The Evolving Habitable Zone of Brown Dwarfs

For a period of time, brown dwarfs indeed possess a habitable zone where conditions might support the existence of liquid water. This article delves into the changes in these zones over the age of a brown dwarf, with a focus on a 70 Jupiter mass brown dwarf (0.0668 solar masses).

Understanding Brown Dwarfs

Brown dwarfs span a mass spectrum from 13 to 74 Jupiter masses. A mass of 70 Jupiter masses, equivalent to 0.0668 solar masses, is a significant benchmark for studying their habitable zones. With this mass, the brown dwarf has a radius of 0.775760 Jupiter radii (55,461 kilometers), an average density of 185,990 kilograms per cubic meter, and a Roche limit of 455,435 kilometers against a satellite with a density of 5,000 kilograms per cubic meter.

Evolution and Luminosity

At an age of 3.5 billion years, a 70 Jupiter mass brown dwarf exhibits an effective temperature of 1,396.5 Kelvin and a bolometric luminosity of 8.33608e21 watts. These characteristics define the potential for a space station to maintain a habitable temperature. Using a Bond albedo of 0.5 for matte-surfaced metal, the space station would have an equilibrium temperature of 255 K, similar to Earth's at an orbital distance of 588,091 kilometers (1.2913 times the Roche limit). The orbital period would be 8.3575 hours.

Over time, as the brown dwarf ages, so does its luminosity and effective temperature. At 5.19 billion years, the effective temperature drops to 1,229.2 Kelvin, and the luminosity decreases to 5.00285e21 watts. At this age, a space station with a Bond albedo of 0.5 would require an inner orbital distance of 455,588 kilometers (1.0003 times the Roche limit) to maintain equilibrium at 255 K. An outer distance of 671,760 kilometers (1.4750 times the Roche limit) would be needed for 210 K.

Adapting to the Changing Environment

As the brown dwarf continues to age, its habitable zone experiences significant changes. Techniques to adapt include altering the space station's surface albedo. Painting the outside with black paint to reduce the Bond albedo to 0.05 can help maintain a closer inner habitable zone. For example, at this albedo, a space station would have an equilibrium temperature of 255 K at an orbital distance of 627,984 kilometers (1.3789 times the Roche limit), and 210 K at 925,957 kilometers (2.0331 times the Roche limit).

Despite these adaptations, the Roche limit continues to encroach, ultimately leading to the loss of the inner habitable zone. At 8.52 billion years, the same parameters result in equilibrium temperatures at orbital distances of 455,463 kilometers (1.0001 times the Roche limit) for 255 K and 671,575 kilometers (1.4746 times the Roche limit) for 210 K. As the brown dwarf ages, the remaining habitable zone becomes both smaller and colder.

In summary, a 70 Jupiter mass brown dwarf can provide a habitable environment for extended periods, with strategic adaptations allowing human habitation for over 5 billion years. However, over tens of billions of years, the relentless expansion of the Roche limit will eventually render the remaining habitable zone uninhabitable, marking the end of any potential civilization on such a celestial body.

Further Reading

For a deeper understanding of similar topics, such as the habitable zones around white dwarfs, see the related article.