Astrobiology 101: Where are all the aliens?

This post was created as part of my crest project in 2020. You can read the original here or watch it as a youtube video!

Our Galaxy, the Milky Way is over 100,000 light years across. To put in the unit favoured by every science documentary maker- That’s a whole lot of football pitches!

Filling this space is at least 100 billion stars (with some estimates reaching all the way to 400 billion!) and the range in estimates for the number of planets in the galaxy is from ‘just’ 50 billion to trillions and trillions!

So, why aren’t we one of many advanced civilisations in a galactic association, like the plot of many popular science fiction films? 

How Many Planets?

First of all, why don’t we have a definitive answer for the number of planets or stars in our galaxy? Because that could make a lot of difference when estimating the chances of life 2.0!

We can’t physically count every star in the galaxy, it’s too difficult to see past the super bright core to the other side clearly, and other stars get in the way! Anyways, it would take way, wayyy too long: even if you found one every second, and it turned out that there were only the lower limit of stars, it would still take over 3 millenia! (3171 years to be more precise)

And that’s not even factoring in analysing for planets..

Astronomy, Constellation, Dark, Daylight, Exploration
Our galaxy’s core, visible from the southern hemisphere

So if it isn’t possible to count every single one, we have to figure out another way. One idea is to figure out the mass of the whole galaxy, which we can figure out using how fast stars rotate around the centre vs their distance to the centre, and then adjusting for the insane amount of dark matter!

From this measurement, we can then divide by the average mass of stars in our galaxy, et voila: The number of stars in our galaxy… right?

Actually, we don’t know the average mass of a star, because we don’t know what the average star is like! Perhaps our sun is an average star, or perhaps red dwarfs like Proxima Centauri, our closest star after the sun, with masses ranging between 1/10 and a ½ of our sun’s, or what if the average is actually massive short-lived stars? All give wildly different answers when substituted in!

From looking out at other galaxies, we can also see that the average type of star changes depending on the age of the galaxy, but looking around us, it is likely that red dwarfs are the most common type of star, at least in our galaxy: out of the 20 closest stars to our sun (excluding brown dwarfs- failed stars that only emit light dimly from leftover heat and a little fusion), 13 are red dwarfs!

Now, it is possible that our area of the milky way is a hotspot of low mass, long-lived stars, as we know that different types of stars tend to be found in different areas of galaxies (eg. in the galactic centre red giants seem to be most common), but as they are so low mass (so it is more likely for them to be created) and last a long time, it is likely that red dwarfs are the most common, especially in the future! 

Red Dwarf Stars

Being the longest-lived type of star, you would think that it would be a hotspot for extraterrestrial life!…

Unfortunately, young red dwarfs are incredibly active, releasing large amounts of radiation that could hinder, or completely wipe out life on any surrounding planet. They are also incredibly variable in terms of luminosity: sunspots can reduce the amount of light emitted by 40%, and at other times can release flares of energy that can double their brightness in a very short period of time! Despite this, life could survive long enough if they had a strong enough magnetic field, or large oceans for life to proliferate in! 

Another problem that red dwarf planetary systems have is down to their size: red dwarfs are a lot cooler than stars like the sun, so their habitable zones are a lot closer to the star than in our solar system. Luckily, it appears that planets in a red dwarf system, particularly the earth-sized one we have discovered so far, seem to form/reside within this zone!

Which would be great, until you realise that the proximity also means that some of the planets are probably going to be tidally locked (meaning the same side always faces the sun, like the moon with the earth), creating a whole host of problems from the difference in temperature on either side!

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Comparison between Jupiter, a red dwarf (M-type), the sun (yellow dwarf/ G-type) and Sirius (A-type)

Take TRAPPIST-1 for example, the system that hit the headlines as it has 7 earth-sized rocky planets, 3 of which are within its habitable zone! The star is an ultra-cool red dwarf, and is only slightly larger than Jupiter, although it has far more mass.

Artist’s conception of the system via JPL

Its low temperature and mass means that all of its discovered planets orbit within the orbit of mercury- if you were to lay the two systems over each other. Their proximity also means it’s likely that all are tidally locked 😦 Despite this, it has relatively few flares compared to other red dwarfs! 

via JPL

The TRAPPIST-1 system was found by a combination of telescopes, both on the ground and in space, and is just one of the many that have been discovered! There are over 4,000 confirmed exoplanets (planets outside our solar system), over 2000 more potentials, and many more are being discovered imminently!

Obviously, not all of these are habitable: as far as we know life only exists on one planet in our solar system, so it appears that the conditions have to be just right for life to exist. For a planet to be considered habitable, it has to orbit at a distance that allows for the presence of liquid water, as well as being terrestrial, earth sized, and so on.

According to Wikipedia, there are 17 exoplanets that exist in the middle of its star’s habitable zone, and 30 more that are towards the edges (although a few in both categories are unconfirmed). Unfortunately, this doesn’t guarantee the existence of E.T. I have already touched on the problem of radiation/flares, especially in red dwarf systems (although the volatility/activity decreases as it ages). 

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The structure of the Milky Way: Our Sun (yellow dot) is in the Orion Arm (orange)

A Galactic Habitable Zone?

