An unusually bright, nearly circular storm system near Neptune’s equator

Astronomers like it dark. That’s why they go to the ends of the Earth to build telescopes, far away from city lights. Unfortunately, that also means that these expensive observation tools have limited viewing hours. But a team at UC Berkeley is working on extending those hours, developing an efficient method for twilight observing. They’ve been observing at these odd hours at the local Lick Observatory and this summer, graduate student Ned Molter tried out twilight observing at the W. M. Keck Observatory on Mauna Kea, Hawaii.

Molter was one of eight students in the new Keck Visiting Scholars Program. Launched this summer, the program gives graduate students and postdoctoral researchers experience working at the telescope, while contributing to the observatory and its scientific community. In his proposal, he wrote that he wanted to set up twilight observing at Keck. Since most observers at Keck peer deep into the night sky and cannot observe their targets during twilight, for Molter, it was an exciting proposal “because we can get lots of observations on a world class telescope essentially ‘for free’ during times that other observers can't use,” he said by email.

Exciting, indeed! Through Molter’s twilight observations in June and July, he discovered a new feature on Neptune—a storm system nearly the size of our own planet! This massive system, which was found in a region where no bright cloud has ever been seen before, is about 9,000 kilometers (5,600 miles) in length, or one-third Neptune’s radius, spanning at least 30 degrees in both latitude and longitude.

“Seeing a storm this bright at such a low latitude is extremely surprising,” he said. “Normally, this area is really quiet and we only see bright clouds in the mid-latitude bands, so to have such an enormous cloud sitting right at the equator is spectacular.”

His advisor agrees. “Historically, very bright clouds have occasionally been seen on Neptune, but usually at latitudes closer to the poles, around 15 to 60 degrees north or south,” said Imke de Pater, a UC Berkeley professor of astronomy. “Never before has a cloud been seen at or so close to the equator, nor has one ever been this bright.”

“This shows that there are extremely drastic changes in the dynamics of Neptune’s atmosphere, and perhaps this is a seasonal weather event that may happen every few decades or so,” de Pater continued.

Neptune orbits the Sun every 160 years, and one season is about 40 years. It’s also the windiest planet in our solar system, with the fastest observed wind speeds at the equator reaching up to a violent 1,600 kilometers per hour (1,000 miles per hour). To put this into perspective, a Category 5 hurricane has wind speeds of 250 kilometers per hour (156 miles per hour).

So what else might we be able to observe in the twilight? “The other objects we’re looking at in this project are Uranus, Io, and Titan, and we're thinking about ways to do Jupiter and Saturn,” Molter remarked. “Anything that is bright enough and changes over timescales of weeks and months has potential to work, though; perhaps variable stars or exoplanet transits would be interesting.”

In the meantime, Molter and de Pater will analyze the Neptune data collected this summer and try for more twilight observing time at Keck Observatory this fall so they can learn more about the nature of this storm and get an idea of what it will be doing over time.

Image: N. Molter/I. De Pater, UC Berkeley/C. Alvarez, W. M. Keck Observatory

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