Planet Earth & Our Place in the Universe

Carl discusses the way photos of our planet, and our universe, have changed the way we think about them, and how we have come to learn about our place in a universe that is full of weird, wonderful and surprising things…

The universe is big. Massive. In fact, we don’t really know just how big it is. We live on one of the eight planets in our solar system that orbit our star, the Sun. The Sun is one of over 200 billion stars in our Milky Way galaxy, all orbiting the galactic center. The galaxy we look into and out of every night is just one of over 170 billion galaxies – and those are just the ones we can see! In just four decades we humans have peered curiously into the cosmos and in doing so we have not just expanded our understanding of it, but also our place in it.

Three Photographs of Earth

With the Apollo missions at the end of the 1960′s, the opportunity to view the Earth from outside low orbit was presented to the lucky astronauts who flew to the moon. These missions gave the astronauts a view of the Earth that most human’s alive at the time had never seen and would never see themselves. These views changed the way that humans thought about their planet, their place on it and their place in the universe.

The first photograph of the Earth was taken during the Apollo 8 mission by astronaut William Anders in 1968. It is a photograph of the Earth rising above the moon’s surface as they orbited the moon. This photograph not only profoundly affected the astronauts aboard Apollo 8, but the entire human population. From that point on the Earth was viewed as a beautiful but fragile ball of rock.

The second photograph to change the way we viewed our planet was taken just a few years later in 1972 by the crew of Apollo 17. On their way to the moon, the astronauts turned towards the Earth and captured what is still one of only a handful of images to show a fully illuminated Earth. The name given to photograph is ‘The Blue Marble’ because the Earth looked like a blue marble from the astronauts’ perspectives. This photograph reinforced the view of Earth as a fragile, vulnerable and small and became the pin-up for the environmentalist movement.

The two photographs above made us think about our place on planet Earth and how small it really is. In a single picture of the Earth you could see half the population – it really puts things into perspective. The third and final photograph of Earth makes us realise how isolated we are. Within our solar system, within our galaxy and within the universe. Snapped by Voyager 1 in 1990, the ‘Pale Blue Dot’ picture was taken at a distance of 6 billion kilometres (or 3.7 billion miles, or 40 AU, or 0.0006 light years) from Earth.

If you are wondering where Earth is in the above photo, it’s in the top band of orangey light near the top right of the image. That pale blue glow is planet Earth and home to everyone that has ever lived and everything we humans have ever done.

Our place in the Milky Way

Our planet is orbiting our star, the Sun, which itself is orbiting the center of our galaxy, the Milky Way. We believe this to be a barred spiral galaxy with over 200 billion stars just like our Sun and it is over 100,000 light years across! If you look up at the sky on a clear night, you can actually observe the plane of our galaxy, with the huge dark dust lanes and the bright galactic center clearly visible.

Source: Digital Sky LLC

Our solar system is located about half way from the galactic center, about 25,000 light years away. We are located in one of the many spiral arms of our galaxy and complete one orbit of the galaxy about once every 200 million years! The Milky Way is massive and to put this into perspective, our nearest star is just a little over 4 light years away which is a unthinkably large distance in its own right. But there are are over 200 billion stars in this galaxy, each one that bit further away from us.

The Milky Way’s Place

When we look up at the night sky we not only see stars twinkling back at us, but also millions of galaxies that appear as faint fuzzy blurs that we can almost resolve. There is one famous image, taken by the Hubble Space Telescope, that solidifies the view of our galaxy as just one of hundreds of billions in the observable universe alone and lets us humans know our galaxy, at least, is not unique. The image in question is the Hubble Deep Field and is a mosaic of images taken by Hubble in 1995. All but a few of the 3,000+ objects in this image are galaxies, some still being the most distant galaxies ever imaged.

Source: R. Williams (STScI), the Hubble Deep Field Team, NASA

This image was followed by the Hubble Ultra Deep Field in 2004 which managed to snap over 10,000 galaxies and provide evidence for high rates of star formation in young galaxies less than a billion years after the big bang.

One question asked by astronomers is what does the large scale structure of our universe look like? Is it homogeneous and isotropic on large scales as most cosmologists like to believe? In 2003, the 2dF Galaxy Redshift Survey, which mapped out the structure of galaxies in local part of the universe, made their results public and the images that were subsequently released showed that our part of the universe was filled with hundreds of thousands of galaxies that clustered together. It also showed that these clusters of galaxies also interacted with each other and formed filaments in the image. Each point in the image is a galaxy.

Source: The 2dFGRS Image Gallery

Increasing radial distance outwards relates to increasing redshift of the galaxies, or rather looking further back towards the big bang. The filaments of galaxy clusters are easily seen at lower redshifts (closer to center) with a less structured distribution at high redshift (further away from center). This image should shock and awe you at the same time. What you are looking at is essentially a map of nearly every single galaxy in just our local neighbourhood. Our tiny planet, nor even our solar system, registers in this image.

Down to Earth

By now, it wouldn’t be uncommon to be thinking about how insignificant our blue planet really is in the grand scheme of things. We are a small blue ball of rock in a solar system that we are beginning to realise is just one of many such systems in our own galaxy alone. Our solar system orbits the center of our galaxy, the Milky Way, which is just one of hundreds of billions of galaxies in the observable universe. There may also be hundreds of billions more outside of the observable universe that we may never be able to detect.

Even though all of this is true, there is something that, as far as we know, makes our tiny blue ball of rock and water unique, special and rather interesting: us. Our planet teems with life and some of that life has the ability to look out into the cosmos and be amazed by the beauty and wonders that our universe beholds. We humans have the unique ability to comprehend just how big our universe really is and have only just begun to understand our place in it.

Further Your Knowledge

The universe is a big place with many beautiful and strange things in it. Here are just a few things you could research to satisfy your curiosity about our weird and wonderful home:

• The Apollo missions
• Sloan Digital Sky Survey
• Hubble tuning fork diagram
• How many planets are there in the Milky Way?