Our part of the galaxy- about midway between the centre and the edge, on the Orion arm- is fairly quiet: our closest black hole is small, and 1000 light years away, our nearest neutron star is around 500 light years away, and the nearest stars possible of ‘going supernova’ are far further away than the 50-100 light year danger zone, so we are pretty safe… I hope!

Our relative security is due to the fact that we are in the Galactic Habitable Zone (GHZ). This doesn’t have totally fixed borders, and would change over time. The conditions and borders for the GHZ have also fluctuated over the short history it has been investigated into, as scientists discover and understand more about our galaxy, and how the universe works! Currently, it is estimated that 0.3-1.2% of stars in the galaxy could support complex life, from a 2011 paper that took past studies, catastrophic event probabilities, distances to the galactic centre and height above the galactic plane all into account. 

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The General Galactic Habitable Zone

What does life need to evolve?

Returning to the smaller scale of individual planetary systems, there are even more hurdles that any life must face, even after the chaotic beginnings of the early solar system! Here are 6 more factors life requires.

  • Essential Chemicals
    • Life probably started off as contained chemical reactions, so chemicals such as Carbon, Nitrogen, Oxygen, Hydrogen and others would be crucial for life!
  • An Energy Source
    • Chemical Reactions require energy- in cells this is ‘provided’ through reactions of ATP. But energy can’t be created, so it has to come from somewhere. For consumers like animals, this comes from the digestion of glucose, which has been transferred up the food chain from autotrophs- the producers that turn pure energy into biomass, like plants, other photosynthesisers, chemotrophs (organisms that obtain energy from chemical reactions in their environments, like around hydrothermal vents)! undefined undefined
  • Liquid Water
    • Although it is used as the main factor when deciding upon habitability, it’s importance is contested. On earth, water is vital for life: without it, our cells wouldn’t function! It would be possible for life to evolve in another liquid with similar properties to water, even though it is very unlikely. In addition, finding liquid water isn’t as simple as finding the correct temperature: liquid water has been found in subsurface oceans of the outer moons, kept liquid due to high salt contents and tidal forces!  undefined undefined
  • A Large Planet Nearby
    • It is thought that the gravitational influence of Jupiter was key to Earth’s development of life, from possibly causing the re-alignment of planets that created the moon (which keeps the earth’s axis stable and causes tides), to pulling asteroids and other projectiles away from a collision course with us! undefined undefined
  • Plate Tectonics
    • Although large volcanic events may have caused past mass extinctions, plate tectonics are important for renewing not only the surface, but the atmosphere too. It keeps our planet balanced, though we as humanity seem to be doing our best to mess that up. Also, the warmth that life needed to begin was probably from a geothermal source like hydrothermal vents!
  • An atmosphere and magnetic field to protect from radiation, whether that is the UV from the sun or cosmic rays. This is one of the problems we face with spaceflight- how to protect the astronaut’s who leave this protective bubble!

Of course, there are cases where organisms survive without water, or an atmosphere, such as the resilient tardigrades, which can survive the vacuum of space (!!), but they most probably evolved from less resistant organisms, so life would always need good conditions, at least to start with! 

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Tardigrades look pretty alien, but they evolved here on earth!

Even though a lot of this makes it seem like we will never find life elsewhere, I have hope! And even if we don’t find anything else, doesn’t that just make it even more amazing that we are here!? And, it makes it even more important for us to protect the Earth, and every organism that lives on it!


Galaxy: A system of millions or billions of stars, gas and dust, held together by gravity

Light Year: The distance light travels in a year. ~9.5 trillion km

Star: A ball of plasma that (undergoes fusion and) emits light after forming from a collapsed cloud of dust and gas. Eg the sun

Planet: A large spherical object that orbits a star and has cleared its orbit of debris.

Dark Matter: material that does not interact with light and can only be detected by its gravitational pull. Has mass. Unknown what it is made of.

Red dwarf: the smallest, least luminous type of main sequence star

Brown Dwarf: An object bigger than a planet but smaller than a star. Does not emit optical light as doesn’t undergo nuclear fusion of hydrogen.

Tidally Locked: An object that turns once on its axis for every orbit. Eg our moon, or a planet very close to its star. One side always face the object it orbits.

Habitable Zone: The typical area where planets that orbit within it could harbour life on its surface. Often designated by where liquid water could exist on its surface.

Exoplanet: A planet that doesn’t orbit the sun/ A planet outside of our solar system, usually orbiting a different star.

Neutron Star: What is left after the supernova of a star 10-25x mass of the sun, made mostly of very high density neutrons

Black Hole: A region of space with so much mass that no matter or radiation can escape it, but is trapped by its gravity. Found at the centre of galaxies or formed after the supernova of a very massive star

Massive: Has a lot of mass

Supernova: The dramatic end to a large stars life where most of its mass is ejected in a huge explosion.

ATP: Adenosine Triphosphate, a key molecule in respiration. Carries energy

Autotroph: An organism that produces its own food. A producer eg plants

Chemotroph: An organism that obtains energy from chemical reactions in its environment. A type of autotroph

Photo Credits:


Star Comparisons: via Wikipedia, by Dave Jarvis, Creative Commons License

Milky Way structure: via Wikipedia, licence:

Tardigrade- doi:10.1371/journal.pone.0045682.g001 licence:

Moon- via Picsart

Volcano- via PNGimg