planets of size 2–4 have proved to have a wide range of densities , implying a diversity of compositions4,5 , but these measurements of the mass of the earth .
it has approximately the same size .
planets with sizes between that of earth that 's been discovered outside our solar system , " said andrew howard , of the university of hawaii at manoa 's institute for astronomy and lead author of one of the studies . "
given a radius of 1.20 ± 0.09 and a mass of 1.69 ± 0.41 , the planet kepler-78b , which orbits a sun - like star 400 light - years from earth .
the planet is 20 percent wider and 80 percent more massive than earth .
credit : david a. aguilar ( cfa ) view full size image a puzzling alien planet is the closest thing to an earth twin in size and composition known beyond our solar system , though it 's far too hot to support life , scientists say .
 
measuring the planet ’s mass — and hence its density , which is a clue to its composition — is more difficult .
most such planets have been discovered through the transit technique , by which the planet ’s size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star .