A medida que las técnicas de detección de planetas extrasolares aumentan su precisión, cada vez tenemos más y más Hot Neptunes (¿por qué no se les llama también Hot Uranuses?, supongo porque en inglés el término no suena muy bien, precisamente). El último, GJ 436 b, detectado mediante el método del tránsito. Con 23 veces la masa terrestre, se trata del planeta menos masivo descubierto hasta la fecha por este método y orbita una pequeña enana roja tipo M 2.5. Precisamente, se ha podido detectar gracias a al escaso brillo de su estrella. Según los propios descubridores:

The mass and radius that
we measure for GJ 436 b indicate that it is mainly com-
posed of water ice. It is an “ice giant” planet like Uranus
and Neptune rather than a small-mass gas giant or a very
heavy “super-Earth”. It must have formed at a larger or-
bital distance, beyond the “snow line” where the proto-
planetary disc is cool enough for water to condensate, and
subsequently migrated inwards to its present orbit.
The temperature profile inside the planet is not ex-
pected to modify this qualitative picture. The atmosphere
of GJ 436 b must be hot: the equilibrium temperature is
520 K to 620 K depending on the albedo, and a greenhouse
effect may heat it to much larger temperatures. Tidal ef-
fects from its eccentric orbit must also inject energy in its
interior, but the iron, rock and water equations of state
are not very sensitive to temperature at high pressure.
We can ponder whether the planet has an H/He en-
velope like the ice giants in the Solar System, or if its at-
mosphere is composed mainly of water vapor. Our best-fit
radius value places it slightly above the “pure ice” com-
position mass-radius line of Fortney et al. (2007). A small
H/He envelope may thus be required – even more if an
iron/rock core is present as expected. At the upper end
of the radius error bar, the H/He envelope would have to
represent up to 10% in mass according to the models of
Fortney et al. 2007 (see Figure 3). The lower end of the
range is close to the mass-radius line for pure ice plan-
ets. Water ice mixed with methane and ammonia is less
dense than pure ice under high pressures, so the presence
of a significant amount of these compounds within the ice
could make the planet large enough despite a rock/iron
core to account for the observed radius without invok-
ing an H/He envelope. GJ 436 b could therefore be an
“Ocean Planet” (L´eger et al. 2004).

Características del planeta:

Más info, aquí.