Aug 25 2016

When Will Life Exist?

proximabThe Drake Equation is a thought experiment identifying which variables are needed to calculate the number of intelligent civilizations in the universe. Some people criticize the equation because we can only guess at the values of those variables, but that is not the point. The point was to identify the variables. This allows us to take the next step in the thought experiment, to plug in possible values and see what answers we get. Also, over time we will get better and better estimates of those variables.

Recently astronomers Loeb, Batista and Sloan published a paper in which they did a similar thought experiment, but instead of asking how common life and intelligent life is in the universe right now, they asked how common life is likely to be over the lifetime of the universe.

Their conclusion:

We find that unless habitability around low mass stars is suppressed, life is most likely to exist near ~ 0.1M stars ten trillion years from now.

Ten trillion years is a long time, given that the universe is only 13.82 billion years old. What they are saying is that there are many low mass stars, or red dwarfs, in the universe. Further, low mass stars have a very long lifespan, hundreds of billions and even trillions of years. About 76.45% of all the stars out there are red dwarfs, and a star with 0.1 solar masses (a tenth the mass of our sun) could survive for 10 trillion years according to our models of stellar physics.

Ten trillion years is a long time for life to evolve and do its thing.

So, if life can develop and survive around red dwarf stars, then, the authors calculate, most life in the universe will exist in the distant future around such stars. Specifically they calculate that life will be 1,000 times more common in 10 trillion years than it is now.

This further means that life on earth evolved relatively early in the history of the universe. Some news outlets reported this as: What If We Haven’t Found Aliens Because Humans Came First? This is not what the paper concludes, however. Loeb is careful to point out that their calculations are relative. They are not asking how likely life is now, but the relative likelihood of life over time.

Life Around a Red Dwarf

The key question here is, how likely is life around a red dwarf star. The primary advantage of such stars is that they are stable over trillions of years. They are a great and reliable source of heat and energy. Our own star has a lifespan of about 10 billion years, so we have about another 5 billion years left. Red dwarf stars are just getting started at 10 billion years.

There are a couple of disadvantages, however. One is that red dwarf stars tend to be unstable in their youth. They tend to flare, dramatically increasing their energy output in minutes, and scouring any nearby planets with radiation. They also may have large sunspots that dim their light output by up to 40%.

Eventually red dwarf stars calm down, but in the meantime they would likely rip the atmosphere off a close world and boil away any water.

In order to be in the habitable zone, a planet around a red dwarf would likely be tidally locked, or at least in a resonance orbit like our own Mercury. If tidally locked then one side would always face the star and the other would always face away.

There may, however, still be plausible scenarios in which such planets could support life. Planets may have originated farther out from the star and then migrated in to the habitable zone after the star became more stable, so they would still have their volatiles which would then heat up, forming an atmosphere and liquid oceans.

A strong magnetic field could protect such a world from radiation, but being tidally locked would mean they would rotate slowly and this might limit the strength of their magnetic field.

If tidally locked, there may still be a strip around the border zone between night and day that is just right for life. A thick atmosphere could also protect the day side and distribute heat to the dark side.

At this point there are just too many variables to make any serious predictions. However, the vast number of red dwarf stars means that even low probability planetary scenarios would still happen frequently and increase the probability of life.

Proxima b

The European Southern Observatory (ESO) just confirmed yesterday that they have discovered a planet around the closest star to our solar system, Proxima Centauri. This is a red dwarf star of 0.123 solar masses – exactly the scenario proposed by the authors.

The planet, Proxima b, is 1.3 earth masses and is in the potential habitable zone of Proxima Centauri. The planet is only 7 million miles from the star and orbits in 11.2 days. This means it is almost certainly tidally locked.

The big question is – could this planet have life? Because it is so close, we may be able to observe whether or not it has an atmosphere, and if it does whether or not there are the tell tale signs of life (like oxygen).

This is what we need – actual observations to better estimate the value of the variables in these equations. We can debate endlessly about the likelihood of life on planets around red dwarf stars, but when we can actually start counting them then we obviously will have a much better idea what the actual percentage is.

If life is common around red dwarf stars, then life is much more common in the universe because most stars are red dwarfs and they survive for a long time. Further, if this is the case, then life’s heyday in the universe will not come for 10 trillion years.

Life around a yellow sun like our own, early in the life of the universe, may be the exception rather than the rule. Again – this is relatively speaking. We still have no real idea how common life is in the universe right now.

Now that we have found lots of exoplanets, so that we can start to form a statistical idea of what typical solar systems are like, the next step is to look for the signs of life so that we can begin to see empirically how common life is in the universe.

Like this post? Share it!

24 responses so far