There are three types of planets in our solar system. On the one hand we have rocky worlds like Earth and Mars have a solid surface on which to put your feet and consist of 'rocks' (silicates and metals). Then there are gas giants like Jupiter and Saturn, composed mainly of hydrogen and helium (and actually should be called "Giant fluid" because most of its interior hydrogen is a liquid or metallic). Finally we have the ice giants,
Uranus and Neptune, composed of hydrogen and helium gas giants, but also a lot of "ice" (ice water, ammonia, methane, etc...). Until a few years ago we thought the rest of the galaxy stellar system follow a roughly similar pattern. Not so. There are planets out there totally different to those we know.
First came the super-Earths, a type of rocky planet that is abundant in the universe. Depending on its orbit, some super-Earths are believed to have huge lava fields on the surface, while other global oceans would be covered by water hundreds of kilometers deep. Then he discovered the minineptune, planets-as the name suggests, are large balls of ice with a rocky core with a size similar to the larger super-Earths. A mini Neptune is basically a small ice giant and, in fact, to this day do not know how to distinguish the super-Earths mini Neptune from observational data. The mini Neptune is a mystery; hence their study is a priority. Why it is so interesting to the latest discovery of the Kepler space telescope, a planet called Kepler-68b and having characteristics make it one of the super-Earths and mini Neptune.
Kepler-68b is part of a three-star system whose minimum mass planets have been determined by terrestrial observations using the radial velocity method (the Kepler telescope discovers planets by the transit method and can only determine the size of the worlds discovered). It has a mass of between 6 and 10.3 times that of Earth, while its radius is 2.3 times that of our planet. With these data we can calculate its average density, which, with 3.32 grams per cubic centimeter, it being understood right between the density of the ice giants like Neptune and the terrestrial planets. Its period-that is, their year-is only 5.4 days, so that the temperature 'surface' of this world must be very high. Whether Kepler-68b is actually a dense mini Neptune would be the hottest known so far.
Now, what does Kepler-68b? With this density should have lots of volatiles (ices and gases), but also a significant percentage of rock. So, how is its internal structure? Well this is where things get interesting, because nobody knows. For a mini Neptune,is too dense, but obviously cannot rule out that it is. But it could also be an essentially rocky world with a thick atmosphere of hydrogen and helium, or contain huge amounts of water distributed between the ocean surface and a dense atmosphere of water vapor. That is, it would be a super-Earth type world ocean, but why? On the other hand, due to the short distance that separates it from its star, Kepler-68b may be the remains of a gas giant after losing its outer layers of hydrogen and helium. Or what is the same; this would be the core of ice and rock of a gas giant. The only certainty is that there is a world of carbon.
Moreover Kepler-68 star system is a compact, similar to Kepler-11 and Kepler-20, a kind of miniature solar system seems to be very common in the galaxy. The other two planets are Kepler-68c system (a rocky world with the size of Earth and a period of 9.6 days) and Kepler-68d (a giant Jupiter with the same mass as Jupiter and a period of 540 days), the latter discovered by the radial velocity method. Kepler-68b clearly shows the need for theoretical models of the different types of planets according to their composition. Because what is clear is that the number of types of planets is much higher than what we thought until recently. The whole zoos of planets waiting to be discover.