Through the years, Hubble has monitored the weather on the outer planets of our solar system.
Like Earth, planets have seasons; however, Neptune’s seasons last decades rather than months. Regardless, these seasons are far from calm and uneventful and the observations reveal interesting information that we can use to learn more about the atmosphere of other planets in our solar system and how these compare to our own.
Let’s look at the changes:
In 1994, Hubble took the first observations of this blue giant, which gets its color due to the absorption of red light by its methane atmosphere. These observations[1], taken over a period of a few days, revealed the dynamic atmosphere of the planet, probably due to instabilities triggered but large changes in the temperature at different altitudes.
Between 1996 and 2002, observations of Neptune taken at the same rotation period, reveal several interesting features [2].
From these observations, astronomers notice the impact of the seasons on this planet. Although it takes about 165 years for this planet to go around the Sun, Neptune's rotation axis is inclined by about 29 degrees with respect to its orbital plane, giving place to seasons just like on Earth. However, because of its distance from the Sun, each season in Neptune is about 40 years long. Hubble's observations taken from 1996 to 2002 [2] - during the springtime in the southern hemisphere of Neptune, show an increase in brightness, revealing that, even that far, its atmosphere responds to the Sun's radiation.
In 2005, Hubble took more images with a better camera. The image below, taken using methane filters, revealed details not visible to the human eye [3].
In 2007, Hubble took more images, this time showing a dark spot in the northern hemisphere. With other observations, astronomers had found that this dark spot first moved to the equator and later reversed its course back to the north.
To commemorate one full rotation of the planet, to nearly the point where it was discovered 165 years ago by the German astronomer Johann Galle, Hubble observed Neptune in 2011. Unknowingly, Galileo Galilei first observed this planet in 1612, but he thought it was just a star. Galileo, however, noted it had some peculiar movement. It was not until 1846 that its existence was mathematically predicted in France by Urbain Le Verrier and independently in England by John Couch Adams.
Both were astronomers and mathematicians who made this discovery while trying to understand the discrepancy in the rotation of Uranus documented by Herchel in 1781, who discovered the seventh planet of our solar system. Prompted by Le Verrier, the astronomer Johan Galle looked for this planet that same year, finding it within 1 degree of the predicted location.
Figure caption: The clouds are tinted pink because they are reflecting near-infrared light, while the faint dark band near the bottom of the southern hemisphere is probably caused by a decrease of scattered blue light in the atmosphere.
Hubble's 2011 observations showed more clouds than seen in previous years. These also showed that the clouds were moving to the northern hemisphere of the planet, where it was still winter.
In 2016, Hubble observed Neptune again. This time with a more powerful camera (WFC3), which confirmed the presence of a dark vortex in the atmosphere of the blue giant. The images below show how this dark vortex is more apparent when observed in the blue light (left). While the visible light (right) reveals also the bright white clouds [5]
More observations taken by Hubble, on September 2018, uncovered a new mysterious dark storm appearing in the southern hemisphere of the planet. Since 1993, Hubble has detected four dark vortexes like this one, while two more were discovered by the Voyager spacecraft in 1989. It has been estimated that these features appear and disappear every two years, as can be seen in the images below [6]
But monitoring the Neptunian weather brought another interesting result. In 2013, a new moon of Neptune was discovered and called S/2004 N[7]. This moon is so small and faint that was missed by the spacecraft Voyager 2 when it flew by in 1989. It was not but after comparing Hubble observations taken between 2004 and 2009, that a small object orbiting Neptune every 23 hours was determined to be a moon. This moon, one of the 14 moons of the planet, was later called Hippocamp, a half-horse half-fish from Greek mythology.
Finally, in 2022, the James Webb Space Telescope (JWST) put the cheery on the pie. JWST took a magnificent view of this planet using its Near Infrared Camera (NIRCam)[9]. The observations captured the high-altitude methane-ice clouds that appear as bright spots. This image [8[ also shows a subtle brightness emanating from the poles. Looking closely at the southern hemisphere, we can see light beams emanating from the vortex, indicating the presence of high-altitude clouds surrounding it.
But what makes this image even more fascinating? The view of two of its moons, Despina and Galatea, moving along the path of the rings of the planet. These rings, made of dust particles, are easily seen by JWST in the infrared.
Credits:
[1] H. Hammel (Massachusetts Institute of Technology) and NASA
[2] NASA, L. Sromovsky, and P. Fry (University of Wisconsin-Madison)
[3] NASA, ESA, E. Karkoschka (University of Arizona), and H.B. Hammel (Space Science Institute, Boulder, Colorado)
[4] NASA, ESA, and the Hubble Heritage Team (STScI/AURA);
[6] , ESA, and M.H. Wong and A.I. Hsu (UC Berkeley)
[9] NASA, ESA, CSA, STScI
To learn more about Neptune, visit the NASA Science Solar System Exploration site at https://solarsystem.nasa.gov/planets/neptune/overview/