The Solirad (So) as a Convenient Unit for Quoting Astronomical Irradiances for Planetary Insolations and Exoplanetary Instellations

TL;DR

The paper tackles the inconsistency in quoting astronomical irradiances across subfields by introducing a dedicated unit, the solirad (So). It proposes anchoring So to the IAU's nominal total solar irradiance ${S}^{\rm N}_{\odot}=1361$ W m^-2 and discusses naming, usage, and practical conversions to SI and bolometric magnitudes. The key contributions include defining So, rationalizing a concise name, and illustrating usage through examples and standard conversions (e.g., $1\ \mathrm{W\,m^{-2}}\approx7.34754\times10^{-4}\ \mathrm{So}$). The practical impact is improved clarity and consistency in reporting planetary insolation and instellation values, facilitating cross-field communication and habitability analyses.

Abstract

Measurements of physical parameters for stars and (exo)planets are often quoted in units normalized to the Sun and/or Earth. The nominal total solar irradiance, ${S}^{\rm N}_{\odot}$, while based on a current best estimate with uncertainties, was adopted to be an exact reference value of 1361 W m$^{-2}$ by IAU 2015 Resolution B3, corresponding to ``the mean total electromagnetic energy from the Sun, integrated over all wavelengths, incident per unit area per unit time at distance 1 au''. In the planetary and exoplanetary science literature, the units employed for ``flux'', ``insolation'', ``instellation'', etc., are often cumbersome or inconsistent. To simplify the quoting of irradiance units for astronomical applications, we introduce the portmanteau solirad, short for solar irradiance, as an abbreviated version of the longer IAU term ``nominal total solar irradiance''. The solirad (So) is a unit of irradiance, where 1 solirad = 1 So = 1361 W m$^{-2}$, equivalent to the IAU nominal total solar irradiance, and to an apparent bolometric magnitude of $m_{bol}$ = -26.832 mag (per IAU 2015 Resolution B2).