THE PLANETS (chapter 7 in the book; this is a general overview) This picture (below) depicts the orbits of the planets around the Sun to scale.

This Picture (below) depicts the size of the Planets and Sun to scale. The distance between them is NOT to scale, only their relative sizes.

When looking at the size of the planets and distance from the sun, one separates the planets between Terrestrial Planets and Jovian Planets (and also Dwarf Planets). The Terrestrial Planets:

- rocky with a hard surface, - small in size, - close to the Sun, - with a density of about 4 to 5 g/cc, or 4000 to 5000kg/cubic meter (5 times denser than water)

The Jovian Planets:

- gaseous, icy, with a denser core (they have a liquid surface) - larger in size, - far from the Sun, - with a density of about 1 (like water).

The dwarf planets are small planets, smaller than Mercury. Pluto used to be considered as a “planet” (the most distant planet from the Sun) and has been recently classified as a “dwarf planet”. The Orbital Period is the time it takes for the planet to go around the sun once, it corresponds to the year on Earth. The orbital period of a planet is the planet’s year. So Mercury’s year is only 88 days (88 times 24h). Jupiter’s year lasts 11.9 of Earth’s years. The closer a planet is to the Sun the shorter is the Orbital Period. The rotational period is the time it takes for the planet to spin on its axis once, it corresponds to the day on Earth. The rotational Period of a planet is the planet’s Day. So Mercury’s day last 59 Earth’s days. The day on Mars is 24.6hr and is almost the same as a day on Earth. The shorter is the rotational (spinning) period of a planet, the faster is spins on its axis. So Jupiter, the largest planet, has actually the fastest spin of all the planets.

The Nice Model (not in your book, it was covered in class)

In this model the migration of the giant planets from an initial compact configuration into their present

positions, long after the dissipation of the initial protoplanetary gas disk. In this way, it differs from

earlier models of the Solar System's formation. This planetary migration is used in dynamical simulations

of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar

System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies

including the Kuiper belt

Around 3.8 or 3.9 billions of years ago the Giant planets were closer to the Sun and further away from

the Sun was a huge amounts of small bodies (like debris, left-over from the solar systems) of the order

of 1000km in size and smaller. The giant planets moved outwards (“migration”) and in their movements

they perturbed the small bodies. Some of the small bodies fell inwards and produced a rain of bodies

impacting the planets which is responsible for the Late Heavy bombardment that formed so many

craters on the face of the Moon, Mercury, Mars, etc… Many of these small objects were pushed away

and can still be found beyond the orbit of Pluto. Some still remain today closer to the Sun and are

known as the Asteroid Belt between the orbit of Mars and Jupiter.

Early and Late Heavy Bombardments: (see chapter 10 and possibly 8 for

more on the late heavy bombardment)

The Early Heavy Bombardment was when the planets were forming and many impacts took place then like 4.5 billion years ago. Another heavy bombardment happened later, some 3.8-4.0 billion years ago, and it is known as the late heavy bombardment. The reason for the late heavy bombardment is the migration of the giant planets as explained in the Nice model. The Highlands on the Moon date from before the late heavy bombardment: 4.0-4.3 billion yrs The Maria date from the late heavy bombardment: from 3.1 to 3.8 billion yrs Every Planet will show sign of the Heavy bombardment if it has a hard surface that is not eroded by its

atmosphere (rains, wind, …) and renewed by plate tectonics.

The Terrestrial Planets Interior (chapter 7)

The core is made of Iron and Nickel mainly The Mantle is made of “rock” (silicates)

The Jovian Planets Interior

Jupiter and Saturn are the Gas Giants, made of gaseous Hydrogen and Helium mainly

Uranus and Neptune are the Ice Giants made mainly of a Mantle of water ice, ammonia ice and methane

ice.

They all have a rocky core (silicates) of about 10 earth masses.

Mercury (chapter 11) - The closest planet to the Sun - The smallest planet (since Pluto has been now classified as a dwarf planet) - The largest eccentric orbit - Because of its high temperature and low gravity (small mass) it has no atmosphere, as gas on

the surface of Mercury can very easily escape to space.

Mercury is just a little larger than the Moon The ratio of the rotation period around the sun (Mercury’s year) to the spin period (Mercury’s day) is exactly 3/2. While Mercury goes around the Sun once, it spins on its axis 1 and a half times.

Mercury’s orbit compared to the Earth’s orbit. Mercury as observed from the Earth. Mercury orbit is very eccentric, so Mercury distance to the Sun changes as Mercury orbits the Sun in 88 days.

