ASTR 330: The Solar System
Tuesday 11/07/06
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Picture credit: solarviews.com
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• All the planets we have discussed so far were and are visible to the unaided eye , and therefore have been known since ancient times.
• Two thousand years after the five ‘ wandering stars ’ were named by classical civilizations of the Mediterranean, the first new planet was discovered: Uranus .
• Uranus led astronomers directly to Neptune , and thence to Pluto : about one ‘planet’ a century.
• Since the first in 1992, a flood of Edgeworth-Kuiper Belt Objects have now been discovered culminating in 2003 with the largest to date: Eris, and object bigger than Pluto.
• But for now let’s look at the discovery of Uranus and Neptune.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Uranus was discovered by a professional musician and amateur astronomer, William Herschel (1738-1822), in Bath, England on the night of March 13, 1781 . (What was happening in North America at this time?)
• Herschel with his 6 in. home-made telescope had noticed a strange object during his methodical charting of the skies: a star which appeared circular , rather than point-like.
• Herschel was knighted by King George III for his discovery, and went on to become a great astronomer: discovering 2 moons of Saturn and
2 of Uranus , the true nature of binary stars , and the disk of the Milky Way, plus 1000s of galaxies .
• Herschel wanted to dedicate the star to his patron, but fortunately was over-ruled. Uranus was father to Saturn in classical mythology: who in turn was father to Jupiter – a nice touch.
Picture credit: Royal Astronomical Society
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Astronomers began to keenly observe the new planet , but soon noticed that it did not hold to its expected movement on the sky.
• By 1830 , Uranus was 0.004 degrees off course: more than 4 times the apparent size of the planet on the sky.
• The path of Uranus could not at all be fit by an elliptical orbit , a fact which seemed to defy Newton’s laws (and Kepler’s).
• As there was no reason to expect all planets except Uranus to obey
Newton’s gravity, there must be something else going on.
• The most likely explanation was a unseen, eighth planet having a gravitational effect on Uranus.
• Using a guessed distance of 39 AU (from numerology) several mathematicians began the arduous task of computing the eighth planet’s whereabouts.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Neptune alone of all the planets was tracked down by mathematics, not by observation.
• The first person to finish the calculation was John Couch Adams (1819-
1892), in September 1845 not long after his graduation in mathematics from Cambridge University.
• Adams gave his calculations to the director of
Cambridge Observatory , who didn’t follow through with the needed observations.
• Adams went on to become a distinguished mathematical astronomer, and is also renowned for his calculation that the Leonids meteor shower was due to the remains of a comet.
Dr Conor Nixon Fall 2006
Picture credit: St Andrews

ASTR 330: The Solar System
• Meanwhile in Europe, Urbain J J Le Verrier (below left) published his own calculations in June 1846 , and followed up in August that year by asking the German astronomer Johann Galle in Berlin to look for the new planet.
• Galle , armed with an accurate set of celestial tables, was able to find the quarry within one hour of searching, on the very first attempt, on September 23, 1846.
• The disk of the planet was too small to be resolved, but its motion as a wanderer with respect to the fixed stars was apparent.
Picture credit: St Andrews
• After some discussion, the planet was eventually named after Neptune , the Roman god of the ocean, which is well fitted by
Neptune’s color.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• The successful hunt for Neptune by analyzing the orbit of Uranus led in turn to a detailed study of the orbit of Neptune.
• Neptune’s orbit too seemed to show perturbations , which inspired
Percival Lowell , the Martian canal sketcher in Arizona, to devote much energy and telescope time to hunting for planet #9.
• Eventually, a 9 th planet was discovered, as a result of Lowell’s persistent campaigning, and at Lowell Observatory in 1930, Pluto was first observed. In fact, the discovery was fortuitous, and the supposed perturbations in the orbits of Neptune and Uranus were never there!
• In France, Le Verrier spent much of the remainder of his life analyzing perturbations in the orbit of Mercury, in the hope of locating an innermost planet, unseen against the glare of the Sun .
• The Mercurian motions in fact were real, but eventually explained by
Einstein’s General Relativity , not the hypothesized ‘Vulcan’.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• This figure shows the planets to scale. Neptune and Uranus , although considered gas giants due to density and composition , are really intermediate worlds in terms of size, between J&S and the terrestrials.
Picture credit: NASA
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Uranus and Neptune are very similar sizes: Uranus is slightly larger, but
Neptune is heavier, due to greater density .
Diameter (km)
Mass (Earth=1)
Density (g/cm
3
)
• Their spin period is intermediate between Jupiter and Saturn which are about 7 hrs less , and Mars and Earth which are about 7 hrs more .
Semi-major axis (AU)
Orbital Period (yrs)
Spin Period (hrs)
• However, Uranus rotates backwards compared to most other planets, with a spin axis inclined at 98 ° to the plane of the solar system – i.e. it is lying on its side.
Number of Moons
Uranus Neptune
51118
14
1.3
19.2
83.75
17.3r
27
49528
17
1.6
30.1
163.72
16.1
13
Dr Conor Nixon Fall 2006
Picture credit: NASA

