Modeling Proxima Centauri b with ReflectX and Picaso/Virga
Can we distinguish Proxima Centauri b atmosphere cases with ELTs like GMT/GMagAO-X?
Known Prox Cen/ Prox Cen b parameters:
Param |
Value |
Ref |
---|---|---|
semi-major axis |
0.048 au |
Faria et al. 2022 |
period |
11.19 d |
Faria et al. 2022 |
eccentricity |
0.02 |
Faria et al. 2022 |
Mp*sin(incl) |
1.07 Mearth |
Faria et al. 2022 |
equil. temp. |
230 K |
Anglada-Escude et al. 2016 |
Star Teff |
3000 K |
Approx value from several refs |
Star radius |
0.14 Rsun |
Faria et al. 2022 |
Star SpT |
M5.5V |
Faria et al. 2022 |
Star log(Luminosity) |
-2.8 +0.1/-0.2 |
Faria et al. 2022 |
log(g) |
5.2 cm s^-2 |
Faria et al. 2022 |
metallicity |
0.21 dex |
Schlaufman and Laughlin 2012 |
No Atmosphere Models
The first case is an airless rocky planet. In this case, we will simply observe the star’s spectrum reflecting from the surface, with a planet-star contrast determined by the albedo of the planet’s surface composition. We used Eqn 1 from Cahoy et al. 2010 for the contrast of a given Lambertian sphere with albedo \(A_g\) , radius \(R_p\) , separation \(\rho\) , and phase angle \(\alpha\)
Since Prox Cen b is an RV detected planet, the orbital inclination is unconstrained. The value of orbital inclination will affect the true mass of the planet, and consequently the radius, and also the phase angle as a function of orbit phase.
The figure on the left shows the observer’s phase angle as a function of orbital phase (expressed in degrees, where 0 degrees is the inferior conjunction and 360 degrees is one complete orbit) as a function of five values of orbital inclinations.
Each inclination will correspond to a different true mass of the planet and thus different radius. We computed mass/radius for a range of inclinations. To estimate radius we used an emperical mass-radius relation.
Incl (deg) |
Mass (Mearth) |
Radius (Rearth) |
---|---|---|
10 |
6.2 |
2.1 |
20 |
3.1 |
1.46 |
30 |
2.1 |
1.30 |
45 |
1.5 |
1.15 |
60 |
1.2 |
1.07 |
70 |
1.1 |
1.04 |
80 |
1.0 |
1.0 |
Placing these four Mass/Radii on a density plot:
The four above estimated mass/radii are plotted as the black triangles. The solid curves represent theoretical density curves from Zeng & Sasselov 2013 (downloaded from Harvard CfA) for airless planets of varying compositions: pure iron (100% Fe), Earth-like rocky (32.5% Fe, 67.5% MgSiO3), pure rock (100% MgSiO3), pure water (100% H2O), and 50% H2O 50% Earth-like rocky core. The red dashed lines show models of an Earth-like rocky planet with varying percent H2 envelope by mass. We see that the three least massive planets fall nicely on the Earth-like density line, while the most massive is consistent with an Earth-like planet with a 1% H2 envelope. We did not estimate uncertainties on the Prox Cen b densnity estimates, so this analysis is used as a rough estimate of what is likely to be found on this planet.
Albedo
We used the wavelength-dependent albedos as a function of surface type for airless rocky planets from Hu et al. 2012 (excluding surfaces unlikely to exist at these temperatures). The plot below shows the relevant surface-type contrast curves for a planet with inclination = 60 deg and viewed at quadrature (phase = 90 deg), with broadband filters \(g^\prime\), \(r^\prime\), \(i^\prime\), \(z^\prime\), \(J\), and \(H\) shown below.
The metal-rich and ice-rich surfaces show the most variation across the observing bands, and will be distinguishable from the rest in color-color space. The filter combinations with the highest distinguishing power involve comparisons between optical and NIR bands. Below shows each surface type in \(J - H\) vs \(i^\prime - H\) color, in which metal-rich and ice-rich are separated from the rest by over half a magnitude.
A color-magnitude diagram provides even more distinguishing power. Below is shown an \(H - i^\prime\) vs \(z^\prime\) contrast CMD in which metal-rich is separated from clay/feldspathic/granitoid by 2 magnitudes, and the rest by over half a magnitude. Clay/feldspathic/granitoid would likely not be distinguishable in CMD or color-color space.