Brown dwarfs are objects whose masses fall below the limit
required for stable hydrogen fusion to occur in their cores. A brown dwarf
cools continuously from the gradual release of gravitational potential energy.
The timescale over which a brown dwarf cools depends on its mass, with more
massive brown dwarfs taking longer to cool. There is good evidence to suggest
that terrestrial-mass planets can form around brown dwarfs. One compelling
discovery is the object Kepler-42, a very low mass star with a trio of planets
measuring 0.7, 0.8 and 0.6 Earth-radii respectively. These planets orbit
Kepler-42 with periods no longer than 2 Earth days. The Kepler-42 system very
much resembles a scaled up version of Jupiter and its 4 large moons - Io,
Europa, Ganymede and Callisto. Since brown dwarfs cover the mass domain between
Jupiter-like planets and the lowest mass stars, it is reasonable to expect that
the process of forming planets around brown dwarfs is somewhat robust.
+-+1.jpg)
Figure 1: This artist’s concept compares the Kepler-42 (KOI-961)
planetary system to Jupiter and the largest four of its many moons. The planet
and moon orbits are drawn to the same scale. The relative sizes of the stars,
planets and moons have been increased for visibility. Credit: NASA/JPL-Caltech.
Because a brown dwarf is so much fainter than the Sun, a
terrestrial-mass planet has to orbit much closer in to receive as much warmth
as the Earth gets from the Sun. In fact, a habitable planet around a brown
dwarf is likely to have an orbital period of not more than a few Earth days.
The habitable zone of a brown dwarf is a region of space around a brown dwarf
where temperatures are neither too high nor too low for liquid water to exist
on the surface of a terrestrial-mass planet. As a brown dwarf cools and fades
over time, its habitable zone will likewise shrink inwards. A planet around a
brown dwarf may start out too hot to support life. But as the habitable zone
shrinks around a cooling brown dwarf, the planet will subsequently find itself
within the habitable zone where temperatures are just right. As the habitable
zone continues to shrink, the planet will eventually find itself exterior to
the habitable zone where temperatures become too cold for surface life.
+-+2.jpg)
The development of life or even complex life on a
terrestrial-mass planet around a brown dwarf is expected to depend a lot on the
amount of time the planet spends within the habitable zone or the
"Goldilocks zone". On Earth, it appears that the development of life
required at least 0.5 billion years, while the development of complex
multicellular life took perhaps ~3 billion years. As a result, a planet has to
reside long enough in the shrinking habitable zone of a brown dwarf in order
for life or even advanced lifeforms to develop. Andreeshchev and Scalo (2002)
show that a planet in a close-in orbit around a 0.07 solar-mass brown dwarf can
reside within the habitable zone for a duration of up to 10 billion years. The
duration of habitability decreases for a brown dwarf of lower mass. For
instance, a planet around a 0.04 solar-mass brown dwarf can remain habitable
for no longer than 4 billion years.
Nevertheless, a planet outside the habitable zone of a brown
dwarf does not necessarily mean that it cannot support life. One can imagine,
in a close-in orbit around a brown dwarf, a hot Venus-like planet with a think
and steaming atmosphere of water vapour. Although the planet's surface is too
hot for life, it may be possible for life to develop in the cool layers of the
planet's upper atmosphere. Over time, the brown dwarf cools, causing the
habitable zone to shrink inwards until the planet eventually finds itself
within it. Temperatures now become cool enough for the atmosphere to rain out,
forming oceans on the planet's surface and creating a planet that more
resembles the Earth. The planet can remain in this state for billions of years.
Slowly but surely, as the brown dwarf continues to cool, the
planet will finally find itself exterior to the habitable zone. Temperatures
now become sufficiently low for the oceans to freeze. Although the ocean
surface freezes, deeper down, the ocean can still be kept liquid due to the
feeble input of geothermal heat from the decay of radioisotopes in the planet's
interior. Eventually, even the deep ocean will start to freeze as the
geothermal heat flux from the decay of radioisotopes dwindles. However, if
other planets exist around the brown dwarf, they can provide sufficient
perturbation to keep the freezing planet in a slight non-circular orbit around
the brown dwarf. This will allow tidal heating to operate for eons, pumping
energy into the planet's interior and keeping the oceans liquid under a surface
layer of ice. The planet now more resembles Jupiter's moon Europa.
Reference:
Andreeshchev and Scalo, "Habitability of Brown Dwarf
Planets", Bioastronomy 2002: Life Among the Stars. IAU Symposium, Vol.
213, 2004