Nobody gives out awards for Worst Planet in the Galaxy—and it would be a dubious honor at best. Somewhere out there may be other garden planets like Earth. Somewhere too may be near-miss worlds like Mars. And somewhere out there are planets like 51 Pegasi b which, if it didn’t win the Worst Planet nod, would surely make it to the medal round.
Located about 50 light years from Earth, 51 Pegasi b is a gas giant like Jupiter, with a mass about 150 times that of Earth, circling its host star at a distance of just 7 million km (4.3 million mi.). Orbiting so close, 51 Pegasi b has a surface temperature estimated at 1,000º C (1,800º F). It’s also tidally locked, meaning one side is always facing that solar oven.
But 51 Pegasi b is more than just an overheated gas ball. In fact, it’s one of the most significant planets ever discovered—so important that, in 2019, the astronomers who found it were awarded the Nobel Prize in Physics. Why? Because whatever its shortcomings, it was the first planet ever discovered outside our own solar system orbiting a main sequence star like the sun. With that, the field of exoplanets was born.
51 Pegasi b was discovered on October 6, 1995, and 25 years later, the exoplanet count has grown tremendously. By the most recent tally, there are 4,354 known exoplanets, including 712 multi-planet systems, for a total of 3,218 solar systems beyond our own. Virtually every one of the hundreds of trillions of stars in the universe is now believed to harbor at least one world—and many are home to whole litters. The Earth, which once sat at the center of humanity’s map of the cosmos, is now known to be an impossibly tiny part of an impossibly vast planetary census.
Astronomers didn’t spot 55 Pegasi b directly—it’s no more possible to visualize a planet in the glare of its parent star from a distance of 50 light years than it is to see a moth fluttering near a streetlight from half a dozen blocks away. Instead, the investigators used the radial velocity method, which involves looking at the slight gravitational wobble a planet causes in its star as it makes its orbit. It’s a nifty method for discovering planets, but it’s slow, as only one star can be observed at a time.
With the 2009 launch of the Kepler Space Telescope, astronomers switched to a technique that can scoop up whole netfuls of worlds at once. Known as the transit method, the technique involves looking for the slight dimming in light that occurs when a planet passes across the Earth-facing side of its parent star. The greater the dimming, the bigger the diameter of the planet. The more frequently the dimming occurs, the faster the planet orbits. Both Kepler and and its follow-on observatory, the Transiting Exoplanet Survey Satellite (TESS), stare unblinking at entire patches of sky at once, observing thousands of potential transits for astronomers to study.
The transit method had turned up far more promising worlds than 55 Pegasi b. There’s Proxima Centauri b, an Earth-like planet just 4.2 light years away, which orbits its star in the “Goldilocks zone,” where temperatures are just right for liquid water to exist. There’s Ross 128 b, another Earth-sized world just 11 light years away, where surface temperatures are estimated to be a balmy 23º C (73º F). There’s the Trappist-1 solar system, just 39 light years distant and home to no fewer than seven Earth-like worlds, at least six of which could orbit in the Goldilocks zone.
The 25 years in which we went from knowing of no other exoplanets to identifying thousands of them (and being certain of trillions more) is a long time when measured against a human lifespan. On the scale of cosmological time, it’s hardly a flicker. But in that flicker, our grasp of our place in the universe was transformed forever. Earth may seem far less special than it did when it was one of only just eight known planets. But it’s home to a species both wiser and humbler for the knowledge it has gained.