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  • Writer's pictureMatt Russell

#135 - Neptune Special

Although Uranus and Neptune are superficially twin planets, they are different enough to remind us - as do Venus and Earth - that we still have a lot to learn about the mix of natural laws and historical accidents that formed the planets and fashioned their destinies.

Timothy Ferris - Seeing in the Dark, 2002.

Jean-Pierre Christin

BORN May 31st, 1683 was a French physicist, mathematician, astronomer and musician. His proposal in 1743 to reverse the Celsius thermometer scale (from water boiling at 0 degrees and ice melting at 100 degrees, to where zero represents the freezing point of water and 100 represented the boiling point of water) was widely accepted and is still in use today.

Interesting facts about the Celsius

  • It was not until February 1985 that the weather forecasts issued by the BBC switched from "centigrade" to "Celsius"

  • Centigrade from the Latin centum, which means 100, and gradus, which means steps, not helpful for the French where it means angular measurement,

  • The more modern Celsius is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty.

  • Anders Celsius in 1742 came up with idea but wrong way round to modern way

  • Working independently Christin came up with the right way round in 1743

  • 1744 Swedish botanist Carl Linnaeus reversed Anders system

  • Classic example of lots of scientists coming up with the same idea at the same time, something inconceivable in the modern communication era

  • Really the unit should be the Linnaeus but Celsius preserves the C symbol better.

  • America is one of the few countries who don’t follow the Celsius scale

  • Over the years the Celsius has been fixed the triple point of water and the absolute zero temperature and as a result as accuracy increases the original 0 and 100 degree marks are slightly out, only important for scientists

  • 2019 redefinition of the SI base units came into force on 20 May 2019

  • Celsius being tied to Kelvin

  • Previous definition: The kelvin, unit of thermodynamic temperature, is 1/273.16 of the thermodynamic temperature of the triple point of water.

  • 2019 definition: The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10−23 when expressed in the unit J⋅K−1, which is equal to kg⋅m2⋅s−2⋅K−1, where the kilogram, meter and second are defined in terms of h, c and ΔνCs.

  • One consequence of this change is that the new definition of the kelvin depends on the definitions of the second, the meter, and the kilogram.


Into the deep ocean of the solar system


Uranus was first seen by Hershel in 1781. Neptune wasn't seen for almost another 50 years, in 1846.

Neptune is very mysterious indeed, we know very little, virtually all we know is from the Voyager 2 flyby in August 1989, Hubble and the large ground telescopes with adaptive optics.

How many times do you think Neptune has been the furthest away planet?

After Pluto's discovery it was the second furthest, but in 1979 Plutos elptical orbit took it closer to the sun so it became the furthest away planet, then in 1999 Pluto reclaimed her place, then by 2006 Pluto was demoted and once again Neptune reclaims the title and still has it.

Neptune is often called a gas giant along with Jupiter, Saturn an Uranus, but like Uranus it is probably better referred to as an Ice Giant. Jupiter and Saturn are mostly Hydrogen and Helium, but Uranus and Neptune have icy and rocky cores, so are significantly different. Despite being the smallest of the 4 giant planets, it actually more massive than Uranus, and the densest of all of them.

Apart from Jupiter, Neptune is the only planet where the gravity is more than earth, Neptune is bizarrely only a bit more than earth despite being 17 times more massive, it’s has much bigger volume so it all cancels out to anly being about 14% stronger, but you would still need twice the escape velocity to get off the planet, something even SLS couldn’t achieve!!

Who discovered the Planet?

When Neptune was discovered the Solar System grew by 50%’s really far out. If earth was a tennis ball, Neptune would be a football about 15 miles away!! Or 23km

It’s impossible to see with the naked eye, but you can see it with binoculars and resolve a blue disc with a great telescope, but with an apparent size of about 2.3 arc seconds you need to know where you are looking as a result it’s the only planet in the Solar System found by mathematical prediction rather than by empirical observation.

So the main players

  • Alexis Bouvard: Student of the great Leplace, realised that the unexpected changes in the orbit of Uranus led the Frenchman to deduce that its orbit was subject to gravitational perturbation by an unknown planet, sometime after 1821

  • John Couch Adams the cornishman began work on the orbit of Uranus at cambridge using the data he had in 1843. with extra data from Sir George Airy, the Astronomer Royal in 1844. produced several different estimates of a new planet by 1846

  • In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations but aroused no enthusiasm in his compatriots

  • George Airy in June 1846, upon seeing Le Verrier's estimate and its similarity to Adams's estimate, persuaded James Challis the director of Cambridge obsovatory to search for the planet. "in the hope of rescuing the matter from a state which is ... almost desperate" Challis didn’t find it.

