Isn't it amazing that astronomy – humanity’s oldest science – continues to generate such a delightful amount of new knowledge?
Seeing as we’ve been studying the motion of the stars for a good long while, you’d be forgiven for thinking that, by now, we would be long past the point of saying “Astronomy? Completed it, mate.”
Fortunately, the Universe is vast and unknowable. However big our telescope mirrors and however sensitive our detectors, there will always be some light that’s too faint to see and signals that are too weak to detect.
This is good; for one thing, it means I still have a job; more importantly, however, it means the sky is still full of wonder.
Planets are one such wonder. The word ‘planet’ comes from the Greek ‘planētēs’, meaning ‘wanderer’. They were so named because our closest planetary siblings in the Solar System appeared to wander across the night sky.
Alas, detecting our distant planetary cousins – the exoplanets that lie beyond the Solar System – takes far more effort than glancing skyward once the Sun has set. It requires enormous telescopes, long observing campaigns and a lot of computing.
But believe me when I tell you, it’s worth the effort. Exoplanet detection is booming. At the time of writing, astronomers have discovered over 6,000 of them and we think that most stars have them.
But of course, in a field characterised by so many discoveries, occasionally truly extraordinary things turn up.
There have been planets that flirt with the line separating science and science fiction; planets that force us to revisit our theories of how they form; and planets that, in all fairness, feel like they shouldn’t exist.
Planets like the 10 that follow…
The planet too big for its star
TOI-6894 b
The discovery of TOI-6894 b was only announced in June 2025 and it presents a genuine challenge to our understanding of planet formation.
Its star is a tiny red dwarf (roughly 20 per cent the size of the Sun), but TOI-6894 b is almost as big as Jupiter.
It’s a problematic combination because our current best theory of planet formation says that very small stars can’t have very big planets.
The planet-forming discs around nascent red dwarf stars simply don’t have enough material. And yet, here is TOI-6984 b.
It’s not the first problematic pairing of this kind to be found, but it is a record breaker: TOI-6894 b’s star is the smallest known to host a giant planet.
Two Suns are better than one
2M1510 (AB) b
The ‘(AB) b’ part in this exoplanet’s name tells us something crucial: it’s circumbinary, meaning the planet orbits a double star system.
Roughly 50 per cent of stars in the Milky Way are in binaries of some sort, but 2M1510 (AB) b orbits both stars at once. That’s uncommon in itself, with only 16 circumbinary systems discovered to date. But there’s more.
This isn’t just any binary, it’s a binary pair of failed stars known as brown dwarfs and that really is a rarity. Only one other pair like it has ever been found.
But wait, there’s still more. The planet is on a polar orbit. This means it’s orbiting at 90° to the plane the brown dwarfs are orbiting in, looping over the top of them. It’s the first of its kind ever detected.
Astronomers had an inkling that they might be a thing since we’ve found protoplanetary discs in polar orbits, but for the first proper one to be found orbiting a pair of brown dwarfs is wild.
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Worlds built from the ashes of a dead star
PSR B1257+12 b, c and d
Exoplanets PSR B1257+12 b and c were the first to be discovered, back in January 1992, with d being found a couple of years later. This system really set the scene for the field, because it’s ridiculous.
The planets orbit a stellar remnant called a pulsar – a rapidly rotating neutron star.
Neutron stars are very dense, have very strong magnetic fields and emit all sorts of electromagnetic radiation. But PSR B1257+12 b – the first exoplanet discovered – is still the lowest-mass planet ever detected.
Plus, there’s the fact it was found at all, as planets orbiting pulsars are rare. Only 0.5 per cent of these long-dead stars are thought to host planets.
That’s because the process of becoming a neutron star is such a violent end-of-life affair that there’s almost no chance any existing bodies would’ve made it through the tumultuous late stages of stellar evolution.
PSR B1257+12 b, c and d are probably the result of a second round of planet formation. It’s kind of nice that their star decided to have a second crack at parenthood in later life. It’s never too late!
Sapphire rain over a boiling planet
WASP-121 b
The first exoplanet discovered orbiting a regular star – 51 Peg b – is a kind of planet we don’t have in the Solar System. It’s a hot Jupiter.
These planets are so named because they’re gas giants (like Jupiter), but much closer to their stars. They’re silly enough, what with their super-inflated sizes, blisteringly hot temperatures and painfully short orbits, but WASP-121 b is actually an ultra-hot Jupiter.
So the extreme conditions are just a touch more extreme. For one thing, WASP-121 b orbits its star in little more than 24 hours, but it doesn’t actually have days because it’s tidally locked – the same side always faces the host star, so it has a permanent nightside and dayside.
The sizeable temperature difference between the two hemispheres (1,500°C-ish/about 2,730°F) causes wind speeds of over 16,000km/h (10,000mph). It’s hot enough on this planet to vaporise iron, aluminium and titanium.
Clouds containing these elements and their oxides can then be blown from the toasty dayside to the cooler nightside, causing rain of iron and gemstones, including sapphires.
There’s an episode of Doctor Who where the Doctor visits a spooky planet called Midnight and goes on an ill-fated train ride to see some sapphire waterfalls.
Planets like WASP-121 b somehow make even the most bonkers sci-fi locations seem plausible.
The dancing planet where you might not get a birthday
TOI-4504 c
If you have one planet orbiting a star and there’s absolutely nothing else in the vicinity, you can likely get a full understanding of the system, with enough observational data.
