Early discoveries in exoplanet detection used the radial velocity (RV) technique to discover exoplanets. This relies on the Doppler effect, whereby a planet orbiting a star tugs on it slightly. When astronomers split the light into a spectrum, we can see the tug of an exoplanet in the shift of lines from red (away from us) to blue (toward us).
Credit: ESO/L. Calçada
When astronomers first started finding planets around other stars (extra solar planets AKA: exoplanets), we did not find planets in configurations like our own Solar System. Instead, among these first discoveries were very short period gas giant planets orbiting very close (within the orbit of Mercury) to their parent star. These planets have been dubbed Hot-Jupiter planets. It makes sense that these were among the first planets discovered, as they are close in and massive, thus providing the largest amount of tug on their parent planet over a short period of time. This made them the easiest to discover in the early days of RV work.
Finding planets in these very close orbits threw into question theories of planet formation. Most planetary formation models have smaller rocky planets forming closer in to the parent star, and larger gas giant planets forming much further out near what astronomers call the “snow line”. This is a distance from the young star where it becomes cool enough for gas to condense onto rocky bodies and accrete large amounts of mass.
Although it has been suggested that these large gas planets may be able to form close to their parent star, it is still largely believed that they must form out past the snow line. They must migrate inward in some planetary systems to end up in close in orbits. This migration scenario is still not well understood.