Drake's equation redux

A quite startling five years ago (have I really been blogging that long?) I wrote a post on the Drake Equation, the formula proposed by Dr Frank Drake in 1960 to try to guess the number of intelligent species ("Intelligent Life Elsewhere") in the galaxy.

Since that post, there have been a number of developments:

• Many more extrasolar planets have been discovered; there are now 1070 planets in 810 systems (some systems have multiple planets). Most of these planets are large, some even larger than Jupiter, but a few Earth-sized ones have been detected. When I wrote that post five years ago, it was just 339 planets. When I was a child, some scientists claimed that our own solar system might be unique - they have been proved utterly wrong.
• A couple of dozen planetary atmospheres have been detected, although most of these belong to gas giant planets like Jupiter. This allows temperatures and atmospheric composition to be detected in some cases.
• The first direct pictures of an extrasolar planet have been taken by the Gemini telescopes. It is of Beta Pictoris b, a gas giant several times the size of Jupiter. It orbits the young star Beta Pictoris, which is 63.4 light years from us.
• As we discover more extrasolar planets, we are able to classify them. This has led to a list of planet types. My favourites have to be the puffy planet and Super Earth, the latter of which brings to mind a planet populated with the likes of Clark Kent and Kara Jor-El.
• Latest estimates show that there might be 11 billion Earth-size planets in our galaxy. 
• Only a small fraction of these planets may be in the so-called 'habitable zone', where life in our solar system can exist. However recent research shows that life may exist much further from the sun that we previously thought.

This progress can only continue. I find it an absolute wonder: the galaxy is turning out to be so much more interesting than I could ever have guessed as a child.

And the conclusion must be that we are not alone. The chances of life only developing on our planet in our solar system, when there are so many similar planets, must be minute.

So why are we not hearing them? Why are we not conversing with them? My views have not changed from five years ago, and that one of the following options is true:

• We are the only intelligent lifeform in the Galaxy;
• We are not listening for messages in the right way;
• Other ILE is too far away from us, and their signals too weak for us to detect;
• Other ILE has developed, perhaps several times over, but died out many years ago (disease, nuclear devastation etc).
• They are around us as we speak, watching us and waiting for the right moment to intervene...

Let's see how things change in the next five years.