According to the standard Big Bang model, the universe was born during a period of inflation that began about 13.7 billion years ago. Like a rapidly expanding balloon, it swelled from a size smaller than an electron to nearly its current size within a tiny fraction of a second.

Types of galaxies:

• Barred spiral
• Irregullar
• Spiral
• Peculiar
• Elliptical
• Lenticular

Mercury resembles nothing so much as a larger version of the Moon. This planet is so close to the Sun that it is actually difficult to observe. The Hubble Space Telescope cannot look at it because it would permanently damage the lens.

Venus’ atmosphere of thick, toxic clouds hides the planet’s surface from view. Scientists and amateurs alike used to think that the planet was covered with thick forests and flora like tropical rainforests on Earth. When they were finally able to send probes to the planet, they discovered that Venus’ surface was actually more like a vision of hell with a burning landscape that is dotted with volcanoes.

Mars has very diverse terrain. One of the planet’s most famous features is its canals, which early astronomers believed were “man”-made and contained water. These huge canyons were most likely formed by the planet’s crust splitting. Mars is also famous for its red color, which is iron oxide (rust) dust that covers the surface of the entire planet. The surface of Mars is covered with craters, volcanoes, and plains. The largest volcanoes of any planet are on Mars.

Jupiter is a gas giant, so it has no solid surface just a core of liquid metals. Astronomers have created a definition for the surface – the point at which the atmosphere’s pressure is one bar. This region is the lower part of the atmosphere where there are clouds of ammonia ice.

Saturn is also a gas giant so it has no solid surface only varying densities of gas. Like Jupiter, almost all of Saturn is composed of hydrogen with some helium and other elements in trace amounts.

Uranus and Neptune are also gas giants, but they belong to the subcategory of ice giants because of the “ices” in their atmospheres. Uranus’ surface gets its blue color from the methane in the atmosphere. Methane absorbs light that is red or similar to red on the color spectrum leaving only the light near the blue end of the spectrum visible.

Neptune is also blue due to the methane in its atmosphere. Its “surface” has the fastest winds of any planet in the Solar System at up to 2,100 kilometers per hour.

One way to remember the difference between the geocentric model and the heliocentric model is to look at the etymology of the words.

Geocentric means the Earth is at the center. The prefix geo means of the Earth. Thus geometry is the measure of the earth, geology is the study of the earth, etc...

Heliocentric means the sun is at the center. The prefix helio means of the sun. One can speak of the heliosphere,...

As we are part of the solar system Earth pretty much follows the path of the sun as it goes through its own orbit around the galaxy. The Milky Way is a spiral galaxy type so it has arms sort of like an octopus. The Sun is located near the outward tip of the Sagittarius arm of the Milky Way.If you are not an astronomy enthusiast you not have thought much about what galaxy do we live in. So depending on that the answer may surprise you. If you know anything about galaxies you know that they are groupings of stars that number in the hundreds of billions. The most famous is the Milky Way. It is from this galaxy that we even have the term. The simple point is that the Earth is part of the Milky Way even though if we see it in the sky it looks like we are observing it from the outside. Why is that? To understand you need to know exactly where we live in neighborhood of the Milky Way Galaxy.

As we are part of the solar system Earth pretty much follows the path of the sun as it goes through its own orbit around the galaxy. The Milky Way is a spiral galaxy type so it has arms sort of like an octopus. The Sun is located near the outward tip of the Sagittarius arm of the Milky Way. This makes Earth about 28,000 light years from the galactic core of our home galaxy.

The Solar System also has a galactic year that it follows. It takes around 200 million to 250 million years for the solar system to orbit the Sun. Another indicator of our position is where the galactic equator. While our star system is considered to be on the outskirts of the Milky Way this is only an estimate. It is believed that the Milky Way is larger than first estimated. There is also suspicion that our galaxy is in the process of absorbing other smaller galaxies. However, there is not enough empirical evidence available to support the claim.

So what would be so important about knowing what part of the galaxy we live in? One reason is space exploration. Some time in the future mankind may find a way to achieve faster than light space travel. This can provide a new set of challenges for engineers and astronomers to tackle. For example how would an astronaut keep from getting lost in space? Detailed mapping and computer programming in the future could help galactic wayfarers know where they are going and more importantly how to get home.

The other reason is that it never hurts to know our place in the scheme of things. Just thinking of the challenge of finding earth if we were so far way helps us to understand how truly vast the universe is.

We have written many articles about the Milky Way galaxy for Universe Today. Here are some facts about the Milky Way, and here’s an article about the closest galaxy to the Milky Way.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.

The outer planets are filled with gas. So they are called gas planets and the inner planets are the planets that doesn't have invisible or visible rings around the planet.

In our solar system all of the planets in the solar system orbit around the sun while the moon is orbiting the earth.

Earth:Closest: 147 million km / 91 million miles (.98 AU)

Farthest: 152 million km / 94 million miles (1.01 AU)

Average: 150 million km / 93 million miles (1 AU)

Mars:Closest: 205 million km / 127 million miles (1.38 AU)

Farthest: 249 million km / 155 million miles (1.66 AU)

Average: 228 million km / 142 million miles (1.52 AU)

Closest to Mars from Earth: 55 million km / 34 million miles

Jupiter:Closest: 741 million km /460 million miles (4.95 AU)

Farthest: 817 million km / 508 million miles (5.46 AU)

Average: 779 million km / 484 million miles (5.20 AU)

Closest to Jupiter from Earth: 588 million km / 346 million miles

Mercury:Closest: 46 million km / 29 million miles (.307 AU)

Farthest: 70 million km / 43 million miles (.466 AU)

Average: 57 million km / 35 million miles (.387 AU)

Closest to Mercury from Earth: 77.3 million km / 48 million miles

Venus:Closest: 107 million km / 66 million miles (.718 AU)

Farthest: 109 million km / 68 million miles (.728 AU)

Average: 108 million km / 67 million miles (.722 AU)

Closest to Venus from Earth: 40 million km / 25 million miles

Saturn:Closest: 1.35 billion km / 839 million miles (9.05 AU)

Farthest: 1.51 billion km / 938 million miles (10.12 AU)

Average: 1.43 billion km / 889 million miles (9.58 AU)

Closest to Saturn from Earth: 1.2 billion km /746 million miles

Uranus:Closest: 2.75 billion km / 1.71 billion miles (18.4 AU)

Farthest: 3.00 billion km / 1.86 billion miles (20.1 AU)

Average: 2.88 billion km / 1.79 billion miles (19.2 AU)

Closest to Uranus from Earth: 2.57 billion km / 1.6 billion miles

Neptune:Closest: 4.45 billion km /2.77 billion miles (29.8 AU)

Farthest: 4.55 billion km / 2.83 billion miles (30.4 AU)

Average: 4.50 billion km / 2.8 billion miles (30.1 AU)

Closest to Neptune from Earth: 4.3 billion km / 2.7 billion miles