The search for outlying planets began with the telescopic discovery of Uranus by William Herschel in 1781. It became apparent that some force was affecting the orbit of Uranus, and in a triumph for mathematics, the position of the planet Neptune was deduced, and almost at once its existence was confirmed in 1846.
In 2012, when this article was first drafted, there were spacecraft orbiting or en route to all the planets except Uranus and Neptune. The US Messenger probe was orbiting Mercury, and is now down, but Europe’s Bepicolombo has just made its first pass in 2021. Venus Express was circling that planet, and it too has gone down, but Japan’s Akatsuki is orbiting now. Mars Express, Mars Climate Observer and Mars Reconnaissance Orbiter are still in orbit, where they have been joined by India’s MOM, the US MAVEN, Europe’s Trace Gas Orbiter, China’s Tianwen-1 and the United Arab Republic’s Hope. On the surface, the Opportunity rover succumbed to a dust storm in 2018, but the nuclear-powered Curiosity survived and has now been joined by Perseverance, with its Ingenuity helicopter, and the Chinese Zhurong, operational again after the planet’s passage behind the Sun. Insight failed to drill into the Martian crust as intended, but the lander continues to return valuable seismic and atmospheric data.
The Dawn probe completed its mission to the asteroids Vesta and Ceres, and Hayabusa-2 and OSIRIS-Rex are now on their way back to Earth with samples from near-Earth asteroids. The LUCY mission is on its way to explore asteroids in the orbit of Jupiter, and the DART and PSYCHE asteroid missions are poised for launch. At Jupiter itself, the Juno orbiter’s mission has been extended to 2025. The Cassini orbiter’s Saturn mission ended in 2017, with no successor in sight; but New Horizons passed Pluto in 2015, and the smaller object Ultima Thule (now called Arrokoth) in 2019, and may yet find another target. Europe’s Rosetta probe completed its mission to Comet 67-P in 2016, but was so successful that a follow-up mission is highly likely.
Apart from that, ‘there’s nothing much happening in planetary exploration’, as any sceptic would tell you. But the one area in which they’d be right is the two remaining gas giant planets. Uranus and Neptune have been visited only once each, by Voyager 2 in 1986 and 1989. In 2011 the European Space Agency shortlisted a proposal for a Uranus Pathfinder mission, for launch in 2021 to go into orbit around the planet in 2037, using chemical propulsion and Venus slingshots. It wasn’t selected for development despite extensive scientific support. Sigrid Close of Stanford University has proposed a SCATTER mission using small probes linked by laser to an orbiting ‘Mothership’, for launch in 2030 or 2031, but whether it will happen remains to be seen.
Originally the ‘Grand Tour’ missions to the outer planets were to begin with the 1977 launch of the Thermoelectric Outer Planet Spacecraft, by Titan III booster. There were to be two of them, to photograph all the known planets of the outer Solar System. One probe would go to Pluto by way of Jupiter and Saturn, the other from Jupiter and Saturn to Uranus and Neptune. The TOPS probes would have pioneered an entirely new type of computer system called STAR, for Self Testing and Repair, a degree of autonomy which computers have yet to achieve today. though similar results have been achieved by other means. Keeping Voyager 2 operational without it, though compromised by receiver and transmitter faults since a week after launch, and imaging Uranus and Neptune despite overworking the camera platform during the Saturn flyby, has been an astounding cumulative achievement.
The less ambitious Voyager probes which were launched in 1977 fulfilled many of the TOPS objectives, though with lesser capabilities. Until Voyager 2 reached Uranus and Neptune, we knew very little about either planet – even the rotation periods were uncertain. Both were known to have large amounts of methane and ammonia in their atmospheres; Uranus was green, Neptune was named for its blue colour. Uranus had a lighter-coloured equatorial zone and occasional white spots, but virtually no features were seen on Neptune until infra-red studies in 1975-76.
One extraordinary thing about Uranus is that it’s the only planet to rotate on its side, with its axis almost in the plane of its orbit. The favoured explanation for that involves one or more collisions in its early history, and the late Prof. Fred Whipple believed that the Final Bombardment of the inner planets had been triggered by the formation of Neptune in the outer Solar System, 800 million years later than the other planets. The formation of Uranus and Neptune may have been triggered by a passing brown dwarf star, before the cluster in which the Solar System formed was broken up (‘Did a Passing Dwarf Star Form the Ice Giant Planets?’, Astronomy Now, December 2005.)
