Download Introduction to Astronomy, Lecture 7

Introduction to
Astrophysics
Lecture 6: Inventory of the Solar System
This shows the planets to scale
(though obviously not the orbit sizes to scale!!)
Basic facts
 There are
known planets, plus comets and asteroids.
 99.8% of the mass of the Solar System is in the Sun.
 Jupiter is more massive than the other planets put together.
 Only planets out to Saturn were known in ancient times, and
Pluto was not discovered until 1930.
 The planets are usually divided into the terrestrial planets
(Mercury, Venus, Earth, Mars) and the giant planets (Jupiter,
Saturn, Uranus and Neptune). Pluto is a rogue which doesn’t
fit this classification …
… and it’s not the only one!
Mercury
The closest planet to the Sun, with an
orbital radius about 40% of the
Earth’s and a mass about one
twentieth of the Earth’s.
It is heavily cratered and geologically
inactive.
It used to be believed that its rotation
was locked to its orbit around the
Sun, like our moon with one face
always pointing towards the Sun. In
1965 this was established not to be
true; the rotation period is two-thirds
the orbital one.
Venus
Venus is often thought of as
the sister planet of the Earth,
due to its similar size.
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However, it is a victim of a
runaway greenhouse effect
with a surface temperature
around 450 Celsius.
Magellan probe radar image animation
Earth
You know this one quite well ...
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One of the peculiarities of
the Earth is that its moon
is so large; only Pluto has
a relatively bigger moon.
Mars
The most famous planet,
thought by many to be the
mostly likely planet (other
than the Earth!) to host
life.
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Movie created from Viking probe images.
Mars has two small
moons, Phobos and
Deimos, which are
basically just lumps of
rock about 10 km
across.
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Jupiter
The largest planet in the solar
system. It is mostly gaseous,
with an inner shell of metallic
hydrogen and a core of ice and
rock.
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Sometimes said to be a failed
star, but it would need to be at
least ten times heavier to burn.
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The great red spot
is a storm which
has persevered for
at least 300 years!
Jupiter’s moons
Jupiter is famous for its moons.
There are four large ones
discovered by Galileo, plus at
least 59 smaller ones (most of
which were found only in the
last few years).
Ganymede is the largest moon
in the solar system, just beating
Saturn’s moon Titan.
Jupiter also has a modest
system of three or four
discernible rings.
Saturn
Everyone knows of Saturn
because of its rings. It also
has 48 recognised and
named satellites,
including the large moon
Titan which has its own
atmosphere.
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It is currently under intensive
scrutiny by the Cassini satellite,
which was launched in 1997 and
arrived there in July last year
after a complex series of fly-bys.
Shadows cast by Saturn’s
rings, and its moon Tethys
The cratered
icy surface of
Tethys
Uranus
Uranus has 27 listed moons,
and also has quite a substantial
ring system.
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Uranus has an unusual rotation,
where the axis is almost in the plane
of its orbit. It therefore appears to
rotate `on its side’.
Neptune
Not too much is known
about Neptune, though it
is the third most massive
planet and has at least 13
moons including the large
moon Triton. It too has a
modest ring system.
It was discovered in 1846
after its position was
predicted from its
influence on Uranus.
Pluto
Pluto is the rogue planet which fails
to fit into the classification. It has
several oddities. Its orbit is by far
the most eccentric of the planets,
and indeed it was closer to the Sun
than Neptune for 20 years until
February 1999 (something that
won’t happen again in our lifetime).
Its orbit is also at a much higher
angle of inclination to the plane of
the solar system than the other
planets, 17 degrees.
Its moon Charon is over half
its diameter, and was only
discovered in 1978. It’s the
only case in the solar system
where the centre of mass is
outside the parent object, and
argues against Pluto being a
lost moon of Neptune.
Miscellaneous `junk’
Asteroids: For the most part located between the orbits of Mars
and Jupiter. 10,000 asteroids are classified, and it is
believed that there are at least 100,000. Ceres is the
largest, and only Vesta is naked-eye visible. Many have
orbits which cross the Earth’s orbit.
Comets: Basically icy rocks from the outer
solar system whose disturbed orbits
send them into the inner solar system
to burn up. Some are seen many times,
such as the famous 76 year Halley’s comet.
Comet Kohoutek
Kuiper Belt objects: A swarm of minor bodies orbiting the Sun in
the outer solar system. The first such object was
discovered in 1992, and there are probably at least tens of
thousands of them.
Quaoar and Sedna:
new planets?
Quaoar is a Kuiper belt object discovered by Trujillo and
Brown in 2002 with the Palomar Telescope.
It orbits outside Pluto and was the largest Solar System object
discovered since Pluto in 1930. Its diameter is about 1300km
(half the size of Pluto), and it is on a very circular orbit
currently one billion miles outside Pluto.
Sedna is a similar object that is even further away, and takes
over 10,000 years to orbit the Sun. It was discovered in 2004
by the same astronomers.
2003UB313, aka Xena
Earlier this year, the
inevitable finally happened;
a Kuiper-belt object was
found which is bigger than
Pluto. It even has its own
moon! Its orbital period is
560 years on a highlyinclined orbit.
Although colloquially
known as Xena, it is called
2003UB313 until an official
name is decided.
Xena and its moon Gabrielle,
imaged by the Keck telescope.
Quaoar, Sedna and 2003UB313:
new planets?
So will they be called new planets? The International Astronomical Union (IAU)
is currently in a tizz trying to decide. Apparently under consideration are …
 Pretend it all hasn’t happened and
there are still nine planets.
 Demote Pluto to a minor planet and
henceforth have an eight-planet solar
system.
 Give Pluto an honorary title as a
`historical planet’.
 Define some criterion for inclusion as
`planets’ and accept all newcomers to
give a solar system of who-knows-how-many
planets.