Mercury can be viewed either directly after sun set or just before the sun rises, because it is so close to the Sun. Mercury is always in the vicinity of the Sun as seen from Earth. Mercury can never be seen in the middle of the night, it is always either after sunset or before sunrise above the horizon. With a telescope and a special filter, Mercury can be observed during the Day in the vicinity of the Sun.

MERCURY’S SURFACE: - It looks like the Moon - It is heavily cratered

Mercury has low lying PLAINS (lying about 2km lower than craters) similar to the Moon’s Maria, but not as dark as they have a lower iron content. The low lying plains are 3.8 billion yrs old, slightly older than on the Moon. They formed in a similar manner due to meteorites/asteroids impacting and cracking the surface and lava pouring out.

MERCURY HAS “SCARPS”, THESE are ~1km high and run for 100s of km near the equator (see full image). These formed as the planet cooled down, after the lava solidified (after the plains formed). Basically the planet “contracted” as its interior cooled down.

Mercury has also some faults, though not many.

THE CALORIS BASSIN IS A HUGE BASSIN THAT WAS CREATED WHEN THE PLANET WAS HIT BY A HUGE METEORIOTE, it has rings of mountains around it. IT is similar to a large “Mare” on the Moon.

Mercury’s Core and Magnetic Field (chapter 11) - Mercury has a very high density with a large nickel-iron (Ni + Fe) core. - The density is as high as Earth’s density, but on earth it is due to the mass/gravity of the earth

compressing the core. Since Mercury does not have such a mass the high density is due mainly to a high content of iron (Fe; a large iron core).

- The percentage of Iron in the planet is the highest in the solar system. - It looks as if the planet somehow lost its mantle. - This could be due to a collision (here too there is a “collision” theory for Mercury’s formation) - It could be that Mercury is the planet that impacted earth, lost some of its mantle to form the

Moon and its iron core was not affected by the impact ? - Clues about the iron core: Mercury’s Magnetic field (1 percent that of Earth’s). - This is evidence that part of the core of Mercury is still liquid, however the spin (rotation) of the

planet is possibly too slow to produce a “dynamo”. We still don’t understand the mystery of the magnetic field of Mercury. It should not really have one.

Magnetic Field of a Magnet :

Magnetic field lines around a magnet can be visualized with “iron dust” on a bar magnet. A compass needle aligns itself with these lines.

Magnetic Field of Mercury would look like this …

… if it wasn’t for the Sun “wind” (solar wind is made of high energy charged particles interacting and pushing on the magnetic field lines of the planet).

TRANSIT OF MERCURY ACROSS THE DISK OF THE SUN (covered in class;

this is not in your book)

The Transit of Mercury (as viewed from Earth- Picture) occurs about 13 to 14 times in 100 yr.

In a Similar way to the Solar and Lunar eclipses happening only near the line of node (where the plane of

the orbit of the Moon and the ecliptic intersect) every 6 months or so, Transits of Mercury can happen

also near the line of nodes of the orbit of Mercury in November or May, but not every year.

The Last 3 transits of Mercury happened in 1999, 2003 and 2006.

The Next transits will happen in 2016, on May 9; 2019 Nov 11; …

The transit of a planet in front of its “parent” star is detectable from far away as the light of the star is

attenuated when the planet is large. This is how some exoplanets have been discovered recently.

Exoplanet: it is a planet around a different star than the sun.

Precession of the Perihelion of Mercury: it was used to validate the

General Theory of Relativity (covered in class, not in your book)

The General Theory of relativity has to be taken into account when the gravity is large, which is the case

for Mercury as it is so close to the Sun that the Gravity of the Sun is important and “distorts” space.

The General Theory of relativity when written down for the motion of Mercury around the Sun predicts the rate of precession of the orbit of Mercury in very good agreement with the observations HOMEWORK #1: What are the similarities and the differences between the Moon and Mercury ? If you can, prepare a Table, if you cannot then present 2 paragraphs with bullets or other symbols. For example: Similarities:

- Similarity #1 - Similarity # 2 - Etc…

Differences: - Difference #1 - Difference #2 - Etc…

(hint: consider the surface and the interior and the general characteristics separately). HOMEWORK #2: What are the differences between the terrestrial planets and the Jovian planets. Here too follow the same presentation as in question #1. Concentrate on the general characteristics of the (terrestrial and Jovian) planets, then on their surface and interior. BOTH HOMEWORKS ARE DUE NEXT WEEK, YOU HAVE TO DO THEM BOTH NOT JUST ONE.

