Astrophysicists think that Jupiter-like planets did need to form past the snow-line of the protostellar nebula, where the temperature was low enough for ices to condense. It is believed that early in the formation of the system, before fusion began in the star and swept away the dust and gas, that the Hot Jupiters migrated inward toward the star.
Drag forces from the material and pressure waves that pushed in more strongly than the pressure pushed outward would provide a mechanism to drive the massive gaseous planets inward. It is possible that some gas giant planets even migrated inward so rapidly that they crashed into the star. This pressure and drag would cease to affect the planet when fusion began in the star, creating stellar wind that cleared the dust and gas away.
Other effects, like pressure exerted by the magnetic field, could supply a braking mechanism. We now think that our own gas giants could have formed even farther away from the Sun than they are today, and migrated inward.
Computational simulations like the one above from Phil Armitage's research page explore possible scenarios for planet migration. Please visit this page to see a movie of this simulation. As you can see, the planet has cleared a gap in the protostellar nebula. This simulation included a high mass planet, while other simulations include lower mass planets.
Small, rocky planets like the terrestrial planets in our Solar system are much harder to detect than gas giants. Terrestrial planets are much smaller and the low mass means they do not offset the star to a great degree. Even so, quite a number of terrestrial exoplanets have been discovered.
Terrestrial exoplanets are very interesting to us in our search for extraterrestrial life, especially if they are found within the habitable zone of the parent star.
The habitable zone of a star is the region where a terrestrial planet could have liquid water. Since it depends on temperature, you might expect that a hotter star would have a habitable zone that lies farther from the star, compared to that of a cooler star. The habitable zone of a cool, M-type star lies close to the star and is generally narrower.
The image above depicts the inner edge of the habitable zone with a cloud of gas, where water would not stay liquid. The outer edge of the habitable zone is the snow-line, where water would freeze. The habitable zone of a star is often referred to as the "Goldilocks zone" where the temperature is not too hot, not too cold, but just right.
Another kind of habitable zone is the galactic habitable zone. Here, the concern is that planetary systems would have a harder time forming and life would have a harder time arising near the center of the galaxy, where stars are closer together and strong radiation could be detrimental to life.
This artist's depiction of the Trappist 1 system indicates that there are seven terrestrial planets orbiting this small, red dwarf star. It is believed that three of these planets lie within the habitable zone of the star. The Trappist 1 system is also relatively close to us, only about 40 light years away, in the constellation Aquarius.
The James Webb telescope, scheduled to be placed in orbit in October 2018, will have the capability of performing a detailed analysis of the atmospheres of the Trappist 1 system. We will be able to tell the atmospheric content of such gases as water, methane, carbon monoxide, carbon dioxide and oxygen.
One question that is of interest to everyone is what life could be like on the planets of Trappist 1. There are considerations beyond jut the temperature and atmospheric makeup of the planets, such as the proximity of the planets to the star and the effect that strong radiation would have on developing life forms.
The planets around Trappist 1 are so close to their star that their orbital periods are very short. The closest planet completes an orbit in just 1.5 Earth days, and the farthest planet takes 20 days to orbit the star. This close to the star, they could also be tidally locked, which would mean that one side would be very much hotter than the other side.
It may be that despite the rough conditions that life would face so near to a red dwarf star, astrobiologists speculate that living organisms might find a way to survive. Life could possibly adapt to bursts of stellar radiation, and even thrive in the environment.
Clearly, the more a star is affected by its planetary system, the easier it is to indirectly detect the planets. This tends to observationally choose for gas giant planets that lie close to a low mass star. We have detected very many planets that fit this description; we call them Hot Jupiters.
The gas giant planets in our Solar system: Jupiter, Saturn, Uranus and Neptune, lie quite a distance from our star. This fact led us to develop our theory of solar system formation according to the model that gas giant planets form outside the snow-line of the Solar system. With the detection of so many hot Jupiters, we need to rethink our theory.