The Origins of The Solar System

We live in a universe of almost unimaginable size. To give you an idea of the scale, we can only use optical and radio methods to observe objects out to about 13,000 million light years away and nobody knows what lies beyond that. Closer to us we can see clusters of galaxies at distances out to about 750 million light years. The other galaxies of what is know as the Local Group of galaxies are all within 2.5 million light years of us. These galaxies are composed of millions and billions of stars.

Our own galaxy, the Milky Way, is a typical spiral galaxy. This means is looks like a great swirling whirlpool with spiral arms extending out from the center. The Milky Way galaxy is 100,000 light years in diameter and our own star, the Sun, lies on the edge of one of the spiral arms. The Sun is about 30,000 light years from the center of the Milky Way galaxy and takes 225 million years to complete one orbit around the galactic center.

According to modern scientific theory the star we call the Sun was formed about 4,500 million years ago. The Solar System originated when gravitational forces caused what is called the solar nebula to collapse and coalesce into a spinning disc with the central mass forming the Sun. The planets formed from large clouds of gas and dust which gradually built up over million of years as they orbited the central mass of the disc.

The central mass continued to build up in size and as it did the gravitational field it exerted grew stronger. As the gravitational field grew stronger the central mass pulled in more gas and dust and grew even larger.

Eventually, the gravitational force became so powerful that the temperature and pressure at the center of the mass rose high enough to cause hydrogen atoms to undergo fusion into helium atoms. This fusion reaction releases enormous amounts of energy and is the source of the Suns power.

Now illuminated by the Suns rays, the planets of our solar system continued to form and transform. All of the planets have undergone a great deal of change since they first accreted. These changes were brought about in a variety of ways, including violent collisions which resulted in craters that can still be seen today. The planets also went through changes caused by volcanism, melting, structural deformation and the release of gases from deep within.

The planets of our Solar System are generally divided into two groups. The inner planets, which include Mercury, Venus, Earth and Mars and the outer planets, Jupiter, Saturn, Uranus, Neptune and until recently the Dwarf Planet Pluto. The inner planets are all characterized by having solid surfaces and are relatively small compared to the outer planets. The outer planets are all characterized by not having a solid surface and are often referred to as gas giants. The inner and outer planets are also separated by a band of asteroids orbiting between Mars and Jupiter.

Comets, meteors and asteroids can also be found orbiting the sun in large numbers from the outer reaches of the Solar System to very close to the Sun. Comets are balls of frozen liquids and dust which often follow eccentric orbits for outside the orbit of Dwarf Planet Pluto and only display their characteristic tails when their orbit brings them closer to the Sun. Asteroids and meteors are both made mainly of rock. The difference between asteroids and meteors is their size, meteors can be as small a grain of dust, asteroids can be miles in diameter and are sometimes called planetoids.

At the outer fringes of the Solar System beyond the orbit of Neptune is the Kuiper belt where at least three dwarf planets orbit. They are Ceres, Pluto and Eris and they are accompanied by millions of smaller objects about which little is known. At this time a variety of spacecraft are either on their way to the far reaches of the Solar System or are planned for the near future. It will be fascinating to see what wonders these space probes will tell us about the Solar System we call home.

Haley's Comet and the Eta Aquarid Meteor Shower

The Eta Aquarids meteor shower was first isolated (and pinned to an orbital event) in the 1870s, when Lieutenant-Colonel G. L. Tupman observed them while sailing in the Mediterranean Sea. As with many astronomical observations of the era, it was catalogued and annotated; it wasn't until William Herschel corroborated the shower with the recent passing of Haley's Comet (and did an orbital plot) that the shower was tied to Halley's comet in 1876.

When observing meteor showers today as then, you will notice they appear to originate from a hidden point in the sky, this is called a radiant. The radiant for the Eta Aquarids is near the star Eta Aquarius, and varies in inferred size by 5 to 7 degrees depending on celestial mechanics. The meteors streak across the sky from a point in space near the star; tracing the paths they make will take you back to this point of origin.

Due to the Earth's orbit around the Sun, the angle the meteors appear at, shifts by a few degrees each night, moving generally eastward and towards the ecliptic (south in the Northern hemisphere, north for the Southern hemisphere.).

One of the unique aspects of the Eta Aquarids is that they're more readily observable from the Southern hemisphere; this resulted in a paucity of observations from their confirmation until the mid 1920s, when Australian and New Zealand amateur astronomers started tracking them. Most years, the Aquarids peak around the 5th or 6th of May, with roughly 30 meteors per hour showing up at the peak from the southern hemisphere, and roughly half that visible to people north of the Equator. (The reason for this is simply that the part of the sky the meteors radiate from is higher in the sky for the Southern hemisphere.) The predicted peaks are usually 15-20/hr for the Northern Hemisphere and for those of us fortunate enough to be in the Southern Hemisphere up to 35/hour on the morning of the 5th and 6th of May! The activity for 2011 is yet to be mapped.

Most meteor showers are gravel and sand-grain sized specks that are left over from the nucleus of a comet as it passes the Sun. Remember that a comet's tail is always going to point away from the sun, and that's where the "train" of gravel and sand comes from. You can think of the entire ellipsoidal path that Haley's comet makes around the sun as being filled with a streamer of sand and dust, all moving at its orbital velocity; over the course of decades, this orbit and the Earth's change, and eventually the Earth passes through this debris field. When it does, these objects impact the upper atmosphere and burn up.

If you're willing to stay up late (or get up early), this is a naked eye observation event. You'll have best viewing around 3 to 5 AM local time. If you're just getting into the fascinating hobby of watching the night skies, this is a good place to start. The objects are bright, move noticeably across the heavens, and only require your naked eyes to see them. More than a few will leave visible streaks that will linger for a half second or more, making them quite memorable and noticeable. The Eta Aquarids, like their sister shower the Orionids in October (also from Comet Haley), are one of the better annual showers to observe from the Southern Hemisphere, because of where they show up in the sky. In comparison to the Geminids and Leonids, they're much more visible for the Southern hemisphere observer.

If you're worried about getting hit by something from the sky, you should be! However it's not going to be from a meteor shower. The big hazard is earth-crossing asteroids, like Apophis (but that's a story for another day). While the Earth takes on about 14 million tons of meteoric dust in a year, the grand total, when compared to the mass of the Earth's atmosphere, is negligible.

What is potentially interesting is that some say the seeding blocks for life came from meteoric dust; this is part of the theory of "PanSpermia" by Sir Fred Hoyle. While not widely trafficked in astro biological circles, it hasn't been ruled out entirely. Certainly, anything that could survive both the vacuum of space, crossing the Van Allen belts, and survive the thousand degree temperatures of crashing through the atmosphere would be quite durable and would likely out compete anything it found on the ground. On the other hand, it's great fodder for science fiction stories, particularly down here in Australia, where our experiences with invasive species have been so colorful.