The Solar Cipher Trivia Quiz

Welcome aboard the Solar Express spaceship. I am the mission commander. Thanks to an unprecedented 10-planetary parade alignment, we will be delivering care packages to all the planets in the Solar System. We will also be visiting two planets of the dwarf variety; they deserve our love too. Mission control needs you to input our destinations into the navigation console in the correct order, working from the Sun outwards. Just one hitch; due to an AI hallucination in the mainframe computer, all of the planet names have been scrambled. Luckily it only managed to convert them into descriptions of their homonyms before I pulled the prompt. I trust you will still be able to sort them out for us. I could totally do it myself, but I have, erm, lunch to attend to. Ahem.

*You dutifully input all the correct navigation details with aplomb*
*Mission commander returns, oblivious to your presence, mumbling and frowning at the navigation hub*
Oh, so that one was Ceres; I could swear ... Ah, I didn't see you there, FunTrivian! Well done on that ... trivial task. So now, at a speed of 17 km/s (11 miles/s), we'll be arriving at Pluto in ... 11 years? Blast! I thought we'd be back by dinner!

At least we're en route to our first destination. Good old elemental mercury, number 80 on the periodic table, bearer of the mysterious symbol "Hg". The ancient Greeks favoured the name "hydrargyros" meaning "water-silver", which was later corrupted into "hydrargyrum" by the Romans, then abbreviated into the chemical symbol we still use today.

Mercury is a little miracle of an element; the only metal that is liquid at room temperature. As such, it has seen numerous practical applications throughout history; in thermometers, barometers, manometers, float valves, dental fillings, electrical switches, relays and fluorescent lamps. What a shame, then, that many of the compounds it forms are highly toxic to humans. This is a lesson that humans were slow to heed. Victorian age medicines containing mercury caused untold disfigurement and grief in children, milliners exposed to mercury-based compounds in the manufacture of felt hats succumbed to "mad hatter disease", and other industrial workers have fallen prey to its toxic effects, including weakness, loss of coordination, kidney disease and cognitive impairment.

Much more can be said about this wondrous yet dangerous element, but we are now arriving as its namesake in the Solar System. The element's English name was actually taken from the planet. Being the fastest planet when moving across the sky as seen from Earth, the Romans named it after the swift-footed messenger god Mercury. When Medieval English alchemists drew parallels between the seven known metals and the seven known planets, it only made sense to name the slippery liquid after the swiftest of the planets.

Ironically, there is very little mercury on Mercury. It's much the same back on Earth, at just 0.08 parts per million. Luckily, we do have some minerals like cinnabar containing up to 2.5% mercury, so we have been able to amass more than we need to run the starship. Thus, I'd like you to alight onto the surface and present this package of surplus mercury to Mercury as a sign of our goodwill. Make haste! I will be busy prospecting for resources to exploit... erm, local business development opportunities. Keep Mercury occupied; I'd rather it doesn't cotton on to our plans just yet.

Onward to Venus! At a speed of 17 km/s (11 miles/s), we should be arriving in about 33 days. This gives us much time to admire the fine art in the corridor. See this one here? "Nascita di Venere", the Birth of Venus. Probably the most famous of Sandro Botticelli's works. Don't ask how it found its way onto the ship, I'm sure the transaction was all above board. There is much to be said about its history, subjects, elements, style, techniques, interpretation, influences and derivative works. Alas, I am not a disciplined scholar of the fine arts, and the AI malfunction has corrupted the "wall_art_interpretation.pdf" file. What fortune that someone had the foresight to install this little engraved plaque on the frame, so at least we have some basic information.

*Plaque reads:*
[Sandro Botticelli, "Primavera", circa 1480]

Blast! Someone has mixed up the "Birth of Venus" and "Primavera" plaques. I shall see to it that the one responsible is disciplined appropriately. Involuntary jettisoning ought to serve as an appropriate deterrent. But never mind that now.

I do happen to be quite the planetary expert, so I can prattle on about Venus for days. It's fortunate we have 33 of them - that should be just the right amount of time. Pull up a chair and pour yourself a glass of your choice of poison, it's on the house. I'd probably avoid the Mercury though.

