Chapter 13 of 15
Population
With thousands of planets, patterns appear. Two of them: a valley in the sizes of small planets, and the zone around each star where liquid water could last.
Small close-in planets come in two groups: rocky super-Earths below about 1.5 R⊕ and gas-wrapped sub-Neptunes around 2–3 R⊕, with fewer in between (Fulton et al. 2017 found at least a factor-of-two deficit at 1.5–2.0 R⊕ in a carefully measured sample). The leading explanations strip thin envelopes from small cores: photoevaporation (Owen & Wu 2017) or core-powered mass loss (Gupta & Schlichting 2019).
These are raw catalogue counts (planets with a stored radius and period < 100 d; 40 log bins from 0.5 to 20 Earth radii), not occurrence rates: nothing corrects for which planets are easier to find, and catalogue radii carry the stars' radius errors. The dip is shallower here than in Fulton et al.'s corrected, precise sample. pi Men c's stored radius is 1.87 R⊕.
The habitable zone is the range of starlight in which an Earth-like planet could keep liquid surface water. Its limits come from climate models as a flux that depends on the star's temperature (Kopparapu et al. 2014). Being in it is necessary, not sufficient: it says nothing about a planet's atmosphere, and many planets inside it are giants.
| Quantity | Value | From |
|---|---|---|
| Sun's conservative habitable zone | 0.95–1.68 AU | Kopparapu et al. 2014, Table 1 [calc] |
| pi Men's conservative habitable zone | 1.13–1.97 AU | L = 1.45 L☉ from exoplanet.eu R★ and Teff [calc] |
| Starlight on pi Men c | 322 × Earth's | a = 0.06702 AU: far inside, a hot planet [calc] |
| Catalogue planets in the conservative zone | 170 | of 4,680 with the needed stored values (exoplanet.eu) [calc] |
| …in the optimistic zone | 266 | [calc] |
| Earth-size planets in the zone per Sun-like star (η⊕) | 0.37–0.60 | conservative zone, 0.5–1.5 R⊕, with wide errors (Bryson et al. 2021) |