LECTURE 20: THE OUTER SOLAR SYSTEM

• How do Uranus and Neptune differ from Jupiter and Saturn?
• How do we determine Pluto's mass? How does it compare to that of other planets?
• Where do comets originate?
• What does it mean to say that Pluto is the "largest known Kuiper Belt Object"?

URANUS AND NEPTUNE: DISTANT TWINS

• Uranus: 19 AU, 15 M_earth, 4 D_earth, 1300 kg / m³.
• Neptune: 30 AU, 17 M_earth, 3.9 D_earth, 1100 kg / m³.
• Spectra show outer layers mainly hydrogen and helium, like Jupiter and Saturn.
• But less massive than Saturn, so should be lower density if had same composition and structure.
• Must have larger rock/ice cores, hydrogen/helium only in outer layers.
• Details of structure uncertain.
• Neptune generates heat by contraction; Uranus doesn't. Neptune shows more atmospheric activity.
• Both planets have thin, dark rings.
• Both have numerous small moons (20 km - 1500 km diameter).
• Neptune has one large moon, Triton, which orbits opposite to Neptune's rotation. Probably captured.
• Uranus rotates on its side (poles point to Sun), unique in solar system. Moons orbit same way. Giant impact?

PLUTO

• Discovered as moving object in extensive telescope survey.
• Very eccentric orbit. Crosses Neptune's orbit but never hits, orbiting exactly twice for every three Neptune orbits.
• Moon, Charon, discovered in 1978.
• Eclipses allowed measurements of diameters, separation, masses.
• Orbit now better measured with images from Hubble Space Telescope.
• Pluto 2300 km across, Charon 1200 km, separation 20,000 km.
• Closer together than any other planet/moon pair. Really a binary system of two similar objects.
• Combined mass only 1/5 of Earth's Moon.
• Density ~ 2000 kg / m³, implies rock/ice composition.
• More like Neptune's captured moon, Triton, than any other planet.

COMETS AND THE OORT CLOUD

• Comets are "dirty snowballs," few km across, largely ice, perhaps with rocky core, on elongated elliptical orbits.
• As comet approaches Sun, ice evaporates, dust escapes.
• Pressure of Sun's light and "solar wind" (particles streaming from Sun) stretch evaporating gas and dust into long, reflective tail.
• Long period comets, periods of millions of years, have semi-major axis of ~ 1000-50,000 AU.
• Come from distant, spherical reservoir surrounding Sun, called the Oort Cloud.
• Deflected into inner solar system by gravity of passing stars (nearest star today is 200,000 AU from Sun).
• Asteroids and comets are debris left over from early solar system. Oort cloud is material kicked out of the inner solar system by gravitational interactions with Jupiter.

THE KUIPER BELT

• Short period comets, periods of 20 - 200 years, orbit in about the same plane as the planets.
• In 1950s, Gerard Kuiper proposed that the "reservoir" of short period comets is a belt of debris beyond Neptune.
• Kuiper belt comets are deflected into inner solar system by Neptune's gravity.
• First (third?) Kuiper Belt Object discovered by direct observation in 1992.
• Hundreds since.
• Could be others as large as Pluto, not yet discovered.
• Now seems that Pluto and Charon are probably just the largest Kuiper Belt Objects found to date.