Where to start? The basics I suppose. Venus is a rocky planet of a very similar size and mass to Earth, often considered its twin. To be honest though, the comparison is rather simplistic, as the similarities end there. At its surface, its dense atmosphere of 96.5% carbon dioxide exerts 92 times the pressure of the Earth's atmosphere at sea level, enough to crush you alive. That is if the surface temperature of 464°C (867°F) due to intense greenhouse warming doesn't get you first.

Venus has no moons. It takes 224.7 Earth days to complete its orbit around the Sun. But most remarkable is its rotation on its own axis. A Venusian day is 116.75 Earth days long, meaning a Venusian year passes every 1.92 Venusian days. What's more, it rotates in the opposite direction compared to all the other planets in the Solar System, in what's called "retrograde rotation". We think it originally formed spinning in a "prograde" or forward direction, but over time the tidal forces from the Sun slowed it down, which in isolation would mean that eventually the same side of Venus would always face the Sun in a "tidally locked" arrangement. This was the fate of Earth's moon. However, Venus also has such a thick atmosphere that thermal tides also develop from the difference in temperatures of the side facing the Sun compared to the other side. This tends to push Venus in a retrograde direction. Thus its present day retrograde motion is an equilibrium between the opposing tidal and thermal forces.

From our perspective back on Earth, Venus is a remarkable planet. It is the brightest natural object in the night sky, after Earth's moon of course. We see it go through phases just like Earth's moon. It also travels in front of the Sun in two transits eight years apart, with each pair separated by over 100 years. A bit like a solar eclipse, but visible only as a tiny black spot as it passes over the Sun. The next transit isn't scheduled until 2117...

...

... and they all lived happily ever after. Ah, jolly good, we have arrived! Don't fret, we'll be staying up in the clouds for this delivery. Your space suit will protect you from the sulfuric acid, and the pressure and temperature up here at 50km altitude are remarkably similar to the Earth's surface, so you'll actually be rather cozy. It's here in the atmosphere that scientists back on Earth have made measurements that suggest the presence of phosphine, which may just be a signature of life. However, we have yet to confirm this hypothesis. So our delivery this time is not a package. Instead, I want you to spray these organic phosphates into the atmosphere. If the phosphine we've previously observed in the Venusian atmosphere is really produced by micro-organisms, we should see a big spike in phosphine levels over the next few days. But we have no time to waste waiting around, so once you're done, it's onwards to our next destination! Fret not, our spectroscopes will be able to look back and measure phosphine levels en-route in a couple of days.

Ahhhhh, Earth. Taker of energy, saver of life.

No, I'm not talking about the planet. Unplug Bender's life support system over there and I'll show you. What's that? Don't look so shocked, there's enough battery backup to power him until we plug it back in.

This is the standardised AS/NZS 3112 AC power plug, also called the Type I plug. Used by Australia, New Zealand, Papua New Guinea, and various Pacific island countries. Even Argentina and China use a similar design. And of course, we use it here on the ship. Two pins in a V shape form the active and neutral connections through which the AC oscillates and delivers power to the appliance plugged in.

But this last pin here, called the earth (or sometimes ground) pin, is the real hero. It connects the conductive parts of the appliance to the literal Earth (or in our case, the spaceship frame). If a circuit fault energises these conductive parts, the earth connection will take all the load and trigger a protective device such as a fuse or circuit breaker. That way, there's less risk of getting electrocuted due to loose wiring inside the appliance. You can get away without an earth pin for some appliances, but only if they're double insulated so there's no chance someone could accidentally touch the conductive parts. Pretty neat, huh? Far from my reputation amongst the crew, I actually prioritise safety over all else. What? Don't give me that look!

Anyway, as for Earthly (planetary) matters, there's not much to say that hasn't already been covered ad nauseam on that FunTrivia website you're so fond of, so I won't trouble you with my monologue.

Now, our gift for Earth. When we were at the Sun, we had the foresight to bottle a bit of sunlight, along with its cosmic ray particles, in this Penning trap. By my calculations, this could be fashioned into some sort of peace ray. Humanity could really use that right about now. Unfortunately, by my calculations, there's also a 50% chance that it would actually turn out to be a death ray. Schrodinger's ray, if you will. Still, I feel it is our best hope. I'll leave it to you to use as you see fit. I'll leave the engines on so we can make a, erm, prompt departure, if required. It's only "Thirty Seconds to Mars" from here ... I mean the songs on the music playlist. It's 53 days to Mars in terms of travel time.

