Planet Jupiter's Moons Remain Lit Even in the Face of an Eclipse

Astronomers have learned a bit more about Jupiter's moons using the Subaru Telescope and the Hubble Space Telescope. They've found that these satellites remain slightly bright, even when in the shadow of the massive Jupiter.

The researchers first saw this strange phenomenon  as they were attempting to measure diffuse light from the distant universe. To do this, the scientists planned to use the moons in eclipse as "occultuers" in order to block distant background emissions. This would allow for an extremely accurate separation of the background light from the very bright foreground radiation from the solar system. That's when they saw that the moons remained bright, even during an eclipse.

"It is a serendipitous discovery made as a by-product of a cosmological study," said Tsumura Kohji, one of the researchers, in a news release. "It is very interesting that it provides us a new method to investigate the atmosphere of Jupiter and of exoplanets. I will keep studying from nearby space (the solar system and exoplanets) out to the farthest universe through this project."

In fact, the researchers found that Ganymede and Callisto in particular remained slightly "bright" even when eclipsed. Their eclipsed luminosity was one millionth of their un-eclipsed brightness, which explains why the phenomenon hasn't been detected before now.

Currently, the researchers aren't sure why this occurs, but it does show that there are findings still being made even when it comes to well-researched planets like Jupiter. Researchers plan to make further observations to potentially understand why the moons remain luminous, despite the lack of sun. Yet these studies won't be easy. The Galilean satellites are extremely faint since they're located next to the bright disk of Jupiter. In addition, eclipses only happen at very specific times. This means that the scientists will have to do quite a bit of planning when conducting future observations.

The findings are published in The Astrophysical Journal.