  • Johann Gottfried Gall at the Berlin Observatory had been persuaded by Le Verrier by letter to search with the observatory's refractor. Heinrich d'Arrest, a student suggested they could compare a recently drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, he discovered Neptune within 1° of where Le Verrier had predicted it to be, about 12° from Adams' prediction

  • James Challis ealised that he had observed the planet on 4 and 12 August, but did not recognise it with no up-to-date star map and was distracted by his concurrent work on comet observations

  • Then Starts the massive French/British rivalry, and Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name came from Galle, who proposed the name Janus. In England, Challis put forward the name Oceanus

  • Le Verrier quickly proposed the name Neptune, but the tried to call it Le Verrier, and the sneaky French even stated calling Uranus Herschel to justify this,

  • Neptune eventually stuck by the end of 1946 as all the other planets are named after greek and roman gods.

  • In modern Greek the planet is called Poseidon and all other languages refer to Neptune as some form of mythical sea god.

  • the Royal Society awarded Le Verrier the Copley medal in 1846 for his achievement, without mention of Adams

  • Even being argued about to this day A pretty recent Scientific American article by Sheehan, Kollerstrom and Waff claimed more boldly "The Brits Stole Neptune" and concluded "The achievement was Le Verrier's alone."

  • But the controversy doesn’t end there, it’s very likely that Galileo's drawings show that he observed Neptune on December 28, 1612, and again on January 27, 1613;[8] on both occasions, Galileo mistook Neptune for a fixed star when it appeared very close (in conjunction) to Jupiter in the night sky.[9] Historically it was thought that he believed it to be a fixed blue star, and so he is not credited with its discovery. At the time of his first observation in December 1612, it was stationary in the sky because it had just turned retrograde that very day; because it was only beginning its yearly retrograde cycle, Neptune's motion was thought to be too slight, and its apparent size too small, to clearly appear to be a planet in Galileo's small telescope.[10] However, in July 2009 University of Melbourne physicist David Jamieson announced new evidence suggesting that Galileo was indeed aware that he had discovered something unusual about this "star". Galileo, in one of his notebooks, noted the movement of a background star (Neptune) on January 28 and a dot (in Neptune's position) drawn in a different ink suggests that he found it on an earlier sketch, drawn on the night of January 6, suggesting a systematic search among his earlier observations

  • John Herschel almost discovered Neptune the same way his father, William Herschel, had discovered Uranus in 1781, by chance observation. In an 1846 letter to Wilhelm Struve, John Herschel states that he observed Neptune during a sweep of the sky on July 14, 1830. Although his telescope was powerful enough to resolve Neptune into a small blue disk and show it to be a planet, he did not recognize it at the time and mistook it for a star

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.

— Adams (1846)

Any way what is Neptune Like!

The strongest winds in the Solar System! Why? Very cold in the upper atmosphere and insanely hot in the core. winds reaching speeds of almost 600 m/s (2,200 km/h; 1,300 mph)—nearly supersonic

1991 paper by V. E. SUOMI

Neptune receives only 1/900th of the earth's solar energy, but has wind speeds of nearly 600 meters per second. How the near-supersonic winds can be maintained has been a puzzle. A plausible mechanism, based on principles of angular momentum and energy conservation in conjunction with deep convection, leads to a regime of uniform angular momentum at low latitudes. In this model, the rapid retrograde winds observed are a manifestation of deep convection, and the high efficiency of the planet's heat engine is intrinsic from the room allowed at low latitudes for reversible processes, the high temperatures at which heat is added to the atmosphere, and the low temperatures at which heat is extracted.

You heard right Most of the winds on Neptune move in a direction opposite to the planet's rotation. Very different to Uranus whose core seems to have cooled down to much for these crazy weather systems, Uranus only radiates 1.1 times as much energy as it receives from the Sun, whereas Neptune radiates about 2.61 times as much, no one really knows what the source of the energy is. Neptune lies over 50% farther from the Sun than Uranus, and receives only 40% its amount of sunlight

Neptune's outer atmosphere is the coldest place on any Planet in the Solar System, with temperatures at its cloud tops approaching 55 K (−218 °C; −361 °F) you would flash freeze in a second! . Temperatures at the planet's centre are approximately 5,400 K (5,100 °C; 9,300 °F)

The heat left over from Neptune's formation may be sufficient to explain its current heat flow, though it is more difficult to simultaneously explain Uranus's lack of internal heat. Maybe it’s easy to generate winds in the super cold temperatures and the fluid gasses in the planet’s atmosphere might reduce friction to make the process easy.