Incidentally, we have received the results from the Venus spectroscopy. It seems there was no change in phosphine levels. In fact, there is now some doubt that phosphine even accounts for the spectroscopy data, which could be caused by other compounds. And if it is phosphine, it's not guaranteed that life created it. Come to think of it, I'm not sure why we didn't just collect a sample of the Venusian atmosphere to drop off at a laboratory for microbiological culture and microscopy...

*Spaceship speakers blaring "Cause Uptown Funk gon' give it to you"*

Blast! Who put Bruno Mars in the Thirty Seconds to Mars playlist?!? That is the WRONG Mars!

I will be upfront with you, I am not particularly fond of Bruno Mars's creative output. Yet now I have "Uptown Funk" stuck in my head. Call that cosmic karma, if you will.

*Beeping sound intensifying as a metallic sphere rolls into the room*
Oh good, here's TrivBot, our resident trivia robot. Powered by good old fashioned quantum intelligence. None of this gimmicky "large language model" AI. I'll let TrivBot say a few things about Mars while I find out who has been messing with the playlist. Say, did you still have that death ray you made from the essence of the Sun? Oh, you made the peace ray? Nevermind, I'll be back in a few minutes.

*Robotic voice*
"Bleep Bloop, I am TrivBot. Paragon of all wisdom. You have requested 'Bruno Mars' trivia. He was born Peter Gene Hernandez in Honolulu, Hawaii. He has Puerto Rican, Ashkenazi Jewish, Hungarian, Ukrainian, Filipino and Spanish ancestry. A true globe trotter! His mother was a hula dancer, and his father a percussionist. One of six children, his family was highly musical. He began performing as early as age three! Now that is a recipe for success if you ask me! He moved to Los Angeles at age 18 to pursue his career and the rest is history, as humans say. Bleep bloop."

*Spaceship speakers now playing "Come, break me down. Bury me, bury me. I am finished with you."*
*Mission commander returns*
Ok that's enough TrivBot.
*TrivBot trundles away*

Did TrivBot just spend the last few minutes prattling on about Bruno Mars? I meant for it to talk about the planet Mars! Curse! Nevermind that now, I can't spend all day disciplining. And we have arrived, so I won't have time to ramble about the planet either.

Now, when you exit onto Mars, the goal is pretty simple. Spring cleaning time! There are six Martians scattered around the surface, and they could all do with some TLC. Of course, I'm not talking about organic Martians, but rather the six rovers that humanity has landed successfully on Mars. Five were sent by NASA, starting with Sojourner (landed in 1997), then Spirit and Opportunity (2004). Sojourner requires some fresh batteries and re-establishment of communications with its lander. Spirit is stuck in a soft sand trap, simply dislodge it, replenish the power reserve and it should be alright (plus you can retrieve my golf ball from the sand trap while you're there). For Opportunity, we simply need to sweep off the abrasive Mars dust that has blocked its solar panels. Curiosity (2012) and Perseverance (2021) are persevering in their curiosity, as NASA had the foresight to power them with nuclear generators that aren't affected by all the dust storms and day/night cycles. All the same, I think they'd appreciate a visitor after all these years, so do drop by. Finally, we also have top-secret orders from the Chinese Communist Party to resuscitate the Zhurong rover (2021). It too fell prey to a dust storm blocking its solar panel, so it should be a quick sweep and then we're ready for our next destination.

Travel times are really starting to increase now - it's 282 days to Ceres from here. In fact, it continues to almost double with each step from here on out.

Ceres is largely forgotten in popular science these days, but it wasn't always that way. You'll have to forgive me, I can't help but indulge myself in spreading the good word about this forgotten but fascinating dwarf planet. And we do have the better part of a year before we arrive, so I should be able to get down to some of the finer points.

Ceres was discovered in 1801 by Italian priest and astronomer Giuseppe Piazzi. This was 46 years before Neptune was discovered, and 129 years before Pluto, its more well-known dwarf planet brother, was found.

