In the mid-1990s, Andrew Strominger performed work on another type of brane, called p-branes, which were solutions to Einstein’s general relativity field equations. The p represents the number of dimensions, which again can go from zero to nine. (A 4-dimensional p-brane is called a 4-brane.)
The p-branes expanded infinitely far in certain directions but finitely far in others. In those finite dimensions, they actually seemed to trap anything that came near them, similar to the gravitational influence of a black hole. This work has provided one of the most amazing results of string theory — a way to describe some aspects of a black hole.
In addition, the p-branes solved one problem in string theory: Not all of the existing particles could be explained in terms of string interactions. With the p-branes, Strominger showed that it was possible to create new particles without the use of strings.
A p-brane can make a particle by wrapping tightly around a very small, curled-up region of space. Strominger showed that if you take this to the extreme — picture a region of space that’s curled up as small as possible — the wrapped p-brane becomes a massless particle.
According to Strominger’s research with p-branes, not all particles in string theory are created by strings. Sometimes, p-branes can create particles as well. This is important because strings alone did not account for all the known particles.