--- title: Exotic worlds canonical_url: https://paxabyssi.com/wiki/Exotic_worlds markdown_url: https://paxabyssi.com/wiki/Exotic_worlds.md type: wiki-page revision_id: 108 revision_view: stable last_updated: 2026-09-27 license: CC BY-SA 4.0 license_url: https://creativecommons.org/licenses/by-sa/4.0/ science_status: - observed - speculative - sim summary: Four kinds of planet that physics allows but the Solar System lacks. Iron-rich super-Mercuries are observed; carbon planets, helium-atmosphere planets and chthonian planets (the stripped cores of giants) remain hypotheses with candidates but no confirmed member. categories: - Exotic planets - Planet classes aliases: - Exotic planets - Exotic world - X series - Carbon planet - Carbon planets - Carbon world - Diamond planet - Carbide planet - XCB - X1-C - Iron planet - Iron planets - Iron world - Super-Mercury - Super-Mercuries - XFE - X2-Fe - Helium planet - Helium planets - Helium world - XHE - X3-He - Chthonian planet - Chthonian planets - Chthonian world - CHT - X4-Ch infobox: type: planet_class code: XCB, XFE, XHE, CHT mass: unit: M_Earth value: Carbon 0.5 to 8; iron 0.3 to 3; helium 2 to 10; chthonian 5 to 40 source: sim name: Exotic worlds image: File:55_Cancri_e_Webb_artist_concept.jpg level: Series (four sim types) radius: unit: R_Earth value: Observed super-Mercuries 0.7 to 1.6; TOI-849 b 3.44 source: observed series: X (exotic) caption: "Artist's concept: 55 Cancri e, once proposed as a carbon-rich world and now thought to hold a thick atmosphere over a magma ocean. Credit: NASA, ESA, CSA, R. Crawford (STScI)" subtypes: "XCB: TA Tar, DM Diamond Mantle, GR Graphite. XFE: SC Scorched, OX Oxidized, CR Cratered. XHE: SH, BN, PL by temperature. CHT: IR ice-rich, MTL and MN molten, TR transitional, SO solid (sim names)" sim_source: Carbon, iron, helium and chthonian physics engines and properties modules; disc chemistry model; the exotic world science references bond_albedo: unit: dimensionless value: "Sim: carbon 0.03 to 0.07; iron 0.10 to 0.15" source: sim legacy_code: X1-C, X2-Fe, X3-He, X4-Ch bulk_density: unit: g/cm³ value: Super-Mercuries about 8 to 13; TOI-849 b 5.2 source: observed last_verified: 2026-09-27, writer B real_examples: Super-Mercuries K2-229 b, Kepler-107 c, GJ 367 b; chthonian candidate TOI-849 b; carbon and helium candidates disputed (55 Cancri e, GJ 436 b) typical_orbit: Close to the star for iron, helium and chthonian candidates; anywhere a carbon-rich disc allows for carbon planets dominant_gases: "Carbon: CO, CH4 and hydrocarbons predicted; helium: He with depleted H2; iron and chthonian: thin or none" science_status: observed (super-Mercuries); speculative (carbon, helium and chthonian planets); sim (subtypes) frequency_in_sim: "275 of the 8,742 planets in the 5,159 generated systems: 107 carbon, 37 iron, 29 helium, 102 chthonian" rendered_example: None yet (shot list) defining_criteria: "Carbon: interior rich in carbides or graphite, from a disc with carbon-to-oxygen ratio above about 0.8. Iron: metal core well above Earth's one-third of the mass. Helium: envelope enriched in helium by escape of hydrogen. Chthonian: the dense core of a giant whose envelope was stripped" interior_structure: "Carbon: carbide or graphite mantles, possibly diamond at depth; iron: core of 60 to 90% of the mass; chthonian: the rock-and-ice core of a former giant" literature_equivalent: Carbon (carbide) planet; iron planet or super-Mercury; helium-atmosphere planet; chthonian planet (remnant giant core) equilibrium_temperature: unit: K value: Candidates mostly 1,000 to 2,000 (close-in, ultra-short periods) source: observed related: - https://paxabyssi.com/wiki/Hot_Jupiter.md - https://paxabyssi.com/wiki/Ice_giant.md - https://paxabyssi.com/wiki/Lava_world.md - https://paxabyssi.com/wiki/Planet_classification.md - https://paxabyssi.com/wiki/Arid_world.md - https://paxabyssi.com/wiki/Barren_rock_world.md --- # Exotic worlds > Source: