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May 15, 2003 09:00 AM (EDT)

News Release Number: STScI-2003-17

Brighter Neptune Suggests a Planetary Change of Seasons

May 15, 2003: Springtime is blooming on Neptune! This might sound like an oxymoron because Neptune is the farthest and coldest of the major planets. But observations by NASA's Hubble Space Telescope reveal an increase in READ: Junior version of this article Amazing Space Learn about this story in the Star Witness, a science newspaper available on our sister site, Amazing Space.   Neptune's brightness in the southern hemisphere. Astronomers consider this increase a harbinger of seasonal change. The observations, made over six years, show a distinct increase in the amount and brightness of the clouds encircling the planet's southern hemisphere.

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Q & A: Understanding the Discovery

  1. 1. Describe Neptune's springtime weather.

  2. Bring your parka. Neptune's springtime weather brings tempestuous storms, temperatures of minus 400 degrees Fahrenheit at the cloud tops, and ferocious winds that sometimes gust to 900 miles per hour. What is remarkable is that Neptune - the farthest and coldest of the major planets - exhibits any evidence of seasonal change. The Sun is 900 times dimmer than it is on Earth.

  3. 2. What causes seasonal changes on Neptune?

  4. Seasons on Neptune occur for the same reasons as on Earth. The seasonal changes on both planets occur because their axes tilt slightly. Earth is inclined 23.5 degrees. Neptune is tipped at an even greater angle: 29 degrees. As both planets spin on their axes and travel around the Sun, their southern and northern hemispheres are alternately tipped toward or away from the Sun.

  5. 3. How long are Neptune's seasons?

  6. Unlike Earth, Neptune's seasons last for decades, not months. A single season on the planet, which takes almost 165 years to orbit the Sun, can last more than 40 years.

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Credit: NASA, L. Sromovsky, and P. Fry (University of Wisconsin-Madison)