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\begin{document}

\title{Have there been aliens?  Will there be aliens?}


\author[ ]{Matteo Cantiello}

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The fact we are alive and pondering a vast Universe from spaceship Earth raises a number of fascinating questions. 
Astrophysicist are now asking why ``\textbf{here}'' and why ``\textbf{now}''. What is the chance of life emerging around a star like the Sun, about 12-13 billion years after our Universe was born?



A recent work looked at the past, and set a lower bound on the probability that one or more technological species have evolved anywhere and at any time in the history of the observable Universe \cite{Frank_2016}.
The authors conclude that is very likely that humanity is not the first technological intelligence that has evolved, so far, in the Universe. ``\href{http://www.nytimes.com/2016/06/12/opinion/sunday/yes-there-have-been-aliens.html}{Yes, There have been aliens}'' seems to be the conclusion of this work, although the \href{https://www.authorea.com/25815}{Fermi paradox} seems to point out that either intelligence is a very rare avenue of life and/or alien civilizations do not last very long, or that aliens like to play hide-and-seek with us.\selectlanguage{english}
\begin{figure}[h!]
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\includegraphics[width=1.00\columnwidth]{figures/6---drake-equation/6---drake-equation}
\caption{{\href{https://www.authorea.com/users/2/articles/11048/_show_article}{Drake equation}- a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way. \href{https://www.authorea.com/users/2/articles/28104/_show_article}{Click here to interact with the Drake Equation}.
image source: University of Rochester%
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But let's forget intelligence for a second and just focus on the likelihood of Life\footnote{Life as we know it, that is life involving carbon-based chemistry in liquid water} in the Universe.
In a \href{http://arxiv.org/abs/1606.08448}{paper} just posted on the arXiv and submitted to the Journal of Cosmology and AstroParticle Physics, Avi Loeb (Harvard), Rafael Batista and David Sloan (Oxford), discussed the relative probability of Life emergence in the Universe as function of time. This is, in a sense, similar to asking the question ``What is the likelihood of being born on Earth on a certain year?'' assuming some knowledge of the number of humans alive as function of time across the full history of our planet. 



This is an interesting question, as in the standard cosmological model (called ``$\Lambda$CDM'') our Universe is only about 13 billion years old and expected to exist and expand for a much longer (infinite?) amount of time. So the number of stars that are yet to be born is still large. Obviously, even in an eternal Universe the amount of gas available to form stars is limited. In an astrophysical sense, we recently passed the baby boom phase: The peak in star formation happened a few billion years ago, and the cold gas in galaxies is expected to be effectively exhausted in about 20-30 billion years or so from now. Long story short, it is possible to count how many stars have been born so far and what is their mass, and also how many more will be born in the future. Yes, astrophysics rocks!



Now, it turns out that the most common type of star in the Universe is not a star like the Sun, but one about 1/10th its mass. The particularity of these very-low mass stars is that they are much dimmer: They burn their candle very slowly and live about 1000 times longer than a star like our Sun. Thermonuclear reactions allow the Sun to shine for about 10 billion years. A star 0.1 times the mass of the Sun, on the other hand, lives for 10 trillion years. This offers plenty of opportunity for life to emerge in the future around the \href{https://www.authorea.com/10997}{habitable zone} of stars smaller than the Sun. These stars will still be shining when no more star formation is ongoing in the Universe and stars like our Sun are long gone.\selectlanguage{english}
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\includegraphics[width=1.00\columnwidth]{figures/low-mass-star/low-mass-star}
\caption{{source: cmglee, NASA Goddard Space Flight Center%
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Therefore, assuming these \href{https://www.authorea.com/users/2/articles/173/_show_article}{low-mass stars} provide a not-too-dissimilar environment for Life compared to our Sun, the relative probability of Life-emergence has to peak in the very far future. This is the main result of Loeb and collaborators. Note they can not tell absolute numbers, that is how many planets hosting Life are present at a certain time in the Universe. They just calculates a relative probability, which turns out to be much higher in the distant future than it is now. 

As the authors point out, ``\textbf{The question is then, why do we find ourselves orbiting a star like the Sun now rather than a lower mass star in the future?}''

There are a number of possible answers to this question, here my 2-cents: 
\begin{enumerate} 
\item \textbf{Due to a statistical fluke we arrived very early to the cosmic-life gathering}.
We can only sit here and watch the stars, mostly alone, and imagine what an amazing party it will be. Bummer. Stars less massive than the Sun provide a hostile environment for Life on a planet. They tend to be more ``active'', which could prevent long-lasting planetary atmospheres. Sun-like stars might then be the best compromise for Life, and that's why ``here'' and ``now''.  
\item For some unknown reason, \textbf{the Universe is not gonna last as long as we think}. This is a sort of cosmological equivalent of the \href{https://en.wikipedia.org/wiki/Doomsday_argument}{Doomsday argument}. 
\end{enumerate}\selectlanguage{english}
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\caption{{image source: Justinas Vitkus.%
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