National Geographic News
Uranus picture: The bright spot of Uranus.

A Hubble picture of Uranus in 2005, shortly before the planet's equinox.

Image courtesy NASA/ESA and M. Showalter, SETI

Rachel Kaufman

for National Geographic News

Published November 2, 2011

In a surprise to astronomers, Uranus recently presented onlookers with a new spot on its northern hemisphere.

Near-infrared pictures from the 8.1-meter Gemini North Telescope in Hawaii have revealed a region on the giant planet that's much brighter than its surroundings.

The spot is likely a tall methane cloud that reaches high enough for us to see sunlight scattered by its icy particles, said Uranus expert Heidi Hammel, executive vice president of the Association of Universities for Research in Astronomy (AURA).

The Uranian cloud is probably similar to an anvil cloud, the type of towering cumulonimbus cloud (picture) that's associated with severe thunderstorms on Earth.

The cloud is also at a lower latitude on Uranus than any that have been observed before. That could mean the spot is a storm that has migrated south.

(Read about a proposed mission to Uranus.)

Uranus Spotty in the Spring

Hammel first saw bright spots on Uranus a few years before the planet's spring equinox in 2007. (Related: "Autumnal Equinox 2011—Sky Show Caps First Day of Fall.")

She was turned on to the spots' presence thanks to a photo of Uranus in another researcher's presentation on the ice giant's moons.

"I said, Wow, what's this? And he said, I don't know, it's just the way Uranus looks. I said, No it's not!"

Subsequent observations made with the Hubble Space Telescope and ground-based telescopes revealed that the spots were most likely storms, similar to Jupiter's Great Red Spot.

Storms are unusual on Uranus because the planet has very little large-scale atmospheric circulation—movements that are driven mostly by temperature differences.

That's because, unlike the other seven planets in our solar system, Uranus's axis of rotation is tilted on its side. (See "Uranus Got Knocked Over by One-Two Punch.")

In addition, at an average of 1.7 billion miles (2.8 billion kilometers) from the sun, the planet completes an orbit every 84 years. That means a day on one of Uranus's poles lasts 42 years, so one half of the planet bakes while the other half stays cold.

But during an equinox on Uranus, the planet is "completely sideways to the sun," Hammel said.

Uranus's exposure to light and dark is therefore more similar to that experienced by the other planets, and the resulting temperature differences allow the icy giant's atmosphere to "turn on" and see more circulation.

Hopes for Hubble to Study Uranus

Hammel and other astronomers have been studying the planet's cloud activity since the last equinox to track how seasonal changes affect the weather.

Until this new spot was observed, researchers had thought Uranus's spring period was over. Now astronomers aren't sure just how long the planet will continue to form such clouds.

Hammel is hoping more astronomers will turn their telescopes toward Uranus's new spot, and that enough independent confirmation of the feature will prompt Hubble managers to once again train the orbiting telescope on Uranus and watch the spot as it evolves.

(Related: "Hubble Reveals New Moons, Rings Around Uranus.")

After all, the last time this planet experienced a change in seasons was 1965.

"This is the first opportunity in modern astronomy to look at Uranus with this detail," she said.

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