https://paxabyssi.com/wiki/Exotic_worlds > > Licence: [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/). Text by Pax Abyssi Wiki contributors; history at https://paxabyssi.com/wiki/Exotic_worlds/history > > Revision 108, 27 September 2026 **Exotic worlds** are planets whose make-up has no match among the planets of the Solar System. Four kinds are discussed most. **Iron planets**, or super-Mercuries, have metal cores far larger than Earth's, and several are known. **Carbon planets** would form from carbon-rich material and be built of carbides and graphite instead of silicate rock. **Helium planets** would be Neptune-like worlds that have lost so much hydrogen that helium dominates their atmospheres. **Chthonian planets** would be the exposed cores of giant planets whose gas envelopes were stripped away. Only the first kind is observed; the other three are hypotheses with candidates, and each candidate so far has another explanation too. ## Iron planets and super-Mercuries Mercury is the Solar System's iron planet: its metal core makes up about 70% of its mass, against about 30% for Earth, Venus and Mars [1], and the top of the core lies about 2,020 km from the centre of a planet only 2,440 km in radius [2]. The favoured explanation is a giant impact that blasted away much of an originally larger planet's rocky mantle [3]. Exoplanet surveys have now found planets like Mercury but larger. K2-229 b, orbiting its star every 14 hours, has 1.17 Earth radii and 2.6 Earth masses, a density that requires a Mercury-like core [1]. In the Kepler-107 system two neighbouring planets have almost the same radius, but the outer one, Kepler-107 c, is more than twice as dense (12.6 against 5.3 g/cm³). Starlight cannot explain the outer planet being the denser, so a giant impact that stripped its mantle is the likely cause [4]. GJ 367 b, smaller than Earth and circling a red dwarf every 7.7 hours [5], has a density of about 10 g/cm³, implying that iron makes up around 90% of its mass [6]. Rocky exoplanets broadly follow the iron content of their host stars, but the super-Mercuries do not: they form a separate, iron-rich group that the chemistry of their stars cannot explain, which points to collisions or to sorting of metal and rock in the disc rather than to iron-rich birth material [7]. A world like this would have no thick atmosphere, a dark, rocky surface, and quite possibly a strong magnetic field from its large molten core. ## Carbon planets In the Solar System, oxygen outnumbers carbon about two to one, so carbon is locked into gases such as carbon monoxide and the rock-forming elements combine with oxygen into silicates. In a disc where carbon outnumbers oxygen, the chemistry flips. Carbon and silicon form silicon carbide, carbon condenses as graphite, and planets could form with carbide mantles, graphite crusts and, under pressure deep inside, diamond. Marc Kuchner and Sara Seager proposed such carbon planets in 2005, predicting water-poor, hydrocarbon-rich atmospheres and, for cooler planets, tar-covered surfaces [8]. Condensation models show that carbides take over once the carbon-to-oxygen ratio (C/O) of the planet-forming material rises above about 0.8, compared with the Sun's 0.54 [9] [10]. Laboratory shock experiments on silicon carbide to 1.5 terapascals suggest that carbide planets would be about 10% less dense than rocky planets of the same mass [11]. No carbon planet has been confirmed, and the best-known candidates have faded. The hot super-Earth 55 Cancri e was proposed as a carbon-rich world in 2012 [12], but its star's C/O was then measured at 0.78 ± 0.08 rather than above 1 [13], and JWST found a thick atmosphere, probably rich in carbon dioxide or carbon monoxide, over a surface much cooler than bare rock would be, most likely outgassed from a magma ocean [14]. A carbon-rich atmosphere reported for the hot Jupiter WASP-12 b [10] was ruled out at more than three standard deviations when