The Kuiper Belt

By Duncan Lunan

Planet nine, with Neptune orbit shown as distant ring

Into the Kuiper Belt

Kohoutek’s Comet in January 1974 was visually a disappointment, partly because it failed to brighten as expected, but also because the weather was atrocious.  Very few people saw it in the UK and I may have been the only one in Scotland, because the Glasgow Herald reported nobody had.  I would probably have missed it if I hadn’t been laid up with the flu in my workroom, facing west at my parents’ house in Troon.  Only a day or so before the comet disappeared into the sunset, I noticed a gap in the clouds travelling in that direction.  A quick look through binoculars found it, and I brought my telescope across the landing and aimed it through the opened window, with enough time to call my mother and sister to see the comet as well.  Robert Law, now at the Mills Observatory in Dundee, saw it from Paisley through what may have been the same gap.

Reginald Turnill’s book The Moonlandings, an eyewitness account is actually an account of his full experience as a commentator on manned spaceflight, including Project Mercury, Gemini, Skylab and the Apollo-Soyuz rendezvous.  He describes the difficulties which the astronauts on Skylab had trying to photograph the comet, which might give you the impression that they didn’t see it at all.  But even today, the traditional astronomer’s skills of naked-eye observation and draughtsmanship can deliver the goods, and detailed portrayals by astronaut Edward Gibson appeared in Sky & Telescope  (‘Comet Kohoutek Drawings from Skylab’, October 1974).  

But the reason why the comet didn’t grow as bright as initially expected was because it emitted large volumes of dust, and that, along with its mean distance of 1800 Astronomical Units from the Sun, much nearer than the Oort Cloud of comets, caused George W. Harper to suggest that it was a member of the nearer ring of objects, beyond the orbit of Neptune, which was postulated by Gerard Kuiper and is now called the Kuiper Belt  (‘Kohoutek;  A Failure That Wasn’t’, Analog, July 1974).  It is probably also the source of the ‘Centaurs’, Chiron and Hidalgo, in erratic orbits among the outer planets, and of Phoebe, the captured outer moon of Saturn.    

With the discovery of Pluto in 1930, the sequence in The Planets Suite by Gustav Holst was no longer complete.  In 2000 British composer Colin Matthews added ‘Pluto, the renewer’, whose musical themes hark back to many of the previous movements.  Quite how the god of the Underworld became ‘the renewer’ wasn’t explained.  But a new world was then added to the Solar System, fully three times further out than Pluto.  It’s about the size of our Moon, and the old arguments started again about whether Pluto should no longer be classified as a planet but as an asteroid, a member of the Kuiper Belt.  The new world, however, was much further out, among the Oort cloud of comets, ranging from 76 to 937 Astronomical Units from the Sun.  (Pluto’s orbit is eccentric, but its range is only 39.5 to 160 AU.)  In September 2004 the new world was named after Sedna, the Inuit goddess of the sea, who was thrown from a kayak ‘by her fearful father’ – whether he was afraid of her, pack-ice, polar bears or killer whales my source doesn’t say.  Sedna’s fingers became seals, and ‘she became the spirit of the sea and all its creatures’.

A name like that is very well for an asteroid but doesn’t suit a planet, all of which are named after Greek and Roman gods.  It also has slightly unfortunate resonances in Glasgow, where ‘Senga’  (Agnes, reversed)  is a common pet-name.  By analogy with ‘Mars, the bringer of War’, ‘Venus, the bringer of peace’ and ‘Jupiter, the bringer of jollity’, one couldn’t help thinking that the next movement of the suite should be ‘Sedna, the talk of the steamie’.

Matters were made worse by the discovery of another object, closer than Sedna but apparently larger than Pluto  (though it turned out not to be).  Amid great controversy, in August 2006 the International Astronomical Union ruled that Pluto and other objects including Ceres, largest of the asteroids, and the new 2003 UB313, ‘Xena’, should be reclassified as ‘dwarf planets’.  With that decided, 2003 UB313 was been officially named Eris after the Greek goddess of discord, while its moon, ‘Gabrielle’, was named Dysnomia, after Eris’ daughter, the spirit of lawlessness in Greek mythology.

