Almost five billion kilometres from Earth is a powerful storm that once was so large it could stretch across the North Atlantic Ocean. Now, it has almost disappeared.

The storm has been raging on Neptune, the eighth planet in our solar system. Just as we get hurricanes here on Earth, storms have been seen on other planets, such as our largest, Jupiter. Its storm, known as the "Great Red Spot," has been around for at least 350 years.

As if an enormous hurricane big enough to stretch across the Atlantic wasn't bad enough, astronomers also believe the storm on Neptune had been pulling up material from deep inside the planet, possibly hydrogen sulphide. That's right: it smelled like rotten eggs.

But Neptune's storms just don't have the staying power of Jupiter's. A storm known as the "Great Dark Spot" was recorded on the planet by Voyager 2 in the 1980s. In 1989, the Hubble Space Telescope was launched into space. A few years later, it turned toward the planet to check on it, only to find it had all but disappeared.

Hubble is the only telescope capable of providing sharp images of Neptune in blue light. Since the 1990s, it has captured two other dark spots. But Neptune storms had never been tracked to understand their short lifespans.

So when a new one was spotted in 2015, researchers decided to monitor it. They soon noticed it weakening.

"It looks like we're capturing the demise of this dark vortex, and it's different from what well-known studies led us to expect," Michael H. Wong, a planetary scientist at the University of California at Berkeley and co-author of a new study published in The Astronomical Journal, said in a statement.

The dark spot was first seen on the planet at mid-southern latitudes. Since 2015, it drifted southward where the researchers believe wind shear may have contributed to its fizzle.

The researchers hope to further study storms on Neptune.

"No facilities other than Hubble and Voyager have observed these vortices," Wong said. "For now, only Hubble can provide the data we need to understand how common or rare these fascinating Neptunian weather systems may be."