On this day in 1995, two Swiss astronomers announced that they had discovered the first extrasolar planet orbiting a star like the Sun. Didier Queloz (left in photo) and Michel Mayor from the Geneva Observatory had observed the star 51 Pegasi, which is located about 50 light-years away in the constellation Pegasus. By analyzing the star's light the researchers determined that 51 Pegasi was wobbling. That wobble is due to the gravity of a massive object near the star: a planet that has about the same mass as Jupiter yet has an orbit that is much smaller than Mercury's around the Sun. Within a week of the Swiss announcement, an American team confirmed the existence of 51 Pegasi b. It's worth noting that 51 Pegasi b is not the first planet outside the solar system to be discovered: In 1992 researchers had spotted planets orbiting dead stars called pulsars. But Mayor and Queloz's discovery proved that stars that resemble our own can support planets. It also revealed that other stars host worlds that are nothing like those in our solar system. Most importantly, the discovery ushered in an exoplanet boom. Today, thanks to instruments such as the Kepler space telescope, astronomers have discovered more than 8000 likely exoplanets. Because we can detect only a tiny fraction of the planets out there, it's estimated that there are more planets than stars in our galaxy, and that billions of planets in the Milky Way are roughly the size of Earth. (Photo credit: L. Weinstein/Ciel et Espace Photos)
