Two decades ago, the planets of our Solar System were the only ones we knew to be orbiting a Sun-like star. Of course, we had theorized there would be many others around other stars – but none had been found.
So October 6 this week marked a momentous occasion, as NASA heralded the 20th anniversary of the first planet to be found orbiting a Sun-like star, 51 Pegasi b. This was arguably not the first exoplanet discovery; that honor belongs to PST B1257+12 A, a planet orbiting a pulsar that was discovered almost by accident three years earlier, although there is some cause for contention. But 51 Pegasi b was the first spotted around a “normal” star like our Sun.
Its discovery, by astronomers Michel Mayer and Didier Queloz, was announced in Nature on October 6, 1995. They found it using the radial velocity method, which detects the gravitational influence of a planet on its star. 51 Pegasi b is half the mass of Jupiter but orbits closer to its star than Mercury does to our Sun, a prototype for so-called hot Jupiters.
Since then, planet hunting has moved from a fringe science to one of the major fields in astronomy. Thousands of exoplanets have now been found, most using NASA’s hugely successful Kepler space telescope. Upcoming space observatories, including the James Webb Space Telescope (JWST) and the Transiting Exoplanet Survey Satellite (TESS), will continue to study and hunt for these strange new worlds.
The “holy grail” for planet hunters is to find another planet exactly like Earth. That is, one with the same age, size, and mass, orbiting a star like our Sun at exactly the same distance. The closest we’ve come so far is Kepler 452b (second from right in the image above) earlier this year, but at 1.6 times the size of Earth, it doesn’t quite fit the bill.
We can look back 20 years, though, and see just how far we’ve come. Along the way, we’ve discovered planets we never thought possible: hot Jupiters in tight orbits, rocky worlds made of diamond and even super-Earths, terrestrial planets much larger than our own. Who knows what else is out there, waiting for us to find.