Terrestrial-sized moons can exist around gas giant planets.
This is especially interesting for gas giant planets that occupy the habitable
zone around their host stars because terrestrial-sized moons around such
planets can support Earth-like conditions. A terrestrial-sized moon requires a
minimum mass of at least 0.1 to 0.2 Earth mass in order to hold on to an
atmosphere for billions of years. This is about 4 to 5 times more massive than
the largest moons in our Solar System and is comparable to the mass of Mars.
The largest moons in our Solar System presumably formed
through accretion of material in the circumplanetary disks around Jupiter and
Saturn. This poses a problem for forming a terrestrial-sized moon because there
is not enough material and/or accretion efficiency in the circumplanetary disk
around Jupiter or Saturn to form anything more massive than ~ 0.025 Earth mass.
The moons Callisto and Ganymede around Jupiter, and Titan around Saturn show
that a formation process via accretion in a circumplanetary disk is unlikely to
form a moon larger than 1/10,000th the mass of its host planet.
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Williams (2013) proposes that terrestrial-sized moons can
exist around gas giant planets through a formation process known as binary-exchange
capture. This process occurs when a binary comprising of two terrestrial-sized
objects is tidally disrupted during a close encounter with a gas giant planet.
One member of the binary gets ejected (escaping mass) while the other remains
behind as a moon (captured mass) around the gas giant planet. Binary-exchange
capture can only take place if the ratio of captured mass to escaping mass is
not too large.
An example of a binary exchange capture involves a
Jupiter-mass gas giant planet in orbit around a Sun-like star at a distance of
1 AU and a binary comprising of one object with the mass of Mars and one object
with the mass of Mercury. If the binary approaches as close as ~ 5 planetary
radii from the gas giant planet, the object with the mass of Mars remains
behind as a moon (captured mass) while the object with the mass of Mercury gets
ejected (escaping mass). In this scenario, the ratio of captured mass to
escaping mass is ~ 2:1.
However, if the binary is comprised of one object with the
mass of Earth and one object with the mass of Mars, then capture of the
Earth-mass object as moon around the gas giant planet is not possible because
the binary’s mass ratio of ~ 10:1 is simply too large. Nevertheless, if the
distance of the Jupiter-mass gas giant planet is doubled to 2 AU, binary
exchange capture of the Earth-mass object as moon around the gas giant planet
becomes possible.
In our Solar System, Neptune’s moon Triton is believed to
have been placed into orbit around Neptune via binary exchange capture. This is
because Triton’s orbit around Neptune is retrograde, which means it orbits in
the opposite direction to the planet’s spin and could not have accreted out
from a circumplanetary disk around Neptune. Although Triton in nowhere as
massive as a terrestrial-sized moon, its existence does supports binary
exchange capture as a viable mechanism.
Reference:
Williams (April 2013), “Capture of Terrestrial-Sized Moons
by Gas Giant Planets”, Astrobiology, vol. 13, issue 4, pp. 315-323