Using data from NASA's James Webb Space Telescope, a team of researchers has shown that the hot, rocky planet 55 Cancri e, located 41 light-years from Earth, is likely surrounded by a gaseous atmosphere. To date, it is the closest rocky exoplanet to our solar system to have an atmosphere detected. This exciting and surprising result was published by Rignot Hu and colleagues in the prestigious journal Nature.
Superhot Earth: 55 Cancri e
55 Cancri e, also known as Janssen, is one of five planets orbiting the star 55 Cancri in the constellation Cancer. With a diameter twice that of Earth and a slightly higher average density, this planet can be classified as a subclass of super-Earth exoplanets: its size is between that of Earth and Neptune, and its likely composition is similar to that of the rocky planets in our Solar System.
However, it should be noted that the name rocky planet can be misleading in the case of 55 Cancri e, since the planet orbits very close to the system's central star: the distance between the star and planet e is 1/25 the distance between Mercury and the Sun. Consequently, the planet's surface is mostly composed of molten magma oceans. The proximity to the central star also results in a closed orbit in the case of 55 Cancri e, so that one side of the planet is always pointed toward the star, while the other side is forever in darkness.
Although the planet was discovered in 2011 and has been measured since, it has not been studied in sufficient depth to answer the question of whether it has an atmosphere, even though it orbits much closer to its star than our planet. Particles from the stellar wind are constantly flowing towards it.
“I’ve been working with 55 Cancri e for more than 10 years,” said exoplanet researcher Diana Dagomer. “I’ve found it incredibly frustrating that so far no single measurement has been enough to solve the mystery surrounding the planet. Now I’m almost happy that we’ve finally found answers to our questions.”
It is relatively easy to prove that there is an atmosphere around gas giant exoplanets, but proving that there is a thin, dense atmosphere surrounding rocky planets is more difficult.
According to previous studies based on data from NASA's Spitzer Space Telescope, the spectrum of 55 Cancri e is rich in volatiles found in Earth's atmosphere, such as oxygen, nitrogen, and carbon dioxide. However, the physical process that created these elements can be explained by several theories: In addition to the presence of an atmosphere, it is also possible that the detected molecules were created during the evaporation of rocks on the planet's hot surface. Which of these scenarios is correct in the case of 55 Cancri e has so far been a mystery.
infrared color change measurement
The research team used images from the James Webb Space Telescope’s NIRCam, a near-infrared camera, and MIRI, a mid-infrared camera, to choose the scenario that best fit the observations of 55 Cancri. Although the James Webb Space Telescope cannot directly image the planet, it can accurately measure the change in brightness caused by the planet passing in front of and behind the star.
In this way, the researchers were able to calculate the infrared radiation coming from the day side of the planet. Scientists have already used this method in the case of other exoplanets, such as the famous TRAPPIST-1 b, to detect the possibility of an atmosphere.
The planet is cooler than expected.
The first sign that 55 Cancri e might have a thick atmosphere was the planet’s infrared radiation. If the planet were covered in dark, molten rock, the evaporation of which alone would create a thin atmosphere around the celestial body, the dayside temperature should be around 2,200 degrees Celsius. In contrast, MIRI measurements show a much lower temperature, around 1,540 degrees Celsius. This suggests that the planet is surrounded by an extensive atmosphere rich in volatile gases, which could transfer a significant amount of heat from the dayside to the nightside. The presence of lava flows would also have some effect on the heat, but not to the extent that would be observed in the case of 55 Cancri e.
After examining the NIRCam data, the research team became more certain that the planet has an atmosphere, as they were able to identify the characteristic absorption lines of carbon monoxide and carbon dioxide in the measured spectra, which certainly could not have come from evaporation of molten rocky material.
Ocean bubbles of magma
According to scientists, 55 Cancri e did not have an atmosphere at the time of its formation, but most likely it was created as a result of physical processes occurring inside the planet.
“The primary atmosphere must have evaporated long ago due to the hot radiation from the star,” said a co-author of the study. “The planet’s current atmosphere is likely a secondary atmosphere, constantly fed by magma oceans bubbling up to the surface. The magma is not only molten rock, but also contains a lot of gases in a molten state.”
Although 55 Cancri e is too hot to be habitable, its research provides a unique opportunity to study the interactions between a rocky planet’s atmosphere, surface, and interior, providing insight into earlier periods in the evolution of Earth, Venus, and Mars. Ultimately, by gaining a deeper understanding of the processes at work on 55 Cancri, we can also answer the question of what conditions are necessary for a rocky planet to have the thick, gas-rich atmosphere that is essential for a planet to be habitable.
Article source: https://webbbtelescope.org/contents/news-releases/2024/news-2024-102
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