The planets

• Let us not worry about the death of the sun for now! And instead turn our attention to the planets in the solar system.

• The planets do not have light of their own

• They all revolve around the Sun and their orbits are all tilted at various angles to the Sun’s equator.

• We will now take a look at each of these planets (except earth) Solar System to Scale

Mercury• Mercury is the closest planet to the Sun and

the 2nd smallest planet in the solar system.• It comes 28M miles closest to the Sun at one

end of its orbit and 43M miles at the other end• The surface of Mercury is very similar to the

Moon and has numerous craters.• The core of Mercury is composed 60 to 70%

of Iron• As we know the earth takes 365 days to

complete a revolution around the Sun. In contrast Mercury takes only about 88.5 days.

• We also know that the earth rotates on its own axis in 24 hours. Compared to this Mercury is really slow. It takes approximately 59 days to rotate on its own axis. Do you see what this means?

Mercury’s rotation• By the time Mercury completes 1 round around the Sun (it is 88 earth

days) it would’ve turned on its own axis 1.5times approximately (88/59 ~1.5)

• So on Day 0 let us assume that we are standing at a point on Mercury facing the Sun at Noon. After 1 year (I.e 88 earth days) where do you think we will be w.r.t the Sun?– We would be in darkness as it would be midnight because Mercury turned on

us!

• After one more complete orbit around the Sun (I.e after another 88 days have passed) we would be back facing the Sun at Noon

• So Mercury presents the same face to the Sun at any given point once in 2 revolutions around the Sun

Venus• Venus is the next planet closest to the Sun and it formed

at the same time as the earth and is often called earth’s sister planet

• But Venus is very different from the earth – it has no oceans, no water vapor (unlike the earth’s the clouds of Venus are made of Sulphuric acid) and has an atmosphere that has so much Carbon Dioxide.

• Although Mercury is closest to the Sun, Venus is much hotter than mercury. Can you guess why?

– It is because Mercury does not have an atmosphere while Venus has one that is full of Carbon Dioxide. The carbon dioxide traps the heat radiated by the surface of the planet. Does this effect sound familiar??? (Greenhouse effect can be explained here)

• Venus also turns very slowly in its axis and takes about 243 earth days to turn once on its axis; Just like Mercury its days are longer than its years

• Need to write about retrogade rotation????

Mars• Following the Earth is the red planet called Mars. It is about 141 Million

miles from the sun• When we think of Mars we cannot avoid talking about the possibility of life

in Mars. Despite having 6 Billion of us in earth we humans are constantly looking to see if there are other humans in other planets. The movie industry and science fiction writers have made use of their imagination to create species called Martians.

• So why do we suspect there exists life in Mars. Some of the reasons are– The atmosphere of Mars has traces of water vapor

– During the early days of the telescope some astronomers thought they saw long straight lines criss crossing Mars surface. They mistook those for water canals and concluded there must be some life on earth.

– Also the avg temperature on Mars is –60 deg C and the max temp is 20 deg C – which although cold is still manageable for humans

• But when space exploration began and space crafts sent to Mars started transmitting pictures and conducting biology experiments it was concluded that there was no life on Mars not even microorganisms. But this doesn’t rule out the possibility that life could’ve existed sometime in the distant past

• The atmosphere of Mars is very different from earth and comprises mainly of Carbon dioxide which freezes to form polar caps (seen in pic2). There is also evidence that water could have flown in the planet in the distant past as there are some physical features on the planet which resemble riverbeds and islands and gorges

• The planet also has 2 satellites which give it a little bit of moonlight in the night

The Asteroid Belt• Beyond Mars for about 340 million miles there is no other planet in space but just islands of

hard rocks and metals called asteroids.• They are especially seen in the belt between little Mars and the giant Jupiter. Ceres was

the first asteroid to be discovered and soon a number of others were seen.• 30,000 such asteroids are believed to exist out of which 3000 are observed and tracked• Because Jupiter is the biggest planet in this region it acts like a big bully with enormous

control over these tiny asteroids. Their motions are controlled by Jupiter.• Due to Jupiter’s pull these asteroids are kept from falling into the Sun but some of these

asteroids might escape Jupiter’s pull and be sent towards the sun and can come very close to the earth. Thousands of small asteroids routinely strike the earth’s atmosphere although the chances of directly colliding with earth is very small

• Another interesting aspect in the asteroid belt is the presence of Trojan asteroids

The Trojan Asteroids• The Trojan asteroids is the name given to a group of asteroids sitting at the Lagrange

points. Jupiter has the most Trojan asteroids.• Wonder what are Lagrange points? This is a name given to those places where a lighter

body can sit “motionless” between 2 heavier objects thanks to gravity!• In the case of Jupiter and Sun (the two heavyweights!) there are 5 such points out of which

3 are unstable (due to the presence of other planetary bodies). In the figure below points marked L4 and L5 are the 2 stable points and that is where we find collection of asteroids called the Trojan asteroids

• If you note carefully, the sun, Jupiter and L4 form an equilateral triangle. Similarly the Sun, Jupiter and L5 also form an equilateral triangle. At these points gravitational stability is achieved. The Trojan asteroids move in the same orbit as the planet either leading or following the planet.