Things get trickier if you add one more planet in, as now you’re into ‘3-body problem’ territory – so-called because going from two to three celestial bodies in a system means it’s no longer possible to get a full solution for the orbits via mathematical equations.
In most cases though, it’s possible to get a decent approximation. Unless, of course, the two planets are in a special little dance called a ‘mean motion resonance’ (MMR).
An MMR (not the life-saving vaccine) means the planets’ orbital periods are very close, in a ratio like 2:1 or 3:2, and it leads to frequent gravitational interactions between the bodies.
As is the case with TOI-4504 c: it’s locked in a resonance with its neighbour, completing two orbits for every one by its counterpart.
The result of these gravitational kicks? The length of a year isn’t fixed. In fact, in this record-breaking case, a year can vary in duration by up to 2.4 per cent.
Can you imagine if that happened on Earth? A year could vary in length by eight days either way. People born in late December would never know if they were getting a birthday or not!
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Hotter than the surface of a star
KELT-9 b
No list of extraordinary exoplanets would be complete without KELT-9 b, the second ultra-hot Jupiter to make this list.
Its parent star is one of the hottest stars known to host a planet and, typically, we wouldn’t expect to see planets orbiting anything at the upper end of the temperature scale.
Hot stars live fast and die young, burning through their nuclear fuel so quickly that planetary formation doesn’t really stand a chance. But here’s KELT-9 b, defying the odds.
The result is a planet so hot that, at over 3,700°C (6,700°F), it’s actually hotter than most stars in the Milky Way. That’s because roughly 75 per cent of our galaxy’s stellar population are red dwarfs with typical temperatures under 3,200°C (5,800°F).
Hell on (another) Earth
55 Cancri e
We’ve known about this planet for more than two decades and it’s been a conundrum the whole time. 55 Cancri e is a super-Earth, meaning it’s a bit larger than our home planet.
But if you’re picturing a slightly enlarged version of here, complete with greenery and oceans, think again. It’s a literal hellscape.
55 Cancri e orbits in just 18 hours – this means it’s very close to its star. So close, in fact, that the star’s gravitational forces are continuously pulling the planet’s surface apart. It’s such a horrendously violent place, that it’s thought to rain lava there.
In fact, this planet is so close to its star that the intensity of the radiation it receives is immense, so much so that astronomers thought there was almost no chance of it harbouring an atmosphere. Atmospheres don’t generally get on with outrageously high temperatures; they tend to escape.
Imagine, then, astronomers’ collective surprise when new observations from the James Webb Space Telescope revealed that 55 Cancri e appears to have a tenuous atmosphere containing carbon dioxide and/or monoxide.
“How?”, we all asked. Constant replenishment, apparently: the gases evaporate from magma oceans on the surface, keeping that atmosphere nicely stocked.
Where in the worlds are all the moons?
PDS 70 c
The PDS 70 system has something that absolutely should exist, but has so far eluded us: evidence of exomoons.
Moons are fairly ubiquitous in the Solar System, with Saturn and Jupiter boasting about 350 between them. Mars has two of them and we’ve got one of our own. Even demoted dwarf planet Pluto has a posse of them.
So, there’s no reason to believe other planetary systems wouldn’t also be riddled with them. But astronomers don’t have the proof to back up that belief.
There have been plenty of ‘potential exomoons’, and countless journal articles on how it might be possible to spot them, but there’s not a single confirmed moon around any of the nearly 6,000 known exoplanets.
There’s still hope, however, thanks to PDS 70; an infant star system, still in the throes of formation.
In the primordial muck of this particular corner in the vastness of space, you’ll never guess what they’ve spotted around planet PDS 70 c. An exomoon? Well, no… not quite. But astronomers have detected a moon-forming disc.
No smoke without fire, and no moon without moon-forming discs.
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Rogue planets
Rogue planets, also known as free-floating planets (a less cool, but more informative name), are the planetary orphans of our galaxy.
So far around 100 of them have been discovered, but it’s thought there could be as many as a trillion of them in the Milky Way. That would mean rogue planets outnumber stars by about 20 to 1.
It’s a baffling thought because… well, where are they all coming from? There are two ways rogue planets form.
The more massive ones (a few times heavier than Jupiter, say) are thought to form in a similar way to stars – from an interstellar gas cloud collapsing under its own gravity.
To get a new star out of this process, the resulting ball of gas and dust would need to be hot and dense enough for hydrogen fusion to kick in. When it fails, we’re left with a brown dwarf or a free-floating gas planet.
The less massive rogues would’ve formed in a cushy planetary system only to be kicked out by one of their siblings during a close encounter.
A planet boiling away to nothing
BD+054868A b
Unfortunately, BD+054868A b isn’t long for this world. It has been flying a bit too close to its sun – this rocky planet is literally boiling away.
Conditions have become so incomprehensibly hot, that the rock is being vaporised and the resulting plumes of material have formed a comet-like tail around the star.
The system was discovered in February 2025 and while it’s the fourth disintegrating planet to be found, it’s the first time one has been detected around a star bright enough for some proper follow-up observations.
In astronomical terms, it’ll all be over soon for BD+054868A b, as the more material it loses, the faster it’ll meet its demise. Best estimates suggest that it’ll all be over in about two million years.
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