At the time of the discovery of Ceres, astronomers had been searching the skies for an undiscovered planet between Mars and Jupiter, predicted by the Titius-Bode law. This was an arbitrary formula created by retrofitting the locations of the known planets. It had already predicted that a planet existed at the distance where Uranus was subsequently discovered, some 20 years earlier, thus astronomers had high confidence that a planet would also be found between Mars and Jupiter.

Piazzi was selected as part of a co-ordinated and methodical search for this fabled planet. However, before he even received his invitation, he had already found Ceres while searching the skies for a completely different reason. He observed it to be a starlike object, perhaps a comet. However, after observing its motion across 24 separate sightings, he deemed it was moving much too slowly for it to be a comet. The next sightings came almost a year later, as Ceres was lost in the Sun's glare for some time. Astronomers soon decided that they had indeed found a new planet, naming it Ceres after the Roman goddess of agriculture, grain, and fertility.

Ceres certainly had its time in the Sun (metaphorically speaking; literally speaking it is always in the sunlight) after its discovery and confirmation. It received an astronomical symbol like the other planets (the sickle), had the rare-earth metal Cerium named after it, and even received an adjectival form - "Cererean". But it didn't take long for things to fall off the rails. 15 months later, another object called Pallas was found in the same region. What's worse, neither Ceres nor Pallas resolved into a disc even under the highest magnifications, unlike other planets. They remained mere points of light, indistinguishable from stars aside from their rapid movement. This led William Herschel, discoverer of Uranus, to propose they be placed in a new category called "asteroids", meaning "star-like".

By 1807, two more objects were found in the region - Juno and Vesta. It took another four decades before the next object Astraea was found, but the dam soon burst after this. More and more objects were being found, and it became unworkable to consider them all to be planets. Eventually Herschel's nomenclature stuck, with Ceres et al. being recategorised as asteroids.

And so began Ceres' long march into obscurity. Perhaps mentioned in passing when learning about the asteroid belt in school, but otherwise forgotten. Until 2006, that is. With Pluto having faced a similar fate to Ceres, becoming just one of innumerable objects in its region, a new category of "dwarf planet" was created (more on that later, I promise). Ceres fulfilled the criteria for this categorisation, making it the only object to hold the dual categorisation of dwarf planet and asteroid. And now, we celebrate Ceres with its rightful ... just kidding, it's still just as obscure as ever.

It really is a shame, because beyond its interesting history of discovery and categorisation, Ceres is quite a fascinating object. Modern measurements put it at 939 km (583 miles)- about a quarter the diameter of Earth's moon, or roughly the distance from Paris to Berlin, or Chicago to Atlanta. It is about four times as massive, and twice the diameter, of the next largest object in the asteroid belt. In fact, it contains 40% of the total mass of the asteroid belt, and is the only object in the belt that is large enough to become roughly spherical under its own gravity.

Its composition is even more remarkable. From the observations we have gathered, it is thought to have a crust of some 70-190 km (40-120 miles) thickness, with a briny (salty) liquid and rock underneath, followed by a core that may be either liquid or solid. It is 50% water by volume, compared to Earth being just 0.1% water. All that buried salty water allows for cryovolcanism on the surface, similar to our hot volcanoes back on Earth, but with eruptions of icy material instead.

Most excitingly, Dawn, the only space probe to visit Ceres (aside from us now, of course), detected water ice on the surface, hidden in at least one of its craters. With a subsurface ocean reminiscent of those hypothesised on Europa and Enceladus, Ceres may well be a suitable habitat in which we could find life. And so, at last, I present our gift for the Cerereans, if there are any. A simply placard. Here, take it and mount it where you see fit.

*Plaque reads:*
[I'm thinking about you]

Cough drops? Lemon drops? Eye drops?
Ah, Train's "Drops of Jupiter", you say? Of course, I remember that classic tune. A little overplayed on the radio, I will admit, but it remains a guilty pleasure of mine. As I recall, the song was inspired by lead singer Pat Monahan's late mother who had died from cancer. She came to him in a dream, giving him some of the lyrics, and the imagery of "swimming through the planets ... with drops of Jupiter in her hair". Though Train has had a respectable career spanning many decades since the 1990s, "Drops of Jupiter" is likely to remain their magnum opus.

