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Planets: Neptune

The Moon
March 21, 2019

Neptune's birthday and a beautiful piece of maths

Neptune is about to celebrate its first birthday. On 12 July it will be exactly one Neptunian year - or 164.79 Earth years - since its discovery on 24 September 1846. But why do we still know so little about the distant planet?

About 4.4 billion kilometres away from Earth lies Neptune, the first planet in the solar system to be discovered deliberately.

After the classification of the planet Uranus in the 1780s, astronomers had been perplexed by its strange orbit. Scientists came to the conclusion that either Isaac Newton's laws were fundamentally flawed or that something else - another planet - was pulling Uranus from its expected orbit.

And so the search for the eighth planet began.

"It was such an incredible mathematical business, it makes searching for a needle in a haystack look like a 10-minute job for a child," says Dr Alan Chapman, author of the Victorian Amateur Astronomer.

While mathematical predictions had been made over the previous decades, it was not until French mathematician Urbain le Verrier's theories were tested at the Berlin observatory by Johann Gottfried Galle that the planet was first seen.

After only an hour or so of searching, Neptune was observed for the first time on the night of 23 September 1846. It was found almost exactly where le Verrier had predicted it to be.

Independently, British scientist John Couch Adams also produced similar results, and now he and Verrier are given joint credit for the discovery.

But many claim it was not Galle who documented the planet first, but the famous astronomer and mathematician Galileo. In his famous work The Starry Messenger, some evidence points to his discovery.

"If you look at the drawings for January 1613, you can see a fantastic drawing of Jupiter and its moons," says Dr Robert Massey of the Royal Astronomical Society. "It even includes an object labelled as 'fixed star' which is the first telescopic drawing of the planet Neptune."Controversy aside, comparatively little is still known about the planet.

Unfriendly lump

Part of the problem is that there is no way for the planet to be viewed with the naked eye and until the Hubble telescope, scientific observation was very difficult. So what is Neptune like?

"It's a frozen lump of frozen gases and I suppose not a terribly friendly place," says Dr Chapman. "Let's wish it a happy birthday but perhaps let's keep as far away from it as we can as it won't give you a welcome."One of the things most interesting to scientists about Neptune is the weather.

"Cloudy with a chance of methane" is how planetary scientist Heidi Hammel, of the Association of Universities for Research in Astronomy (Aura), describes it. Winds can reach 1,930km/h (1,200mph), creating storms unimaginable on Earth. These huge storms are seen as dark spots in a similar way to the Great Red Spot on Jupiter.

The reason astronomers know so little is because the planet has only been photographed once from close range - on the Voyager 2 mission in 1989. And because its seasons last 40 Earth years, only Neptune's spring and early summer have been closely documented. "Every time we go to a telescope and look at this planet, it's doing something new, it's doing something we hadn't thought of before," says Dr Hammel.

What Dr Hammel found was that storms were appearing and forming and changing much more quickly than had previously been thought. She was looking at a planet very different from the photos taken by Voyager 2.

"We really have only been observing Neptune with big telescopes since shortly before 1989," she says."We haven't been looking long enough. This planet is not for the impatient."

Pluto's place The chance to find out more about the planet close-up still seems a long way away - more than just the billions of kilometres in distance.Nasa missions to discover more about the planet have been sidelined for the moment due to budget constraints.

The Neptune Orbiter mission, once planned to launch sometime in 2016, no longer features on Nasa's proposed mission list. We've never had a mission that's been dedicated to Neptune," says Dr Robin Catchpole, of Cambridge University's Institute of Astronomy

"We know how it fits into the sequence of planets as far as composition goes but we don't know a lot." Even the New Horizons mission to discover more about Pluto and the outer reaches of the solar system - due to pass through the orbit path of Neptune on 24 August 2014 - has not been organised to closely monitor Neptune.

Instead, photos are being taken of it and its moon to test the imaging equipment rather than for any major scientific purposes.And this mission is allowing some to question whether Pluto could be reclassified as the ninth planet in the solar system after its primary planet title was taken away in 2006.If it was reinstated, Neptune would lose the honour of being the furthest planet from the Sun.

"Whether Pluto is called a planet or not is a matter of semantics," says Dr Catchpole."The situation with the classifications is that Pluto doesn't fit into the [current] system very well. I don't think it's ever going to change again."So happy birthday Neptune. Though lighting any candles on a birthday cake might be tricky in those high winds.

