The exoplanet GJ 504b stands out in the catalog of discovered planets because it glows a distinctive magenta or pink. Located about 57 light-years away in the constellation Virgo, this massive gas giant was one of the first exoplanets detected through direct imaging. This method captures the planet’s light directly, unlike techniques that infer existence from the wobble or dimming of its parent star. The unusual color results directly from its atmospheric composition and relatively young age, offering astronomers a glimpse into the physics governing the atmospheres of giant planets that are still cooling.
Context: The Super-Jupiter GJ 504b
GJ 504b is classified as a “Super-Jupiter,” placing it at the boundary between a massive planet and a small brown dwarf. It is about four times more massive than Jupiter but comparable in size to our solar system’s largest planet. It orbits the G0-type star GJ 504, which is similar to the Sun but considerably younger.
The system is estimated to be only about 160 million years old, a mere fraction of the Sun’s 4.6-billion-year age. This youth is a factor in the planet’s appearance, as it is still radiating immense heat generated during its formation. This residual thermal energy causes the planet to glow faintly in the infrared spectrum, which allowed its detection by the Subaru Telescope’s SEEDS survey.
The planet’s effective temperature is relatively cool for a directly imaged exoplanet, hovering around 510 Kelvin (237 degrees Celsius or 460 degrees Fahrenheit). This temperature is significantly lower than the nearly 1,000 Kelvin temperatures of many other imaged gas giants. This lower temperature profile allows for its unique atmospheric chemistry.
Its T8 spectral classification indicates the presence of methane in its atmosphere, a characteristic shared with the coolest brown dwarfs and outer solar system gas giants. The combination of its mass, temperature, and spectral type makes GJ 504b valuable for studying the transition between planets and brown dwarfs. Detecting the planet was technically challenging, as its faint glow had to be isolated from the intense light of its parent star.
The Atmospheric Chemistry Behind the Pink Color
The magenta color of GJ 504b is an interpretation of its infrared light, arising because the atmosphere acts as a filter for the light emitted from its internal heat source. This filtering effect results from specific gases absorbing certain wavelengths of light while allowing others to pass through.
Two primary chemical components sculpt its spectrum: methane and alkali metals. Hotter gas giants often have thick atmospheric haze that scatters light, making them appear dark red or brown. GJ 504b appears “bluer” in the near-infrared than these hotter planets, suggesting a clearer upper atmosphere with fewer obscuring clouds.
This lack of thick, high-altitude clouds allows light to penetrate deeper and escape more easily. The planet is cool enough for methane gas (CH4) to form, which absorbs light strongly in the red and near-infrared spectrum. The removal of red light, combined with the absorption of blue and green light by other chemical species, creates the magenta appearance.
The other key contributors are neutral alkali metals, specifically gaseous sodium and potassium. These metals are powerful absorbers of light in the blue and green regions of the optical spectrum. By absorbing blue and green light, the atmosphere allows redder wavelengths and the residual thermal glow to dominate the visible color. The resulting light profile, deficient in blue/green and some red wavelengths, produces the dark magenta hue.
Implications of Its Unusual Orbit
The orbital configuration of GJ 504b challenges long-standing theories of planet formation. The planet orbits its star at approximately 43.5 astronomical units (AU), nearly nine times the distance between Jupiter and the Sun. This wide separation is far beyond where most models predicted such a massive planet could form.
The prevailing core accretion model suggests that giant planets form close to their star in the inner protoplanetary disk. This involves a rocky core forming first, which then gathers a massive gaseous envelope. This model struggles to explain how a planet four times the mass of Jupiter could form so far from the central star.
The existence of GJ 504b supports the alternative gravitational instability model. This theory proposes that massive gas giants can form rapidly in the outer, cooler regions of a protoplanetary disk. If the disk is massive and cool enough, gas and dust can collapse directly under gravity to form a planet quickly.
The wide, distant orbit is related to the planet’s distinctive color because it receives very little external heat, allowing it to cool slowly over time. This slow cooling rate maintains the internal temperature of 510 Kelvin. This temperature permits the specific atmospheric chemistry—the thin cloud layer and gaseous alkali metals—that filters the light to produce the magenta color.