lesson 1:Introduction to the Universe

Introduction to the Universe

Solar System

  The Solar System is a collection of planets, moons, asteroids and comets and other rocky objects orbiting the Sun. The Solar System is believed to extend out to at least 150 000 million km from the Sun, although the planets are all found within about 6000 million km.

Our Solar System is thought to have formed 4.6 x 10⁹ years ago from a vast, rotating cloud of gas and dust known as the solar nebula. As the solar nebula rotated, its gravity began to attract gas and dust towards the centre, eventually forming our Sun.

The Sun

The Sun is the powerhouse of the Solar System. Without it, life on Earth simply would not exist. Despite burning its hydrogen fuel for the best part of 5 billion years, the Sun is still only half way through its life cycle.
The study of the Sun, its environment and how the material it discharges interacts with other worlds in the Solar System is of great interest to us all. ESA has a number of mission that analyse various aspects of this solar emission.

Planets and Moons

The formation of the Sun had a dramatic effect on the temperatures across the solar nebula, introducing a temperature range that stretched from about 2000K near the Sun to less than 50K at the outer regions. The heat in the inner Solar System only allowed materials with high condensation temperatures to remain solid. These particles eventually gathered to form the four terrestrial planets: Mercury, Venus, Earth and Mars.
A similar process formed the outer planets of the Solar System: Jupiter, Saturn, Uranus, and Neptune. Yet, they are different because icy materials such as frozen water, carbon dioxide and methane were also available. Consequently, these outer Jovian planets are much larger than the terrestrial planets. In addition these giant planets were able to enhance their atmospheres by capturing gas atoms moving more slowly due to the colder temperatures.
Each planet travels around the Sun in an elliptical orbit that is held in place by the gravitational attraction between the Sun and the planet. Some of the planets, including, of course, Earth, have moons orbiting them. Mars has just two moons in orbit around it, while Jupiter has 63 moons known to be orbiting it. Kepler's three laws of planetary motion define the motion of the planets around the Sun, and the movement of moons around their parent planet.

	Distance		Year	Day	ESA Mission
	AU	106 km
Mercury	0.387	57.9	87.969d	58.65d	BepiColombo
Venus	0.723	108.2	224.701d	243.01dR	Venus Express
Earth	1.000	149.6	365.365d	23.934h	Earth Observation
Mars	1.524	227.9	686.980d	24.623h	Mars Express
Jupiter	5.203	778.3	11.862y	9.842h	-
Saturn	9.539	1427.0	29.457y	10.233h	Cassini-Huygens
Uranus	19.182	2869.6	84.010y	17.24hR	-
Neptune	30.058	4496.6	164.793y	18.4 h	-

Table 1.1: Distance and Orbital Parameters for the Planets

	Observational Parameters		Moons	Physical Data
	Mean Angular Diameter (")	Mean Visual Magnitude		Radius	Mass	Density	Gravity
Mercury	7.8	0.0	0	0.38	0.06	0.98	0.37
Venus	25.2	-4.4	0	0.95	0.86	0.95	0.90
Earth	-	-	1	1.00	1.00	1.00	1.00
Mars	17.9	-2.0	2	0.53	0.15	0.71	0.38
Jupiter	46.8	-2.7	63	11.19	1323	0.24	2.69
Saturn	19.4	0.7	47	9.41	752	0.13	1.19
Uranus	3.9	5.5	27	3.98	64	0.24	0.93
Neptune	2.3	7.8	13	3.81	54	0.32	1.22

Table 1.2: Observational Characteristics of the Planets

Asteroids and Comets

There was some material left over from the solar nebula once the Sun and the planets had formed. Some of this debris remains in our Solar System in the form of asteroids and comets.

Asteroids

Asteroids, which are sometimes called minor planets, are rocky bodies mostly found in the planetary region between Mars and Jupiter. This region is known as the asteroid belt, and it stretches from about 250 million km to about 600 million km from the Sun. The largest known asteroid is Ceres with a diameter of roughly 1000 km. Only around a dozen are more than 250 km across. Over 100 000 asteroids larger than one kilometre in diameter are known to exist, with more being discovered all the time.
We often hear of asteroids on the news, when near-Earth asteroids pass close enough to our planet to cause concern of a potential impact either now, or in the future. These near-Earth objects have highly elliptical orbits, which bring them into the inner Solar System, crossing the orbit of Mars and occasionally coming close to Earth.

Comets

   Comets are often referred to as 'dirty snowballs', as they are made up of ice and dust. The ones we can see travel around the Sun in highly elliptical orbits taking from a few years to thousands of years to return to the inner Solar System. Typically comets are just a few kilometres across, which makes them very difficult to spot for most of their orbit. As they approach the Sun, however, solar radiation vaporizes the gases in the comet and the characteristic comet 'tail' is formed. The tail of a comet consists of two parts: a whiter part made of dust, which always points away from the Sun, and a blue part consisting of ionised gas. Comets are mainly found in two regions of the Solar System: the Kuiper belt, a region that extends from around the orbit of Pluto to about 500 AU from the Sun, and the Oort Cloud (from the Kuiper Belt to about 50 000 AU from the Sun).

Meteors

Occasionally small rocks or dust particles enter the Earth's atmosphere. The dust particles and small rocks burn up in the atmosphere leaving behind brief trails in the sky witnessed as meteors. It is estimated that more than 200 million kg of meteoritic material is swept up by the Earth each year, with around 10% reaching the ground.

   Much of this material orbits the Sun in distinct streams, usually as debris from different comets. At various times throughout the year the Earth crosses these streams and for a few nights an observers can witness a meteor shower.