Working with the Berlin Philharmonic, Simon Rattle commissioned and released a recording of an extended Planets Suite. It includes the Pluto movement, Mark-Antony Turnage’s ‘Ceres’, Brett Dean’s ‘Komarov’ and Kaija Saariaho’s ‘Toutatis’, among others. (Review and interview, Peter Culshaw, ‘The Planets Re-aligned’, Telegraph Review, Sept. 9th 2006).  Whether ‘Eris, the Bringer of Discord’ will be added remains to be seen, but I don’t suppose they’ll take up my suggestion for Sedna.  Thousand of KBOs  (Kuiper Belt Objects)  have now been discovered, eight of them large enough to be considered ‘dwarf planets’ in terms of size, and the total population may be 100 times that or more.  We don’t know if it has an outer edge or if it merges into the Oort Cloud of comets, which extends for 1.9 light-years to the gravitational limit of the Solar System in the direction of Alpha Centauri. 

Ultima Thule New Horizons

After the New Horizons probe survived its Pluto encounter in 2015, it was hoped that it might obtain images of at least two more KBOs as it recedes into the Belt.  Almost immediately potential targets were found, and the most accessible was an object named 2014 MU69, nicknamed ‘Ultima Thule’.  That name was attached at one time to one of the Trojan asteroids in Jupiter’s orbit, to which the Lucy space probe is now on its way. They were then thought to be the most distant of the asteroids, but the discoveries of Centaurs and the Kuiper Belt have out paid to that. 

A bonus was that the object, designated PT-1  (Potential Target 1)  was due to pass in front of a star in 2017.  The occultation data would be very helpful in planning the encounter, and indeed, observations from Patagonia indicated that the object was binary, with a satellite like the Eris-Dysnomia pair, or the triple KBO Haumea, which were originally nicknamed Santa Claus, Rudolph and Blitzen.  (Interestingly, the artist Gavin Robert had painted a similar paired object in the outer Solar System, in 1979, for my book Man and the Planets, long before the existence of the Kuiper Belt was confirmed.)  

Gavin Roberts, binary object in outer Solar System, 1979, for Man & the Planets,

If PT-1’s moon were a collision fragment, then PT-1 might have a ring, like Haumea has, but by the time that risk was discovered, the die was cast.  To rendezvous with PT-1, New Horizons had to make rocket burns in late October and early November 2015, before the extended mission could be authorised by Congress and Senate, as the US Constitution requires for each year’s government expenditure.  

The encounter with PT-1, now officially named ‘Arrokoth’, took place on January 1st, 2019.  As so often happens, the object proved to be a complete surprise:  not an asteroid with a moon, formed by a collision, but a binary object formed by a merger, of a spherical object with one shaped like a discus.  Back in the 1970s, an astronomer named Rudge had suggested that many comets could have that shape, due to rotating very close to the speed at which they’d break up.  More recently, a similar shape has been suggested for the interstellar object ‘Oumuamua, and Dr. William Hartmann has suggested that if it’s composed of ices, that shape might be due to erosion by cosmic rays.  But it’s very hard to see how two objects with such different rotation rates could have merged without shattering, or how an interstellar object could have merged with one in the Kuiper Belt.  The mystery is still unsolved.

course changes, PT-1 2014 MU69 Jan 1 2019

Two other potential targets for New Horizons had to be discarded, and no new ones have been found.  Its radioactive power source was expected to give out in 2019, four years after the Pluto encounter, but on 17th April 2021 the spacecraft was still going strong when it reached a distance of 50 Astronomical Units from the Sun  (50 times the Earth’s average distance of 93 million miles).  It’s now thought that New Horizons will remain operational till the 2030s, another in the list of spacecraft which have lasted for unexpectedly long.  It began with the Pioneer 6 solar wind observer, for which NASA used to release bulletins at 5-year intervals headed ‘Pioneer 6 May Go On Forever’.  The Opportunity rover on Mars was expected to last 6 months but kept going for 14 years and would be driving yet, but for a particularly severe dust storm in 2018.  But the record is held by Voyager 1 and 2, launched in 1977, both of which have now crossed the boundary of the Solar Wind and are expected to survive in interstellar space at least until 2030.

