If it orbits a star, as opposed to orbiting a common object, binary stars it must be a planet. My interpretation is that dwarfs are failed stars and not planets. I quote "if orbits star, clears path, round and other requirements not remembered at the time of this typing it is a PLANET.

Whats good for the goose is good for the gander.

Excalibur – Thanks for the explanation. This new super-dense planet is a lot less smelly now.

Heavily degenerate objects like White Dwarfs, and Neutron Stars, are the endproduct of extreme pressures.

The reason densities starts to rise when under these tremendous pressures are that the electrons are beginning to cave in – they begin to occupy states that are closer in to the nucleon even when its a higher energy state, and eventually they simply fall loose – that is what degeneracy is. And the more the electrons cave in, the deeper the degeneracy – White Dwarfs and Neturon Stars are the extremes. Even Brown Dwarfs, and smaller Red Dwarfs are expected to have highly degenerate cores.

But i dont believe this 'planet' is a pure gas-giant, sounds more reasonable that it does have a large rocky core, surrouded by huge amounts of gas. Im quite convinced its not a rock through-and-through though.

Ofc if the object also have a large rocky/metal based core, this scenario will only be stronger.

So it shouldnt really come as any surprise at all that a planet with 21.6 times the mass of Jupiter turns out to be just about the same size.

Bill – There have been a few images of objects similar to this new "planet", but if you look at how hard it is to image these things, you won't be wondering why for long. Don't you think astronomers would rather see the actual planets than just their effects?

Just one more data point trying to tell us Newtonian estimates of mass, based upon orbital velocities without correctly factoring in the mass of the system are wrong.

Likewise, Mercury is less dense, Jupiter and all outer planets more dense, and Titan is just what it looks like: A rocky, sandy desert.

A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals (wikipedia, fine definition).

What is a brown dwarf?

Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth and, in discussion, that have experienced fusion at some point in their history. (again, a good definition in Wikipedia).

I think that that is the first criterion to consider about the nature of the objects: is it a star or a planet?

Now, two stars circling a common gravity center that is outside both bodies make a double star system… but what if that center of gravity is inside the more massive one?

Well, that doesn't mean that de less masive one is not an stellar object, for sure.

And so? Well, let's say that it is a close double star system.

Anyway, we'll have to wait until we resolve the first question.

But I cannot help but retain the feeling that we (or at least astronomers with big scopes0 are staring at artifical objects out there all the time and don't know it – or are too afraid of ridicule and job loss to even suggest it out loud.

Some day we will grow up, then we'll be allowed into the Galactic Empire.

Its possible to be a brown dwarf, but a neutron star is not an option, since an object the size of the earth likely contains more mass than our sun.

Im sticking with huge terrestrial planet, or small brown dwarf

With a four day orbital period It's probably much to close to its star to image with anything we have now.