Exolife: an interesting picture is emerging

Protoplanetary disk (NASA).

There have a been a number of interesting articles coming out recently about alien solar systems. When you consider three articles in particular, the sense that the conditions necessary for life are widespread in our own galaxy gets very strong. The three articles I am talking about are:

Once again, astronomy is revealing how litte special we are. We first thought the Earth was flat and made the entire Universe. Then, we thought the Earth was at the core of some vast space containing the sun, the moon and the planets. Then, we learned the Earth circumnavigates an astral body immensely more impressive than the Earth itself, on which all life on Earth depends. Then, we learned that this sun is one among many, a small dot in a galaxy of stars. We eventually learned that galaxies are also incredibly abundant, and that the Universe is vast way beyond imagination.

One of the last hiding place for our ego was the special configuration of our solar system. Now that we are gathering more and more data on other star systems, we are discovering that our solar system is not so special after all. Here is the thing that strikes me the most about these 3 articles.

It is an interesting fact of our solar system that with the exception of the Earth, the inner solar system is exceedingly dry. This can easily be explained by looking at the history of the formation of the solar system, according to the nebular hypothesis.

In the nebular hypothesis, planets are a natural result of star formation. You have a large cloud of gases and dust which starts collapsing under its own gravity. Eventually, matter piles up more at one particular point, creating an agglomeration of hydrogen so dense that it starts a reaction of nuclear fusion. The star is born, and it warms up the rest of the gases and dust surrounding it.

Of course, the closer you get to the star, the warmer it becomes. Close to the sun, gases like water vapour will remain in gaseous form and will not be able to condense. Beyond a certain distance from the star which is called the snow line, it gets colder and water vapour condenses and agglomerates. If you ever wondered where that cloud of comets and all those icy moons orbiting the gas giants came from, ask no more. It’s simply water that is given the freedom to agglomerate beyond the snow line, and there can be a lot of water in a forming solar system.

So what happens inside the snow line? The sun does not only warm up its proximity, it bombards it with light and other particles, which we call the solar wind. Gases like water vapour that do not manage to agglomerate are pushed away, beyond the snow line. According to this model, the inner solar system should be entirely dry. So why is there so much water on Earth?

The leading hypothesis is that comets placed on funny orbits eventually brought that water back into the inner solar system. Venus eventually lost all its water, poisoned by heavy gases that caused a runaway greenhouse effect. Most of the water of Mars was also lost, although large quantities remained, frozen. The Earth’s story is obviously different, and the reasons for this are numerous. But the point is, if you have comets, you can bring back water in the Goldilocks zone where rocky planets can have water in liquid form for long periods of time.

And now this is also possible with moons of gas giants, making even more solar systems potentially habitable. It looks pretty clear to me from the current science that the astronomical conditions necessary for the emergence of life are easily met in our galaxy. These conditions might happen with an astonishing frequency. We are even less special than we thought.

Is there life elsewhere in the Universe? This is one of the deeper questions driven by pure human curiosity that is in the process of being answered by science. We are investigating the specifics of that question which are well illustrated by the Drake equation. Now, it seems like the focus is shifting away from astronomy to biology. The big mystery of whether other planets can harbour life is being answered. It’s just a matter of time before we have telescopes powerful enough to inspect the atmospheres of rocky exoplanets. The deeper mysteries now are the emergence of life and the evolution of complex life. There are a few places for our ego to hide, but they are going away. They are going away fast, and a Universe that leaves our ego in the dust turns out to be extremely fascinating.

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