Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

This light curve shows the change in brightness of the 55 Cancri system as the rocky planet 55 Cancri e, the closest of the five known planets in the system, moves behind the star. This phenomenon is known a secondary eclipse.

When the planet is next to the star, the mid-infrared light emitted by both the star and the dayside of the planet reaches the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease.

Astronomers can subtract the brightness of the star from the combined brightness of the star and planet to calculate how much infrared light is coming from the dayside of the planet. This is then used to calculate the dayside temperature and infer whether or not the planet has an atmosphere.

The graph shows data collected using the low-resolution spectroscopy mode on Webb’s Mid-Infrared Instrument in March 2023. Each of the purple data points shows the brightness of light ranging in wavelength from 7.5 to 11.8 microns, averaged over intervals of about 5 minutes. The grey line is the best fit, or model light curve that matches the data most closely. The decrease in brightness during the secondary eclipse is just 110 parts per million, or about 0.011 percent.

The temperature of the planet calculated from this observation is about 1,800 kelvins (around 2,800 degrees Fahrenheit), which is significantly lower than would be expected if the planet has no atmosphere or only a thin rock-vapor atmosphere. This relatively low temperature indicates that heat is being distributed from the dayside to the nightside of the planet, possibly by a volatile-rich atmosphere.

Provider: Space Telescope Science Institute

Image Source: https://webbtelescope.org/contents/news-releases/2024/news-2024-102

Curator: STScI, Baltimore, MD, USA

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