Super-Earth

A super-Earth is a planet more massive than Earth but well below the mass of Neptune, roughly 1 to 10 times Earth's mass. The name refers only to size: a super-Earth can be a lava world, an ice world or anything between. The Solar System has none, yet super-Earths and their slightly larger cousins, the mini-Neptunes, turned out to be among the commonest planets in the Galaxy's inner planetary systems. They are the best targets for studying rocky planets outside the Solar System, and some of them are candidates for habitable worlds. Pax Abyssi sorts super-Earths by what the extra mass does to them: internal heat, water, ice, and how close their star is.

Image pendingFile:Super-Earth_mass_radius.svgA log-log plot of planet mass against radius with composition curves; rocky super-Earths cluster along the Earth-like rock line below 1.6 Earth radii.

Characteristics

What counts

Definitions vary. NASA describes super-Earths as planets more massive than Earth but lighter than Neptune, typically 2 to 10 Earth masses, which may or may not have atmospheres ^[1]. Surveys that measure only size often use radius: the Kepler mission's super-Earth bin ran from 1.25 to 2 Earth radii ^[2]. Since the discovery of the Radius valley, a shortage of planets between about 1.5 and 2 Earth radii, many astronomers reserve the term for the rocky population below it, and call the planets above it sub-Neptunes or mini-Neptunes ^[3].

Size, mass and composition

Rock compresses under its own weight, so a larger rocky planet is denser. The radius of a rocky planet rises only slowly with mass, roughly as

$$R \approx R_\oplus \left(\frac{M}{M_\oplus}\right)^{0.27\text{ to }0.28},$$

a relation seen both in interior models and in the planets themselves ^{[4] [5]}. A planet of Earth's composition and 5 Earth masses is only about 1.5 to 1.6 times Earth's radius, and its density is around 7 to 8 g/cm³. Measured planets follow this pattern up to about 1.5 Earth radii, where density peaks; larger planets become less dense again, because they carry lighter material ^[6]. By about 1.6 Earth radii, most planets are too large to be pure rock ^[7]. Fitting the known planets separately, Otegi and colleagues found a rocky population following $R = 1.03\,M^{0.29}$ in Earth units and a volatile-rich one that is much larger at the same mass ^[8].

The same mass and radius can be matched by different interiors: an iron-rich rocky planet, a lighter rocky planet, or rock with a layer of water, or with a thin hydrogen envelope. Water-rich super-Earths, if they exist, would be a distinct kind: models put planets of half rock and half water on a curve well above the rocky one ^[9], and a group of planets around red dwarfs with densities in that range has been proposed as water worlds, though the interpretation is contested ^[10].

Gravity and surface

Surface gravity scales as $M/R^2$. For a rocky planet following the relation above, that grows roughly as $M^{0.46}$: a 5 Earth-mass rocky super-Earth has about twice Earth's surface gravity, and a 10 Earth-mass one nearly three times. Stronger gravity holds an atmosphere more tightly and squeezes the planet's mantle.

Plate tectonics

Whether super-Earths have plate tectonics has been argued both ways. One model found that larger planets have thinner, weaker plates driven by stronger convection, making plate tectonics inevitable ^[11]; another, published the same year, found that the extra heat and pressure make a stagnant lid, a single rigid shell as on Venus or Mars, more likely ^[12]. The answer matters for habitability, because plate tectonics recycles carbon and drives the climate thermostat described on Mixed world. Heller and Armstrong suggested that a planet of about twice Earth's mass could be more hospitable than Earth, with longer-lived tectonics and a stronger magnetic field ^[13].

Formation

Many close-in super-Earths are probably not born bare. The shape and slope of the radius valley suggest that most of them began with a thin envelope of hydrogen and helium, and lost it to their stars' radiation or their own internal heat, leaving the rocky cores seen today ^[14] (see Radius valley). Others may have formed after the gas in their stars' discs had largely gone, too late to gather much hydrogen. Planets that assembled farther out, beyond the snow line, could include large amounts of water ice and migrate inward as water-rich super-Earths. Far from their stars, super-Earths would keep their ices, like the cold planets of about 5 Earth masses that gravitational microlensing has found beyond the snow lines of their stars ^[15].

How we know

Super-Earths are found in large numbers by transits and radial velocity. Kepler showed that planets of 1 to 2 Earth radii orbit about a quarter of Sun-like stars on orbits of 5 to 100 days ^[16], and that small planets are about as common around F, G and K stars alike ^[2]. From Kepler data, somewhere between about a third and two-thirds of Sun-like stars may host a rocky planet in the habitable zone ^[17].

The first rocky super-Earths with measured sizes and masses were hot. CoRoT-7 b, announced in 2009, was the first super-Earth with a measured radius ^[18]; Kepler-10 b was Kepler's first confirmed rocky planet, in 2011 ^[19]. Both orbit their stars in less than a day and are lava worlds. JWST is now measuring their atmospheres: 55 Cancri e, a super-Earth of about 8 Earth masses on an 18-hour orbit, shows evidence of an atmosphere of carbon dioxide or carbon monoxide rather than bare rock ^[20].

Temperate super-Earths are harder to find, because their longer orbits give fewer transits. The best studied is LHS 1140 b, 5.6 Earth masses and 1.73 Earth radii, in the habitable zone of a red dwarf 49 light years away. Its density is too low for pure rock, making it either a water world or a planet with a thin hydrogen envelope ^[21], and JWST has ruled out a thick hydrogen atmosphere ^[22]. TOI-1452 b, about 4.8 Earth masses and 1.67 Earth radii around a red dwarf, is another temperate candidate whose density leaves room for a large fraction of water ^[23].

