Pluto has shown signs of global warming because it is near perihelion. Because of its highly elliptical 250 year orbit, it was actually closer to the sun than Neptune between 1980 and 1999, and it has been steaming in the more torrid parts of the outer system for some time now, and will continue to do so for a while longer. It's not surprising that it shows signs of warming, especially since it's period of closest solar approach coincided with our improved capability for observing it. Because of its long orbital period, this is the first perihelion passage we have observed since its discovery in 1930.

By the way, Mars also has a highly elliptical orbit and it is at perihelion every 1.88 years. The last time Martian perihelion coincided with opposition with Earth was in 2003. What this means is that when the planet is best placed for visual observation from Earth, perihelion solar warming kicks up dust storms and we can't get a good look at the surface anyway.

Variation in solar energy output is tiny, and probably averages out to zero, at least at human timescales. Much more important are variations due to planetary kinematics and their perturbations. If global warming does indeed exist, I'd look elsewhere for the causes.

It's amazing that even though global warming here on Earth is still a topic of controversy, evidence of it throughout the solar system is now suddenly popping up all over the place. If the alleged global warming of our familiar home planet is that difficult to prove conclusively, why should it be so easily detectable in alien environments at astronomical distances? It sounds like wishful thinking to me.