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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 planet-forming 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.

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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.