ASTR 330: The Solar System
• Once again, let us consider planetary density as an insight into composition .
• Uranus and Neptune are smaller than Jupiter and
Saturn, so if they had the same composition , they should be less compressed , and have lower densities.
• In fact, Uranus has the same density as Jupiter, but Neptune is even higher. This tells us straight away that Uranus and Neptune are likely to contain greater proportions of heavy elements than Jupiter and Saturn.
• We believe that much of these planets is water ice and rock (oxygen and silicon being common elements). They are surrounded by a relatively thin layer of liquid and gaseous hydrogen , mixed with some other gases.
Picture credit: Kaufmann and Comins
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Model interiors of Neptune and Uranus are depicted below, in comparison with Saturn and Jupiter.
• Note that neither Uranus or Neptune is massive enough to have a liquid metallic hydrogen layer, unlike their larger counterparts.
Picture credit: Bennett et al
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• The Voyager 2 passes of Uranus (1986) and Neptune (1989) were the first detailed views we had of these worlds. Uranus was proved to be featureless at first glance, whereas Neptune had visible spots like Jupiter.
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Both planets appear green to blue in color. The high, white clouds we see on Saturn and Jupiter are mostly absent.
• Therefore, sunlight penetrates further into the atmosphere, before being back-scattered to space. Absorption by methane gives the characteristic blue-green color.
• The spectra of both planets show much stronger methane features than at J&S as expected: we expect a higher proportion of heavy elements and ices .
• Hence, N&U are depleted in hydrogen and helium relative to J&S. The molar gas abundances are about: both N&U.
H
2
(84%), He (14%) and CH
4
(2%) for
• The He/H ratio is more similar to the Sun for these worlds than for J&S: the lack of a metallic hydrogen core means that the helium ‘rain’ effect does not exist, to deplete the He concentration.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• At Jupiter and Saturn , we found very substantial heat excesses: the fact that the planets were radiating much more heat in the infrared than they were absorbing in the visible.
• This led us to the conclusion that Jupiter and Saturn are generating heat internally somehow: which turned out to be from helium precipitation.
• What about Uranus and Neptune ? In fact, Uranus has no heat excess, but Neptune, further from the Sun, does have an excess. This was noticed when the two planets showed the same temperature at 25 micron wavelength in the infrared , when Neptune should be colder.
• The conclusion is that Neptune, being slightly larger, is still radiating primordial heat from its formation. The high proportion of rock and ice to overall mass has led to a very slow cooling.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• The hydrogen-helium atmospheres of N&U are the natural product of the two-stage accretion process we have discussed before.
1. The core forms, of ice and rock , about 10-15 Earth masses.
2. Hydrogen and helium are captured as a secondary atmosphere, from the remaining gases in the solar nebula.
• At the same time as the capture (2), there is also outgassing from the core, mostly of N
2
, CO and CH
4
. However, the amounts are much smaller than the H
2 and He captured, so the atmospheres will be dominated by these lighter gases.
• Apparently, Neptune and Uranus were able to attract much less H
2 and He than Jupiter and Saturn.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• The atmospheres of N&U are much colder than J&S: about 73K (-200 °C) at the 1 bar level, so ammonia and water are completely frozen out. Also note that Neptune has a much stronger temperature inversion .
Picture credit: Eric Weisstein
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• On planets with a tropopause – a temperature inversion between the troposphere and stratosphere - we see emission lines of various gases in the infrared: methane and ethane for example.
• Neptune does have such an inversion, and hence allows detection of these gases spectroscopically . But Uranus lacks a significant inversion, and its IR spectrum is essentially blank.
• Note that the tropopause regions of both planets are so cold ( 55 K ) that only H
2
, He and Ne will not condense, although remain partially in the vapor state.
CO, N
2 and CH
4 can
• All these gases have been detected on Neptune so far except Ne
(which is hard to detect), and additionally HCN has been found.
• It is interesting that nitrogen and carbon are found as N
2 they are not completely hydrogenated to NH
3 and CH
4 and CO : i.e.
, as on J&S. A lack of suitable catalyst for the reactions is probably to blame.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• At Uranus and Neptune, we do not see the decametric ( 10s of meters ) or decimetric ( 10s of cm ) wavelength non-thermal radio emission which
Jupiter and Saturn produce.
• However, at shorter wavelengths we are able to probe the atmosphere at depth, and measure the temperature . As the wavelength increases , we see deeper into the atmosphere, so we can build up an idea of the temperature profile .
• Normally, atmospheres get warmer as we go deeper into the troposphere. This is true of Jupiter, Saturn and Neptune as confirmed by radio observations.
• But Uranus does not show an increasing temperature with depth. This is attributed to the fact that Uranus does not have an internal energy source , unlike the other three worlds, and so there is no significant convection taking place. This is comparable to the deep oceans on Earth.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• On Neptune , rotating with a axial tilt of 27 ° similar to the Earth, seasons occur as on Earth, except 165 times as long!
• But what about Uranus , rotating on its side? For 42 years it has one pole sunlit, and then for 42 years the other pole is sunlit.
• We expect some sort of global Hadley cell to arise, with warm gas rising at one pole and streaming to the other pole.
• However, this type of circulation does not in fact arise. The rapid rotation of
Uranus dominates the global circulation, and so Uranus exhibits a banded pattern parallel to latitude lines, like
Saturn and Jupiter.
Dr Conor Nixon Fall 2006
Picture credit: NASA. Voyager 2 false color image showing banding.

