The researchers assumed the Galilean satellites would be dark while immersed in Jupiter's shadow. As such, they "planned to use the Galilean satellites in eclipse as 'occulters' to block distant background emissions," said lead study author Kohji Tsumura, an astronomer at Tohoku University in Japan.
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Instead, using the Subaru Telescope and Hubble Space Telescope, the researchers found an unexpected surprise: The Galilean satellites were still slightly bright even when eclipsed. This effect is especially pronounced for Ganymede and Callisto.
"This is a serendipitous discovery, which makes us surprised and excited," Tsumura told Space.com.
Making this discovery was very challenging because the Galilean satellites are extremely faint while eclipsed, and the incredibly bright face of Jupiter near them can blind attempts to see them. Furthermore, the eclipses only take place at specific times, and Jupiter and its moons are continuously in motion, which makes observations very complex, the researchers said.