Notable examples

In Pax Abyssi

A super-Earth is a size, not a kind of world, so we split the series by what the extra mass does. Our generator has three families of super-Earth, all of 2 to 10 Earth masses and all without a thick hydrogen envelope (planets that keep one are mini-Neptunes). A heavier rocky planet keeps its internal heat longer, so volcanism gets a family of its own; a water-rich one grows deep oceans or high-pressure ice; a cold one freezes. Real exoplanets in the catalogue are sorted into the same families by mass, temperature and radius.

Each one is a destination. Lock its star, jump, and it is there on its true orbit at its true size, wearing the surface plate its subtype calls for.

See also

• Radius valley
• Mini-Neptune
• Mixed world
• Ocean world
• Lava world
• Ice world
• Superhabitable worlds
• Planet occurrence
• Planet classification

References

1. 1NASA Science. Exoplanet types. NASA Science: Exoplanets. science.nasa.gov/exoplanets/planet-types/
2. 2Fressin, F. et al. (2013). The False Positive Rate of Kepler and the Occurrence of Planets. The Astrophysical Journal 766, 81. doi:10.1088/0004-637x/766/2/81
3. 3Fulton, B. J. et al. (2017). The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets. The Astronomical Journal 154, 109. doi:10.3847/1538-3881/aa80eb
4. 4Chen, J. and Kipping, D. (2017). Probabilistic Forecasting of the Masses and Radii of Other Worlds. The Astrophysical Journal 834, 17. doi:10.3847/1538-4357/834/1/17
5. 5Zeng, L., Sasselov, D. D. and Jacobsen, S. B. (2016). Mass-radius relation for rocky planets based on PREM. The Astrophysical Journal 819, 127. doi:10.3847/0004-637x/819/2/127
6. 6Weiss, L. M. and Marcy, G. W. (2014). The Mass-radius Relation for 65 Exoplanets Smaller Than 4 Earth Radii. The Astrophysical Journal 783, L6. doi:10.1088/2041-8205/783/1/l6
7. 7Rogers, L. A. (2015). Most 1.6 Earth-radius Planets Are Not Rocky. The Astrophysical Journal 801, 41. doi:10.1088/0004-637x/801/1/41
8. 8Otegi, J. F., Bouchy, F. and Helled, R. (2020). Revisited mass-radius relations for exoplanets below 120 M⊕. Astronomy & Astrophysics 634, A43. doi:10.1051/0004-6361/201936482
9. 9Zeng, L. et al. (2019). Growth model interpretation of planet size distribution. Proceedings of the National Academy of Sciences 116, 9723-9728. doi:10.1073/pnas.1812905116
10. 10Luque, R. and Pallé, E. (2022). Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars. Science 377, 1211-1214. doi:10.1126/science.abl7164
11. 11Valencia, D., O'Connell, R. J. and Sasselov, D. D. (2007). Inevitability of Plate Tectonics on Super-Earths. The Astrophysical Journal 670, L45-L48. doi:10.1086/524012
12. 12O'Neill, C. and Lenardic, A. (2007). Geological consequences of super-sized Earths. Geophysical Research Letters 34, 2007GL030598. doi:10.1029/2007gl030598
13. 13Heller, R. and Armstrong, J. (2014). Superhabitable Worlds. Astrobiology 14, 50-66. doi:10.1089/ast.2013.1088
14. 14Van Eylen, V. et al. (2018). An asteroseismic view of the radius valley: stripped cores, not born rocky. Monthly Notices of the Royal Astronomical Society 479, 4786-4795. doi:10.1093/mnras/sty1783
15. 15Beaulieu, J. P. et al. (2006). Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing. Nature 439, 437-440. doi:10.1038/nature04441
16. 16Petigura, E. A., Howard, A. W. and Marcy, G. W. (2013). Prevalence of Earth-size planets orbiting Sun-like stars. Proceedings of the National Academy of Sciences 110, 19273-19278. doi:10.1073/pnas.1319909110
17. 17Bryson, S. et al. (2021). The Occurrence of Rocky Habitable-zone Planets around Solar-like Stars from Kepler Data. The Astronomical Journal 161, 36. doi:10.3847/1538-3881/abc418
18. 18Léger, A. et al. (2009). Transiting exoplanets from the CoRoT space mission. VIII. CoRoT-7b: the first super-Earth with measured radius. Astronomy & Astrophysics 506, 287-302. doi:10.1051/0004-6361/200911933
19. 19Batalha, N. M. et al. (2011). Kepler's first rocky planet: Kepler-10b. The Astrophysical Journal 729, 27. doi:10.1088/0004-637x/729/1/27
20. 20Hu, R. et al. (2024). A secondary atmosphere on the rocky exoplanet 55 Cancri e. Nature 630, 609-612. doi:10.1038/s41586-024-07432-x
21. 21Cadieux, C. et al. (2024). New Mass and Radius Constraints on the LHS 1140 Planets: LHS 1140 b Is either a Temperate Mini-Neptune or a Water World. The Astrophysical Journal Letters 960, L3. doi:10.3847/2041-8213/ad1691
22. 22Cadieux, C. et al. (2024). Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS. The Astrophysical Journal Letters 970, L2. doi:10.3847/2041-8213/ad5afa
23. 23Cadieux, C. et al. (2022). TOI-1452 b: SPIRou and TESS Reveal a Super-Earth in a Temperate Orbit Transiting an M4 Dwarf. The Astronomical Journal 164, 96. doi:10.3847/1538-3881/ac7cea