Neptune Is Cooling, and No One Is Sure Why

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The planet Neptune is about 30 times farther from the sun than Earth, and as you’d expect, it’s a frigid world. As it turns out, it’s getting even colder when it should be heating up. That’s the latest from an international team of astronomers that just completed a 17-year analysis of the gas giant. The temperature swing is so significant that scientists are uncertain of the cause. 

On Earth, the seasons can come and go in the blink of an eye, but not so on Neptune. A single Neptunian year lasts 164 years on Earth, and we only discovered the planet 176 (Earth) years ago. Because it takes so long for Neptune to complete a single orbit, its seasons last decades. It’s been summertime in Neptune’s southern hemisphere since 2005, and astronomers were anxious to see how the atmosphere warmed during this period. However, they found mostly the opposite. 

The necessary technology to accurately take the temperature of a planet with an average global temperature of -220 degrees Celsius has only existed for about 20 years, but the team gathered as much of it as they could. Using data from (mostly) ground-based telescopes with high infrared sensitivity, including but not limited to the ESA’s Very Large Telescope, NASA’s Spitzer Space Telescope, and the Keck Telescope, it was possible to model Neptune’s atmospheric temperature all the way back to 2003. 

Instead of seeing the planet warm as summer dragged on, the average temperature on the planet dropped 8 degrees Celsius. That might not sound like a lot, but keep in mind the summer lasts about 40 Earth years — the temperature dropped precipitously when it was supposed to be inching upward. At the same time, the team noted a rapid increase in heat over Neptune’s south pole, and that’s where it’s staying. 

Interestingly, no one has a good handle on why this is happening yet. This rapid drop in temperature is not visibly associated with any processes we can see from our vantage on Earth. There is some speculation that Neptune’s atmospheric chemistry might work differently than we currently believe, or perhaps this is the result of random weather patterns. A similar build-up of heat was detected on Saturn before a large storm, but that wasn’t associated with a planet-wide cooldown. So, we don’t know why Neptune is chilling out, but admitting that is the first step toward knowledge.

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