###### The Universe - How Old? How Big?

The Age of the Solar System
Based on the age of the oldest meteorites, a group of research scientists at the Institute of Physical Earth Sciences in Paris has carried out very precise measurements of the decay of Uranium 238 to Lead 206. Using radiometric dating we can calculate the age of the Solar System at about 4.56 billion years.

Formation of the Solar System – Stephen Hawking:

The Size of the Solar System
The universe contains an estimated 100 billion galaxies, of which our Milky Way is one. The Milky Way itself contains an estimated 100 billion stars, of which our Sun is one. The Milky Way is a spiral galaxy with a radius of around 50,000 light-years (a light-year being the distance that light travels in one year), and the Sun and its solar system are around 30,000 light-years from the center of the galaxy.

The figure shows the Sun and planets with the correct sizes. The distances between them are way too small. In general, the farther away from the Sun, the greater the distance from one planet’s orbit to the next.

The figure below shows those distances correctly. In the upper left are the orbits of the inner planets and the asteroid belt. The asteroid belt is a collection of many small objects between the orbits of Mars and Jupiter. In the upper right are the orbits of the outer planets and the Kuiper belt. The Kuiper belt is a group of objects beyond the orbit of Neptune.

In this image distances are shown to scale.

In Figure above, you can see that the orbits of the planets are nearly circular. Pluto's orbit is a much longer ellipse. Some astronomers think Pluto was dragged into its orbit by Neptune.

Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km (93 million miles). Table below shows the distance from the Sun to each planet in AU. The table shows how long it takes each planet to spin on its axis. It also shows how long it takes each planet to complete an orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus!

 Distances to the Planets and Properties of Orbits Relative to Earth's Orbit Planet Average Distance from Sun (AU) Length of Day (In Earth Days) Length of Year (In Earth Years) Mercury 0.39 AU 56.84 days 0.24 years Venus 0.72 243.02 0.62 Earth 1.00 1.00 1.00 Mars 1.52 1.03 1.88 Jupiter 5.20 0.41 11.86 Saturn 9.54 0.43 29.46 Uranus 19.22 0.72 84.01 Neptune 30.06 0.67 164.8

Source: Open Education Group Textbooks - Earth Science