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ASTRONOMY: 4. Details about the objects in our solar system and beyond - a sun, planet, moon, asteroid, true planet, dwarf planet, exoplanet, comets, meteors, meteorites all defined and explained

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INDEX physics notes on ASTRONOMY

4. Details about the objects in our solar system - definitions explained


U represents a sun (star) at the centre of a solar system.

X or Z might represent a planet.

Z might also represent the path of a large asteroid.

Y might represent a moon of plane = X

V might represent a comet.

The Sun is our star at the centre of our solar system.

A sun forms the centre of a solar system and generates its own visible light as a fraction of its enormous energy release from nuclear reactions e.g. the fusion of hydrogen into helium.

A solar system is everything that orbits our Sun (and everything that itself orbits anything orbiting the Sun e.g. moons, asteroids, comets).

Our sun is ~98% of all the mass of our solar system.

Objects orbiting the Sun are held in their circular or elliptical orbits by the force of gravity - the gravitational field force that attracts one object to another.

It decreases (gets weaker) significantly, the greater the distance between the two centres of the objects attracted to each other.

Gravity decreases proportionately to 1 / d2, where d = distance between centres of the objects.

A planet is the largest type of body that orbits a star - 8 major ones orbit our Sun (a star called Sol).

A planet cannot produce its own visible light - they are seen from the reflected light of its star.

Moons are natural satellites of a planet - Earth just has the 'Moon', other planets in our solar system have many moons.

There are eight planets in our solar system - listed in the table below in order of distance from the Sun.

A true planet may be defined has having the following properties-criteria:

It must orbit a star (e.g. our Sun).

It must be big enough to have enough gravity to force it into a spherical shape.

It must be big enough that its gravity has 'pulled in' and cleared away any other objects except for its natural satellites.

8 major PLANETS Distance from Sun in Mkm Mass relative to Earth Size relative to Earth Time to orbit Sun (days or years) Axis rotation time Average surface temperature oC
Mercury 58 0.05 0.4 88 d 58.6 d +350
Venus 108 0.8 0.9 225 d 242 d +480
Earth 150 1 1 365 d 24 h +22
Mars 228 0.1 0.5 687 d 24.7 h -23
Jupiter 778 318 11 12 y 9.8 h -153
Saturn 1430 95 9.4 29 y 10.8 h -185
Uranus 2870 15 4 84 y 17.3 h -214
Neptune 4500 17 3.8 165 y 16 h -225
Pluto (dwarf planet) 5915 0.003 0.2 248 y 153 h -236

Exoplanets are planets that orbit another sun in another solar system beyond ours e.g. in our own galaxy or some even greater distanced galaxies.

Dwarf planets (minor planets)

These objects do not meet the current criteria for 'major' planets - usually because they are not big enough.

A dwarf planet must be spherical and orbit a star but due to a weaker gravity field, has been unable to clear other large objects near its own orbit.

Pluto is now classed as a dwarf planet - it just isn't big enough!

Pluto has a moon called Charon that is half Pluto's radius (and 1/8th mass of Pluto) orbiting it, plus at least four other moons.

Another reason is that some asteroids are bigger than Pluto and could also be classed as minor planets!

Natural satellites - moons

A moon is a relatively large body of material that orbits a planet - they are natural satellites and they have almost circular orbits - actually very slightly elliptical.

We have one moon, Jupiter has dozens and Galileo spotted some of them in 1610 with his new telescope!

A satellite can be defined as any smaller object, due to its speed and resulting force of gravity, that orbits a more massive object

Artificial satellites

Artificial satellites are 'human-made' satellites that launched into space to orbit a planet or one of its moons and they also have almost circular orbit.

The orbit is designed so an artificial satellite can perform a particular function.

More on artificial satellites in the next section and why one object can orbit another in a steady cycle.


Asteroids are irregularly shaped lumps of rock-metal that orbit the Sun and are found in the asteroid belt between the orbits of Mars and Jupiter.

They vary considerably in size, from a two metres to nearly a 1000 km, but are too small to be considered as major planets - though the largest can be considered as minor/dwarf planets.

It is not impossible for a huge asteroid to collide with Earth e.g. the one of 11 to 81 km diameter that created the impact crater in Mexico that is believed to have led to the extinction of dinosaurs and the rise of the mammals like us!!!!

Meteors and meteorites

Meteors ('Shooting stars') are seen when dust particles called meteoroids come down into the Earth's atmosphere, due to the Earth's gravitational pull and 'burn-up' giving out bright visible light due to the high temperature generated by friction of the particles with the Earth's atmosphere.

Small meteor particles completely burn up in the atmosphere, but larger ones fall to Earth as meteorites, often consisting mainly of metal or mainly of stone - the largest known weighed 100, 000 kg.

Most meteors and meteorites are fragments of asteroids that have somehow broken out of the asteroid belt due to a collision and hurtle round the Sun in their own very elliptical orbits. They can also be bits of a comet.

image of meteorite made of iron nickel gcse physics igcse O level

 In the Ulster Museum, Belfast, is a meteorite of nickel and iron, perhaps once part of the metal core of a planet?


Comets are 'cosmic snowballs' of frozen gases, rock and dust that orbit the Sun.

Their orbits are often highly elliptical and some can travel from near the Sun to the outer regions of our Solar System and back!

Unlike the planet orbits, which are all in a narrow plane, comet orbits are at all sorts of angles with respect to the plane of our solar system.

When a comet's orbit brings it near the Sun, it heats up and pours out dust and gases into a huge giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles!


You should realise there is quite an 'overlap' between asteroids, meteors, meteorites and comets!

INDEX of my physics notes on ASTRONOMY

Keywords, phrases and learning objectives for astronomy

Be able to describe and explain what the following are: sun, planet, moon, asteroid, difference between a true planet and a dwarf planet, exoplanet ,comet, meteor and meteorite.

Know the a pattern of planets of our solar system in terms of distance their from the sun and the length of their year and so explain why the planets have different lengths of year further from the Sun, the longer the year..


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INDEX of my physics notes on ASTRONOMY