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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4. Details about
the objects in our solar system - definitions explained
represents a sun (star) at the centre of a solar system.
or Z might
represent a planet.
might also represent the path of a large asteroid.
might represent a moon of plane = X
might represent a comet.
The Sun is our star at the centre of our
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 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.
true planet may be
defined has having the following properties-criteria:
It must orbit a star (e.g. our
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.
||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
|Pluto (dwarf planet)
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
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
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
satellites are 'human-made' satellites that launched into
space to orbit a planet or one of its moons and they also have almost
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
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.
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!
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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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