Astronomers know of more than 3,500 exoplanets—worlds orbiting stars other than our sun—and will probably find thousands more in the next few years. Some of these newfound worlds will resemble our own planet in size, composition and temperature. Yet many of these potential “mirror Earths” will be alien in one respect: they will orbit red dwarfs, also known as M dwarfs, rather than sunlike stars. M dwarfs are the universe's smallest, coolest stars, but they are also the hottest sites for exoplanet discovery, largely because of their sheer abundance. Although none are visible to the naked eye, they make up more than half of the 140 known stars within 20 light-years of the sun and harbor two thirds of the known exoplanets in that region (graphic). Even though M dwarfs are cool stars, planets orbiting close to them would be warm. Earth-like or not, these alien worlds are destined to become the ones we know best.

Credit: Nadieh Bremer; Sources: Jessie Christiansen and Eric Mamajek (consultants); NASA Exoplanet Archive and NASA Exoplanet Exploration Program (data)