water was found in its spectrum [15]. The most carbon-rich planet-mass object known may be PSR J1719-1438 b, a body of about Jupiter's mass orbiting a pulsar every 2.2 hours with a density of at least 23 g/cm³; it is thought to be the remnant of a white dwarf, crystalline carbon and oxygen stripped down by its companion, but that interior is inferred rather than measured [16]. ![Artist's concept of a dark, hot rocky planet with a hazy atmosphere close to a bright star](https://media.paxabyssi.com/public/68d0eebb4126bae1f6403e0aa20018213ace43ade6512d1ab9de3d4bea7b40e3/2560.webp "Artist's concept: 55 Cancri e, once a carbon-planet candidate. JWST's measurements point instead to an atmosphere of carbon dioxide or carbon monoxide over a magma ocean. Credit: NASA, ESA, CSA, R. Crawford (STScI).") *Figure 1.* Artist's concept: 55 Cancri e, once a carbon-planet candidate. JWST's measurements point instead to an atmosphere of carbon dioxide or carbon monoxide over a magma ocean. Credit: NASA, ESA, CSA, R. Crawford (STScI). Licence: CC BY 4.0. ## Helium planets A Neptune-sized planet close to its star loses gas from the top of its atmosphere, heated by the star's ultraviolet and X-ray light. Hydrogen, the lightest gas, escapes more easily than helium, and if the escape is slow enough for the gases to separate, the remaining envelope grows richer in helium over billions of years. Renyu Hu, Sara Seager and Yuk Yung showed in 2015 that warm Neptunes and sub-Neptunes on orbits like Mercury's could end up with helium-dominated atmospheres this way, poor in methane and water; they suggested it could explain the puzzling spectrum of GJ 436 b [17]. Later models that follow both the escape and the planet's cooling are more restrictive. Helium makes up more than 40% of the envelope only for small planets, about three Earth radii or less, that start with thin envelopes (under about 0.5% of their mass) and receive 10 to 1,000 times the sunlight Earth does; GJ 436 b's envelope is too massive to have been enriched [18]. The best places to look are planets of 1.6 to 2.5 Earth radii along the upper edge of the [radius valley](https://paxabyssi.com/wiki/Radius_valley.md), where helium could reach more than 40% of the envelope after several billion years [19]. Helium escaping from exoplanets can be seen. An absorption line of helium at 1,083 nanometres was proposed as a tracer of escaping gas [20], and helium was first detected in an exoplanet atmosphere, the eroding envelope of WASP-107 b, in 2018 [21]. Seeing helium escape is not the same as finding a helium-dominated planet, and none has been confirmed. ![Diagram showing a Neptune-like planet losing hydrogen faster than helium as starlight heats its upper atmosphere](https://media.paxabyssi.com/public/60ad0532a06e8b64a7eac1eacffda99707d20a7fcac6e15fd6ca59825db9d50e/2550.webp "Diagram: how a warm Neptune could slowly turn its atmosphere to helium. Credit: NASA/JPL-Caltech.") *Figure 2.* Diagram: how a warm Neptune could slowly turn its atmosphere to helium. Credit: NASA/JPL-Caltech. Licence: Public domain (NASA). ## Chthonian planets A giant planet that migrates very close to its star could have its hydrogen-helium envelope stripped away by the star's heat and tides, leaving only its dense core. Guillaume Hébrard and colleagues named such hypothetical remnants **chthonian** planets in 2004, from the Greek *khthon*, earth, the root of words for the underworld [22]. They would look like very dense, very hot Neptune-sized or smaller planets on short orbits, the region where Neptune-sized planets are otherwise scarce, the [hot Neptune desert](https://paxabyssi.com/wiki/Hot_Neptune_desert.md) [23]. The strongest candidate is TOI-849 b, found by TESS in 2020 in the middle of the desert. It has 39 Earth masses packed into 3.4 Earth radii, a density of 5.2 g/cm³, an 18-hour orbit, and at most about 4% of its mass in hydrogen and helium. Its discoverers concluded that it is most