I normally don’t write posts about a comic strip, but if you don’t already read xkcd on a regular basis, you should.  The author, Randall Munroe, is a former NASA employee who now spends his time creating comics which are simultaneously hysterical, educational, and thought-provoking.   Often times, the strip is simple and easy to follow, and yet, like today’s entry, very complex and (I imagine) time-consuming to create.

The reason I enjoy xkcd comics so much is because they seem to take a special interest in science topics.  Today’s entry shows us both how much, and how little we know about our universe.  xkcd shares with us the entire known collection of planets as of June 2012 – all 786 of them – and points out that by “known”, we actually know practically nothing about them other than the fact they exist.

Even within our own Milky Way galaxy, we have estimated the existence of upwards of 500 billion planets.  And the Milky Way is only one of over 100 billion galaxies in the universe…at least.  Some estimates are anywhere from several hundred billion to 500 billion galaxies.  Each with potentially at least several hundred billion planets.  Recently, European Southern Observatory’s VISTA telescope snapped a jaw-dropping high-resolution image containing over 200,000 galaxies (below).  This image covers only 0.004% of the sky.  Phil Plait (Bad Astronomy) wrote a great piece on this image, which I encourage you to read.

VISTA image showing 200,000 galaxies – European Space Observatory (3/21/12)

Even more fascinating is the belief that the number of planets out there may exceed the number of stars, which are currently estimated at around 300 sextillion – that’s 300,000,000,000,000,000,000,000.  I’m not even going to try to put that into a perspective you can logically grasp…like the fact that 300 sextillion baseballs could cover the entire surface of planet Earth….over 2.5 million times, which – ignoring that the surface area of this baseball sphere would increase with each layer – would create a coating of baseballs over 115 miles thick!  This information is, of course, meaningless.  It’s impossible to really visualize and understand.   It simply doesn’t register with you.  It doesn’t register with me!  Suffice it to say, there are a lot of stars out there.

We have 8 planets in our solar system (I almost said 9! Sorry, Pluto). Orbiting one of 300 sextillion stars which may host their own planets that we don’t yet know about.  Meanwhile, only 786 planets are known.  Total.  In the entire universe.  This is why this month’s Venus transit was so important to the science community.  Unlike Venus, we cannot see extragalactic planets with our eyes…even when looking through powerful telescopes.  Instead, we rely on minute shifts in a star’s light as a planet passes between it and us.  By detecting these shifts from our sun while Venus crossed in front, we can know what to look for when observing other stars in the observable universe.

Less than a century ago we only just realized that the Milky Way was not the only galaxy in the universe.  With the rate our understanding of how to detect new planets (and stars and galaxies) is increasing, we may soon be able to discover thousands, or millions of new planets.  Monroe summarized all of this pretty well – “This is an exciting time.”

Bonus – One of my favorite xkcd comics (especially the mouseover text):