Planet Building Part 4
The Jovian Problem
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A Problem for the Solar Nebula
Theory (SNT)
• New information about the star formation
process makes it difficult to explain the
formation of the Jovian planets.
• The new information – gas and dust disks
around newborn stars do not last very long.
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Jovian Problem
With our greater
“observational
powers,” we have
discovered that
gas and dust disks,
like this one in the
Orion planetforming region, do
not last very long.
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Jovian Problem
• As it turns out, the rate of evaporation of the
gas and dust disks is faster than the rate at
which a Jovian planet could be formed by the
SNT.
– The SNT, if you recall, depends on a combination
of condensation, accretion, and gravitational
collapse.
– This is a major challenge to the SNT.
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Jovian Problem
• There are indications that the disks cannot last
longer than about 7 million years (which is an
extraordinarily short amount of time – some
evaporate in around 100,000 years).
– The disks are evaporated by intense ultraviolet
radiation.
– In addition, the outer layers are possibly
“stripped” away by gravitational influences of
other stars also be formed.
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Jovian Problem – Direct Collapse
• According to the SNT, Jovian planets should be
rare, though we know they are common (in
our SS and in others).
• To solve the Jovian problem, astronomers
have posited that Jovian formation was from
direct collapse.
– In other words, the condensation and accretion
stages were skipped.
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Jovian Problem – Direct Collapse
Artistic rendition of
what gravitational
collapse might look
like.
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Jovian Problem
• How have they arrived at the direct collapse
hypothesis.
– They have built mathematical models of solar
nebulae, which have been run on super
computers.
– The programs rely on observed data (as all
veritable models should) and take weeks to run
the algorithms.
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Jovian Problem
– The results of the runs show that the rotating gas
and dust of a solar nebula could have become
unstable and formed Jovian planets by direct
collapse.
– Instead of forming a dense core through
condensation and accretion of sold materials, the
formation of Jovian planets would have jumped
immediately to the gravitational collapse stage
• As a reminder, gravitational collapse is the rapid
accumulation of large amounts of in-falling gas from
the solar nebula.
9
Jovian Problem – Uranus and
Neptune
• If the direct collapse is true, that would help
to explain a puzzle about the formation of
Uranus and Neptune.
– Both planets are so far from the sun that accretion
could not have built them rapidly.
– The solar nebula’s gas and dust would have been
sparse that far out.
– Furthermore, Neptune and Uranus orbit so slowly
they would not have swept up material very
rapidly.
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Jovian Problem – Uranus and
Neptune
• Conventional view of the formation of Uranus
and Neptune is they grew by accretion very
slowly.
– Their growth was so slow that they never became
quite massive enough to begin accelerated growth
by gravitational collapse.
– The SNT has never adequately explained how the
got to be the size they are – it has become clear
that they could not have started growing by
accretion so far from the sun.
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Jovian Problem – Uranus and
Neptune
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Jovian Problem – Uranus and
Neptune
• One hypothesis is that Uranus and Neptune
began growing in the region of Jupiter and
Saturn and then somehow were shifted
outward by gravitational interactions.
– This seems to be a rather complex explanation.
– Remember, in science, the more complex an
explanation, the less likely it is true.
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