gavran wrote:I don't understand this. If the planet was 4 times bigger than Jupiter, shouldn't it radiate heat? The pressure would be so huge that the temperature of the core should radiate more than enough fo us to detect. Every outer planet in our solar system radiate more heat than it absorp, why would this be an exception? Especially if it's bigger than every outer combined.
The internal heat is a valid argument. 4 times the size of jupiter likely isn't big enough to cause any fusion. I'm not sure what the fusion starting point is, but I think it's a good deal higher than that, however, internal heat can also be generated from debris falling into the planet, in fact, the simple matter of gas collecting and forming into a planet generates a lot of heat which cools over time, as well as radioative decay. I think, most likely a planet 4 times the size of jupiter would radiate thermal energy, but at that distance, whether it would get lost in the background of space, that's harder to say.
So I think your point is valid. Planets do tend to cool down over time, but 4 times the size of jupiter is very large. That would take a long time to cool. But if it's half a lightyear away and if it generates such a small amount of energy in comparison to the dimmest star, it might not be detectible. So, long and short of it is, you may be right, but I don't know enough to say conclusively.
gavran wrote:
Still cloudy to me. We can detect long dead brown dwarfs from lightyears away, because their gravitational pressure provides us all the radiation we need. Same with the gas planets, which radiate more than they absorp, which means that they don't need the Sun to generate heat. If this planet is as big as they say, it should easily do the same.
Tight orbits are more easily detected than longer orbits because we can see wobbles and we can detect when an object is between the earth and a visible star.
A planet that's 1,000 times further away from the sun than Neptune would orbit so slowly that the wobble would be more difficult to detect. So this theory might be possible, but that doesn't mean it's likely. It reads more like some preposterous proposal than an actual well researched proposal.
God invented war so Americans would learn geography.
But, okay. 