After simulations with gas cannon at the Lawrence Livermore Laboratory in California, Dr. Marvin Ross suggested that inside Uranus and Neptune, methane could be stripped to pure carbon and clumping could produce diamonds the size of the Earth (Nature, 31st July 1981, ‘Planet-Sized Diamonds’, Daily Telegraph, 31st July 1981; ‘Uranus and Neptune: Diamond-Studded Interiors?’, Sky & Telescope, October 1981). In September 1981 Tomorrow’s World suggested that instead the material could be graphite, and in 1988, Dr. Andrew Prentice of Monash University suggested that the outer planets and their moons consisted of up to 30% carbon in the form of graphite, rather than methane. (Richard Pree, ‘Planet Theory Harks Back to History’, The Australian, October 11th 1988). Arthur C. Clarke immediately jumped on the diamond idea for his portrayal of Jupiter in 2010: Odyssey Two (Granada, 1982), but it wouldn’t work there because Jupiter is still contracting and hasn’t begun concentrating heavier elements yet. In the event, the Uranus and Neptune interiors were mapped in sufficient detail by Voyager 2 to rule out the diamond hypothesis.
It may be that each giant planet requires one huge ‘soliton’ storm to stabilise its atmosphere. Jupiter’s Great Red Spot has persisted since the 17th century, and Saturn has a huge hurricane with a central eye at the south pole, with another in the form of a hexagon at the north pole which has been there since the Voyager flybys, at least. Only Uranus isn’t known to have one, but because of its bizarre ‘on its side’ rotation, the Sun was overhead at the south pole when Voyager 2 passed in 1986. By 2007 a quarter of a Uranus year had passed and the Sun was overhead at the equator, where the Hubble Space Telescope saw a considerable increase in storm activity. As the north pole becomes illuminated a bright cap formed over the northern hemisphere in 2014, but seems to have dispersed with seven years still to go till the solstice.
In 1971 the late Ed Buckley gave a talk to ASTRA titled ‘An Artist’s View of the Solar System’, which stimulated the formation two years later of the Interplanetary Discussion Project, generating my books New Worlds for Old and Man and the Planets. One of his many original ideas was that we were asking the wrong question about Saturn, not ‘Why is it the only gas giant with rings?’ but ‘Why can’t we see the rings of the other gas giants?’ He was right about the rings of Jupiter being narrow and close to the planet, and almost right about the rings of Uranus being dark, chunky and scattered. Part of his dramatic 1975 painting on that theme, ‘Golgotha Moon’, formed the cover of New Worlds for Old. In 1977, during a stellar occultation observed by NASA’s Kuiper Airborne Observatory, the answer turned out to be that the rings are chunky, dark and narrow; their origin has still to be explained.
Voyager 2 found spectacular and puzzling features on the moons of Uranus. All the moons are named after Shakespearian characters except Umbriel, which is named after the dark sprite in The Rape of the Lock by Alexander Pope. That’s because Umbriel is much darker than all the other moons, even though it’s in the middle of the large ones, in order outward from the planet; but it has a large bright ring, 90 miles across, near the north pole, possibly the ice rim of a recent impact crater.
Ariel and Titania have huge valleys and fault systems, as yet unexplained and not like anything on the other planets or moons. Oberon has a mountain which simply shouldn’t exist: the mixture of rock and ice of which it appears to be composed should not support a structure of that height. Miranda, the innermost of the large moons, has extraordinary surface features including huge cliffs which indicate that the moon has been pulled apart and reassembled. Features like chevrons indicate that enormous distorting forces have been at work in the past, and all over the surface we see geologically incompatible terrain jammed together, so roughly that Miranda isn’t even properly spherical, and at the site of the cliffs, the level is three miles out of line where two chunks of the moon have been ground together.
There’s plenty of ice on the moons of Uranus and Neptune, but even at that huge distance from the Sun, there may be liquid water in large amounts. Immediately after the Voyager flybys, it was suggested that within Uranus there may be, not just a water vapour layer like Jupiter’s, but an actual ocean much larger than the entire Earth. (National Commission on Space, Pioneering the Space Frontier, Bantam, 1986). Now that we know there’s subsurface water in large amounts under the crusts of Europa, Ganymede, Enceladus and possibly Titan, Pluto and Charon, it’s been suggested that the valleys and faults on the Uranus moons may also be due to subsurface water movement (Ian O’Neill, ‘Uranus Pathfinder:
Mission to the Mysterious Ice Giant’, Discovery News, January 25th, 2011; Chris Arridge et al, ‘Uranus Pathfinder’, Experimental Astronomy, 2011.) Although we still have only the Voyager 2 1986 photos to go on, Ted Stryk of the Jet Propulsion Laboratory (known for his reprocessing of Russian images from the surface of Venus) has enhanced the photos of Miranda, Ariel and Titania to show extensions of those features into the night side of those moons, lit by reflected light from the others, revealing an X-shape of ‘transecting valleys’ on Ariel and giving a glimpse of what future missions might show. (Emily Lakdawalla, ‘Ted Stryk: Revealing the night sides of Uranus’ moons’, The Planetary Society, Mar 13, 200.)
