10 of the weirdest exoplanets in the Universe
Facts about the strangest, most unusual and downright cool exoplanets astronomers have discovered over the past few decades.
Since the discovery of the first exoplanets in the 1990s, all manner of weird and wonderful worlds have been found orbiting distant stars beyond our Solar System.
Who wouldn’t want to leave Earth for the promise of an interplanetary trip to a strange exoplanet? With thousands confirmed and the number of exoplanets discovered growing daily, you may get lucky: about half of the Sun-like stars out there are thought to have the potential to host life
That’s about 300 million potentially habitable worlds in our galaxy (for more on this, find out what makes a planet habitable) Don’t pack your bags just yet though – as these weird planets show, you may be in for the holiday from hell.
And, with a wealth of new exoplanet-hunting missions in the pipeline, there could be plenty more strange worlds discovered over the coming years.
10 of the strangest exoplanets discovered
HD 189773b – where it rains glass sideways
This nightmare world is only 64 lightyears away and the closest ‘hot Jupiter’ to Earth. It may look like a gorgeous deep-blue marble floating serenely in space, but if you had the misfortune to visit this massive gas giant, you’d soon regret it.
As well as being spun furiously by winds blowing at 8,700 km/h, you’d be cut to shreds by glass rain. The planet’s delightful blue colour is the reflection of silicate in its atmosphere – silicate that, when heated by the planet’s deathly 1300°C temperature, forms grains of glass.
TOI 849 b – a world stripped bare
Discovered in 2020 by NASA’s Transiting Exoplanet Survey Satellite (TESS), it’s no fun at all on TOI 849 b. This exoplanet orbits so tightly to its star that a year passes in 18 hours. Don’t bother with constant birthday parties though, as there’s no atmosphere and the 1530°C heat would melt the cake.
What makes TOI 849 b particularly weird though is its strangely hybrid nature. While it’s around the size of gas giant Neptune, it’s dense and rocky not gaseous – in fact, it’s the largest rocky world yet discovered, 40 times as massive as Earth. It may even be the first Chthonian planet to be detected: the exposed remnant core of a gas giant that has had its atmosphere blasted away.
WASP-12b – puffed up planet in a death spiral
Just three million or so years from its eventual fiery demise, WASP-12b is spiralling inexorably inwards towards doom at the hands of its yellow dwarf host star.
New research has shown that the planet, located 600 lightyears away in the Auriga constellation, is now so close that it’s begun wobbling and distorting under the spell of the star’s gravity, while intense stellar radiation has caused it to swell up so much that it’s falling apart.
Read more about exoplanet WASP-12b
Rogue worlds: exoplanets on the loose
Many exoplanets may be scary and inhospitable, and they may come in different sizes, colours and densities, but at least they all reliably do one thing: orbit a star. Or do they? While most planets are locked in orbit around their sun, some worlds are actually roaming the galaxy untethered. With no parent star to light and warm them, life is dark and cold on these nomads adrift in the vastness of space.
Catching sight of these hard-to-detect ‘rogue planets’ will be one of the tasks for NASA’s Nancy Grace Roman Space Telescope, coming online around 2025. One such planet is OT44, located 550 lightyears away in the constellation Chamaeleon. This cosmic wanderer is eleven times more massive than Jupiter and thought to have a circumstellar disc of dust, rock and ice. The recently identified OGLE-2016-BLG-1928 is another. Likely smaller than Earth, it’s one of the lowest-mass objects ever found using microlensing techniques.
55 Cancri e – a diamond planet
This exoplanet in orbit around Sun-like host star 55 Cancri A may be a real gem. The first super-Earth discovered around a main sequence star, it was thought to be so abundant in carbon that, thanks to immense pressure and 2,700°C temperatures, its interior was made of diamond.
More recent research has taken the shine off the diamond theory, revealing less carbon than previously thought, but the nature of 55 Cancri e remains enigmatic and hotly contested.
Read more about exoplanet 55 Cancri e
TrES-2b – the darkest exoplanet
“It's so…black! You can hardly make out its shape…light just seems to fall into it!” Hitchhiker’s’ Ford Prefect may have been describing Hotblack Desiato’s limoship, but he could just as easily have been talking about TrES-2b. Identified by NASA’s Kepler Space Telescope in 2011, it’s the darkest known exoplanet, reflecting less than 1% of any light that hits it.
TrES-2b orbits a star some 750 lightyears away in the direction of the constellation Draco and is the darkest planet or moon ever discovered. “It’s darker than the blackest lump of coal, than dark acrylic paint you might paint with. It’s just ridiculous how dark this planet is,” said study lead-author David Kipping from the Harvard-Smithsonian Center for Astrophysics.
KELT-9b – the hottest exoplanet
Ultra-hot Jupiter-type exoplanet KELT-9b is so scorching that it’s even hotter than many stars. It orbits so close to its sun that its surface sizzles at 4,300C – so hot that it has atomic iron and titanium in its atmosphere – and a year lasts less than a day and a half.
Using data from the Spitzer Space Telescope, researchers have found that the extreme temperatures on the planet’s dayside cause molecules of hydrogen gas to tear apart, only to recombine when they flow to the relatively cooler eternal nightside, before being torn apart once more when they move back into the furnace.
Read more about exoplanet KELT-9b
HR 5183b – the planet with the maddest orbit
HR 5183b is in no hurry to orbit its star. Discovered in 2019, it’s a galumphing giant, three times more massive than our biggest planet, Jupiter, that ambles round in a leisurely 74 years (far more than Saturn’s 29 Earth years, but close to Uranus’s 84 years).
What’s strange, though, is its bizarre orbit, which sees it loitering on the outer reaches of its system before slingshotting into the centre, passing a hair’s breadth from its host star before peeling away again. This mad behaviour has earned it the nickname the ‘whiplash planet’. It’s also been likened to a wrecking ball for its likely devastating effect on any other planets in the system that are trying to quietly orbit in a more orthodox fashion.
K2-18b – where a swim may vaporise you
K2-18b, twice the radius and eight times the mass of Earth, has been a top contender for an Earth-like planet for years, so there was huge excitement when it was announced in 2019 that water had been discovered in K2-18b's atmosphere. For the first time, we’d found a rocky planet orbiting in the habitable zone of its star, in which liquid water could potentially pool on the surface.
Before we could dream of luxuriating in exotic oceans, though, came the view that K2-18b may be more like the far less friendly mini-Neptunes – planets with a thick hydrogen atmosphere, a watery layer and a rocky iron core, where temperatures and pressures are far too high to support life.
New research suggests K2-18b could hover in a third zone: planets that looks like a gaseous mini-Neptune but are actually rocky planets covered in superheated, super-compressed seas, where the water exists somewhere on the threshold between liquid and gas, and is topped by a steamy water vapour atmosphere. Sadly, we have to put our swimming costumes away for now.
Read more about exoplanet K2-18b
HAT-P-7b: an ultra-hot Jupiter, dark as charcoal with a sapphire sky
HAT-P-7b is larger than Jupiter and an orbital plane titled 108° relative to its host star, meaning it's almost in a polar orbit.
It orbits the star so close that a year on this world is less than 3 days. In fact, it's over 20 times closer to the star than Earth is to the Sun.
During daytime on HAT-P-7b, temperatures reach over 2,200°C - that's hotter than many stars. Accordingly, the exoplanet is of a kind known as an 'ultra-hot Jupiter'.
But it's also one of the darkest known worlds, about as dark as charcoal, absorbing over 97% of visible light that shines onto it.
Read more about HAT-P-7b
Jane Williamson is a science journalist and writer.