The planet Neptune, as seen by Voyager 2
Neptune, as seen by Voyager 2 (Wikimedia Commons)

Just after midnight on September 23rd, 1846, Johann Gottfried Galle stood at the telescope in the Berlin Observatory, calling out coordinates one by one to his assistant, Heinrich Louis d’Arrest. Despite the lateness of the hour, and the apparent tediousness of the task, there was an uncharacteristic tension in Galle’s voice, a certain expectancy. D’arrest, in the dim candlelight, dutifully checked off each entry on a map of the heavens. “Right ascension: 21 hours, 53 minutes, 16 seconds,” called out Galle. “Declination: negative 13 degrees, 24 minutes, 15 seconds. Magnitude 7.8.” There was a pregnant pause. “Sir,” came the reply, “that star is not on the chart.” The tiny speck of light would turn out to be the last undiscovered planet in the solar system.

Today (July 12, 2011) Neptune will finally celebrate its first anniversary, as it returns to the exact point in its orbit where Galle and D’arrest spotted it all those years ago. Although our knowledge of Neptune has improved greatly in the last 165 years, it’s still the furthest official planet in the solar system, and the only one that can never be seen with the naked eye. The existence of Neptune had to be deduced indirectly, by carefully analyzing the perturbations caused by its gravity to the orbit of Uranus, its nearest neighbour. The sheer mathematics involved in working out the size and location of the unknown planet from these slight variations must have been daunting in an age without computers. Nevertheless, in 1846 both John Couch Adams of Britain and Urbain Joseph Le Verrier of France came up with independent solutions within a few months of each other. After the discovery was announced, both British and French national pride were inflamed by the audacity of the other’s claim to priority. Scientists, however, are a magnanimous bunch, and today Adams, Le Verrier, and Galle generally receive joint credit.

While the war of words was still raging in the press, William Lassell, a British brewer with a rabid enthusiasm for astronomy, sat down at his own hand-made telescope to have a look at the new planet for himself. Just 17 days after its discovery, Lassel announced that Neptune had a moon. Triton was (and still is) the largest object in the solar system to exhibit what’s known as retrograde motion, meaning it orbits in the opposite direction to all the planets. This makes it extremely unlikely that Triton formed from the same cloud of gas and dust that created Neptune. Instead, it must once have been a free-orbiting body like Pluto, which it resembles closely in both size and composition. Both are a bit smaller than our own moon, and are believed to have a rocky core covered in a thick layer of ices, including water, methane and nitrogen.

But if the surface of Triton is frozen, it’s anything but boring. Those transparent ices can still trap what little heat they receive from the sun using a kind of solid-state greenhouse effect. At negative 196 degrees C, the heat is enough to vaporize nitrogen. Pressure builds, and eventually the surface explodes, spewing ice and dust in plumes several kilometres high. These geysers have erased most of the craters on Triton, and created an extremely wispy nitrogen atmosphere

We know this because of photos taken in 1989 by NASA’s Voyager 2 probe, the only man-made object ever to visit the Neptunian system. It found six new moons and evidence of five rings too faint to be seen from Earth. It also provided the first shots of Neptune’s brilliantly blue atmosphere, made of hydrogen and helium with clouds of methane, ammonia and hydrogen sulphide. The weather on Neptune is lively: clouds have been clocked at over 2000 km/h, faster than the speed of sound. When Voyager 2 went by, Neptune was sporting a storm about the size of the Earth, nicknamed the “Great Dark Spot.” However, when the Hubble Space Telescope photographed Neptune in 1994, the storm had blown itself out.

Of all that we now know about Neptune, the most intriguing may be its crucial role in the early life of the solar system. “One of the problems with Uranus and Neptune is that they’re really far away from the sun, and the density of gas that was thought to exist out there at that time is really insufficient to form them,” says John E. Moores, Planetary Scientist and Post-Doctoral Fellow at the Centre for Planetary Science and Exploration at the University of Western Ontario. “The theory that’s currently popular is that all the four gas giants formed much closer to the sun, and later migrated outward.” As they did so, they would have scattered any rocky material left over from the formation of the solar system. This meant that the constant bombardment which characterized earth’s early history – a hellish period appropriately called the Hadean – would finally have ceased. “And the remarkable thing about life on earth,” says Moores “is that we see it almost as soon as these conditions are present.” If this is true, we may owe our very existence to the gravity of a planet whose existence was beyond our knowledge until only a year ago, Neptunian standard time.

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