The entire abundance of alien worlds we have discovered: super-Earths, mini-Neptunes, lava worlds, and more.
1. From rare cases to common phenomena
Just a few decades ago, planets beyond our Solar System were only a hypothesis. Since the first confirmed detections in the 1990s (e.g., 51 Pegasi b), the field of exoplanet research has expanded dramatically – we now know over 5000 confirmed planets and many more candidates. Kepler, TESS, and ground-based radial velocity studies have revealed that:
- Planetary systems are very common – most stars have at least one planet.
- Planetary masses and orbital structures are much more diverse than we initially imagined; here we find classes of planets that we do not have in our System.
This exoplanet diversity – hot Jupiters, super-Earths, mini-Neptunes, lava worlds, ocean worlds, sub-Neptunes, very short orbit rocky bodies, and distant giants – shows how inventive planet formation can be in different stellar environments. These new types also challenge our theoretical models, forcing improvements in migration scenarios, disk substructures, and alternative formation pathways.
2. Hot Jupiters: Massive Giants Close to Stars
2.1 First Surprises
One of the first stunning discoveries was 51 Pegasi b (1995) – a hot Jupiter with a mass similar to Jupiter's but orbiting just 0.05 AU from its star, completing an orbit in about 4 days. This challenged our understanding of the Solar System, where giant planets “live” in cold distant regions.
2.2 Migration Hypothesis
Hot Jupiters likely form beyond the frost line, like typical jovian planets, and later migrate inward due to disk-planet and disk interactions (Type II migration) or later dynamical processes (planet-planet scattering and tidal circularization). Radial velocity studies still find many such giants close to their stars, although they make up only a few percent of Sun-like stars, showing that hot Jupiters are not very common but remain an important phenomenon [1], [2].
2.3 Physical Characteristics
- Larger Radius: Many hot Jupiters have “inflated” radii, possibly due to strong stellar radiation or internal thermal mechanisms.
- Atmosphere Studies: Transit spectroscopy reveals sodium and potassium lines, and in especially hot cases sometimes even evaporated metals (e.g., iron).
- Orbit and Spin Axis: Some hot Jupiters have significantly tilted orbits at a large angle relative to the star's spin axis, indicating a dynamic history of migration or scattering.
3. Super-Earths and Mini-Neptunes: Planets with Intermediate Parameters
3.1 Discovery of Medium-Sized Worlds
One of the most common types of exoplanets discovered by Kepler are those with radii around 1–4 Earth radii and masses ranging from a few Earth masses up to ~10–15 Earth masses. These planets, called super-Earths (if mostly rocky) or mini-Neptunes (if they have a noticeable hydrogen/helium envelope), fill a niche absent in our Solar System – since our Earth (~1 R⊕) and Neptune (~3.9 R⊕) leave a significant gap. But exoplanet data show that many stars have precisely such medium-radius/mass planets [3].
3.2 Diversity of Main Compositions
Super-Earths: Likely dominated by silicates/iron, with a thin or no gaseous envelope. They may have formed near the inner disk and be large rocky bodies (some have water layers or thick atmospheres).
Mini-Neptunes: Similar mass but with more H/He or volatile layers, resulting in lower density. They may have formed slightly beyond the snow line or accreted more gas before the disk dissipated.
The transition from super-Earth to mini-Neptune shows that even small differences in formation time or location can cause significant differences in atmospheres and final density.
3.3 Radius gap
Detailed studies (e.g., California-Kepler Survey) identified the "radius gap" around ~1.5–2 Earth radii. This means some smaller planets lose their atmospheres (becoming rocky super-Earths), while others retain them (mini-Neptunes). This phenomenon is likely related to stellar radiation photoevaporation or different core sizes [4].