likely the remnant core of a giant planet, though they also considered a planet that simply failed to capture gas [24]. Other dense planets have been proposed as remnants too [25], among them TOI-1853 b, 73 Earth masses in a Neptune-sized body [26]. Earlier candidates such as CoRoT-7 b could equally have formed rocky from the start [27]. Models of how cores respond when their envelopes are removed suggest that most very dense exoplanets are unlikely to be stripped giant cores [28], so the class remains unconfirmed. ## How we know Every exotic world is identified the same way: a mass from radial velocities and a radius from transits give a bulk density, and the density is compared with models of iron, rock, carbides, water and gas. Densities alone often allow more than one answer, which is why only the super-Mercuries, whose densities no other plausible mixture reaches, count as observed. Spectra add chemistry. JWST's measurement of 55 Cancri e [14] and helium escaping from warm Neptunes [21] show how atmospheres can confirm or rule out a proposed class. ## Notable examples | Planet | Mass | Radius | Period | Class and status | | ---------------- | -------------------- | --------------------- | ---------- | -------------------------------------------------------- | | Mercury | 0.055 Earth masses | 0.38 Earth radii | 88 days | Iron-rich, about 70% metal core [1] | | K2-229 b | 2.6 Earth masses | 1.17 Earth radii | 14 hours | Super-Mercury (observed) [1] | | Kepler-107 c | about 9 Earth masses | about 1.6 Earth radii | 4.9 days | Super-Mercury, likely from a giant impact [4] | | GJ 367 b | 0.63 Earth masses | 0.70 Earth radii | 7.7 hours | Iron about 90% of its mass [5] [6] | | 55 Cancri e | 8.8 Earth masses | 1.95 Earth radii | 17.7 hours | Carbon-planet candidate, now disfavoured [13] [14] | | PSR J1719-1438 b | about 1 Jupiter mass | | 2.2 hours | Probably a stripped white dwarf, carbon and oxygen [16] | | TOI-849 b | 39 Earth masses | 3.4 Earth radii | 18 hours | Chthonian candidate [24] | > **In Pax Abyssi** > > The sim's exotic series has four types, each tied to the chemistry of the disc a system forms from. **Carbon worlds** (XCB, 0.5 to 8 Earth masses, very dark with Bond albedos of 0.03 to 0.07) become far more likely when the disc's carbon-to-oxygen ratio exceeds 1, somewhat more likely between 0.8 and 1, and are ruled out below 0.4; their subtypes are Tar (below 500 K), Diamond Mantle (at least 2 Earth masses and 500 K) and Graphite. **Iron worlds** (XFE, 0.3 to 3 Earth masses) are allowed where the disc's iron-to-silicon ratio is high, and are split by temperature into Scorched, Oxidized and Cratered. **Helium worlds** (XHE, 2 to 10 Earth masses) and **chthonian worlds** (CHT, about 5 to 40 Earth masses, split by temperature and by whether they are molten, tidally locked or ice-rich) complete the set. All the names are the sim's own. Of the 8,742 planets in the 5,159 generated systems, 275 are exotic: 107 carbon, 37 iron, 29 helium and 102 chthonian. No render of an exotic world is published yet. ## See also - [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md) - [Radius valley](https://paxabyssi.com/wiki/Radius_valley.md) - [Hot Neptune desert](https://paxabyssi.com/wiki/Hot_Neptune_desert.md) - [Atmospheric escape](https://paxabyssi.com/wiki/Atmospheric_escape.md) - [Lava world](https://paxabyssi.com/wiki/Lava_world.md) - [Ice giant](https://paxabyssi.com/wiki/Ice_giant.md) - [Hot Jupiter](https://paxabyssi.com/wiki/Hot_Jupiter.md) ## References 1. Santerne, A. et al. (2018). An Earth-sized exoplanet with a Mercury-like composition. Nature Astronomy 2, 393-400. 2. Hauck, S. A. et al. (2013). The curious case of Mercury's internal structure. Journal of Geophysical Research: Planets 118, 1204-1220. 3. Benz, W. et al. (2007). The Origin of Mercury. Space Science Reviews 132, 189-202. 4. Bonomo, A. S. et al. (2019). A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system. Nature Astronomy 3, 416-423. 