Weird worlds: These are the most extreme exoplanets we’ve discovered so far

While more than 5,000 exoplanets is a lot to sort through, we’ve gone ahead and pulled out some of the most gnarly worlds we’ve found so far to show you just how weird it can get out there and hopefully make us all a little bit more grateful for the world we have.

How exoplanets are discovered, and how we can tell what their conditions are like

You might be wondering how exactly we’ve been finding all these exoplanets in so short a time and how we’re able to tell so much about them from so far away. Fortunately, the science behind exoplanet discovery is pretty straightforward (even if you need some pretty specialized equipment), and like most science, it is largely a matter of math and observation.

The two main ways we detect exoplanets are using the transit method and the radial velocity method.

With the transit method, we simply stare at a candidate star using a special telescope that can measure tiny variations in a star’s luminosity. Whenever something passes in front of a light source, the amount of light that we see dims (the most obvious case being a shadow or, better yet, a solar eclipse or the transit of Venus).

If we record that dimming in the luminosity of a star, we can make a pretty good estimate as to how large the object was. What’s more, if we see that same dip in luminosity occur with a regular period, we’ve established that the object is an exoplanet orbiting around the star.

The other method, the radial velocity or “wobble” method (also formally known as Doppler Spectroscopy), measures the Doppler redshift or blueshift in the spectral lines from the light from the star, indicating that the star is “wobbling” towards or away from us. If this shift is periodic and the shift is slight enough that it isn’t attributable to the star actually approaching or receding from us (which would be much more gradual and sustained), then something locally must be pulling on the star in its own local star system. The only thing that could do this would be one or more exoplanets. 

Our own Sun, for example, also has a wobble thanks to the gravitational pull of Jupiter, so if aliens on Alpha Centauri or elsewhere looked our way, they could detect Jupiter by detecting the amount of wobble in our Sun’s light and working backward mathematically to the planet causing the perturbation.

This can also tell us a lot about the nature of the planets involved as well. If we see an especially large dip in luminosity corresponding to a Jupiter-sized planet, but its period is measured in days (or even hours) rather than years, we know that it is a hot Jupiter orbiting very close to its host star.

And, believe it or not, with infrared sensors, spectrochemical analysis, and other instruments, we can measure the temperature of the planet, the composition of its atmosphere, its mass, and much more. Using these tools has allowed exoplanet hunters to identify all of these utterly bizarre worlds that we didn’t even realize were possible. 

TOI-2109b

The exoplanet TOI-2109b is not just any hot Jupiter; it is an absolutely record-smashing one.

Weighing in at 1.35 times the size and five times the mass of our own Jupiter, TOI-2109b has an orbital period of just 16 hours. For context, a hot Jupiter is defined as a roughly Jupiter-sized gas giant with an orbital period of 10 days or less. The closest planet to our Sun, Mercury, orbits once every 88 days, and Jupiter takes a leisurely 12 years to make its way around the Sun.

The way planetary physics works, the closer you are to a star, the faster you are going to orbit, so at 16 hours per orbit, this puts TOI-2109b perilously close to its host star – so much so that it is likely in the last stages of a death spiral into a fiery doom. It is estimated that it is orbiting at a distance of just 1.5 million miles (2.4 million kilometers). That’s just under 2% of the distance between the Earth and the Sun.

“In one or two years, if we are lucky, we may be able to detect how the planet moves closer to its star,” said NASA Goddard Space Flight Center astronomer Ian Wong. “In our lifetime, we will not see the planet fall into its star. But give it another 10 million years, and this planet might not be there.”

Understandably, TOI-2109b is also ridiculously hot; the second-hottest exoplanet ever found. It is orbiting a yellow-white star about 1.7 times the size and 1.4 times the mass of the Sun.

On its daytime side, TOI-2109b is believed to get as hot as 5,840 degrees Fahrenheit (3,227 degrees Celsius, or 3,500 Kelvin). There are stars that don’t even get that hot. What’s more, at its orbital distance, it’s almost certainly tidally locked with its star, so only one side of the planet is ever facing the star, with the nightside of the planet existing in permanent darkness.

This latter feature might make for even more extreme conditions in the planet’s atmosphere. 

“The planet’s night side brightness is below the sensitivity of the TESS data,” said Avi Shporer, an astrophysicist at MIT’s Kavli Institute for Astrophysics and Space Research, “which raises questions about what is really happening there. Is the temperature there very cold, or does the planet somehow take heat on the day side and transfer it to the night side? We’re at the beginning of trying to answer this question for these ultrahot Jupiters.”

Ian Wong, an astronomer at the NASA Goddard Space Flight Center, agrees. “Ultrahot Jupiters such as TOI-2109b constitute the most extreme subclass of exoplanet,” he said. “We have only just started to understand some of the unique physical and chemical processes that occur in their atmospheres – processes that have no analogs in our own solar system.”