4. Lava worlds: ultra-short orbit rocky planets
4.1 Tidal locking and molten surfaces
Some exoplanets orbit extremely close to their star, rotating in less than 1 day. If they are rocky, the surface temperature can greatly exceed the silicate melting point, turning their star-facing side into a magma ocean. These are called lava worlds, examples include CoRoT-7b, Kepler-10b, K2-141b. They may even form an atmosphere of evaporated minerals [5].
4.2 Formation and migration
It is likely that these planets did not form so close to the star (it would be too hot for the disk there) but migrated similarly to hot Jupiters, except these have lower mass or did not accrete gas. By observing their unusual composition (e.g., iron vapor lines) or phase curve changes, we can test theories of high-temperature atmospheres and surface evaporation.
4.3 Tectonics and atmospheres
Theoretically, lava worlds can have intense volcanic or tectonic activity if volatiles still remain. However, most lose their atmospheres due to strong photoevaporation. Some may form iron "clouds" or "rains," but this is difficult to verify directly. Studying them helps understand extreme cases of "rocky exoplanets" – where rocks evaporate under the star's influence.
5. Multi-planet resonant systems
5.1 Tight resonant chains
Kepler studies found many star systems with 3–7 or more closely packed sub-Neptunes or super-Earths. Some (e.g., TRAPPIST-1) show nearly resonant chain connections between adjacent planets, such as 3:2, 4:3, 5:4, etc. This is explained by disk migration, which brings planets into mutual resonances. If they remain stable, the result is a tight resonant chain.
5.2 Dynamic stability
Although many such multi-planet systems orbit stably in resonant orbits, others likely experience partial scattering or collisions, leaving fewer planets or larger distances between them. The exoplanet population ranges from several compact super-Earths to giant planets on highly eccentric orbits – reflecting planet-planet interactions that can create or disrupt resonances.
6. Giants on distant orbits and direct imaging
6.1 Distant gas giants
Since the 2000s, direct imaging studies (Subaru, VLT/SPHERE, Gemini/GPI) occasionally find massive jovian or even superjovian planets tens or hundreds of AU from the star (e.g., four giants in HR 8799). They can form via core accretion if the disk was massive, or due to gravitational instability in the outer disk.
6.2 Brown dwarf or planetary mass?
Some distant satellites approach the ~13 Jupiter mass boundary separating brown dwarfs (capable of fusing deuterium) from exoplanets. Determining whether such massive "companions" are planets or brown dwarfs sometimes depends on formation history or dynamic environment.
6.3 Impact on outer debris disks
Giants orbiting on wide orbits can form debris disks, clear gaps, or create ring structures. For example, HR 8799 has an inner debris belt and a distant outer belt, with planets positioned in between. Studying such systems helps understand how giant planets reshape remaining planetesimals – like Neptune influenced the Kuiper belt in our system.
7. Unusual phenomena: tidal heating, disappearing planets
7.1 Tidal heating: the "Io" effect or super-Ganimes
The existence of strong tidal forces in exoplanet systems can cause intense internal heating. Some super-Earths in resonance may experience volcanism or cryovolcanism (if farther from the star). Observing any possible gas emissions or unusual spectral signatures would confirm that tidal geology exists beyond just the example of Io.
7.2 Evaporating atmospheres (hot exoplanets)
Stars' UV radiation can "tear off" upper layers, creating evaporating or "hthonic" remnants. For example, GJ 436b shows flowing helium/hydrogen "tails." This can form sub-Neptunes that lose part of their mass and become super-Earths (this is linked to the mentioned radius gap).
7.3 Ultra-dense planets
Detected are also very high-density exoplanets – possibly iron or mantle-stripped. If a planet experienced an impact or stripping event removing volatile and silicate parts, a “iron planet” would remain. Studying such extreme cases helps understand the diversity of disk chemistry and dynamics.
8. Habitable zone and potentially life-supporting worlds
8.1 Earth-like analogs
Among many exoplanets, some orbit within their star's habitable zone, receiving enough but not too much radiation for water to remain liquid if the atmosphere is suitable. Many such planets are super-Earths or mini-Neptunes; whether they truly resemble Earth is unclear, but this question is of great interest due to potential life.
