Stonehenge to Hubble Unit 9
Stonehenge to Hubble Unit 9 and Beyond
Unit 9 – Solar System
Planetary Configurations
Inferior Planets
Mercury
Venus
Superior Planets
Mars
Jupiter
Saturn
Conjunction: sun and planet are in the same directions in our sky
Opposition: sun and planet are in opposite directions in our sky
The best time to view an inferior planet is when it is at greatest elongation
The best time to view a superior planet is when it is at opposition
Elongation angle: the angle between a planet and the sun as seen from the earth
If sun is setting and an inferior planet is visible the planet is east of the sun and is in a position of eastern elongation
If sun is rising and an inferior planet is visible the planet is west of the sun and is in a position of western elongation
Magnitude Scale: In the 2nd century BCE, Greek astronomer Hipparchus ranked the stars he could see into 6 groups according to their brightnesses. He called the brightest stars 1st magnitude and the faintest 6th magnitude. (large magnitude = faint star!)
This system has been modernized and adapted to measure apparent brightness (flux). It is called the magnitude scale.
A difference of 1 magnitude is a factor 2.5 in flux. A difference of 5 magnitudes is a factor 100 in flux (a 6th magnitude star appears 100 times dimmer than a 1st magnitude star).
This is an example of a logarithmic scale.
Discovery of Uranus: The five 'classic' planets are easy to see with the naked eye. Venus has a maximum magnitude of -5, Jupiter -3, and Mercury -2.6. Mars and Saturn at their faintest are still 1.8 and 1.5, brighter than most stars (Polaris is magnitude 2).
Uranus has a maximum magnitude of 5.3, so it is visible to the naked eye. It was included in several star catalogs (by Flamsteed in 1690 and Lemonnier in 1750). However, its orbital period is 84 years, so its motion against the background stars is very slow, and it was not recognized as a planet
Uranus was discovered by Sir Frederick William Herschel (1738-1822) German-born English astronomer. Discovered Uranus, two of its moons, and two moons of Saturn.
Trained as a musician, he played many instruments, was an accomplished organist and conductor, and wrote 24 symphonies. Herschel's music led to an interest in mathematics and lenses, and he started to build his own telescopes when he was in his thirties. By the 1780s, Herschel had become the best astronomer in England. He made precise catalogs of the sky because of his interest in 'nebulae' (more on this later). He often worked with his sister Caroline. Frederick William Herschel
Herschel’s 40 foot telescope D=1.2m, built in 1789 was the largest in the world for 50 years
Herschel first observed Uranus on March 13, 1781 while in the garden of his house in Bath. He immediately realized that it must be a new object, but initially reported it as a "comet"
Discovery of Neptune: The apparent magnitude of Neptune is ~ 8, so it is not visible to the naked eye. Its orbital period is 165 years. As with Uranus, several astronomers (including Galileo) saw Neptune with their telescopes, but did not identify it as a planet.
By the 1820s the orbit of Uranus was being mapped in great detail. The orbital path of Uranus showed clear deviations from the predictions of Newton's theory.
In the 1840s, John Adams and Urbain LeVerrier independently concluded that the deviations must be due to an undiscovered planet. Le Verrier sent a letter to Johann Galle in Berlin, who found the planet the next night (September 23, 1846), within 1 degree of the predicted position – the Scientific Method at work
Ceres, Pluto, Sedna, and Eris:
In 1801, Guiseppe Piazzi discovered Ceres, a small body (~900 km, ¼ of the Moon) between Mars and Jupiter. Initially hailed as a 'new planet', enthusiasm faded quickly in 1802 with the discovery of Pallas, a similar (but even smaller) body. Herschel proposed the name 'asteroids' for Ceres and Pallas, which belong to the asteroid belt.
After Neptune's discovery in 1846, the tally of planets was closed at 8. Then Clyde Tombaugh discovered Pluto in 1930 from Lowell Observatory in Arizona. But Pluto was strange – smaller than the Moon (but larger than Ceres), with e=0.2, and an inclined orbit.
In 2003, Mike Brown at Caltech discovered Sedna, a body similar to Pluto, but much further away from the Sun. In 2005, he discovered Eris, which is actually larger than Pluto. These discoveries led to the demotion of Pluto to the status of 'dwarf planet'.
Why is Pluto no longer considered a planet?
Size and distances to scale in the solar system
Pluto groups with either the rocky or gas giant planets depending on property.
So, it was known that pluto didn’t entirely fit in but it still orbited the sun in its own orbit and so it was a planet – at the turn of the century understanding of solar system changes
Found a family of pluto like objects- eris, haumea, makemake
The discovery of Eris in 2005 forced the International Astronomical Union to act on a definition of planet.