WASP-76b

Ultrahot Jupiters may be some of the most extreme exoplanets we’ve seen so far, but WASP-76b takes things to another level entirely.

As a tidally-locked gas giant in a tight orbit around its star, Wasp-76, the planet experiences an extreme temperature differential between its dayside and nightside. Its dayside temperature is 4,350 degrees Fahrenheit (2,400 degrees Celsius) – which has been suggested is enough to turn neutral iron into vapor – while the nightside of the planet is a relatively cooler 2,730 Fahrenheit (1,500 Celsius) – cold enough to condense that iron vapor back into its molten state.

The temperature difference produces a pressure differential that can produce howling winds, which then circulate the vaporized neutral iron from the dayside to the nightside, where the iron condenses and rains out – at least to some extent.

With the help of the ESPRESSO instrument on the European Space Organization’s Very Large Telescope in Atacama, Chile, astronomers at the University of Geneva, Switzerland (UoGS) and the Centre for Astrobiology (CfA) in Madrid, Spain, identified the chemical variations on an ultra-hot gas giant planet along the border between its dayside and nightside.

Evidence of iron gas in the atmosphere at the border separating the planet’s dayside from its nightside implies some very unusual weather patterns, to say the least. “Surprisingly, however, we do not see the iron vapor in the morning,” said UoGS’s David Ehrenreich, a professor at UoGS who led the study documenting WASP-76b. 

“The observations show that iron vapour is abundant in the atmosphere of the hot day side of WASP-76b,” said María Rosa Zapatero Osorio, a CfA astrophysicist and the chair of the ESPRESSO science team. “A fraction of this iron is injected into the night side owing to the planet’s rotation and atmospheric winds. There, the iron encounters much cooler environments, condenses and rains down.”

Yep, that’s right, it rains molten iron at night – or at least that’s what some have argued. Followup observations didn’t see the same iron vapor signatures, so whether it rains iron on WASP-76b is not 100% settled. 

However, recent evidence suggests the presence of barium high in the planet’s atmosphere an element nearly 2.5 times as heavy as iron. It is not known how such a heavy element can remain in the atmosphere without sinking, but some researchers speculate that there may be unexpected convection churning heavy elements towards the top of the atmosphere.

Gliese 1132b

This exoplanet about 41 light-years away from us might not look like a very nice place to live, but at least it’s undergoing some refurbishing, particularly where its atmosphere is concerned.

Gliese 1132 b (also GJ 1132 b) is a rocky world that is several times the diameter of the Earth, putting it in a class of exoplanets known as a sub-Neptune. The thick, smoggy atmosphere is being supplied by the molten volcanism on the planet’s surface, and it is highly suspected that this isn’t Gliese 1132 b’s first atmospheric rodeo.

Hydrocarbons and other volatiles suggest that Gliese 1132 b is surrounded by a secondary atmosphere produced after the energetic young star it orbits blasted away its original hydrogen and helium atmosphere, and that trace elements of its original atmosphere can be seen in the second atmosphere.

“It’s super exciting because we believe the atmosphere that we see now was regenerated, so it could be a secondary atmosphere,” said Raissa Estrela with NASA’s Jet Propulsion Laboratory (JPL) and a co-author of a study on the exoplanet.

“We first thought that these highly irradiated planets could be pretty boring because we believed that they lost their atmospheres. But we looked at existing observations of this planet with Hubble and said, ‘Oh no, there is an atmosphere there.'”

As for its primary atmosphere, it was substantially larger than its current one and would have been enough to qualify Gliese 1132 b as a gas giant prior to having the air knocked out of it. Remnants of that ancient atmosphere might remain dissolved in the molten surface and subsurface layers of the planet, perhaps belching out this secondary atmosphere using those initial elements to form more complex compounds. 

HD 189733 b

Another fantastical exoplanet with a wind problem, HD 189733 b just might be the most inhospitable exoplanet we’ve ever discovered.

Its deep blue color is a product of the dissolved silicates in its atmosphere, which break up more blue light than red, and because it is a tidally-locked ultra-hot Jupiter, it has the same temperature differentials between its dayside and nightside as many other exoplanets of its kind, producing extreme weather patterns at the transitional borders.

“On this turbulent alien world, the daytime temperature is nearly 2,000 degrees Fahrenheit,” NASA says, “and it possibly rains glass — sideways — in howling, 4,500-mph winds. The cobalt blue color comes not from the reflection of a tropical ocean as it does on Earth, but rather a hazy, blow-torched atmosphere containing high clouds laced with silicate particles.”

WASP-103b

WASP-103b is an exoplanet not quite like any we’ve seen yet, principally because it’s not your typical sphere-shaped planet.