Uranus also had a storm very similar to Jupiter’s GRS ...the GDS the great dark Spot. Voyager 2 discovered it and it was quickly evolving, probably a cyclone, methane cloud tops developed and over the years the spot has been seen by hubble to slowly dissapear, probably as it migrates to the equator (coreolis effect listen to earlier podcast)

Neptune's atmosphere is made up of 80% hydrogen and 19% helium and a bit of methane. The methane in Neptune's upper atmosphere absorbs the red light from the sun, and so Neptune appears blue, however another chemical must contribute to Neptune's vivid azure differing from Uranus's milder cyan.

Really intersing thermosphere, For reasons unknown, the planet's thermosphere is at an anomalously high temperature of about 470 oC

The planet is too far from the Sun for this heat to be generated by ultraviolet radiation. Maybe caused by atmospheric interaction with ions in the planet's magnetic field. or gravity waves from the interior that dissipate in the atmosphere. The thermosphere contains traces of carbon dioxide and water, which may have been deposited from meteorites and dust from space.

The rotation axis of Neptune is tilted like earths, but slightly more extreme, relative to the revolution axis of the solar system, but nowhere near as extreme as Uranus. It's magnetic axis also has a large tilt relative to the rotation axis. These effects combine to not only give Uranus a more a more variable magnetosphere, but suggest the planet's magnetic field may be generated by a different mechanism than that of Earth, Jupiter and Saturn. This field may be generated by convective fluid motions in a thin spherical shell of electrically conducting liquids (probably a combination of ammonia, methane and water) resulting in a dynamo action, like swilling a magnetic water around a bottle.

Neptune’s orbital eccentricity of .0097 is second smallest behind that of Venus. This small eccentricity means that the orbit of Neptune is very close to being circular

It has Rings!!

Neptune has five rings, all of which are named after astronomers who made important discoveries about Neptune – Galle, Le Verrier, Lassell, Arago, and Adams. The adams rings are more like Arcs, named Courage, Liberté, Egalité 1, Egalité 2 and Fraternité, normally arcs would spread out into a uniform ring over short timescales. But like the spokes and gaps in saturns rings they are probably corralled into their current forms by the gravitational effects of Galatea, a moon just inward from the ring

Unlike Saturns lovely reflective new ice rings, the rings of Neptune are dust and rocks an darkened probably with a reddish hue, from radiation exposure. They are probably quite new and made from moons that has crashed into one another. They can already be seen to be decaying. Liberty arc may be gone within the century.

Neptune is the gate keeper of the Kuiper belt and resonances with it orbit create bands of stable orbits in the Kuiper belts, one of the bodies trapped in these stable resonances is Pluto, and although Pluto crosses the orbit of Neptune as it does 2 orbits for every 3 of Neptune they will never crash. The other bodies trapped in this orbit are also called Plutinos. I’d say this is one of the biggest reasons why Pluto isn’t really a planet.


The largest of the 14 moons is called Trtion, discovered by William Lassell just 17 days after the discovery of Neptune itself. and the pictures of triton are incredible, it’s all crumpled up and looks in some ways very similar to pluto.

It is the 7th largest Moon, and is only a bit smaller than the Moon, and is actually bigger than Pluto. Some would even call it’s Plutos twin. It orbits a little closer than our own moon, so very close in really compared to the planet sizes. 220,405 miles (354,800 kilometers). 6 days to go round Neptune!!! Due to the moon's high albedo and very little sunlight absorbed by the surface, Triton's surface temperature is colder than any other measured object in the Solar System with an average temperature of minus 235 degrees Celsius. Mazomba is Triton's largest known impact crater that was observed by Voyager 2 and is about 27 kilometers in diameter. Larger craters were observed; however, they are generally thought to be volcanic in nature.