Extra-Solar Planets (or “Exoplanets”; covered in class; End of Chapter 8) - In the last decade or so close to 500 planets have been discovered (detected) around stars.

The methods of detection include: - Transit of the planet in front of the parent star, when this happens the light of the star is slightly

reduced. - Measurement of the velocity of the parent star by using Doppler shift of spectral lines explain

with a diagram… The larger mass the planet and the smaller mass the star the larger will be the radial velocity of the star.

- Coronography: masking the image of the parent star to see the dim light of the planet - And more…

It is easier to detect large planets close to their parent star. Most of the planets detected are Jovian-like Planets some even closer to their star than Mercury. Many of these exoplanets have highly eccentric orbits that cannot be explained by current theories. Most theories concentrated on the explaining our Solar system, with terrestrial planets close to the Sun and Large Jovian Planets far from the Sun with little eccentricity. New theories involve “migration” of planets either toward the Sun or away from the Sun… The discovery of 500 exoplanets has created a challenge for theorists to explain the new systems discovered. The Field of Exoplanets is now growing very fast and it is at that points where observations are advancing faster than the theories. More recently some earth-size planets have been detected

From the News: The First Earth-Like Planet in the Habitable Zone –

Gliese 581 g (covered in class; NOT in your book) What is the Habitable Zone ?

The Habitable Zone is that region around the star for which the planet will have a temperature like that of Earth, so the planet could potentially sustain life (if for example it has water, an atmosphere,..). If a planet is too close to its parent star, then it is too hot for life to develop and survive. If a planet is too far from its parent star, then it is too cold for life to develop and survive. An Earth-like planet is a terrestrial planet with a solid surface, a mass of the order of the mass of the Earth. A planet that is too small has a low gravity and might not be able to keep its atmosphere from escaping to space (like Mercury and the Moon).

The Star Gliese 581 - It is a “red dwarf” star in the constellation Libra

- It has a mass of about 1/3 that of the Sun

- It has a radius of about 1/3 that of the Sun

- It is at a distance of 20 light years only! (most we see are like 200 to 2000 light years away!). So

it is (in astronomical terms) in our “backyard”.

- It has an orange red color

An “artist” view of he Sun (left) and Gliese 581 (right)

- Gliese has a brightness (luminosity) only 1percent that of the Sun

- For a planet around Gliese to receive the same amount of flux as the earth from the Sun, it has

to be located about 10 times closer (since the brightness increases like 1/r^2).

- It has a (surface) temperature of 3,480degrees Kelvin (the Sun is 5,770K).

6 Planets have been detected to orbit the Star Gliese 581 as follows:

The order of these planets from the Gliese 581 is: e, b, c, g, d, f

- planet b is a Jovian planet

- planets c and d are “Super-Earth”, very large solid planets, c might be too hot and d might be

too cold, but they are at the edges of the Habitable Zone.

- planet e is terrestrial but much hotter (too close to its star)

- planet f is like half Neptune, not sure if it is a Super Earth or Jovian in Nature

- planet g is a Large Earth-Like planet sitting right in the middle of the Habitable Zone.

The blue dashed line depicts the orbit of planet earth if the sun was in the center of the system. The green dashed line depicts the orbit of Venus if the Sun was in the center of this planetary system. The red dashed line depicts the orbit of Mercury if the Sun was in the center of this planetary system. The 6 concentric black solid lines depict the orbit of the 6 planets orbiting Gliese 581. From the outside in we have the planets: f, d, g, c, b and e. The Earth-like planet in the habitable zone is the first one on the orbit (solid black circle) just inside the dashed red line.

Gliese 581 g (The Earth-like planet in the habitable zone).

- It has a mass somewhere between 3.1 and 4.3 the mass of the Earth

- It must have a radius between 1.3 and 1.5 the radius of the Earth

- The gravity on the planet is somewhere between 1.1 and 1.7 that of the Earth (you would feel

heavier than on earth).

- The temperature on that planet is likely to be similar to the temperature on earth, the planet

has also a greenhouse effect like on earth - if it has an atmosphere.

In our solar system there is evidence of water on Mars, on some of the Moons of Jupiter and Saturn,

on all the comets, and even deep in the bottom of some craters on the poles of the Moon. Water

has also been detected in huge clouds where stars are forming. So it is very likely that the stellar

system Gliese 581 has water (either as gas, liquid or solid) on some of its planets, and most probably

liquid water on planet g. It is very likely that planet g with water in the habitable zone has developed

some form of life. The following text is from the original article in which these findings were

published at the beginning of October 2010.