WASP-103b is about twice the size of Jupiter, with about 50% more mass, and it orbits much closer to its star than Jupiter does. With an orbital period of less than a day, WASP-103b is whipping around its star while under the effect of extreme tidal forces from the star, which is almost twice the size of the Sun and slightly hotter as well.

Researchers with the ESA’s CHEOPS mission were using spectrum analysis to determine the exoplanet’s composition when the light curves from the star and transiting exoplanet revealed a surprising deformity in the planet’s structure – it has an oval shape, like a rugby ball.

“It’s incredible that Cheops was actually able to reveal this tiny deformation,” Jacques Laskar, from the Paris Observatory, Université Paris Sciences et Lettres, and a co-author of the 2021 study announcing the discovery, said in a statement from the European Space Agency (ESA). “This is the first time such analysis has been made, and we can hope that observing over a longer time interval will strengthen this observation and lead to better knowledge of the planet’s internal structure.”

Besides being weird, the deformation of the exoplanet’s shape from the host star’s gravity provides researchers with an important tool to probe the exoplanet’s internal structure.

“The resistance of a material to being deformed depends on its composition,” said Susana Barros, with the Instituto de Astrofísica e Ciências do Espaço and University of Porto, Portugal, who lead the research. “For example, here on Earth we have tides due to the Moon and the Sun but we can only see tides in the oceans. The rocky part doesn’t move that much. By measuring how much the planet is deformed we can tell how much of it is rocky, gaseous or water.”

“In principle, we would expect a planet with 1.5 times the mass of the Jupiter to be roughly the same size, so WASP-103b must be very inflated due to heating from its star and maybe other mechanisms,” says Barros.

“If we can confirm the details of its internal structure with future observations maybe we could better understand what makes it so inflated. Knowing the size of the core of this exoplanet will also be important to better understand how it formed.”

KELT-9b

If you’re looking for one of the most extreme exoplanets out there, you should definitely take a look at KELT-9b – just don’t get too close if you don’t want to get burned.

Of all the exoplanets we’ve confirmed so far, KELT-9b is the hottest one on record. It orbits an A-type star, which can have surface temperatures up to 10,000 Kelvin and is close enough to complete an orbit every 1.5 days.

Measurements of KELT-9b puts its surface temperature at about 4,600 Kelvin (7,820 degrees Fahrenheit), which is hotter than most stars in the universe. “This is the hottest gas giant planet that has ever been discovered,” said Scott Gaudi, an astronomy professor at Ohio State University in Columbus, who led the research into KELT-9b.

The planet is also about 2.8 times more massive than Jupiter, but about the same diameter as Jupiter, based on its transit data. “It’s a planet by any of the typical definitions of mass, but its atmosphere is almost certainly unlike any other planet we’ve ever seen just because of the temperature of its dayside,” said Gaudi.

Recently, astronomers have detected the rare earth metal terbium swirling around in clouds of vaporized metal in KELT-9b’s atmosphere. This is the first time this extremely rare element has been found on an exoplanet.

KELT-9, the exoplanet’s host star, is only about 300 million years old and is expected to burn itself out in a few hundred million years. In the meantime, it is about twice the size of our Sun and twice as hot.

“KELT-9 radiates so much ultraviolet radiation that it may completely evaporate the planet,” said Keivan Stassun, professor of physics and astronomy at Vanderbilt University, Nashville, Tennessee, who led the study with Gaudi. “KELT-9 will swell to become a red giant star in a few hundred million years. The long-term prospects for life, or real estate for that matter, on KELT-9b are not looking good.”

Kepler 51b, Kepler 51c, and Kepler 51d

While “Super Puffs” might not sound like the actual name for a kind of exoplanet, we can assure you it is a very real thing. The Kepler 51 system includes three gas giant exoplanets, designated Kepler 51b, Kepler 51c, and Kepler 51d, and astronomers looking at these systems couldn’t resist the analogy to cotton candy.

These three exoplanets are about the size of Saturn and Jupiter, but they have about a hundredth of Jupiter’s mass and are more comparable to the mass of Earth than any gas giant we’re familiar with.

For reasons that aren’t clear yet, the hydrogen/helium atmospheres of these planets are puffed out, but they also provide astronomers a great way to study the atmospheres of exoplanets since the densities of the Kepler 51 exoplanets are significantly lower than we have seen elsewhere.

J1407b

Saturn might have some of the most majestic rings in our solar system, but it’s got nothing on J1407b.

The ring system around the companion of the star J1407 was spotted in 2012 when its rings eclipsed the star, and further analysis of the data indicates that there might be as many as 30 individual rings around the exoplanet with clear gaps between them. Given the size of the ring system around it, it’s hard to say if J1407b is an exoplanet or a brown dwarf orbiting a companion star. 

