HD 149026 B - Physical Characteristics

Physical Characteristics

The planet orbits the star in a so-called "torch orbit". One revolution around the star takes only a little less than three Earth days to complete. The planet is less massive than Jupiter (0.36 times Jupiter's mass, or 114 times Earth's mass) but more massive than Saturn. The temperature of the planet was initially estimated on the basis of 0.3 Bond albedo to be about 1540 K, above the predicted temperature of HD 209458 b (1400K), which had inaugurated the category of Chthonian "hell planet". Its day-side brightness temperature was subsequently directly measured as 2,300 ± 200 K by comparing the combined emissions of star and planet at 8 μm wavelength before and during a transit event. This is around the boiling point of silicon and well above the melting point of iron.

This planet's albedo has not been measured directly. The initial estimate of 0.3 had come from averaging Sudarsky's theoretical classes IV and V. The planet's extremely high temperature has forced astronomers to abandon that estimate; now, they predict that the planet must absorb essentially all of the starlight that falls on it — that is, effectively zero albedo like HD 209458 b. Much of the absorption takes place at the top of its atmosphere.

Between that and the hot, high-pressure gas surrounding the core, a stratosphere of cooler gas was once predicted but has not been observed. The atmosphere is likely high in carbon monoxide and dioxide.

The outer shell of dark, opaque, hot clouds are usually thought to be vanadium and titanium oxides ("pM planets"), but other compounds like tholins cannot be ruled out as yet.

The planet-star radius ratio is 0.05158 +/- 0.00077. Currently what limits more precision on HD 149026 b's radius "is the uncertainty in the stellar radius", and measurement of the stellar radius is distorted by pollution on the star's surface.

Even allowing for uncertainty the radius of HD 149026 b is only about three quarters that of Jupiter (or 83% that of Saturn). HD 149026 b was the first of its kind: HD 149026 b's low volume means that the planet is too dense for a Saturn-like gas giant of its mass and temperature.

It may have an exceptionally large core composed of elements heavier than hydrogen and helium: the initial theoretical models gave the core a mass of 70 times Earth's mass; further refinements suggest 80-110 Earth masses. As a result, the planet has been described as a "super-Neptune", in analogy to the core-dominated outer ice giants of our solar system, though whether the core of HD 149026 b is mainly icy or rocky is not currently known. Robert Naeye in Sky & Telescope claimed "it contains as much or more heavy elements (elements heavier than hydrogen and helium) than all the planets and asteroids in our solar system combined". In addition to uncertainties of radius, its tidal heating over its history needs be taken into account; if its current orbit is circular and if that had evolved from a more eccentric one, the extra heat increases its expected radius per its model and thereby its core radius.

Naeye further speculated that the gravity could be as high as ten g (ten times gravity on Earth's surface) on the surface of the core.