The minimum temperature of protoplanetary disks around stars located in massive dense star clusters can exceed the temperature necessary for water ice to condense (~ 150 to 170 degrees Kelvin). Massive dense star clusters tend to form in single bursts of intense star formation where temperatures can remain too hot for water ice to condense on a timescale that is comparable to the planet formation timescale. This can inhibit the formation of gas giant planets as they only form in environments where the temperature is cool enough for water ice to condense. Irradiation experienced by a protoplanetary disk in a dense cluster environment is made up of flux from the stars in the cluster and flux from the central star. The minimum temperature of the protoplanetary disk is determined by the total flux it receives from stars in the cluster and if this component of flux is strong enough, it will result in a protoplanetary disk that is too hot for water ice to condense.
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A protoplanetary core with about 10
Earth-mass is required for the formation of a gas giant planet. This is because
only an object that massive is able to initiate the runaway accretion of
hydrogen and helium from the protoplanetary disk to form a gas giant planet. In
a typical protoplanetary disk where the temperature is cool enough for water
ice to condense, the mass of all condensables is a factor of a few times higher
than the mass of all rocky material. If the temperature is too high for water
ice to condense, the protoplanetary disk will lack the surface mass density
required for sufficient material to accrete and form a 10 Earth-mass
protoplanetary core. Since the presence of a large amount of condensables is an
essential requirement for gas giant planets to form, protoplanetary disks that
are too hot for water to condense are expected to form only rocky terrestrial
planets where they too are likely to be devoid of water. As a result, stars
that form in massive dense star clusters may be devoid of gas giant planets and
habitable planets.
There are a number of places which can
have cluster environments that are massive and dense enough to keep
temperatures above the water ice condensation temperature. Examples of such
places include nuclear star clusters and the cores of globular clusters. The
formation of stars in such environments is an exception rather than the norm.
Searches for planets around stars in these places should turn up a paucity of
gas giant planets if temperatures during their formative periods were high
enough to prevent the condensation of water ice in protoplanetary disks around
the clusters' stars. In fact, searches for gas giant planets around stars in
the dense core of a globular cluster named 47 Tucanae turned up zero planets
even though 10 to 15 of them were expected based on the known abundance of gas
giant planets discovered around stars nearer the Sun. Lastly, in the cores of
galaxies, accretion of material by supermassive black holes can put out
enormous amounts of energy which can inhibit the formation of gas giant planets
in protoplanetary disks around stars located up to great distances away.