star other than the Sun: a few times Earth's mass, far lighter than the gas giants that dominated early discoveries. It orbits GJ 876, a red dwarf about 15 light-years away, in under two days, the innermost of the star's four planets.
A team led by Eugenio Rivera announced it in 2005 after eight years of frequent, precise measurements at Keck Observatory of the star's wobble, the slight back-and-forth motion planets' gravity gives it. The star's two known giant planets explained most of that motion, but the leftovers repeated every 1.94 days. Fitting a third planet improved the match significantly. Its minimum mass came out at about 5.9 Earth masses; assuming all the planets share one orbital plane, tilted about 50 degrees to our line of sight, its true mass is about 7.5 Earth masses. The team called it by far the lowest-mass planet then found around a main-sequence star other than the Sun.
The same study measured the star's rotation, once every 96.7 days, and used precise brightness measurements to show that GJ 876 d does not pass in front of its star, so its size has never been measured; the NASA Exoplanet Archive's radius is an estimate. A 2010 follow-up confirmed that its orbit is noticeably stretched. Orbiting so close, it is hot, with an estimated equilibrium temperature, set by starlight alone, of about 600 kelvin, and receives nearly 30 times the light Earth gets from the Sun.