In fact, let's put "Drops of Jupiter" on repeat while we travel to Jupiter, some 248 days away. That ought to test my theory that you can dig a song out of the "overplayed" hole by playing it even more.

And I did rave on about Ceres a bit too much, didn't I? I'll make up for it by keeping it brief with Jupiter. What do you get for a planet that already has it all - over 100 moons, an iconic storm raging for over 100 years (the Great Red Spot), and even its own set of, admittedly insubstantial, rings? Jupiter is the undisputed king of the planets in the Solar System, with 71% of the mass in the Solar System. At least, ignoring the Sun's mass. That is the elephant in the room, however. The Sun contains 99.9% of the mass in the Solar System, and is some 1,000 times more massive than Jupiter. So while Jupiter is a titan amongst its peers, it has always considered itself to be a failed star. It would take 80 times more mass than it presently has for it to begin fusing hydrogen into helium, and ignite itself into stardom. So I'd like you to pump this metric tonne of hydrogen directly into its atmosphere. It's not much, but it's a start. We'll get you there one day, sweet Jupiter.

It's over a year to our next destination Saturn, 446 days. How are your space legs holding up? I'm sad to say we won't have an opportunity to stretch our legs for a few years longer than that. Even when we arrive, there is no solid surface to land on, just as it was on Jupiter, and will be for Uranus and Neptune too. Though we'll fly by many a solid moon, we won't have time to land until we reach Pluto at the end of our journey.

Our speed of 17km/s (11 miles/s) matches the speed of the Voyager 1 probe, the fastest object humans have ever launched in a direction away from the Sun. I wouldn't want to rain on Voyager 1's parade, so I have set a speed limiter on our spacecraft so as not to break that record. Of course, the all time speed record is held by the Parker Solar Probe at a staggering 191 km/s (119 miles/s), or 0.064% the speed of light. As impressive as that is, Parker was on a suicide mission into the Sun's corona. It's not as easy to obtain such velocity when heading away from the Sun.

It's fortunate that we're not travelling in the Saturn V rocket, which only reached max speeds of about 11.2 km/s (7 miles/s). At that rate, we would take about 50% longer to get back home. And yes, it is pronounced "Saturn five" - it's a Roman numeral. Thirteen Saturn V rockets were launched over just a six year period during the Apollo era. They were quite the behemoths - almost 3,000 tonnes (6,000,000 pounds) in mass and capable of carrying 140 tonnes (310,000 pounds) into low-Earth orbit. For over 50 years, Saturn V was the only rocket that had taken humans beyond low-Earth orbit, until 1 April, 2026, when the Space Launch System rocket took humans around the Moon again.

Saturn herself, the planet, needs little introduction. She is blessed with the most spectacular set of rings in the Solar System, visible even through Galileo's pioneering telescope. She has 21% of mass in the Solar System outside the Sun, which is pretty respectable, and yet her density is so low that she would float in a swimming pool (an immensely large one, of course). She has, at most times, been recognised as having more known moons than any other planet in the Solar System, with about 300 of them identified so far. She was the most distant planet known to humans for millennia, until Uranus was finally spotted in 1781.

Our gift for Saturn? A ring polishing kit. Look, it's more of a gag gift than anything. The famous rings are actually comprised of water ice particles with a trace of rocky material, ranging from micrometres to metres in size (0.000039 inches to 3 feet). I don't think there's much we could do to polish them, and they already light up so elegantly anyway. Let's just marvel in Saturn's beauty as we approach.

It will be 2.68 years to Uranus from here ... stop sniggering! No planet deserves to face such scorn and ridicule on a daily basis; we'll have none of that when we arrive, thank you very much! I'll have to call TrivBot back in here to deliver a dry anatomy lesson to you. That ought to dispel you of any childish inclinations upon mention of anuses. TRIVBOT!