5. Lam, K. W. F. et al. (2021). GJ 367b: A dense, ultrashort-period sub-Earth planet transiting a nearby red dwarf star. Science 374, 1271-1275. 6. Goffo, E. et al. (2023). Company for the Ultra-high Density, Ultra-short Period Sub-Earth GJ 367 b: Discovery of Two Additional Low-mass Planets at 11.5 and 34 Days. The Astrophysical Journal Letters 955, L3. 7. Adibekyan, V. et al. (2021). A compositional link between rocky exoplanets and their host stars. Science 374, 330-332. 8. Kuchner, M. J. and Seager, S. (2005). Extrasolar Carbon Planets. arXiv. 9. Bond, J. C., O'Brien, D. P. and Lauretta, D. S. (2010). The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations. The Astrophysical Journal 715, 1050-1070. 10. Madhusudhan, N. et al. (2011). A high C/O ratio and weak thermal inversion in the atmosphere of exoplanet WASP-12b. Nature 469, 64-67. 11. Kim, D. et al. (2022). Structure and density of silicon carbide to 1.5 TPa and implications for extrasolar planets. Nature Communications 13, 2260. 12. Madhusudhan, N., Lee, K. K. M. and Mousis, O. (2012). A Possible Carbon-rich Interior in Super-Earth 55 Cancri e. The Astrophysical Journal Letters 759, L40. 13. Teske, J. K. et al. (2013). Carbon and Oxygen Abundances in Cool Metal-rich Exoplanet Hosts: A Case Study of the C/O Ratio of 55 Cancri. The Astrophysical Journal 778, 132. 14. Hu, R. et al. (2024). A secondary atmosphere on the rocky exoplanet 55 Cancri e. Nature 630, 609-612. 15. Kreidberg, L. et al. (2015). A Detection of Water in the Transmission Spectrum of the Hot Jupiter WASP-12b and Implications for Its Atmospheric Composition. The Astrophysical Journal 814, 66. 16. Bailes, M. et al. (2011). Transformation of a Star into a Planet in a Millisecond Pulsar Binary. Science 333, 1717-1720. 17. Hu, R., Seager, S. and Yung, Y. L. (2015). Helium Atmospheres on Warm Neptune- and Sub-Neptune-sized Exoplanets and Applications to GJ 436b. The Astrophysical Journal 807, 8. 18. Malsky, I. and Rogers, L. A. (2020). Coupled Thermal and Compositional Evolution of Photoevaporating Planet Envelopes. The Astrophysical Journal 896, 48. 19. Malsky, I. et al. (2023). Helium-enhanced planets along the upper edge of the radius valley. Nature Astronomy 7, 57-66. 20. Oklopčić, A. and Hirata, C. M. (2018). A New Window into Escaping Exoplanet Atmospheres: 10830 Å Line of Helium. The Astrophysical Journal Letters 855, L11. 21. Spake, J. J. et al. (2018). Helium in the eroding atmosphere of an exoplanet. Nature 557, 68-70. 22. Hébrard, G. et al. (2004). Evaporation rate of hot Jupiters and formation of Chthonian planets. Extrasolar Planets: Today and Tomorrow (ASP Conference Series 321). 23. Mazeh, T., Holczer, T. and Faigler, S. (2016). Dearth of short-period Neptunian exoplanets: A desert in period-mass and period-radius planes. Astronomy & Astrophysics 589, A75. 24. Armstrong, D. J. et al. (2020). A remnant planetary core in the hot-Neptune desert. Nature 583, 39-42. 25. Mocquet, A., Grasset, O. and Sotin, C. (2014). Very high-density planets: a possible remnant of gas giants. Philosophical Transactions of the Royal Society A 372, 20130164. 26. Naponiello, L. et al. (2023). A super-massive Neptune-sized planet. Nature 622, 255-260. 27. Valencia, D. et al. (2010). Composition and fate of short-period super-Earths: The case of CoRoT-7b. Astronomy & Astrophysics 516, A20. 