I’m interested to learn how it formed. If it was ejected from a solar system or possibly was on it’s way to becoming a start itself and simply ran out of fuel. We tend to think of planets as being gathered remains of dust and rock circling a star over time, but to have one form with no star present really makes you think about the possibilities.

Even though we think of the atmosphere here as empty space, formless, and weightless, the atmosphere actually does have weight and as it piles down to the surface from the influence of gravity it heats up. This explains why it is colder as you increase elevation (like going to the mountains) and its warmer at the surface, at least in the troposphere.

The pressure from massive amounts of gasses is apparent in Jupiter, our sun, and which gas laws can certainly explain as to why this planet is 400 degrees C. Jupiter is more than 300 times the mass of Earth, so you can imagine how much hotter temperatures are on the “surface” of Jupiter compared to our cozy little planet.

If this “homeless” planet is 13 times the mass of Jupiter, its temperature would be very, very great because temperature is proportional to volume and pressure. We must also factor in gravity, which would be that much stronger on a planet of this magnitude based solely on how massive in size this stellar object is. Yet even still, this “wandering star” has no comparison to the average temperature on the surface of our sun: 5,726 degrees Celsius.

They probably were partly or entirely molten.

After an eon or two of cooling down and reheating due to radioactive decay, they have arrived at a temperature that enables them to be detected and measured by an infrared or radio telescope.

It is even possible they may contain some life forms from times before they were ejected from their solar systems.