The issue was put to a vote at the IAU General Assembly meeting in Prague in August 2006. Prof. Turnshek of Pitt was at this meeting and voted with the majority – 237 in favor of making Pluto a dwarf planet, 157 against, and 17 abstentions.
“The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:
(1) A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. (2) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
The Solar System:
The dominant object in the Solar System is obviously the Sun. It holds 99.8 % of the mass, and generates all the available energy.
There are eight planets orbiting the Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), which have dozens of satellites. The smallest planet is Mercury, with R=2400 km (1/3 the Earth). Some moons of Jupiter and Saturn are larger than Mercury.
The inner planets (Mercury, Venus, Earth, and Mars) are small, dense and rocky, with thin atmospheres (except Mercury).
The outer planets (Jupiter, Saturn, Uranus, and Neptune) are gas giants, very large but with very low density. They have small solid cores and huge atmospheres.
Besides the planets, there are thousands of other bodies in the Solar System.
Minor planets orbit the Sun, but do not meet the requirements to be classified as planets (they do not clearly dominate their orbit). Subdivided into:
Dwarf planets, massive enough for gravity to make them round.
Asteroids, not massive enough to be round. Many asteroids (and some dwarf planets) reside in the asteroid belt between Mars and Jupiter.
Comets are small icy bodies that evaporate when they get close to the Sun, leading to distinct tail
Planets and dwarf planets have Satellites,
range from the tiny (Mars' Phobos has R~11 km) to the very large (Jupiter's Ganymede, with R~2600 km, is larger than the planet Mercury).
Jupiter's satellite Europa has an icy surface that could harbor an ocean of liquid water underneath. Scientists have speculated about the existence of life in Europa, perhaps around thermal vents in the ocean.
Saturn's satellite Titan has an atmosphere denser than Earth's, weather, and surface lakes of hydrocarbon compounds. There is also speculation that Titan could harbor life in these lakes, but with a surface temperature of 94º K (-180º C), it would be quite exotic.
Satellites…
The Moon is very large compared to Earth. It is by far the largest satellite by relative size, and the largest satellite of a rocky planet. As we have seen, it is largely responsible for tides, and the lucky coincidence in angular size with the Sun makes for spectacular eclipses.
It is thought that the Moon was formed by the impact of a large (Mars- sized) object on the proto-Earth during the formation of the Solar System. This would explain why no other rocky planets have satellites as large as the Moon.
Asteroids
Nobody knows exactly how many asteroids lurk in the Solar System. Most are concentrated in the asteroid belt between Mars and Jupiter.
Asteroid impacts have affected life on Earth in the past – and will do so again in the future. Small asteroids fall on Earth all the time (meteorites). Larger asteroids (~100 m) can leave impact craters.
The danger of a large asteroid impact is real, but it is very unlikely that it will happen in your lifetime.
Tunguska Meteor Strike Siberia 1908
Chicxulub event 65 million years ago
Chelyabinsk fireball
Comets:
Comets are small icy bodies that evaporate when they get close to the Sun, forming distinct tails
Most comets originate in the Oort cloud. The Oort cloud is VERY far away, about ¼ of the distance to the nearest star
Most comets reside in the Oort cloud, far beyond the orbit of Neptune At R~50,000 AU (1 ly), this is the outermost edge of the Solar System.
Occasionally, comets from the Oort cloud get perturbed and 'fall' to the inner solar system, where they eventually evaporate or fall into the Sun.
The nucleus appears tiny compared to the tail. The tail points away from the Sun due to radiation from the Sun pushing on the gas and dust in the tail
Recent comet exploration:
The Rosetta
Edmund Halley:
His study of comets at the turn of the eighteenth century helped predict the orbit of the comet that now bears his name
Meteor Showers:
are debris from comets
Convergent Point:
Some debris regions can be larger than others, so some passages make “meteor storms.” The Orionids, which peak on Oct 21, are from Halley’s comet
Solar System formation
Where did all this stuff come from?
Planets, satellites, asteroids and comets are essentially left over material from the formation of the Sun.
The origin of meteorites helps determine their composition and structure.
a. Primitive meteorites mix rock with metal flakes.
b. Processed meteorites are either metallic or rocky.
c. Carbonaceous meteorites are rich in carbon and volatiles
Aurorae
The Earth has a magnetic field with 'north' and 'south' poles.
Charged particles move along magnetic field lines that connect the magnetic poles.
The Sun produces lots of charged particles (mostly protons and electrons - this is called the 'solar wind').
When the particles in the solar wind reach the Earth, they are funneled towards the poles.
The charged particles glow when they interact with the atmosphere – this is an aurora.
Aurorae can be seen on any planet with a strong magnetic field and some kind of atmosphere