Interiors of the Giant Planets 415
FIGURE 9 Density as a function of
normalized radius for three Neptune
and one Uranus interior models. The
solid, dashed, and dot-dashed curves
represent the range of possible
Neptune models. Note the wide
variety of acceptable core sizes,
ranging from a model with no core to
a model with a core extending to 20%
of Neptune’s radius. The dotted curve
represents a single Uranus model.
Because of Neptune’s greater mass, it
is everywhere denser than Uranus at
the same relative radius. The inset
shows the region of transition from a
hydrogen-rich atmosphere to the icy
mantle in more detail.Uranus and Neptune likely represent failed gas giant
planets. The time to accrete solid objects onto the growing
ice and rock planetary cores was much longer in the outer
solar nebula than at the orbital distances of Jupiter and
Saturn. Thus, Uranus and Neptune took longer to grow.
By the time the nebular gas was swept away, these planets
had not yet grown massive enough to capture substantial
amounts of hydrogen and helium gas from the nebula. Per-
haps if the nebular gas had persisted for a longer time,
Uranus and Neptune would have grown large enough to
complete the capture of a hydrogen–helium envelope. In
that case, these planets might now more closely resemble
the current Jupiter and Saturn.
5.5 Extrasolar Giant Planets
With current technology, we can learn very little of the
physical state of the over 200 planets (as of summer 2006)
that have been detected around other stars. The minimum
masses of planets are obtained by observing the motion
of the parent star induced by the gravitational tug of the
planet. But just knowing the minimum mass (since orbital
inclinations are unknown) does not tell us much about the
structure of a planet. However, there are 10 planets in or-
bit around other stars for which we have derived accurate
massesandradii. The radii can be measured if the extra-
solar planetary system has a favorable alignment, and the
planet passes in front of its parent star (a transit), blocking
a small fraction of the star’s light. The planet’s orbital incli-
nation is then constrained to be essentially edge on, so the
mass is then also known. As was shown in Fig. 6, with a deter-
mination of only the mass and radius of a planet, we can get
to a reasonable understanding of its interior composition.
New theoretical procedures will have to be developed
to understand the structure of giant planets that are 100–
200 times closer to their stars than Jupiter is to the Sun
and hence receive intense stellar irradiation. This irradia-
tion slows the contraction of a giant planet with time. Our
understanding is progressing, but the results are already
very surprising. Fig. 10 shows the radius and mass (withFIGURE 10 Planetary radius as a function of mass for the 10
transiting extra-solar giant planets, Jupiter, and Saturn. HD
209458b and HD 149026b, the transiting planets with the largest
and smallest radii, respectively, to date, as well as several others
of note, are labeled. Curves of constant bulk density
(mass/volume) are shown.