Breakneck speed' mini moon hurtles around Neptune at 20,000mph

A miniature moon that whizzes around Neptune at breakneck speed has been tracked by astronomers working from the US. The speck of a moon, no more than 21 miles across, hurtles around the distant gas giant at about 20,000 miles an hour,

10 times faster than our own moon circles Earth, scientists said. “This is the smallest known moon around the farthest known planet in the solar system,” said Mark Showalter, a senior research scientist at the Seti Institute, in Mountain View, California.

The scientists first saw the moon in enhanced images from the Hubble space telescope, which showed the tiny body 65,000 miles away from Neptune. It orbits the planet once every 22 hours.The astronomers began the work that led to the discovery in 2004 but required further images, which were taken in 2009 and 2016, to confirm its presence and the nature of its orbit.

The moon is too small and too distant to be seen in individual images taken by the Hubble telescope. But it showed up when Showalter and his colleagues stacked a sequence of images on top of each other. For the process to work the scientists had to calculate how fast any moon would be moving and shift the pixels in each image to take account of the motion.

“We could take eight images with five-minute exposures and turn them into one image with a 40-minute exposure,” Showalter said. “You can’t normally do that because the moons would smear out, but we essentially took the orbital motion out of the images. And when we did that this extra dot showed up. We realised we were looking at a tiny little moon.”

Under guidelines set down by the International Astronomical Union, Neptunian moons must be named after sea creatures and gods from Greek and Roman mythology. Since Showalter is a keen scuba diver and a fan of seahorses, he named the moon Hippocamp, a nod to the seahorse genus, and the name of a horse-fish sea monster often depicted pulling Neptune’s chariot. “It’s always gratifying when you can find a piece of real estate in the solar system and get to name it,” he said.

In 1989 Nasa’s Voyager 2 probe shot past Neptune and photographed six inner moons orbiting the planet in the same direction as it spins. All are thought to have formed around the same time as the planet itself. Seven more moons, which may have been captured by Neptune’s gravity after it formed, lie further out.

Hippocamp circles just inside the orbit of Proteus, the largest of Neptune’s six inner moons. Proteus’s 250-mile-wide body bears a huge crater from a violent collision deep in its past.

Writing in the journal Nature, Showalter and his colleagues speculate that Hippocamp formed from the material that was blasted into space when a comet or asteroid clattered into Proteus.

Showalter said Neptune was a planet shrouded in mystery. Beyond its moons– 14 have been found – the planet has a number of complete and partial rings, some of which appear to be fading away.

“It’s a good reason why Nasa and the European Space Agency need to get together and send a mission to Neptune,” said Showalter. “All we know from close-up studies of Neptune are from Voyager. There’s a great big hole in our knowledge of the outer solar system.”

In retrospect, after it was discovered, it turned out it had been observed many times before but not recognized, and there were others who made various calculations about its location which did not lead to its observation.

By 1847, the planet Uranus had completed nearly one full orbit since its discovery by William Herschel in 1781, and astronomers had detected a series of irregularities in its path that could not be entirely explained by Newton's law of universal gravitation.

These irregularities could, however, be resolved if the gravity of a farther, unknown planet were disturbing its path around the Sun. In 1845, astronomers Urbain Le Verrier in Paris and John Couch Adams in Cambridge separately began calculations to determine the nature and position of such a planet.

Le Verrier's success also led to a tense international dispute over priority, because shortly after the discovery George Airy, at the time British Astronomer Royal, announced that Adams had also predicted the discovery of the planet. Nevertheless, the Royal Society awarded Le Verrier the Copley medal in 1846 for his achievement, without mention of Adams.

Irregularities in Uranus's orbit

In 1821, Alexis Bouvard had published astronomical tables of the orbit of Uranus, making predictions of future positions based on Newton's laws of motion and gravitation. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize some perturbing body


These irregularities or "residuals", both in the planet's ecliptic longitude and in its distance from the Sun, or radius vector, might be explained by a number of hypotheses: the effect of the Sun's gravity, at such a great distance might differ from Newton's description; or the discrepancies might simply be observational error; or perhaps Uranus was being pulled, or perturbed, by an as-yet undiscovered planet.

