Types of Planets
From antiquity, philosophers and scientists have pondered and debated the existence of planets outside of our Solar System. It was established during the Renaissance that the planets revolve around the sun in a perpetual free fall, just as the moon revolves around the earth. But the extent of the sun's gravity well marks the end of our cosmic neighborhood, and the entrance into interstellar space. Any planet found beyond this sphere is classified as an extrasolar planet. The general consensus nearing the end of the 20th Century was that most, if not all, stars in our Milky Way galaxy, and indeed all main sequence stars in the universe, are orbited at some point by at least one planet. However, not a single extrasolar planet, or exoplanet, had been found even by the dawn of the 1990's.
This all changed in 1992 when several detected planets were confirmed orbiting a pulsar. Then, in 1995, a planet was confirmed orbiting a main-sequence star (a sun-like star). As of the most recent survey (March 2013), there are 861 confirmed exoplanets. In the late 1990's when exoplanet hunting was in its infancy, the techniques were relatively crude and led to a sampling bias of large planets revolving very near to their parent star. These were gas giants resembling Jupiter (although several times larger), and orbited closer to their parent stars than Mercury orbits the sun. Representing a new understanding of solar system formation, no gas giants orbit the sun at such a close range in our Solar System, but these new worlds were not revolutionary to our understanding of planetary science.
However, as detection techniques improved, it slowly became apparent to scientists that there is a "world" of possibilities when it comes to the structure and chemical composition of exoplanets. The following are several hypothesized types of planets put forth by scientists and astronomers.
A helium planet is a hypothesized planet composed primarily of helium. It would start as a white dwarf as another, nearby white dwarf drew its mass away in a fairly well understood process called accretion. The white dwarf losing mass could eventually approach planetary mass, manifesting as a gas giant. Ordinary gas giants are primarily hydrogen, but a helium planet might form in an environment where all of the hydrogen gas has been converted to helium through nuclear fusion.
A super earth is a terrestrial planet with a mass several times that of the earth. Super earths are not actually hypothesized anymore as several have been discovered. They are usually between 1 to 10 times the mass of the earth, with a possible atmosphere, plate tectonics, and a solid core (as opposed to gas giants).
This type of world begins life as a gas giant. It becomes Chthonian if its atmosphere is stripped away from wandering too close to its parent star. The metallic or rocky core left over would somewhat resemble a terrestrial world. A rogue planet drifting through space could also come too close to a passing star and lose its atmosphere, becoming Chthonian.
Like the name implies, this hypothetical class of planet has no core. That is, it has no differentiated, metallic core, but it's not hollow. The Earth and the other terrestrial bodies of our solar system have undergone differentiation, which means the heavier elements (metals) have sunk to the core. A coreless planet would be terrestrial, but through a couple of possible unique chemical settings, would never differentiate, and therefore would have no metallic core. The center would be the same compositionally as the mantle, which on earth is just beneath the crust. One interesting aspect of a coreless planet would be the lack of a magnetic field. This is explained by a planet's magnetic field being generated by the circulation of metallic elements (especially iron) in the liquid portion of the core. A coreless planet has no metallic core, and therefore, no magnetic field.
An ocean planet is another single biome planet with water covering the entire surface of the planet. Such a planet would resemble the earth if all of the dry land were submerged. However, scientists have imagined a type of ocean planet that is more than just a flooded world. They hypothesize that a body could exist as simply a gigantic, cosmic drop of water with little or no rocky material. It should be noted that planets like to migrate from time to time. If an ocean planet migrated too far from the sun, it would become instead nothing but ice water.