Latest observations from NASA's James Webb Space Telescope (JWST) show the existence of strange clouds composed of salt around an exoplanet nicknamed the "Pink Planet," providing direct evidence for the atmospheric structure of one of the coldest planet-like companions in the universe and solving a mystery that has puzzled the astronomical community for more than a decade. The relevant research was led by Northwestern University and published in the Astronomical Journal on June 18.
The "pink planet", officially designated GJ 504 b, was first discovered in 2013 and orbits a sun-like star about 57 light-years from Earth. Despite its "planet" moniker, scientists aren't entirely sure whether it should be classified as a true planet - at about 25 times the mass of Jupiter, it's close to the dividing line between giant planets and brown dwarfs, so researchers are more cautiously calling it a "planetary-mass companion," a planet-like object orbiting a star.
GJ 504 b has long been difficult to study because of its extremely low temperature and faint brightness. Most of the exoplanets that have been directly imaged so far have temperatures of about 1,000 to 2,000 degrees Fahrenheit (about 538 to 1,093 degrees Celsius), while GJ 504 b is only about 550 degrees Fahrenheit (about 290 degrees Celsius), about the same temperature as bread baking in an oven. The research team analyzed that this relatively "cold" state reflects its very old age - giant planets are extremely hot when they are born, but gradually cool down over billions of years. The age of GJ 504 b is estimated to be between 2.5 billion and 4 billion years.
Aneesh Baburaj, a postdoctoral researcher at Northwestern University's Center for Astrophysics (CIERA) who led the research, pointed out that "the pink planet is the coldest companion star object discovered using ground-based instruments." Over the past decade or so, many teams have tried to use the world's largest ground-based telescopes to conduct follow-up observations to obtain atmospheric spectra, but they have all failed because the targets are too faint. In comparison, the James Webb Space Telescope, with its highly sensitive infrared observation capabilities, was able to successfully separate the atmospheric spectrum of this companion star in about two hours of observation, becoming a key new tool for studying such "cold dark worlds".
In this observation, the researchers used JWST to perform high-contrast imaging of the main star and its companion star, and used advanced data processing methods to eliminate the strong glare from the parent star, and finally extracted the spectral signal emitted by the companion star itself. By breaking light into different wavelengths, scientists are able to analyze the chemical "fingerprints" in the atmosphere to infer the types of elements and molecules present in it. After successfully obtaining the spectrum, the team quickly realized that the "pink planet's" atmospheric characteristics were "very different from anything analyzed before," Baburaj said.
Spectral analysis results show that GJ 504 b's atmosphere contains water vapor, methane, carbon dioxide, ammonia and a variety of other molecular components. However, when the research team compared these observations with existing atmospheric models, they were initially only able to barely match the data when unrealistic extreme conditions were introduced, which clearly contradicted common sense in physics. The real breakthrough came after scientists started adding clouds to their simulations: When different types of clouds were introduced into the model and their effects on the spectrum were tested one by one, the salt cloud model matched the measured data much better than other options.
The study points out that these salt clouds may have obscured the deeper layers of the planet's atmosphere, causing the spectral signals eventually detected by JWST to come mainly from regions above or near the clouds, thus changing the characteristics of molecular absorption and scattering. Baburaj said, "After we added clouds to the simulation, the results began to be consistent with our theoretical understanding of cold planets; we tried three different types of clouds, and the salt cloud scheme was the best fit." After considering the influence of salt clouds, the characteristics of atmospheric molecules hidden in deeper layers were moderately weakened, and the spectral model finally became physically reasonable.
This work is believed to be the first direct evidence for the existence of salt clouds in the atmosphere of a cold planetary mass object, and also verifies a class of theoretical predictions put forward by the scientific community more than a decade ago. At the same time, observations also show that GJ 504 b is unusually rich in heavy elements—elements other than hydrogen and helium that astronomers collectively refer to as “metals”—which may mean that its formation process is different from that of ordinary giant planets. Based on the existing data, the research team is still unable to determine whether the celestial body is closer to a "giant planet" formed by the aggregation of planetary disks, or whether it is more similar to a small star or brown dwarf formed through gravitational collapse. The issue of its origin still requires further in-depth study.
The researchers emphasized that the method demonstrated by the James Webb Telescope in this study will open a new window for exploring more similar faint and cold objects. Jupiter, for example, has thick clouds of ammonia ice covering its upper atmosphere, but existing instruments are still unable to observe these cloud structures with a similar level of detail as GJ 504 b. Today, the successful detection of salt clouds in the atmosphere of GJ 504 b shows that astronomers are steadily expanding the types of cold worlds that can be studied in detail, providing an important reference for future exploration of clouds and atmospheric structures inside and outside the solar system.
Baburaj pointed out that this is the first time that salt clouds have been found to play a "critical role" in interpreting the spectrum of a cold planet-mass object, which has important implications for building and correcting atmospheric models. He reminded, "This is an important reminder: the presence and impact of clouds must be more systematically considered in simulations." As JWST continues to carry out high-contrast imaging and spectroscopic observations, scientists expect that more cold dark exocompanions like GJ 504 b will be studied in depth, thereby helping humans more comprehensively understand the diverse planets and substellar worlds in the universe.