Table of Contents
- The four outer planets, called the gas giants, are substantially more massive than the terrestrials.
- The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of substances with relatively high melting points (compared with hydrogen and helium), called ices, such as water, ammonia and methane, and are often referred to separately as “ice giants”.
- Outer Planets are Jupiter, Saturn, Uranus, Neptune and the dwarf planet – Pluto.
- The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun.
- All four gas giants have rings, although only Saturn’s ring system is easily observed from Earth.
- The term superior planet designates planets outside Earth’s orbit and thus includes both the outer planets and Mars.
- Surface gravity: 1 kg = 2.53 kg.
- It is composed mostly of gas and liquid swirling in complex patterns.
- Jupiter has no solid surface and hence no record of a geologic history.
- Its moons are, however, solid planetary bodies that contain geologic wonders.
- Number of moons = 67. Planet with highest number of moons.
- Jupiter’s four large moons (Io, Europa, Ganymede, and Callisto), called the Galilean satellites because they were discovered by Galileo in 1610
- Surface gravity: 1 kg = 1.07 kg.
- It is composed mostly of hydrogen, and helium.
- Saturn’s rings for long have been considered as its most dramatic feature.
- The rings are probably made up of billions of particles of ice and ice-covered rocks
- Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere. (Our Moon is the fifth largest natural satellite. Ganymede, a moon of Jupiter, is the largest natural staellite in this solar system. At 5,268 km at the equator, it is larger than Mercury, the dwarf planet Pluto, and three times larger than the Moon orbiting Earth.)
- Number of Moons = 62.
- Surface gravity: 1 kg = 0.92 kg.
- No solid surface.
- Enveloped by a thick atmosphere of hydrogen and helium.
- In contrast to all other planets in the solar system, it is tipped and spun on its sides, that is its axis of rotation lies nearly the plane of its orbit.
- Moons = 27.
- 1 kg = 1.18 kg
- Uranus and Neptune are called the twins of the outer solar system.
- Surrounded by thick atmosphere of hydrogen, helium and methane.
- Moons = 13.
Pluto and Charon
- 1 kg = 0.30 kg.
- The dwarf planet Pluto (39 AU average) is the largest known object in the Kuiper belt.
- When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet.
- Pluto was moved into the list of Dwarf Planets along with Ceres and Eris.
- Charon, Pluto’s largest moon.
- The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.
- It extends between 30 and 50 AU from the Sun.
- A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail.
- These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet.
- Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune.
- Comets, composed of ice and dust, originated outside our solar system. Their elliptical orbit brings them close to the Sun and into the inner Solar System.
- Comets are among the most spectacular and unpredictable bodies in the solar system.
- They are made of frozen gases (water, ammonia, methane and carbon dioxide) which hold together small pieces of rocky and metallic minerals
- One of the larger comets is the Halley’s Comet. The orbit of Halley’s Comet brings it close to the Earth every 76 years. It last visited in 1986.
- Any solid debris origination from asteroids or comets or from outer space that fall to the Earth, the Moon, or another planet in the solar system.
- Meteor is a body of matter travelling at a great speed through space which becomes luminous when enters into the atmosphere (mesosphere) at about 200 km above the Earth’s surface, because it is heated by friction. Generally, this latter process dissipates the material into meteoric dust.
- A meteor is popularly termed a ‘shooting star’ or ‘falling star’.
- Largest Meteor Crater: A meteor crater in Arizona (USA) is 4,200 ft (1,300 m) deep is the largest meteor crater in the world. It was formed over 10,000 years ago.
Solar System – Relevant Facts
Diameter in kms;
Distance from Sun; Distance in Astonomical Units(AU)
1 AU = Distance between Sun and Earth = 149.6 milliom kms
|Mercury||4,878 = 0.38||57.9 mkm = 0.38|
|Venus||12,104 = 0.96||108.2 mkm = 0.72|
|Earth||12,576.3 = 1||149.6 mkm = 1|
|Mars||6,794 = 0.54||227.9 mkm = 1.52|
|Jupiter||143,884 = 11.44||778.4 mkm = 5.22|
|Saturn||120,536 = 9.58||1.426 mkm = 9.57|
|Uranus||51,118 = 4||2.87 mkm = 19.26|
|Neptune||50,538 = 4||4.498 mkm = 30.18|
Relative size of Planets
|Planets in the ascending order of proximity to sun||Temperature in °C|
The reason that Venus is hotter than Mercury is because it has an atmosphere made of carbon dioxide; it also has clouds of acid inside its atmosphere.
|Planets in the ascending order of proximity to sun||Period of Rotation||Period of Revolution|
|Mercury||58 days||87 days|
|Venus||243 days||224 days|
|Earth||23:56 hrs||365d, 5:48|
|Mars||1.05 days||687 days|
|Jupiter||9 hrs||11.86 years|
|Saturn||10 hrs||29.46 years|
|Uranus||17 hrs||84.01 years|
|Neptune||16 hrs||164.8 years|
|Planets in the ascending order of proximity to sun||Density relative to water|
(Taking, density of water = 1)
|Planets||No of Known Moons|
|Planets||Rank according to size|
Inclination angle to Ecliptic
Orbital Velocity in km/s
|Pluto (Dwarf planet)||17°||4|
Heliocentric vs Geocentric
- Heliocentric system is an astronomical model in which the Earth and planets revolve around a relatively stationary Sun at the center of the Solar System. [Remember the name of the man who first suggested this model?]
- Geocentric model (Earth the centre) was proposed by Ptolemy.
Kepler’s laws of planetary motion
- The orbit of a planet is an ellipse with the Sun at one of the two foci.
- A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
- The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.
Why is Venus sometimes called Earth’s twin?
- almost the same size,
- have about the same mass (they weigh about the same), and
- have a very similar composition (are made of the same material).
- They are also neighboring planets.
However, Venus and Earth are also very different
- Venus has an atmosphere that is about 100 times thicker than Earth’s and has surface temperatures that are extremely hot.
- Venus does not have life or water oceans like Earth does.
- Venus also rotates backwards compared to Earth and the other planets.
Mars Compared to Earth
- 53% the diameter of Earth
- 10% the mass of Earth
- surface gravity on Mars is only 38% the gravity on Earth
- A day on Mars lasts 1.03 Earth days
- axial tilt on Mars is 25.19 degrees. Very close to Earth’s 23.5 degree tilt
- a year on Mars lasts about twice as long as an Earth year, the seasons are twice as long.
- The atmosphere of Mars is less than 1% the thickness of Earth’s atmosphere. Furthermore, it’s made up of 95% carbon dioxide
- Most large objects in orbit around the Sun lie near the plane of Earth’s orbit, known as the ecliptic. The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at significantly greater angles to it.
All the planets except VENUS and URANUS rotate in anti-clockwise direction.
- The moon is the only natural satellite of the earth.
- It is now generally believed that the formation of moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”.
- A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space.
- This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.
- Its diameter is only one-quarter that of the earth.
- It is about 3, 84,400 km away from us.
- The moon moves around the earth in about 27 days. It takes exactly the same time to complete one spin. As a result, only one side of the moon is visible to us on the earth.
- Neil Armstrong was the first man to step on the surface of the moon on 29 July 1969.