Astronomers using the European Southern Observatory’s Very Large Telescope in Chile have made a remarkable discovery within Neptune’s atmosphere: a large, enigmatic dark spot, accompanied by an unexpectedly bright companion.
While space-based observatories, such as the Hubble Space Telescope, have previously observed vortex-like storms, known as dark spots, swirling within Neptune’s atmosphere, this marks the first time that an Earth-based telescope has observed one on the distant planet.
The findings, published in the journal Nature Astronomy, provide valuable insights into this phenomenon, which has long intrigued scientists. Lead study author Patrick Irwin, a professor of planetary physics at the University of Oxford, expressed his curiosity about these “short-lived and elusive dark features.”
Dark spots are a characteristic feature of giant gaseous planets in our solar system, including Neptune. These storms, akin to Jupiter’s Great Red Spot, have been observed on Neptune by Hubble and the Voyager 2 probe. However, these storms follow a pattern of appearing and disappearing over approximately two years, making them challenging to study. Voyager 2 glimpsed two dark storms on Neptune during its 1989 flyby, but they had vanished by the time Hubble observed the planet in 1994.
Neptune’s dark spots behave differently from Earth’s hurricanes. They are high-pressure systems that initially form stably and rotate clockwise, whereas Earth’s Northern Hemisphere hurricanes are low-pressure systems that rotate counterclockwise.
The recent discovery on Neptune was made possible when Hubble spotted new dark spots in 2018. Irwin’s team seized the opportunity to conduct Earth-based observations using the Very Large Telescope and its Multi Unit Spectroscopic Explorer (MUSE) instrument. MUSE allows astronomers to observe entire celestial objects in various wavelengths simultaneously, using adaptive optics to correct for atmospheric distortions.
The data collected by the telescope revealed that the dark spots on Neptune are not caused by gaps or clearings in the clouds but rather by darkening air particles gathering beneath the planet’s prominent atmospheric layer, where haze and ice mix. In a surprising observation, MUSE captured a 3D spectrum of light for Neptune and its dark spot, revealing a rare deep bright cloud type that had never been identified before, even from space. This unique cloud appeared as a smaller bright spot next to the larger dark spot, both at the same atmospheric level.
The astronomers expressed their intrigue about this newfound feature and their hopes of learning more through future Earth-based observations. These advances in technology have significantly enhanced humanity’s ability to explore and understand the cosmos.
“It’s an astounding increase in humanity’s ability to observe the cosmos,” said study co-author Michael Wong, a planetary scientist at the University of California, Berkeley. “At first, we could only detect these spots by sending a spacecraft there, like Voyager. Then we gained the ability to make them out remotely with Hubble. Finally, technology has advanced to enable this from the ground.”