Adams learned of the irregularities while still an undergraduate and became convinced of the "perturbation" hypothesis. Adams believed, in the face of anything that had been attempted before, that he could use the observed data on Uranus, and utilising nothing more than Newton's law of gravitation, deduce the mass, position and orbit of the perturbing body.

Though the problem is a simple one for modern mathematics after the advent of electronic computers, at the time it involved much laborious hand calculation. Adams began by assuming a nominal position for the hypothesised body, using the empirical Bode's law.

He then calculated the path of Uranus using the assumed position of the perturbing body and calculated the difference between his calculated path and the observations, in modern terms the residuals. He then adjusted the characteristics of the perturbing body in a way suggested by the residuals and repeated the process, a process similar to regression analysis.

On 13 February 1844, James Challis, director of the Cambridge Observatory, requested data on the position of Uranus, for Adams, from Astronomer Royal George Biddell Airy at the Royal Observatory, Greenwich. Adams certainly completed some calculations on 18 September 1845. Supposedly, Adams communicated his work to Challis in mid-September 1845 but there is some controversy as to how.

The story and date of this communication only seem to have come to light in a letter from Challis to the Athenaeum dated 17 October 1846. However, no document was identified until 1904 when Sampson suggested a note in Adams's papers that describes "the New Planet" and is endorsed, in handwriting not Adams's, with the note "Received in September 1845"

Meanwhile, Urbain Le Verrier, on November 10, 1845, presented to the Académie des sciences in Paris a memoir on Uranus, showing that the pre-existing theory failed to account for its motion.

Unaware of Adams's work, he attempted a similar investigation, and on June 1, 1846, in a second memoir presented to a public meeting of the Académie, gave the position, but not the mass or orbit, of the proposed perturbing body. Le Verrier located Neptune within one degree of its discovery position.

Upon receiving in England the news of Le Verrier's June prediction, George Airy immediately recognized the similarity of Le Verrier's and Adams' solutions. Up until that moment, Adams' work had been little more than a curiosity, but independent confirmation from Le Verrier spurred Airy to organize a secret attempt to find the planet.

At a July 1846 meeting of the Board of Visitors of the Greenwich Observatory, with Challis and Sir John Herschel present, Airy suggested that Challis urgently look for the planet with the Cambridge 11.25 inch equatorial telescope, "in the hope of rescuing the matter from a state which is ... almost desperate".The search was begun by a laborious method on 29 July.

Adams continued to work on the problem, providing the British team with six solutions in 1845 and 1846 which sent Challis searching the wrong part of the sky. Only after the discovery of Neptune had been announced in Paris and Berlin did it become apparent that Neptune had been observed on August 8 and August 12 but because Challis lacked an up-to-date star-map, it was not recognized as a planet.

I mention these dates merely to show that my results were arrived at independently, and previously to the publication of those of M. Le Verrier, and not with the intention of interfering with his just claims to the honours of the discovery ; for there is no doubt that his researches were first published to the world, and led to the actual discovery of the planet by Dr. Galle, so that the facts stated above cannot detract, in the slightest degree, from the credit due to M. Le Verrier.

The criticism was soon afterwards made, that both Adams and Le Verrier had been over-optimistic in the precision they claimed for their calculations, and both had, by using Bode's law, greatly overestimated the planet's distance from the sun. Further, it was suggested that they both succeeded in getting the longitude almost right only because of a "fluke of orbital timing".

This criticism was discussed in detail by Danjon (1946) who illustrated with a diagram and discussion that while hypothetical orbits calculated by both LeVerrier and Adams for the new planet were indeed of very different size on the whole from that of the real Neptune (and actually similar to each other), they were both much closer to the real Neptune over that crucial segment of orbit covering the interval of years for which the observations and calculations were made,

than they were for the rest of the calculated orbits. So the fact that both the calculators used a much larger orbital major axis than the reality was shown to be not so important, and not the most relevant parameter.

The new planet, at first called "Le Verrier" by François Arago, received by consensus the neutral name of Neptune. Its mathematical prediction was a great intellectual feat, but it showed also that Newton's law of gravitation, prevailed even at the limits of the solar system

I could not expect however that practical astronomers, who were already fully occupied with important labours, would feel as much confidence in the results of my investigations, as I myself did.

M I Ro

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