A New Look at Neptune – Skywatching

When the James Webb Space Telescope was launched, we all had great expectations of what the telescope would show us. The instrument meets these expectations and more.

The images of distant galaxies and star-forming regions in our galaxy are beautiful and scientifically exciting. The JWST will play a very large role in finding Earth-like planets orbiting other stars and will help us find out if we are alone. However, one of the most intriguing images to date is of something far more local, actually in our cosmic backyard. It’s from the planet Neptune.

Neptune is the eighth planet outside the sun. It is 30 times farther from the Sun than Earth, so it receives only about 1/900th the amount of sunlight and heat. It’s cold out there.

The planet is nearly 50,000 km across, compared to Earth’s 12,756 km. Despite being almost 60 times the volume of Earth, its mass is only 17 times that of Earth. That means it can’t be a giant stone ball. A cubic centimeter of its material weighs an average of just 1.6 grams.

For Earth, that amount is about 5.5 grams. Like Jupiter, Saturn, and Uranus, Neptune is mostly gas, with a kind of solid in the middle.

Its atmosphere consists mostly of hydrogen and helium with traces of methane, which gives the planet its blue color. Jupiter and Saturn appear beige through telescopes because their atmospheres are laden with ammonia and complex hydrocarbons. Neptune is colder and these materials are frozen out.

The flyby of the Voyager 2 spacecraft showed that Neptune is surrounded by one or two thin rings and that, despite its great distance from the Sun, there is enough energy to control the weather with clouds and strong winds. However, the images provided by the JWST showed a better and more scientifically intriguing view than ever before. The camera is sensitive to the red and infrared ends of the spectrum and much less sensitive to blue. This works well here because the bright, blue disk in the image is much dimmer, which allows the fainter things on the planet to stand out.

The image shows a bluish disk with many clouds, probably made of frozen methane. These are concentrated in belts due to the planet’s rapid rotation. A day on Neptune is about two-thirds the length of a day on Earth, despite the planet’s larger diameter. The thin rings shown in the Voyager images are unmistakable. There are two well-defined rings, and a fainter dust belt extends outward from the inner ring. We can also see seven of Neptune’s cluster of fourteen known moons. There is probably more to discover: mostly very small ones.

The James Webb Space Telescope will play a role in studying planets orbiting other stars – exoplanets. Many of them are probably similar to those in our solar system. These can be rocky spheres like Mercury, Venus, Earth and Mars, gas giants like Jupiter and Saturn or ice giants like Uranus and Neptune.

Some of the exoplanets discovered so far are unlike any in our solar system, but even so, those we can get a close look at help us understand what we see as they orbit other stars.

Even with the best ground- or space-based telescopes, these exoplanets appear only as shadows in the light of the stars. From this we can deduce the size of these worlds, how close they are to their suns, and something about what their atmospheres are made of.

Putting this information into an image builds on what we see in our own solar system.

The JWST is proving to be a powerful tool in answering the questions we all have deep within – are there planets like ours out there and are we alone?

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• Mercury is low in the east before sunrise. In the early evening, Jupiter lies to the east and Saturn to the southeast.

• October 9th is full moon.

This article was written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.