Gas giant worlds, formerly HyperthermalJovians, which have lost their volatiles through evaporation
Image from LordOther
A Chthonian world with an evaporating tail
These worlds, at first glance, are terrestrial in nature, and indeed their atmospheres are composed only of the gases produced by the planet-wide volcanism that wracks the surface. But these planets have a very different evolutionary history from the Terrestrial Hephaestian planets. Rather than being hyper-volcanically active due to tidal flexing, they are so because of their closeness to their parent sun. These worlds are the naked cores of HyperThermicJovians that have lost their atmospheres to the intense solar radiation over a period of hundreds of millions to even billions of years. The loss increases in rate as more of the atmosphere and thus the gravitational pull of the planet decreases.
As the planet loses its gaseous outer layers, it develops a comet-like tail, which can be detected from distant stars with relative ease. The tail may also be easily visible within the local system, or may be difficult to detect.