Are We in a Goldilocks Zone in Time as Well as Space?

A recent finding has just blown my mind and I'm not sure if anyone else has put it together with other results yet. To me at least, putting it together, it makes the universe look much different than I thought at the beginning of the day, so I'd like to share it with you and see what you think. It also suggests that Earth is that much more special. Let me explain.

The Kepler space telescope is exciting stuff, because it's looking for planets around distant stars and finding them all over the place. Here's an orrery of the planets found by Kepler (and even this is more than 18 months old now!):



The real key is to find a planet in the habitable zone (the "Goldilocks zone"), which is the right distance away from a sun so that ice melts and steam condenses, giving liquid water. Note that even this is only one element in the recipe for life, but it's got to be one of the most important ones. Here's one planet recently discovered to be "in the zone."

But, like I mentioned, you need more than liquid water for life. For instance, you need metals far down on the periodic table like iron, molybdenum, etc., to build a rocky planet out of, and to provide important catalysts for life. Basically, you need a decent portion of the periodic table built in order to do complex chemistry. After the Big Bang you start with the simple stuff, hydrogen and helium, and have to build up to iron and company. That's takes a while, past the first generation of stars, at least, according to this article, "a few billion years."

I've been aware of that for a while, and that some people say you need more than "a few" billion years, you need something more like 7 or 8 billion years. (You need at least three because the oldest stars of this type are about 10 billion years old, and our sun's right in the middle of that.) The universe is 13.7 billion years old and the earth is 4.5 billion years old, so by that logic, there's a "few billion"-year window in which an earth-like planet could have been built and produce complex life. Kind of like a "Goldilocks zone" in time rather than space. I've always wondered if the reason why we're not hearing much through SETI or seeing many aliens coming down from the sky (well, I haven't seen them, have you?) is if we're kind of the first kids on the block, because it's taken this long for the periodic table to form and then a planet to form and gestate life. I've assumed there's plenty of time for more planets and stars to form in the universe. I may have assumed wrong.

So, you need water and you need metals. But don't forget, you also need a star to have formed at the same time as the planet. The "photo" part of the whole photosynthesis thing is kind of important. That's why I was shocked to hear that a recent comprehensive study of star formation has suggested that the universe is almost done making new stars. By looking at star formation rates, the astronomers concluded that the golden age for star formation was 11 billion years ago and has been declining ever since, and recently it's just dropped off the table. Check out this graph:

It's not that the universe's biological clock is ticking ... it's more like it's wheezing its last. The universe is not just out of childbearing age, it's close to collecting retirement. [Insert bad "my universe is so old/how old is it?" joke here.] This -- if it holds up, and it looks solid to me -- is one of the most mind-boggling things I've read. It also means that complex life is that much more likely to be rare and precious. If it ain't happened yet, it ain't happening.

Maybe complex life can form in 3 billion years rather than 4 (maybe the "boring billion" didn't have to happen ... but my impression is that it did). Maybe a star has formed 2 billion years ago that will have complex life in 2 billion years. But it looks like the constantly-expanding universe combined with limited energy and matter would suggest that if something hasn't happened yet it will not have much more of a chance to happen. That's a "few billion"-year window that may have already closed. The numbers are adding up to be surprisingly restrictive in time, even in a universe that is huge in space beyond comprehension.

As my physics colleague likes to quote, "There are two possibilities: either we are alone in the universe or there are others like us out there. Either way it blows your mind." For me, the likelihood of the former just got raised, and my mind is suitably blown.