New Horizons is travelling towards the Galactic Centre in Sagittarius, a very interesting direction.  In 1977, a team of French astronomers announced the discovery of a cloud of interstellar dust, travelling towards us from there.  They couldn’t determine its distance, size or thickness, but if it was a close as 100 AU, it could pose a major hazard to life on Earth like the one the late Sir Fred Hoyle had described in his 1955 novel The Black Cloud.  I was notified of this by a colleague in Venezuela, but otherwise the idea seemed to arouse little interest except for a short news item in Sky & Telescope, and a sensational novel by the late Martin Caidin called Exit Earth  (to say the least, not one of his better ones).

Nevertheless, it raised some very interesting possibilities.  The late A.T. Lawton had given the Scottish spaceflight society ASTRA a fascinating lecture on the possible existence and uses of large dust clouds within what he termed the ‘isogravisphere’ of the Sun, generally assumed to extend to around 2 light-years and mark the limit of the Oort Cloud of comets.  Lawton calculated that in some directions it could extend to 10 light-years, and concentrations of dust within it could allow us to build whole new solar systems, if we chose.

Local Interstellar Cloud(s) in Local Bubble (NASA)

I took these ideas a lot further in my books New Worlds for Old  (1979)  and Man and the Planets  (1983), and in a 1985 essay called ‘Shadows on the Milky Way’, which won Honourable Mention in an essay context sponsored by Hughes Aircraft and the Griffith Observer, at the Griffith Observatory in Los Angeles.  I attempted to use those ideas to narrow down the possible orbit and location of another Solar System planet beyond Pluto, on which I based a story called ‘Out of the Ecliptic’ for Sydney Jordan’s Lance McLane comic strip in the Daily Record, and a subsequent lecture to the Astronomical Society of Cardiff.

The Search for the Tenth/Ninth Planet

The search for outlying planets began with the telescopic discovery of Uranus by William Herschel in the 18th century. 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.  That still didn’t account for all of the observed perturbations, and in the 20th century there was a major search for a ninth planet, culminating in the discovery by Clyde Tombaugh of Pluto.

putative ninth planet 5000 times the mass of Pluto, 10x Earth

Still the problem wasn’t solved.   Pluto wasn’t in the predicted orbit and seemed much too small to produce such effects;  after the discovery of Pluto’s moon Charon, which allowed calculation of Pluto’s mass, it was clear that Pluto couldn’t exert much pull on the other planets.  Lowell himself suggested that Pluto should be renamed Loki, the god of thieves, because it had stolen a bigger planet’s thunder.  Pluto has now been demoted and is no longer officially a planet, but that may still change back for historical reasons  (not least because it’s the only planet to have been discovered by an American), and it’s tempting to keep calling the possible undiscovered one the ‘tenth planet’ so it can be designated ‘X’, the unknown.

How I Killed Pluto, Mike Brown

My own ideas about it drew on the 1970s research of Prof. J.D. Fernie in Canada, Brady, Harrington, Van Flandern and A.T. Lawton, and were based on the assumption that the visible concentrations of dust around the Solar System all lay within 10-15 light-years of us, so generating what looked like some significant pattern of movement around the galactic plane as viewed from here, in interaction with the plane of the Solar System.  I thought that the remaining unexplained peak of absorption might be generated by a super-earth planet  (as we’d now call it), with a large moon in retrograde orbit, with tidal forces generating volcanic activity and a broken ring, as we see in the Neptune system.  The planet itself would be in retrograde orbit, highly inclined to the Ecliptic  (hence my story title), grazing Neptune’s orbit every few hundred years and causing the oddities in Neptune’s pre-discovery motion, which I mentioned here in ‘Uranus, Neptune and their Moons’.  It all worked out very well and there even seemed to be possible glimpse of the planet in the point source data from the Infra Red Astronomy Satellite in the early 1970s – no longer there when the area was rescanned by WISE in 2009-11.  Nevertheless, more recent plotting of the interstellar dust surrounding the Solar System puts the dust concentrations further away than I supposed, within the assumed radius but much too far out for ‘Planet X’ to be responsible.  The focus of the search for what’s now called ‘Planet 9’ now lies with Dr. Mike Brown, ‘the man who demoted Pluto’.

