Conclusions and Course Outlook

Over the course of the last two hundred years, the scientific study of the Universe beyond Earth’s atmosphere underwent a dramatic change. Without the ability to move beyond our tiny speck of a Solar System way out on the arm of a spiral galaxy in the middle of nowhere—without the ability to visit another one of the ten thousand billion billion stars in the hundred billion galaxies that we know exist just in the observable part of our Universe—we have been able to unlock so many of its secrets. We’ve achieved this by sorting out how to decipher the fingerprints that the Universe leaves on light it sends out into space for up to billions of years of solitary travel.

That light somehow manages to find its way to our Pale Blue Dot, shown in Figure 5-14 from a distance of 6 billion kilometres. On the scale of even our Milky Way galaxy, that distance might as well be taken as outside your back door—Voyager had barely even crossed Pluto’s orbit at the time; it was still within our Solar System—and yet the Earth is already barely visible. Even so, here on that Pale Blue Dot that the Universe isn’t even aware of, we’ve become aware of it. Miraculously, light bearing the Universe’s fingerprints does find us, and we’re doing everything we can to make the most of it.

Paradoxical as it may seem, our discovery of all that’s happening out there in the vast reaches of space really began when we started understanding the Universe on its smallest scales. It was by understanding the incomprehensibly small that we finally unlocked the incomprehensibly large.

In this module, you saw how that step in our search for understanding was made. It began with an instrument that could spread a source of light out over its constituent wavelengths and measure its intensity throughout that spectrum. When we had finally sorted out how the different features of that spectrum are produced, our understanding of the world we are a part of had completely changed. We now know what colour is and why it differs in everything we see. We have a better understanding of how an object is illuminated, and we see the connection between starlight and a glowing metal rod. We know that everything glows, whether or not we can see it, and we know that in order for everything to glow the way that it does, all matter has to be made up of atoms composed of particles with quantised charges that can only ever be in quantised energy orbitals. We know that this picture simultaneously explains the absorption and emission phenomena which gave the first inklings that new physics may be required. 

In astronomy, this new view of the world presented an opportunity that we could learn much more by analysing the light that is our only source of information about the Universe: we could now use light to see the atoms that make up distant nebulae and galaxies.

Even so, the amount of information we can potentially gain by exploring distant astronomical objects alone is limited. As you’ll see in Module 11, our inability to resolve stars as anything other than points of light means that the current search for extrasolar planets is limited to looking for the slight dimming as planets periodically pass in front of those points of light, or to looking for Doppler shifts that occur as orbiting planets cause those stars to wobble slightly back and forth. But something we have learned is that every bit of information we are able to gather from our own remote observatory has potential to be applied in our exploration of the rest of the Universe. By understanding how the Sun operates, for example, astronomers have been able to extend that description to account for the differences observed in other stars as well.

But while the Sun is unquestionably important to us, it is by no means the only celestial body we have such direct access to. And as we’ve seen already in this course, the planets have also done their share to advance our understanding of the Universe. Indeed, without them we may never have moved beyond the geocentric view of an Aristotelian universe.

Now the question is: if a scientific revolution occurred simply as a result of sorting out the cause of their apparent motions across the sky, what else is there to be discovered by applying this knowledge we now have of the way that light and matter interact, and studying each of the planets up close? In the rest of the course, we will take up a systematic study of the knowledge about the planets that has been gained in the age of modern physics and space exploration. You’ll find that it is indeed quite a lot, and that the discoveries we have made provide a better understanding of our own planet, where it came from, and what its place is in the Universe.