ASTR 330: The Solar System
• This false color image of Uranus was taken in
1998 with the HST in three near-infrared wavelengths.
• The orange-colored clouds near the bright band circle the planet at about 500 km/h .
• The rings of Uranus and 10 satellites are also visible.
Dr Conor Nixon Fall 2006
Picture credit: HST/Arizona

ASTR 330: The Solar System
• Unlike Uranus, Neptune has welldefined high altitude white clouds .
• Neptune also has dark clouds which mark the lower limit of the visible atmosphere.
• The Great Dark Spot (upper right) was a huge eddy the size of the
Earth, and similar to Jupiter’s GRS .
• As with the GRS, the GDS is a southern hemisphere anticyclone , with counterclockwise winds blowing around a high-pressure region. Recent images show that the GDS has disappeared.
Dr Conor Nixon Fall 2006
Picture credit: NASA/HST-APL/Nanjing Univ.

ASTR 330: The Solar System
•
2
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• To measure wind speeds , again we need to know two things:
1. How long a cloud feature takes to circle the planet.
2. The System III rotation period , by measuring the magnetic field, which gives us the rotation speed of the planet interior.
• For example, clouds on Uranus were observed to circle in 16 hours , and the
System III period was measured to be 17.2 hours . Taking the difference gives us a measurement of how fast the clouds are moving relative to the interior.
• Figure 14.11
in the textbook shows the variation of wind speeds with latitude on Uranus and Neptune.
• Note that Voyager measurements indicate that both the illuminated and unilluminated poles are at the same temperature, showing that heat is rapidly redistributed around the planet.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Our initial expectation, from our experience elsewhere in the solar system, was that magnetic fields are generally aligned close to the rotation axis of the planet.
• However, at Uranus, the magnetic field was found not only to be inclined at 60 ° to the rotation axis, but also to be offset from the rotational axis by one third of the planet’s radius.
Dr Conor Nixon Fall 2006
Picture credit: NASA

ASTR 330: The Solar System
• Uranus’s magnetosphere is similar in size to Saturn’s, but simpler in composition. It is composed almost entirely of electrons and protons derived from hydrogen escaping from the planet.
• The magnetotail stretches out 10s of planetary radii behind the planet, and also rotates like a corkscrew due to the inclination between magnetic and rotation axes.
• On the sunlit side of Uranus, there is an ultraviolet glow (the
‘ electroglow ’) emitted by escaping hydrogen atoms.
• Uranus also has aurorae like the other planets, produced by the collision of magnetospheric electrons and ions with the upper atmosphere.
• Due to the inclination difference, the aurorae occur near the equator.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• If we thought that perhaps Uranus’s offset and inclined magnetic field was perhaps due to the planet’s own inclined rotation axis, we would be wrong. The magnetic field of Neptune is in fact quite similar to Uranus.
• Neptune’s magnetic field is offset from center by half the planet’s radius , and inclined at 47 ° to the rotation axis, with a field strength about half that of Uranus.
Dr Conor Nixon Fall 2006
Picture credit: NASA

