While astronomers have been explaining the origin and composition of our Sun’s family of planets for hundreds of years, this story has only come together in the last 30 years or so. Before then, astronomers assumed that planets were born in the location and configuration in which we see them today. The idea of planets moving about while they are forming was only seriously considered once planets in other systems — exoplanets —had actually been found.
The hot Jupiters
In 1995, astronomers studying the nearby star 51 Pegasi found that it appeared to be wobbling back and forth, movement that revealed itself through a regular pattern of Doppler shifts in its spectrum. The observations suggest that the star is in orbit around a position just slightly offset from its center. By measuring the size of the wobble, it was possible to get an estimate of the mass of the perturbing object, which turned out to be less than half the mass of Jupiter, too small to be a star. They had found an exoplanet!
The discovery was puzzling. The new planet completed an orbit every 4.23 days. That placed this giant world, which we would have expected to find in the icy reaches of its outer solar system, seven times closer to its star than Mercury is to the Sun. To find a giant planet so close to its star that its year was measured in just a few days was completely unexpected, calling into question everything we’d assumed about planetary systems.
One such planet might have been an exception, the result of some gloriously unlikely freak accident of planet formation. But “hot Jupiters” like the planet perturbing 51 Peg were found to be common. More than 400 are known to date, and they account for around 10 percent of known exoplanets. This overstates their actual abundance — it is easier to find a large planet close to its star, where it will induce significant wobbles, than to pick out the signal of a puny Earth-sized world. Correcting for these biases, hot Jupiters seem to account for about 1 percent of all worlds. That doesn’t sound like much, but it could easily mean there are a billion hot Jupiters in the Milky Way alone!