Jupiter• Following Mars and the asteroids comes Jupiter by far the largest planet in the solar system

and one of the brightest objects in the sky.Back in 1610 Galileo with his telescope found 4 large moons moving around Jupiter. This was one of the first discoveries of motions not centered around earth and helped confirm Copernicus’ sun-centered theory which got both Copernicus and Galileo into trouble in those days!

• Unlike the 4 inner planets Jupiter is not a sphere of rock and metal instead it is composed of gaseous substances like Hydrogen (90%), Ammonia, Helium(10%) and Methane. Jupiter also has rings around it though not as bright as that of Saturn’s

• A huge red spot called the Great Red Spot(GRS) has been seen near the Southern Hemisphere of Jupiter for nearly 100 years. 3 earths can fit across GRS. No one is sure but the most common theory is that the GRS is a result of an atmospheric disturbance caused by irregularities on Jupiter’s surface

• Another unique feature of Jupiter is the planet’s stormy outer layers which emit short wave radio signals energetic enough to reach the earth!

2 of Jupiter’s moons (Io and europa are seen)

Saturn• Following Jupiter is Saturn. Saturn is also a gaseous planet. Like Jupiter Saturn is about

75% Hydrogen and 25% Helium with traces of methane, water, ammonia • Saturn is most known for its rings. Two prominent rings and one faint ring can be seen from

earth. Though they appear continuous the rings are made of a number of small particles which are in an independent orbit. Their sizes can range from a few centi meters to several meters and even a few kilometer sized objects are seen. The ring particles seem to consist mainly of water and ice and some rock type materials.

• The rings are very thin and although their diameter is about 250,000 km they are about 1km in thickness. The origin of the rings is not known. While they are believed to have existed from the beginning it looks like they are constantly regenerated. The current set of rings are said to be about 100M years old.

• Saturn has about 30 named satellites and a couple discovered in ’03 and ’04. The largest, Titan is as big as mercury (Pic2 shows some of Saturn’s satellites)

Kelvin Helmholtz mechanism

• Jupiter, Saturn and Neptune (oddly not Uranus) all exhibit this phenomenon

• They radiate more energy into space than what they receive from the sun

• This is because when the surface of a planet cools, the pressure drops, and the planet starts to compress in order to compensate. This compression in turn heats up the core of the planet.

• Because of this internal heat source these planets radiate more energy than whatever little they receive from the sun

Uranus• Uranus is the 7th planet from the sun and was the first planet discovered in modern times• The unique thing about Uranus is that its equator is tilted 98 deg out of the plane of its orbit.

(the earth’s is tilted 23.5 deg). Since Uranus year is about 84 earth years long (that is how long it takes to go around the sun) and because of this tilt, the South pole receives warmth from the Sun (whatever little it can get at that distance) for 42 years and the North pole for the next 42 years.

• The greenish blue color is because of absorption of red light and radiation of blue light by Methane in Uranus’ atmosphere

• Uranus also has rings but they are very dark just like in Jupiter. There are a total of 11 rings with Epsilion being the brightest.

• Uranus has 21 named moons and 6 unnamed ones. 11 are very small, 5 are large (seen in picture) and the remainder are very far.

Neptune• After the discovery of Uranus astronomers found many irregularities in its orbit. This led

them to a conclusion there must be another planet beyond Uranus which caused these disturbances. Based on this 2 mathematicians calculated where exactly that planet must lie, and a German astronomer actually found the planet Neptune in 1846

• Neptune is also a gas planet and just like Uranus its blue color is also due to the Methane in its atmosphere

• And just like Jupiter and saturn it also radiates more energy than what it receives from the Sun because of its internal heat source

• Neptune also has 13 moons the largest of which is Triton which is considered bigger than the earth’s moon (seen in picture 2).

Pluto• Pluto is the farthest planet from the Sun and by far the smallest• It was discovered by an accident. Based on some calculations on the orbits of Uranus and

Neptune the existence of another planet was predicted. These calculations were later found out to be wrong. Not knowing this an astronomer in the US was looking at the sky in hopes of finding another planet and by accident, he did find Pluto!

• Pluto is the one planet that has not been visited by a spacecraft yet.• Pluto is a tiny sphere which has an erratic orbit. It sometimes comes closer to the Sun than

Neptune.Its satellite Charon was discovered in 1978.• Not much is known about Pluto. Its temperature is anywhere between –235 to –210C• There is a chance of a NASA space mission to Pluto in 2006. Hopefully that mission will

give us more information on Pluto

Conclusion

• That brings us to an end of the story of the solar system and its planets.

• Beyond Pluto there is still a chance of some undiscovered planets existing, but none have been discovered so far.

• But other objects like comets roam the edges of the solar system.

• In the next part we will discuss more about other galaxies and the cosmos beyond our solar system.