As a young object that is still contracting, GQ Lupi b still retain a fairly large radius and a hot temperature when compared to older objects. Cold substellar objects are predicted to have radii below , while GQ Lupi b is over double or triple this.
Mass In the early years following its discovery, the models of thermal evolution for substellar objects, used to infer a mass estimate for such objects based on their luminosities (or temperatures) and ages, were not calibrated to objects as young as GQ Lupi, which combined with uncertainties on the luminosity, made its mass very uncertain. Taking the age of the companion as 1.1 Myr and the luminosity as , the discovery paper derived a mass of based on the combination of three evolutionary models. One of these models, from Wuchterl & Tscharnuter (2003), were consistent with an object of just . This estimate assumed that the object formed in a
protoplanetary disk and was subsequently challenged as such a formation scenario would need at least a million years, in tension with the estimated age of the system. Janson
et al. (2006) derived masses between based on GQ Lup b's luminosity, and between based on its
effective temperature of . Marois
et al. (2006) then derived masses of and based on different models and an updated luminosity of , hence a range of was adopted. Seifahrt
et al. (2007) came up with a mass derived independently from evolutionary models, based on the relation of
surface gravity, radius, and mass. They arrived at a value of , but due to the uncertainty of both radius and surface gravity, this value is also uncertain, and could be as high as and as low as . The upper range was narrowed down to based on comparisons to the brown dwarf
2M0535-05 B, which it was thought to be coeval with GQ Lupi B at 1
Myr and had its mass () and radius () measured independently from the models. Based on the same calculations of Seifahrt
et al. (2007) and an updated radius, Neuhäuser
et al. (2008) derived a nominal value of with a lower value of a and a upper value constrained by the comparisons with 2M0535-05 B. The nominal value agreed with the expectations of evolutionary models, but the uncertainties on such models still allowed for highly different values. The mass estimates from this epoch were thus consistent with GQ Lupi b being either a lower-mass
gas giant exoplanet (which would give it the designation ) or a higher-mass
brown dwarf (which would give it the designation ). In the years that followed, updated and more robust evolutionary models, precise measurements of the distance by the
Gaia spacecraft resulting in accurate luminosity measurements, and revised age estimates for the system, helped narrowing down the range of estimated masses. Stolker
et al. (2021) came up with a mass of based on a revised
absolute magnitude with the Gaia distance, an assumed age of 3 Myr that was consistent with a range of
Myr derived in 2012, and AMES-Dusty evolutionary models from 2000. While this value was considered uncertain, other parameters derived with the same model showed agreement with those from atmospheric models. Using an
effective temperature of and an age between 2 and 5 Myr, Demars
et al. (2023) obtained a mass of based on ATMO evolutionary tracks (2015). Using the updated luminosity from the Gaia distance () and an age of Myr, Xuan
et al. (2024) derived a mass of based on four evolutionary models which "have been shown to reasonably reproduce the bulk properties of benchmark substellar companions with dynamical masses". Kammerer
et al. (2025), adopting an age of Myr and a luminosity of derived from a single passband, obtained from evolutionary models. While improvements in the evolutionary models and the inclusion of brown dwarf binaries increased the robustness of the mass estimates, all of them are based on differing age estimates and the models still remain largely uncalibrated at the youngest ages and planetary masses. This is well visible in the case of GQ Lup b in differences of up to about 40% in best mass estimates like ~ versus , both published in 2024, partially remaining large error ranges like 10–40 , as well as higher mass despite lower age estimate ( at Myr) versus lower mass despite higher age estimate ( at Myr).
Radius The first estimate of the radius was performed by the discovery paper. By comparing their low-resolution spectrum to atmosphere models and assuming a distance of based on membership to the Lupus I cloud, they found a best-fit radius of . In 2006, Marois
et al. derived a radius of by comparing the obtained spectrum to model atmospheres and assuming the same distance. In 2007, Seifahrt
et al. found a radius of based on the estimated luminosity and
effective temperature, while Neuhäuser
et al. (2008) refined it to based on a more precise luminosity. In 2012, Patience
et al. inferred radii between as a way to match the best-fitting model atmosphere to the observed brightness. Zhou
et al. (2014) arrived at an estimate of by scaling synthetic spectra to the
spectral energy distribution (SED). Stolker
et al. (2021) analysed the full spectral energy distribution of GQ Lupi b and obtained a radius of from an atmosphere model taking in account
interstellar extinction and the surrounding
protolunar disk. Demars
et al. (2023) obtained a mass of from their best-fit atmospheric model, but they note that evolutionary tracks would predict a smaller radius of . This could be due to evolutionary tracks not capturing the physics of GQ Lupi b such as
deuterium burning, but is more likely to be to an inaccurate estimate of the extinction in the atmospheric model. They note this discrepancy is not unique to GQ Lupi b and might indicate that it is a close binary of nearly identical components in terms of luminosity and temperature, which would result in a radius of for each and agree with evolutionary tracks. Radial velocity measurements have not detected such a companion. Cugno
et al. (2024) found radii of and , by analysing the spectral energy distribution of the companion with the best-fitting atmospheric models that take in account the contributions of the circumplanetary disk, which are stronger at longer wavelengths, and
accretion. Xuan
et al. (2024) obtained a radius of based on evolutionary models adopting a system age of
Myr and a luminosity of .
Effective temperature The first temperature measurement was performed by the discovery paper by comparing the spectrum to model atmospheres, which resulted in . Marois
et al. (2006) used the same technique and obtained with their spectra. The same was done by Seifahrt
et al. (2007) who obtained . Stolker
et al. (2021) obtained by comparing the full
spectral energy distribution (SED) of GQ Lupi b to an atmosphere model taking in account
interstellar extinction and the surrounding
disk. Demars
et al. (2023) obtained from their best-fit atmospheric model. Cugno
et al. (2024) obtained and by comparing the SED to the best-fitting atmospheric models that take in account the surrounding disk and
accretion. ==Formation==