Dr. Brown, who discovered Sedna, Eris and other KBOs, heads a team who suggested in 2016 that there might be a gas giant planet orbiting far beyond Neptune, in an elliptical, inclined orbit, with a mass ten times the Earth’s, with an average distance of 400 Astronomical Units and an orbital period of 10-20,000 years.  On Tuesday, 23rd November 2021, Dr. Brown gave a detailed update on it, long-distance from California, in the online David Elder Lecture Series from the Glasgow Science Centre Planetarium.

extended mission

The reasoning rests on outlying Kuiper Belt Objects, all in nearly coplanar orbits with a 6-degree inclination to the plane of the Solar System, and all with their aphelia  (furthest distances from the Sun)  on the same side of the Solar System, in two fairly tight clusters.  Their spacing and current locations suggest two possible areas to search for the proposed planet, with another less probable one on the far side of the Sun, and as the number of objects apparently influenced by it has grown to 12, the search areas are narrowing.  With a probable magnitude of around 21, the planet should be within the reach of existing telescopes, and the search has so far involved the 48-inch Schmidt telescope on Mount Palomar, the southern hemisphere Dark Energy Survey, the Keck and PanSTAARS telescopes on Hawaii, and now the Subaru  (Pleiades)  telescope of Japan, also on Hawaii.  Although none have succeeded so far, Dr. Brown stated that success is likely within two month to two years from now.  If the planet is there, it should lie in a region taking in parts of Orion, Taurus and Gemini.

He was asked whether the effects could be explained by a smaller planet nearer the Sun, as some recent researchers have suggested, but he said they could not – if so, that could rule out the various previous models mentioned above, including A.T. Lawton’s one and my version of it.  Asked if he had given the planet a name, he replied, “If you name something before you find it, you’ll never find it”.  Perhaps Lawton ruled out his planet by suggesting it be called ‘Odin’, and I ruled out my version by suggesting that its hypothetical satellite be called ‘Frigg’.  But to my surprise, and without himself knowing it, Dr. Brown then suggested that some earlier, discarded ideas of ours might have some validity.

Pluto Charon Moon Earth Comparison

At the time of Lawton’s lecture to ASTRA in 1974, the exact size of Pluto was still unknown, and one possible explanation was that astronomers were actually trying to measure the diameter of the Sun’s reflection on a smooth, icy ball.  A dramatic illustration of the idea by Mel Hunter had appeared in the book Nine Planets by Alan E. Nourse  (Harper, 1960).  Lawton’s lecture on interstellar dust around the Solar System suggested to us that Pluto might have a thin methane ocean, overlying a thick layer of methane ice.  Unlike water ice, methane ice sinks when it forms, but over very long periods, the ice might acquire a deep layer of dust just below the liquid, giving Pluto a dark reflecting surface rather than the water ice previously suggested.

Of course, true measurement of Pluto’s diameter ruled that out.  But continuing refinements of the Planet 9 calculations suggest that its mass could be as low as 6 times Earth’s, perhaps putting it into the category of solid ‘super-earth’ now known to exist in substantial numbers around other stars.  “That’s my current nightmare,” said Dr. Brown.  “Maybe Planet 9 exists, but it’s much harder to see than we imagine.”  And maybe our 1974 idea wasn’t so crazy after all.

See also: The Sky Above You – December 2021

Duncan Lunan has written a series of Astronomy articles for The Orkney News suitable for all – beginners and advanced.

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