As for the size of the ring system, the light curves suggest that the entire system might have a diameter of nearly 75 million miles or about 120 million kilometers (this is nearly the distance between the Earth and the Sun, about 93 million miles or 150 million kilometers), and that all the dust and rock in the ring system might have as much mass as the Earth. 

“If you were to grind up the four large Galilean moons of Jupiter into dust and ice and spread out the material over their orbits in a ring around Jupiter, the ring would be so opaque to light that a distant observer that saw the ring pass in front of the sun would see a very deep, multi-day eclipse,” said Eric Mamajek, a professor of physics and astronomy at the University of Rochester who led the team that first discovered the rings in 2012.

“In the case of J1407, we see the rings blocking as much as 95 percent of the light of this young Sun-like star for days, so there is a lot of material there that could then form satellites.” 

“The details that we see in the light curve are incredible,” said Matthew Kenworthy of the Leiden Observatory. “The eclipse lasted for several weeks, but you see rapid changes on time scales of tens of minutes as a result of fine structures in the rings.

“The star is much too far away to observe the rings directly, but we could make a detailed model based on the rapid brightness variations in the star light passing through the ring system. If we could replace Saturn’s rings with the rings around J1407b, they would be easily visible at night and be many times larger than the full moon.”

What kind of planet could sustain such a ring system, though? It’s hard to nail down the mass of J1407b, but the researchers estimate that the exoplanet’s mass is somewhere between 10 to 40 Jupiter masses and that it takes about 10 years to orbit J1407.

“This planet is much larger than Jupiter or Saturn, and its ring system is roughly 200 times larger than Saturn’s rings are today,” said Mamajek. “You could think of it as kind of a super Saturn.”

“The planetary science community has theorized for decades that planets like Jupiter and Saturn would have had, at an early stage, disks around them that then led to the formation of satellites,” Mamajek added. “However, until we discovered this object in 2012, no one had seen such a ring system. This is the first snapshot of satellite formation on million-kilometer scales around a substellar object.”

M51-ULS-1 candidate

For good measure, we wouldn’t dream of leaving off without talking about M51-ULS-1. In October 2021, a multi-institution research team attempted something rather audacious (and ingenious): detect an exoplanet in another galaxy using the transit method.

Using NASA’s Chandra X-ray Observatory, an international team of astronomers turned their sights to the Whirlpool Galaxy (Messier 51, or M51) about 31 million light-years from us and homed in on an X-ray bright binary system with either a neutron star or a black hole paired up with a main-sequence companion star.

“We are trying to open up a whole new arena for finding other worlds by searching for planet candidates at X-ray wavelengths, a strategy that makes it possible to discover them in other galaxies,” said study lead author Rosanne Di Stefano, from Havard & Smithsonian Institution’s Center for Astrophysics (CfA) in Cambridge, Massachusetts.

While the transit method for stars in the Milky Way uses the star’s luminosity to identify exoplanet candidates, bright X-ray sources are produced in the volatile accretion disks surrounding neutron stars and the event horizons of black holes, and as a consequence, these X-ray bright areas are considerably smaller than the light from a star.

This means that it could be possible for an exoplanet in orbit to completely block X-ray emissions rather than just dimming them (as in the standard transit method), and so it should be possible to spot exoplanets in galaxies beyond our own. The researchers searched more than 200 star systems in three galaxies (M51, M101, and M104), and in the end, they saw a single three-hour-long transit in M51-ULS-1 where the bright X-ray source there was blocked entirely for about three hours.

Armed with this and other data, the team estimates that the exoplanet would be about the size of Saturn and orbiting at about twice the distance from the X-ray source as Saturn does from the Sun. Unfortunately, they also determined an orbital period (based on its mass and transit time, among other things) of about 70 years, all but ruling out follow-up confirmation needed to verify exoplanet discovery.

“Unfortunately to confirm that we’re seeing a planet we would likely have to wait decades to see another transit,” said study co-author Nia Imara of the University of California, Santa Cruz. “And because of the uncertainties about how long it takes to orbit, we wouldn’t know exactly when to look.”

If M51-ULS-1 did turn out to be an exoplanet, though, it would have had a rough go of it. The creation of a neutron star or black hole usually involves a very powerful supernova, and the exoplanet candidate would have been bathed in extremely high levels of radiation from the event. If anything was living on it at the time, it’s highly unlikely that it would have survived. What’s more, the star could soon be destabilized due to its feeding material into its companion’s accretion disk and go supernova as well, putting the exoplanet through a rerun of the worse week of its existence.

Still, if the M51-ULS-1 candidate is an exoplanet, there’s a good chance it’s orbiting a black hole, Interstellar-style, and you really can’t get more extreme than that.