The Influence of Clouds and Deuterium-Burning on Brown Dwarf Habitable Zones

Abstract

To better understand the potential habitability of planets orbiting brown dwarfs, this work presents a new set of equilibrium temperature evolution tracks. Unlike most previous work that relied on analytic scaling relationships for brown dwarf luminosity evolution, we use the outputs of modern brown dwarf evolution models that account for the effects of deuterium burning, cloud formation and dissipation, and the most recent atmospheric opacities. While clouds are present, brown dwarfs cool more slowly than if they did not have clouds, allowing orbiting planets to remain in the habitable zone for millions of years longer than previously estimated. Similarly, we find that during the deuterium-burning phase of brown dwarfs, which also slows the evolution, planets at the same orbital radius but orbiting brown dwarfs of different masses can remain in the habitable zone for the same duration, creating deuterium "sweet spots" for habitability around brown dwarfs near the deuterium-burning limit. For example, at 0.01 AU a planet orbiting both a 0.012 solar mass and a 0.020 solar mass brown dwarf stays in the habitable zone for ~170 - 180 Myr because deuterium burning more strongly affects the cooling of lower-mass brown dwarfs. The size of the effect decreases with decreasing orbital radius, with larger orbital radii having a more pronounced deuterium burning influence. These effects are absent from the analytic cooling approximations used in prior studies of substellar habitable zones and are revealed by our application of modern substellar evolution models.

Figures (11)

• Figure 1: Brown dwarf evolution for different masses (lines) and metallicities (panels). The solid lines show the cloudless evolutionary models used in this work, which combine the Red Diamondback and Bobcat models Marley_20212025arXiv251008694D, as described in Section 2. The dashed lines are the analytical predictions using Equation \ref{['eq:1']} by Lingam_2020. The top, middle, and bottom panels are illustrating brown dwarfs of metallicities -0.5, +0.0, and +0.5 respectively.
• Figure 2: Evolution of brown dwarf effective temperature as in Figure \ref{['fig:loeb']}, but comparing cloudless evolution (top panel; Marley_2021) and cloudy evolution (bottom panel; morley2024sonorasubstellaratmospheremodels) evolution as described in the text. Models in both panels have [M/H] = $+0.0$.
• Figure 3: Planet equilibrium temperature versus log age of the host brown dwarf for different orbital radii. Panels in the left column present $T_{\rm eq}$ assuming the cloudless evolution while the right column is for the cloudy evolution. All of the planet temperatures were calculated orbiting a brown dwarf with a [M/H] = $+0.0$ and Bond albedo of 0.25.
• Figure 4: Planet equilibrium temperature versus log age of the host brown dwarf for difference orbital radii. The left panel is the amount of time planets orbiting cloudless brown dwarfs are $175 \le T_{\rm eq} \le 275\,\rm K$ K, or the HZ. The right panel is the same plot but with cloudy brown dwarf evolution. Each line corresponds to a log step orbital radius from 0.001 to 0.01. The solid, dashed, and dotted line correspond to a Bond albedo of 0.125, 0.25, and 0.50, respectively. Both cases have [M/H] = $+0.0$.
• Figure 5: Planet equilibrium temperature (calculated using Equation \ref{['eq:2']}) versus log age of the cloudy brown dwarf in Gyr. The left panel is [M/H] = $-0.5$ and the right panel is [M/H] = $+0.5$. The planets are orbiting at 0.05 AU.