“Jupiter is a painted harlot. Neptune is an Audrey Hepburn planet.”
- Jurry van der Woude, Voyager mission team, at the time of the Voyager 2 flyby, quoted in The Neptune File by Tom Standage.
Pluto’s orbit is eccentric enough to bring it within Neptune’s for part of Pluto’s year, 248 Earth years, and from 1989 to 1999 Neptune was in reality the planet furthest from the Sun.
In 1975 Ed Buckley produced a remarkable painting for Man and the Planets, published in the book and on the back cover in 1983, but never in colour till now. It attracted criticism at the time because it showed cloud structures on Neptune, and a field of derricks on the surface of Triton, the major moon, implying that there was some kind of liquid below the surface. Neither was known to exist at the time, but both turned out to be true.
The Voyager and Galileo missions showed that the Galilean moons of Jupiter do have sub-surface liquid, due to internal heating by tidal forces: there’s molten rock and sulphur within Io, a water ocean below the crust of Europa and another within Ganymede; in the Saturn system, water eruptions on Enceladus driven by similar forces; and there are volcanic plumes on Triton. After being briefed by my friend Mike Urban, one of the Voyager 2 team controllers, at JPL in 1986, I wrote an article called ‘The Neptune Options’ which ended with the line, in block capitals, “WE WANT TO SEE TRITON”. I predicted the volcanoes in an essay which everybody forgot about as soon as they were discovered! (‘Shadows on the Milky Way’, Griffith Observer, December 1986). In 1989 Voyager 2 reached Neptune, revealing both the clouds and Triton’s volcanic vents which Ed and I had predicted. Within weeks a BBC-2 Horizon programme was declaiming, with emphasis, “Nobody predicted active volcanoes on Triton…” I used a similar model for the tenth planet and its moon in ‘Out of the Ecliptic’, one of the stories I wrote for the Daily Record’s ‘Lance McLane’ strip, in 1987-88.
In 1980, the strip prophetically showed the planet with a large circular storm; there was one in the southern hemisphere in 1989, which disappeared, but the Hubble telescope has imaged similar ones on the equator and in the north. Of all the gas giant plants, Neptune has the biggest discrepancy between the temperature of its atmosphere and the heat it receives from the Sun. A student paper at the International Astronautical Federation Congress in the 1970s suggested that the effect was due to tidal drag by Neptune’s large satellite Triton, which is in retrograde orbit around the planet, probably due to some catastrophic event in its early history. The idea hasn’t won general acceptance but seems plausible to me, because of the demonstrable tidal effects which have been discovered since.
The Soviet astronomer Vsekhsvyatskiy thought in the 1960s that eruptions on the giant planets’ moons would be sufficiently violent to propel comet-sized masses into circumsolar orbit. He pointed out that such ‘launches’ would be particularly easy from Triton, because at some points in its retrograde orbit around Neptune Triton would be virtually at rest with respect to the interplanetary medium, when the velocity vector of the Neptune system was cancelled out. If so, I argued in ‘Shadows on the Milky Way’, Neptune should have a broken ring, like the one depicted on the cover of The Tar-Aym Krang, Alan Dean Foster’s first published novel. Sure enough, before I even finished the article in 1985, an occultation of the star SAO 186001 by Neptune revealed what appeared to be an intermittent ring, detected on one side of the planet but not the other. When Voyager 2 flew past Neptune in 1989, the rings turned out to be clumpy rather than broken, but the volcanoes on Triton were confirmed.
The catastrophic event in Neptune’s early history which reversed Triton’s orbital motion, or captured it from elsewhere, could have been a close encounter with a large planet in the outer System which has still to be discovered. Its pull could explain two curiosities in the history of Neptune, which some have taken to indicate the presence of a tenth planet. In May 1795, more than 50 years before its discovery, Neptune was observed twice by Joseph F. Lalande. He failed to realise that it wasn’t a star, putting down its movement to observational error; but the position he did mark is seven minutes of arc from where it should have been, according to modern calculations. Even more remarkably, there was a close conjunction between Jupiter and Neptune in 1613, very shortly after Galileo began observing Jupiter’s moons, and Neptune is marked in one of his diagrams with the caption ‘I’m not going to use this star for reference any more, it seems to have moved since last night’! (Stillman Drake & Charles T. Kowal, ‘Galileo’s Sighting of Neptune’, Scientific American, December 1980.) And although Galileo was using gridded paper, his position is also out, by a full minute of arc. For more notes on the Planet X search, see the Kuiper Belt article to follow.
Check out The Orkney News series of Astronomy articles by Duncan Lunan.