The really intersting thing about Triton though is that it is the only large and circularized moon to travel in the opposite direction of the planets spin (retrograde). This is pretty convincing evidence that it was plucked from the Kuiper belt where it may have been considered a planet like Pluto of course. More reasons to demote Pluto. Triton is an active moon, features include tectonic structures, cryovolcanic land-forms, "cantaloupe terrain", and several particulate plumes and associated deposits, with cryovolcanoes and even a faint atmosphere, what is very interesting is the very capture of Triton would be one of the mechanisms that heats up the interior from all the tidal forces, implying you can MAKE and ocean world, and they don’t have to form in place. The heat may even be sufficient to maintain a global subsurface ocean similar to what is hypothesized to exist beneath the surface of Europa. The black material ejected is suspected to contain organic compounds, and if liquid water is present in Triton, it has been speculated that this could make it habitable for some form of life.

Recent article about gas hydrates insulating the ocean on Pluto worth mentioning.

Basically having an active moon that far out is incredible, potentially expanding the habitable zone to 30 AU

A new paper Tegler et al., “A New Two-Molecule Combination Band as Diagnostic of Carbon Monoxide Diluted in Nitrogen Ice on Triton,”in the Astronomical Journal, shows how lab work can reveal new insights into old data, “While the icy spectral fingerprint we uncovered was entirely reasonable, especially as this combination of ices can be created in the lab, pinpointing this specific wavelength of infrared light on another world is unprecedented.”

Further evidence that triton maybe harbouring an ocean?

Eventually Tritons surprisingly circular orbit will decay and it will pass the Roche limit and will be torn into pieces and will become a beautiful ring system. All the other moons are tiny and hippocamp was only discovered in 2013 by stacking Hubble images.

Neptune has a number of known Trojan objects occupying both the Sun–Neptune L4 and L5 Lagrangian points—gravitationally stable regions leading and trailing Neptune in its orbit, respectively.[107] Neptune trojans can be viewed as being in a 1:1 resonance with Neptune. Some Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being captured. Neptune also has a temporary quasi-satellite, 12,500 years and it will remain in that dynamical state for another 12,500 years

Future Missions

After the Voyager 2 flyby mission, the next step in scientific exploration of the Neptunian system, is considered to be a Flagship orbital mission.

Such a hypothetical mission is envisioned to be possible in the late 2020s or early 2030s.

Another proposal was for Argo, a flyby spacecraft to be launched in 2019, that would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would be on Neptune and its largest moon Triton to be investigated around 2029.

In 2003, there was a proposal in NASA's "Vision Missions Studies" for a "Neptune Orbiter with Probes" mission that does Cassini-level science.

In 2008, the mission was removed from NASA's possible future missions list. According to NASA's 2010 budget, funding to missions to the outer Solar System was aimed at the future Europa Jupiter System Mission. The remaining budget was allocated to ongoing undertakings such as Cassini–Huygens, Juno, and New Horizons, with the Neptune system not being part of any official considerations.

In 2011, NASA's Decadal Survey considered a mission to an ice giant—either Uranus or Neptune—but for feasibility reasons recommended a Uranus orbiter and probe.

In 2019, a Neptune flyby proposal was made by the JPL under the name "Trident" for inclusion in the Discovery program, “The time is now to do this mission,” said Louise Prockter, director of the Lunar and Planetary Institute in Houston and the principal investigator of the proposed mission. “The time is now to do it at a low cost. And we will investigate whether it is a habitable world, which is of huge importance.” bsacically the seasons take so long, 40 years, that to miss one is a disaster and time is running out. In 2026 is an ideal window as you get a straight shot out with a jupiter asssist an you get your best chance of seeing a geeyser.

One of the problems is Nasa is running out of Plutonium for the RTG power supplies they use on these missions where the sun don’t shine much.

Formation of Neptune

The formation of the ice giants, Neptune and Uranus, has proven difficult to model precisely. Current models suggest that the matter density in the outer regions of the Solar System was too low to account for the formation of such large bodies from the traditionally accepted method of core accretion, and various hypotheses have been advanced to explain their formation.

  • One is that the ice giants were not formed by core accretion but from instabilities within the original proto-planetary disc and later had their atmospheres blasted away by radiation from a nearby massive OB star.

  • They formed closer to the Sun, where the matter density was higher, and then subsequently migrated to their current orbits after the removal of the gaseous protoplanetary disc. This hypothesis of migration after formation is favoured, due to its ability to better explain the occupancy of the populations of small objects observed in the trans-Neptunian region. The current most widely accepted explanation of the details of this hypothesis is known as the Nice model, which explores the effect of a migrating Neptune and the other giant planets on the structure of the Kuiper belt.

  • This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies including the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune. Its success at reproducing many of the observed features of the Solar System means that it is widely accepted as the current most realistic model of the Solar System's early evolution

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