In Astronomy, the term metalicity refers to the abundance of heavy elements . In case of stars, the term refers to all other elements but hydrogen and helium. In case of a planet, the term should refer to the presence of heavy elements (majority of them being heavy metals).
Until now, no metal planet has been discovered, but there is a high chance that one day we will find one.
Origin in theory Edit
As we well know, atomic fusion powers up the stars. The fusion is limited to helium in case of M - type stars. Other stars, like Sol, end-up as White dwarfs, releasing heavier elements, but not as far as iron. Larger stars end-up as supernovas, violent explosions, powerful enough to create elements heavier then iron. A supernova is a huge explosion, able to destroy an entire planetary system. However, we know that, around their corpses, the Neutron stars, planets do exist. Somehow, a tiny part of the nebula resulted after a supernova, remained around the star, collapsed and formed small planets. On the other hand, matter from the explosion can mix with other nebulas to form other planets elsewhere. And there is another class, the super-supernovas, created by O - type stars. When these explode, they release black holes. There is a chance that, even around them, planets can form. In case of a massive supernova, there is enough force to create the super-heavy elements, like uranium and beyond. However, as time passes, many radioactive elements will slowly decay.
Another scenario is when a planet gets too close to a star. The extreme heat will force all lighter elements to sublimate. The remaining heavy metals will condensate.
Interactions with another star or with the gravity of another planet can disrupt the metal planet from its orbit and send it into the interstellar space, where it can be captured by other stars.
Since no metal planet has been discovered, we can only speculate what we might find on one.
Rich metallic planets are somehow similar to Mars, where there is enough iron on the ground. Terraforming them should not be a problem, if there is enough light.
Iron planets could originate where there is enough iron. They should also contain manganese, chrome, cobalt and nickel, since these elements are located nearby in the periodic table. Mars is a planet rich in iron. However, an iron planet should be even richer in the element. There is a chance that we can find metallic iron on the surface.
Planets with elements beyond iron should also be found. In this case, we will find huge amounts of heavier elements in large quantities. However, there is a catch. Elements beyond (61) promethium should be very rare, since they are formed only in giant supernovas.
Heavy metal planets should contain high amounts of the heaviest metals, including actinides. Lighter elements, such as carbon or oxygen, should be in low quantities, except if they were added later. On these planets, settlers will find rich uranium ores.
Radioactive planets should be very young. Many radioactive isotopes are supposed to have existed in past and have modeled the structure of planets and asteroids. So, a radioactive planet should be very hot, probably a sphere of lava.
On Earth, only the heavy actinides thorium and uranium are found in nature, all the other elements down to bismuth and lead occur only as decay products from the former two. Heavier elements then uranium also occur only as byproducts in nuclear fission. In nature, formation of these elements occur naturally in supernovas. If matter from a supernova collides fast enough, the newly formed planets might contain some rare trans-uranium elements. Also, the rare, radioactive isotopes of other, more common elements, should be found.
The importance of metal planets is for industry. Industrial colonization will occur around them.
The terraforming processes involved on a metal planet are highly complex. Depending on the amount of heavy metals, it might be possible to terraform them or not. Ores rich in heavy elements can be covered with an insoluble protective layer. In other cases, chemicals can be added to make heavy metals become insoluble, fixed in the ground.
If the amount of heavy elements is too high, terraforming might not be possible.
In many cases, terraforming can simply not be possible. Such planets might occur around neutron stars or black holes.