8.2 M dwarf worlds
Small red (M) dwarfs – the most common stars in the Galaxy – often have several rocky or sub-Neptune planets in close orbits. Their habitable zones are very close to the star. However, this poses challenges: tidal locking, strong stellar flares, possible water loss. Still, TRAPPIST-1 with seven Earth-sized planets showed how diverse and potentially habitable M dwarf worlds can be.
8.3 Atmospheric studies
To assess potential habitability or search for biosignatures, JWST, future extremely large telescopes (ELT), and other missions will analyze exoplanet atmospheres. Subtle spectral traces (e.g., O2, H2O, CH4) may indicate life-supporting conditions. The diversity of exoplanet worlds – from super-hot lava planets to sub-cold mini-Neptunes – means atmospheric chemistry and possible climate conditions are very varied.
9. Synthesis: why such diversity?
9.1 Different formation pathways
Small initial variations – protoplanetary disk mass, chemical composition, longevity – can significantly change the final outcomes: some systems grow large gas giants, others only small rocky or ice-rich planets. Disk migration and planet-planet interactions further shift orbits, so the final picture can differ greatly from our Solar System.
9.2 Star type and environment
The star's mass and luminosity determine the location of the snow line, the disk's temperature profile, and the boundaries of the habitable zone. High-mass stars have shorter-lived disks, possibly quickly forming giants or unable to grow many small worlds. M dwarfs with smaller disks often grow a collection of super-Earths or mini-Neptunes. Additionally, the star's environment (e.g., nearby OB association members) can photoevaporate the disk, erasing the outer system and thus promoting a different planetary outcome.
9.3 Further research
Exoplanet observation methods (transits, radial velocity measurements, direct imaging, microlensing) are continuously improving, allowing better capture of mass-radius relationships, axial tilts, atmospheric compositions, and orbital structures. Thus, the exoplanet “zoo” with hot Jupiters, super-Earths, mini-Neptunes, lava worlds, ocean worlds, sub-Neptunes, and other types is constantly expanding, revealing complex combinations of processes that form such diversity.
10. Conclusion
The diversity of exoplanets encompasses a vast spectrum of planetary masses, sizes, and orbital arrangements – much greater than what our Solar System showed us. From blazing “lava worlds” in extremely short orbits to super-Earths and mini-Neptunes filling gaps absent in our system, and from hot Jupiters close to their stars to giants in resonant chains or wide distant orbits – all these alien worlds reveal how disk physics, migration, dispersal, and stellar environment intertwine.
The study of these “strange” configurations allows astronomers to refine models of planet formation and evolution, gradually creating a comprehensive understanding of how cosmic dust and gas give birth to such diversity of planets. With ever-improving telescope equipment and detection methods, in the future we will be able to delve even deeper into these worlds – exploring their atmospheres, potential habitability, and the physics governing each star's unique planetary family.
Links and further reading
- Mayor, M., & Queloz, D. (1995). “A Jupiter-mass companion to a solar-type star.” Nature, 378, 355–359.
- Winn, J. N., & Fabrycky, D. C. (2015). “The Occurrence and Architecture of Exoplanetary Systems.” Annual Review of Astronomy and Astrophysics, 53, 409–447.
- Batalha, N. M., et al. (2013). “Planetary candidates observed by Kepler. III. Analysis of the first 16 months of data.” The Astrophysical Journal Supplement Series, 204, 24.
- Fulton, B. J., et al. (2017). “The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets.” The Astronomical Journal, 154, 109.
- Demory, B.-O. (2014). “Planetary Interiors and Host Star Composition: Inferences from Dense Hot Super-Earths.” The Astrophysical Journal Letters, 789, L20.
- Vanderburg, A., & Johnson, J. A. (2014). “A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission.” Publications of the Astronomical Society of the Pacific, 126, 948–958.