Table A.1
Model parameters.
| Parameter | Maximum a posteriori values with 16th and 84th percentiles | |||
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| Stellar parameters | ||||
| 2MASS Photometry | J = 9.763 ± 0.028, H = 9.135 ± 0.026, Ks = 8.899 ± 0.019 | |||
| Stellar mass, Ms [M⊙] | 0.359 ± 0.047 | |||
| Stellar radius, Rs [R⊙] | 0.411 ± 0.038 | |||
| Effective temperature, Teff [K] | 3457 ± 39 | |||
| Distance, d [pc] | 34 ± 4 | |||
| Rotation period, Prot [days] |
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|||
| Systemic velocity, γ0 [m s-1] | 653.7 ± 0.9• | |||
| Model 1 | Model 2 | Model 3 | Model 4 | |
| GP hyperparameters | ||||
| Covariance amplitude, a [m s-1] |
|
N.S.° | N.S. | − |
| Exponential timescale, λ [days] |
|
N.S. | N.S. | − |
| Coherence, Γ |
|
N.S. | N.S. | − |
| Periodic timescale, PGP [days] |
|
N.S. | N.S. | − |
| Additive jitter, s [m s-1] |
|
|
|
− |
| K2-18c | ||||
| Period, Pc [days] | 8.962 ± 0.008 | 8.964 ± 0.010 | 8.965 ± 0.010 |
|
| Time of inferior conjunction, T0,c [BJD-2 450 000] | 7264.55 ± 0.46 | 7264.55 ± 0.51 |
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| Radial velocity semi-amplitude, Kc [m s-1] | 4.63 ± 0.72 |
|
4.52 ± 0.82 |
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|
| Semi-major axis, ac [AU] | 0.060 ± 0.003 | 0.060 ± 0.003 | 0.060 ± 0.003 | 0.060 ± 0.003 |
| Eccentricity, ec | < 0.47∗ | – | – | – |
| Minimum planet mass, mp,csinic [M⊕] | 7.51 ± 1.33 |
|
7.33 ± 1.48 |
|
| Equilibrium temperature, Teq,c [K] | ||||
| Bond albedo of 0.3 | 363 ± 14 | 363 ± 14 | 363 ± 14 | 363 ± 14 |
| K2-18b | ||||
| Period, Pb [days] | 32.93963 ± 1.0 × 10-4 | 32.93962 ± 1.1 × 10-4 | 32.93961 ± 1.0 × 10-4 | 32.93960 ± 9.3 × 10-5 |
| Time of inferior conjunction, T0,b [BJD-2 450 000] | 7264.39157 ± 5.9 × 10-4 | 7264.39133 ± 6.4 × 10-4 |
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| Radial velocity semi-amplitude, Kb [m s-1] | 3.18 ± 0.71 |
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| Semi-major axis, ab [AU] | 0.143 ± 0.006 | 0.143 ± 0.006 | 0.143 ± 0.006 | 0.143 ± 0.006 |
| Eccentricity, eb | <0.43∗ | – | – | – |
| Planet mass, mp,b [M⊕]† | 7.96 ± 1.91 |
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| Planet density, ρp,b [gcm-3]‡ | 3.3 ± 1.2 |
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| Surface gravity, g [ms-2]‡ | 14.0 ± 4.3 |
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| Escape velocity, vesc,b [kms-1]‡ | 20.6 ± 2.7 |
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| Equilibrium temperature, Teq,b [K] | ||||
| Bond albedo of 0.3 | 235 ± 9 | 235 ± 9 | 235 ± 9 | 235 ± 9 |
| Model diagnostics | ||||
| Median lnℒ from Cross-Validation | −1.462 ± 0.013 | −1.574 ± 0.017 | −1.600 ± 0.018 | −1.548 ± 0.017 |
| lnℒ1−lnℒi | 0 | 0.112 ± 0.021 | 0.138 ± 0.022 | 0.086 ± 0.021 |
Notes. (•) from the Model 1 RV modelling. (◦) N.S. stands for “no solution” and occurs when the parameter’s marginalized posterior PDF is unconstrained by the data. (∗) upper limit based on the 99% confidence interval from the RV data analysis and conditioned on the dynamical stability constraints from Sect. 7. (†) assuming the measured orbital inclination of K2-18b from B17; 
deg. (‡) assuming the measured radius of K2-18b from B17; rp,b = 2.38 ± 0.22 R⊕. M⊙ = 1.988499 × 1030 kg, R⊙ = 6.955 × 108 m, M⊕ = 6.045898 × 1024 kg, R⊕ = 6.378137 × 106 m.
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