*TrivBot trundles into the room*
*Robotic voice*
"Bleep Bloop, I am TrivBot. Paragon of all wisdom. You have requested 'anus' trivia. Present in humans and many animals, the anus is an opening at the far end of the gastrointestinal tract, where solid waste is excreted. The region about 2.5 to 4 cm (0.98 to 1.57 inches) from the opening to the rectum is called the anal canal. Bleep bloop. Time for the interesting fact of the day. When you were a little embryo, just three weeks after fertilisation, your cells split into three layers. The endoderm went on to form the lining of your gastrointestinal tract, the mesoderm formed muscles, bones and connective tissues, while the ectoderm formed skin and nerves. The pectinate or dentate line sits inside the anal canal, and marks the boundary between the gastrointestinal tract of endodermic origin, and the anal and external skin of ectodermic origin. Thus structures above and below the line have entirely different blood and nerve supply and lymphatic drainage. This is why external haemorrhoids found below the pectinate line are painful, while internal haemorrhoids found above the line are painless! Bleep Bloop."

Very good TrivBot, you may get back to ... what exactly do you do around here anyway?

Now, a little about Uranus. Yes, the planet. As I've mentioned a couple of times, it was first detected in 1781 by German-British astronomer and composer William Herschel. It had actually been spotted numerous times by multiple earlier astronomers, but mistaken for a star each time. While it's generally considered to be visible only by telescope, it can actually be seen very faintly by the unaided human eye in ideal viewing conditions during certain times in its orbit where it is at its brightest. In fact, it seems to have been seen by Greek astronomer Hipparchus in 128 BC, who once plotted a star that does not exist, in a position where Uranus would have been at the time.

Uranus is an ice-giant, made mostly of water, ammonia and methane. It is flipped almost 90 degrees onto its side, meaning its equator is nearly perpendicular to its orbit around the Sun, in contrast to all other planets that orbit and rotate in parallel. This gives it very intense and long seasons. It has its own faint ring system. And finally, it has 29 known moons, most of them named after characters from the plays of William Shakespeare and Alexander Pope's narrative poem "The Rape of the Lock". So the gifts that you will set into orbit as Uranus' honorary 30th and 31st moons, are a compendium of Shakespeare's complete works, and a placard featuring Pope's poem.

Did you know, we're not even half way done on our trip yet? Despite having already visited eight of our ten destinations, the distance from the Sun to Uranus is less than the distance from Uranus to Pluto. The final two trips will really test our patience. Just over three years travel time to our next destination. I do grow weary, if I am being honest. I'll keep it brief, so that we may go out with a bang at our final destination.

Neptune is the most distant planet known to orbit the Sun. There have long been rumblings about a distant "Planet X" beyond Neptune ("X" as in mysterious, not the Roman numeral, though perhaps we could still rename it "Planet IX" after Pluto's demotion), but more on that later. Neptune was discovered due to anomalies in the orbit of Uranus that suggested gravitational perturbation by an unknown planet beyond. Calculations by French astronomer Urbain Le Verrier mathematically predicted its location, subsequently confirmed by direct observation by German astronomer Johann Gottfried Galle in 1846. It's the only planet humans have found by prediction, rather than detection by chance.

Though Le Verrier and the French attempted to name the planet after its discoverer, the name Neptune won out as the internationally accepted name. It was named after the Roman god of the sea, the parallel of the Greek Poseidon. In turn, the chemical element Neptunium takes its name from the planet. Neptunium is sandwiched between Uranium and Plutonium in the periodic table of elements, just as the planet sits between their namesakes in the Solar System. But as far as we know, there is no meaningful amount of any of the three elements on Neptune. So our gift is this sample of Uranium, Neptunium and Plutonium in a lovely display case - a souvenir to help bridge the gap between lonely Neptune and its distant neighbours.

Finally onwards to Pluto, a mere 2.63 years away. The only rations we have left are these deep fried sausages, which should tide us over. Americans call them "corn dogs", as do some Canadians, while other Canadians call them "pogos". In Australia they call them "Dagwood Dogs", "Dippy Dogs", or "Pluto Pups". Variants exist as far and wide as Argentina, South Africa, Japan and South Korea, with unique local twists, and names to match. But on this spaceship we will call them Pluto Pups in honour of our final destination. Dig in!