ASTR 330: The Solar System
• What causes these strange magnetic fields? We believe that an electrically conducting fluid is required, but it cannot be molten rock
(as on the Earth) or liquid metallic hydrogen (as in Saturn and
Jupiter).
• Our best guess for the conducting fluid is some sort of pressureionized ‘ice’: compounds of C, H, O and N ionized by high pressures.
• This could perhaps also explain the offset of the fields from the planet centers: as we expect the molten ice layer to be outside the rocky cores.
• Much more research is needed to obtain a better understanding of these fields and the planet interiors.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Picture credit: NASA
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Pluto was discovered on February 18 th 1930 at
Lowell Observatory by Clyde Tombaugh , a young
Kansan.
• It was named for the Greek god of the underworld, but coincidently, the first letters also honor Percival Lowell who first pursued it.
• Pluto’s moon Charon was discovered in 1978 , and named after the boatman who conveyed the dead across the Styx and into Hades.
• Charon is 1/8 the mass and 1/2 the diameter of its parent. Due this closeness in size, Pluto and
Charon are sometimes considered to be a double or binary planet system.
Dr Conor Nixon Fall 2006
Picture credit: Univ. Northern Iowa

ASTR 330: The Solar System
• Very, very little is known about Pluto, due to its distance from the
Sun ( semi-major axis 39.48 AU ), small size ( 2302 km diameter , 2/3 the size of the Moon), and the fact that no spacecraft has visited it.
• Pluto has other anomalies. It rotates in about 6.4 Earth days , longer than all planets except Venus and Mercury. Also, its elliptical orbit crosses Neptune’s.
• Pluto’s axial inclination of 112 ° also means that it rotates backwards, like Venus and Uranus.
• Was Pluto originally a moon of Neptune which somehow escaped?
In fact, Pluto and Neptune are in a resonance which prevents them from getting closer than 17 AU, making this possibility unlikely.
• Also, the fact that Pluto has its own moon argues for an independent formation.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Picture credit: NASA
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• From 1985 to 1991 Pluto and Charon lined up as an eclipsing binary system, as seen from Earth. This enabled us to better determine their masses and sizes .
• Charon orbits Pluto at just 20,000 km , and both planets are tidally locked , presently the same face to each other at all times.
• Pluto’s mass was uncertain until Charon was discovered, when Kepler’s laws could be applied.
• We now know that its density is 2.1 g/cm 3 , similar to Neptune’s moon
Triton.
Dr Conor Nixon Fall 2006
Picture credit: NASA/APL/HST

ASTR 330: The Solar System
• Pluto’s brightness was observed to change as it rotates, from about
0.3 to 0.5
in reflectivity.
• Infrared spectroscopy showed first the presence of methane ice , and then CO and N
2 ices as well, on Pluto’s surface.
• Charon is different, covered in water ice . Perhaps the energy of its formation event was sufficient to drive off more volatile gases.
• Pluto’s atmosphere was first observed in 1988 , as a dimming before disappearance during a stellar occultation .
• Calculations suggest that the atmosphere is probably 1-20x10 -6 bar of
N
2
, at a surface temperature of 35-40 K .
• Due to Pluto’s eccentric orbit around the Sun ( 30-50 AU ), the amount of solar heating changes by a factor 3 over its year, and hence the atmosphere will soon grow much colder and freeze out on the surface.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
1. Briefly describe how Neptune, Uranus and Pluto were discovered.
Which of the three was not found by accident?
2. What are the similarities and differences between Uranus and
Neptune, in terms of mass, size, rotation and orbit?
3. Are the interiors of Uranus and Neptune similar to Jupiter and Saturn?
4. Which of the two outer gas giants has more visible features? Which have internal heat sources?
5. What is the main difference in the formation of Uranus and Neptune which led to a different composition from Saturn and Jupiter?
6. What gases are found in the atmospheres of U&N apart from H
2
He: why are they not fully hydrogenated?
and
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
7. Is the GDS on Neptune similar to the GRS on Jupiter? In what ways?
8. Which planet does not have a pronounced tropopause? Why?
9. Describe the magnetic fields of Uranus and Neptune. In what ways were they unexpected?
10. What could cause these magnetic fields?
11. Describe the orbital properties of the Pluto-Charon system.
12. What sort of surface and atmosphere might we expect to find on these very outer worlds (P&Ch).
13.
Which planets rotate ‘backwards’ relative to most of the solar system?
Dr Conor Nixon Fall 2006