28. Lin, Z., Cambioni, S. and Seager, S. (2025). Most High-density Exoplanets Are Unlikely to Be Remnant Giant Planet's Cores. The Astrophysical Journal Letters 978, L41. ## Infobox (planet class) | Field | Value | | --- | --- | | Code | XCB, XFE, XHE, CHT | | Mass | Carbon 0.5 to 8; iron 0.3 to 3; helium 2 to 10; chthonian 5 to 40 M_Earth | | Name | Exotic worlds | | Image | File:55_Cancri_e_Webb_artist_concept.jpg | | Level | Series (four sim types) | | Radius | Observed super-Mercuries 0.7 to 1.6; TOI-849 b 3.44 R_Earth | | Series | X (exotic) | | Caption | Artist's concept: 55 Cancri e, once proposed as a carbon-rich world and now thought to hold a thick atmosphere over a magma ocean. Credit: NASA, ESA, CSA, R. Crawford (STScI) | | Subtypes | XCB: TA Tar, DM Diamond Mantle, GR Graphite. XFE: SC Scorched, OX Oxidized, CR Cratered. XHE: SH, BN, PL by temperature. CHT: IR ice-rich, MTL and MN molten, TR transitional, SO solid (sim names) | | Sim source | Carbon, iron, helium and chthonian physics engines and properties modules; disc chemistry model; the exotic world science references | | Bond albedo | Sim: carbon 0.03 to 0.07; iron 0.10 to 0.15 dimensionless | | Legacy code | X1-C, X2-Fe, X3-He, X4-Ch | | Bulk density | Super-Mercuries about 8 to 13; TOI-849 b 5.2 g/cm³ | | Last verified | 2026-09-27, writer B | | Real examples | Super-Mercuries K2-229 b, Kepler-107 c, GJ 367 b; chthonian candidate TOI-849 b; carbon and helium candidates disputed (55 Cancri e, GJ 436 b) | | Typical orbit | Close to the star for iron, helium and chthonian candidates; anywhere a carbon-rich disc allows for carbon planets | | Dominant gases | Carbon: CO, CH4 and hydrocarbons predicted; helium: He with depleted H2; iron and chthonian: thin or none | | Science status | observed (super-Mercuries); speculative (carbon, helium and chthonian planets); sim (subtypes) | | Frequency in sim | 275 of the 8,742 planets in the 5,159 generated systems: 107 carbon, 37 iron, 29 helium, 102 chthonian | | Rendered example | None yet (shot list) | | Defining criteria | Carbon: interior rich in carbides or graphite, from a disc with carbon-to-oxygen ratio above about 0.8. Iron: metal core well above Earth's one-third of the mass. Helium: envelope enriched in helium by escape of hydrogen. Chthonian: the dense core of a giant whose envelope was stripped | | Interior structure | Carbon: carbide or graphite mantles, possibly diamond at depth; iron: core of 60 to 90% of the mass; chthonian: the rock-and-ice core of a former giant | | Literature equivalent | Carbon (carbide) planet; iron planet or super-Mercury; helium-atmosphere planet; chthonian planet (remnant giant core) | | Equilibrium temperature | Candidates mostly 1,000 to 2,000 (close-in, ultra-short periods) K | ## Related pages - [Hot Jupiter](https://paxabyssi.com/wiki/Hot_Jupiter.md): A gas giant orbiting so close to its star that its year lasts days and its dayside is hotter than lava. Hot Jupiters were the first planets found around Sun-like stars and remain the best-studied exoplanet atmospheres, though fewer than one Sun-like star in a hundred has one. - [Ice giant](https://paxabyssi.com/wiki/Ice_giant.md): A giant planet made mostly of water, methane and ammonia rather than hydrogen and helium, like Uranus and Neptune. Their interiors are hot, dense fluids, their magnetic fields are lopsided, and planets of about their mass may be among the commonest in the Galaxy. - [Lava world](https://paxabyssi.com/wiki/Lava_world.md): A rocky planet so close to its star that its dayside rock is molten, often a magma ocean under a thin atmosphere of vaporised rock. Dozens are known, most on orbits shorter than a day. - [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md): How astronomers sort planets by size, mass, temperature and composition, and how Pax Abyssi files every world it generates under one of 37 coded types. - [Arid world](https://paxabyssi.com/wiki/Arid_world.md): A rocky planet with a thin, cold atmosphere and no stable liquid water at the surface, only ice and the traces of ancient rivers. Mars is the Solar System's example. - [Barren rock world](https://paxabyssi.com/wiki/Barren_rock_world.md): A rocky planet with no real atmosphere, whose surface lies bare to starlight, cosmic rays and meteorites. Mercury is the Solar System's example, and JWST has found several around other stars. Categories: [Exotic planets](https://paxabyssi.com/wiki/Category:Exotic_planets.md), [Planet classes](https://paxabyssi.com/wiki/Category:Planet_classes.md)