I've already mentioned the discovery of Neptune based on mathematical predictions arising from anomalies in the orbit of Uranus. Even before Neptune was discovered, some had speculated that more than one planet was required to account for the anomalies. Indeed, the estimated mass of Neptune did not end up accounting for all the discrepancies in the orbits of the gas giants, and so the search continued for yet another "Planet X" even further beyond.

Over the decades, innumerable astronomers had speculated about and searched for Planet X. One of the notable seekers was Percival Lowell, a wealthy American who had founded the Lowell Observatory in Flagstaff, Arizona. Derided for his belief that the channel-like features on Mars were canals built by intelligent aliens, he staked his reputation on finding Planet X. Starting in 1906, he searched in vain until his sudden death in 1916. Sadly, he had in fact captured Pluto on some of his plates, but did not recognise it for what it was. It took 22 year old Kansas farm boy Clyde Tombaugh, who had just started at the Lowell Observatory, to finish the job and at last discover Pluto in 1930. Here it was, Planet X! It was soon dubbed Pluto after the Roman god of the underworld, with the first two letters alluding to Lowell's initials.

However, it did not take long for doubts about its planetary status to set in. Pluto appeared like a star, as only a point rather than a disc, as we saw with Ceres and the asteroids. It was six times dimmer than expected, limiting its potential size to perhaps 90% that of Earth. It had a highly eccentric orbit that was inclined at 17 degrees to the plane that all other planets orbited in, even coming closer to the Sun than Neptune at times. Was this not just another asteroid, comet, or other undiscovered but non-planetary type of object?

In 1978, when Pluto's largest moon Charon was discovered, its mass could finally be pinpointed to around 1/6th of the mass of the Earth's moon. This was far too small to account for the anomalies it had been supposed to explain. The search for Planet X was clearly not over.

When NASA sent the space probe Voyager 2 past Neptune in 1989, its mass was revised down by 0.5%. Using this updated value in simulations, the discrepancies in the orbit of Uranus disappeared. It seemed that they had been a mirage the whole time. Further confirmation came when data from the trajectories of the Voyager 2, Voyager 1, Pioneer 10 and Pioneer 11 space probes did not reveal any discrepancies to support the existence of another Planet X. Some astronomers continue to search, using other apparent anomalies to hypothesise different variations, but most astronomers no longer believe in the existence of any substantial planet beyond Neptune.

So now we were left with Pluto, this accidentally discovered oddball of a planet. Some astronomers had already been uneasy about its inclusion in the list of fully fledged planets, but this became a full-blown panic once thousands of new objects were discovered orbiting in the region of Pluto's orbit, in what we now know as the Kuiper belt. Quaoar and Sedna were two larger of such objects, found to be around half the size of Pluto. Finally, when Eris was discovered in 2005 with more mass than Pluto, the die had been cast. The International Astronomical Union resolved to create a more robust definition of a planet, with Pluto being demoted (and Ceres being "promoted", as we saw earlier) to dwarf planet status. The main criterion where they failed in being designated as a full planet was that they had not cleared other objects in the neighbourhood around their orbits. While all the major planets have very little material remaining in their orbits, the asteroid belt and the Kuiper belt where Ceres and Pluto respectively lie contain significant leftover material.

Pluto has five known moons. The largest is Charon, with 12.2% the mass of Pluto. It is so large relative to Pluto that the two objects orbit each other around a barycentre that sits outside Pluto. While all orbital pairs of objects technically both orbit each other, including the Earth and its moon, the centre of rotation is usually found within the larger object's mass, not outside it. Some thus consider the Pluto-Charon system to be a double dwarf planet. But we could spend forever getting into the weeds of definitions, where arbitrary distinctions become more and more like individual preferences.

Pluto is a lonely and cold outpost, despite the Kuiper belt objects in its neigbourhood. It takes sunlight 4-7 hours to reach this neighbourhood, with less than 0.1% the intensity of sunlight reaching Earth. At this distance, the Sun looks no larger than Venus does from Earth, though it is still 250 times brighter than a full moon on Earth - enough light to illuminate the Plutonian surface to the level of twilight back on Earth. So our final gift is another plaque, but this time it's backlit. As is the case back home, we could always do with a little bit more light in all the darkness.

*Plaque reads:*
[You'll always be a planet in my heart]