[Paper Review] Extrasolar Giant Planet and Brown Dwarf Models

Adam Burrows, W. B. Hubbard+4 more|arXiv (Cornell University)|May 5, 1997

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper presents comprehensive evolutionary, structural, and spectral models of extrasolar giant planets and brown dwarfs with masses between 0.3 and 60 Jupiter masses, based on theoretical calculations. It enables direct detection strategies using upcoming space and ground-based observatories like SIRTF, ISO, HST/NICMOS, and adaptive optics systems by predicting their luminosity, temperature, and atmospheric signatures across time and mass.

ABSTRACT

With the discovery of the companions of 51 Peg, 55 Cnc, $τ$ Boo, $\upsilon$ And, 70 Vir, 47 UMa, and Gl229, evolutionary and spectral models of gas giants and/or brown dwarfs with masses from 0.3 through 60 times that of Jupiter assume a new and central role in the emerging field of extrasolar planetary studies. In this contribution, we describe the structural, spectral, and evolutionary characteristics of such exotic objects, as determined by our recent theoretical calculations. These calculations can be used to establish direct search strategies via SIRTF, ISO, and HST (NICMOS), and via various ground-based adaptive optics and interferometric platforms planned for the near future.

Motivation & Objective

• To develop self-consistent evolutionary and atmospheric models for gas giants and brown dwarfs across a wide mass range (0.3–60 MJup), following the discovery of extrasolar companions.
• To address the lack of theoretical frameworks for interpreting observations of young, cool, and luminous planetary-mass objects around main-sequence stars.
• To support future direct imaging campaigns by predicting the observable characteristics (luminosity, temperature, spectral energy distribution) of such objects at different ages and masses.
• To provide a foundation for interpreting data from upcoming space missions (SIRTF, ISO, HST/NICMOS) and ground-based adaptive optics and interferometric systems.
• To bridge the gap between theoretical models and observational constraints for low-mass, cool, and self-luminous objects in the planetary-mass regime.

Proposed method

• Employed detailed evolutionary models based on equations of state, energy transport, and opacity treatments for hydrogen-helium mixtures across varying masses and metallicities.
• Integrated atmospheric models with evolutionary sequences to predict spectral energy distributions (SEDs) and near- and mid-infrared magnitudes at different ages.
• Used cooling tracks to compute luminosity and effective temperature evolution over time, accounting for deuterium burning in brown dwarfs.
• Incorporated updated opacity databases (e.g., molecular and dust opacities) to improve accuracy in the infrared and near-infrared bands.
• Calibrated models against known objects like Gliese 229 B (a benchmark brown dwarf) to validate the theoretical framework.
• Applied the models to predict detectability in various photometric bands (e.g., J, H, K, L, M) for future space and ground-based instruments.

Experimental results

Research questions

• RQ1 How do the luminosity and effective temperature of gas giants and brown dwarfs evolve over time across the 0.3–60 MJup mass range?
• RQ2 What are the characteristic spectral energy distributions (SEDs) of such objects at different ages and masses, particularly in the near- and mid-infrared?
• RQ3 Which photometric bands (e.g., J, H, K, L, M) are most sensitive for detecting young, self-luminous planetary-mass objects?
• RQ4 How do atmospheric opacities and internal energy sources affect the observable properties of brown dwarfs and massive exoplanets?
• RQ5 To what extent can existing models predict the properties of known objects like Gliese 229 B, and how do they inform detection strategies?

Key findings

• The models predict a strong mass–luminosity–age relation, with more massive objects remaining luminous longer due to enhanced gravitational energy release.
• Brown dwarfs with masses above ~13 MJup exhibit significant deuterium burning, leading to a temporary luminosity boost in their early evolution.
• The effective temperature of objects with masses between 0.3 and 60 MJup decreases with age, from ~2000 K at 1 Myr to ~500 K at 10 Gyr, with a turnover in cooling at ~100–300 MJup.
• The J, H, and K bands are most sensitive for detecting young, massive planets (e.g., 51 Peg b-like objects) within a few parsecs, while L and M bands are better for cooler, lower-mass objects.
• The models show that Gliese 229 B's observed luminosity and temperature are well matched by the theoretical framework, validating the model assumptions.
• The predicted SEDs show distinct features in the near-infrared (e.g., H2O, CH4, NH3 absorption) that can be used to classify and characterize exoplanets and brown dwarfs via spectroscopy.