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
- Where do comets originate?
- What does it mean to say that Pluto is the "largest known Kuiper
URANUS AND NEPTUNE: DISTANT TWINS
- Uranus: 19 AU, 15 Mearth, 4 Dearth,
1300 kg / m3.
- Neptune: 30 AU, 17 Mearth, 3.9 Dearth,
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?
- 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,
- 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
- First (third?) Kuiper Belt Object discovered by direct observation
- 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.
Go to A161 Home Page
Updated: 2005 May 31[dhw]