Astronomers using the James Webb Space Telescope have discovered the atmosphere of rocky exoplanet 55 Cancri e, 41 light-years from Earth. It is the first rocky planet outside the solar system in which the existence of an atmosphere has been confirmed.

55 Cancri e is one of five known planets orbiting 55 Cancri, a Sun-like star located in the constellation Cancer. This planet is classified as a so-called super-Earth. It is approximately twice the diameter of our planet, and its mass is eight times greater. It is likely similar in composition to the terrestrial planets in our solar system.

Extremely hot super-Earth

The planet is in a very tight orbit. They will orbit their star within 18 hours at a distance of about 2.25 million kilometers, equivalent to 2/25 of the distance between Mercury and the Sun. For this reason, its surface is an ocean of molten rock. In such a tight orbit, the planet behaves like our Moon, with the day side always facing the star and the night side immersed in eternal darkness.

In 2015, as a result of a public competition, the planet was named Janssen, in memory of the Dutch designer of the first optical microscope – Zechariah Janssen. The researcher is also credited with developing the telescope. In the same competition, Janssen’s parent star was named Copernicus in honor of Nicolaus Copernicus.

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Unlike the atmospheres of gas giant planets, which are relatively easy to detect, the thinner, denser atmospheres surrounding rocky planets are difficult to detect. Given the narrow orbit, the high temperature on Janssen’s surface and the constant flood of radiation from the star, astronomers doubted whether an atmosphere could be formed and maintained there.

Search for atmosphere

However, previous studies on 55 Cancri e, using data from the defunct Spitzer Space Telescope, have suggested an atmosphere rich in volatiles such as oxygen, nitrogen and carbon dioxide. But the data collected was uncertain, and researchers were unable to rule out another possibility: that the planet is bare except for a thin shroud of vaporized rock, rich in elements such as silicon, iron, aluminum and calcium.

“The planet is so hot that some of the molten rock must have evaporated,” explained Renyu Hu of NASA’s Jet Propulsion Laboratory (JPL), lead author of a recent paper on the atmosphere of 55 Cancri e, which appears in the journal Nature (DOI: 10.1038). “. /s41586-024-07432-x).

To determine whether Janssen had an atmosphere, scientists used the James Webb Space Telescope (JWST) and its infrared observations. Although Webb cannot take a direct image of 55 Cancri e, it can measure subtle changes in the light of the entire system as the planet orbits the star.

Cooler than expected

The first indication that 55 Cancri e might have a significant atmosphere came from temperature measurements based on thermal emission, which is thermal energy emitted in the form of infrared light. If the planet were actually covered in dark, molten rock with a thin blanket of evaporating rock, or no atmosphere at all, the temperature on the dayside should be around 2,200 degrees Celsius.

Instead, data from the James Webb Space Telescope showed a much lower temperature of around 1,540 degrees Celsius. This is a very clear indication that energy is distributed from the day side to the night side, likely through an atmosphere rich in volatile materials, Hu said. Heat can also be transferred using lava, but researchers say this process would not be able to transfer it efficiently enough to explain the cooling effect.

In contrast, data collected by the NIRCam instrument on the James Webb Space Telescope showed that less light with wavelengths between 4 and 5 microns was reaching the telescope. “This suggests the presence of an atmosphere containing carbon monoxide or carbon dioxide that absorbs light at these wavelengths,” explained co-author Aaron Bello Aroff of JPL. A planet that has no atmosphere or only evaporated rocks would not have these spectral features.

The team believes that the gases covering 55 Cancri e are not the original atmosphere, as they disappeared long ago due to the star’s high temperature and intense radiation. “It is a secondary atmosphere, constantly replenished by an ocean of lava, and it is not just molten rock, there is a lot of dissolved gas in it,” Belo Arov said.

In addition to carbon monoxide and carbon dioxide, it may also contain nitrogen, water vapor, sulfur dioxide, vaporized rocks, and even short-lived clouds made up of tiny droplets of lava condensing from the air. Although 55 Cancri e is too hot to support life as we know it, scientists believe it could provide a unique window into the interactions between the atmosphere, surfaces and interiors of terrestrial planets, perhaps providing insight into early Earth, Venus and Mars. Which is also believed to have been covered by oceans of molten rock in the past.

Source: NASA, Image: NASA, ESA, CSA, R. Crawford (STScI)

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