Astronomers have suspected that Ceres, the dwarf planet that inhabits the asteroid belt, might have a very thin atmosphere, which comes and goes over time.

Researchers thought that it could be influenced by how close or far Ceres is from the Sun around its orbit but a new study, published in the Astrophysical Journal Letters, suggests that the atmosphere is created when the Sun is more active.

"We think the occurrence of Ceres' transient atmosphere is the product of solar activity," lead author Michaela Villarreal said in a statement. Energetic particles from the Sun hits water ice present on the surface of the dwarf planet. This process, called “sputtering”, could explain the observations of this vanishing act of an atmosphere.

Hints of an atmosphere were first detected in 1991 by the International Ultraviolet Explorer. Different telescopes and space observatories have looked since then with mixed results. NASA’s Dawn reached Ceres in 2015 and it discovered extensive deposits of ice.

Some assumed that Ceres was behaving like a comet. As its orbit took it closer to the Sun, its outer layer would begin to sublimate. But the detection periods didn’t match with the planet’s orbit.

"Sublimation probably is present, but we don't think it's significant enough to produce the amount of exosphere that we're seeing," Villarreal added.

^{Animation of the Hydrogen content of Ceres. Blue is high and red is low. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI}

In its mission around Ceres, the spacecraft was able to identify the thin atmosphere, technically known as an exosphere. It also measured how many high energy particles were present around the dwarf planet. The team has shown that the detections of the exosphere coincide with a particularly active Sun.

"Our results also have implications for other airless, water-rich bodies of the solar system, including the polar regions of the moon and some asteroids," said Chris Russell, principal investigator of the Dawn mission, also at UCLA. "Atmospheric releases might be expected from their surfaces, too, when solar activity erupts."

The Sun is about to enter a quiet period, so Ceres might say goodbye to its atmosphere for now.