They are called brown dwarves, and they blur the line separating planets and stars. Often, they are referred to as failed stars.
The way solar systems normally form is when a cloud of gas and dust condenses and begins to form pebbles and clumps. These clouds are normally condensed by stellar wind of nearby stars. Our particular solar system, scientists believe, was formed when a nearby cataclysmic event (most likely a supernova) caused the cloud to move and gather upon itself. Mass attracts more mass and it essentially snowballs from there.
However, sometimes these clouds do not contain enough mass in its central region (where a star would typically form). What happens?
Well, let’s look at what normal stars are. A star is mainly comprised of hydrogen gas. When enough gas accumulates, the amount of pass creates more and more inward pressure. This is known as gravity. The more mass it gathers, the higher the pressure in the center of this would-be star becomes. Eventually it reaches a tipping point where the pressure/gravity & the resulting heat are so great that it causes nuclear fusion. Atoms split and form heavier elements, the main one in stars being helium. Stars are essentially balls of gas that contain such high temperatures and pressure that they become nuclear bombs. The Sun is essentially a nuclear bomb that is continuously exploding. It forms a perfect circle where the outward exploding energy is contained by gravity. Even in the randomness of nature, nuclear explosions in space form perfect spheres of heavenly light.
However, this is not always the case. Sometimes there isn’t enough mass to generate the high pressures and heat needed to cause nuclear fusion. These objects are called brown dwarves, many of which are barely larger than Jupiter and can be as cool as your kitchen oven.
Recently, we have found a handful in our galactic neighborhood. This leads to the assumption that there could be billions in our galaxy alone.
This photo is a map generated from NASA’s WISE telescope. It shows a grouping of brown dwarves as seen from a vantage point 100 light years from our own sun, pointed towards the constellation Orion.
They are almost invisible to us as it does not create a lot of light in the visible spectrum (light we see). However, they show appear in infrared, though dimly.