Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy

Is there other life out there?

Requirements for Life

Earth is a pretty special planet. It’s just the right distance away from the Sun that it’s warm enough to sustain life, has plenty of water to help life thrive and has a well behaved orbit around the Sun. But is it unique? Are there other planets out there similar in composure to Earth, or the right distance away from their star to support life? Well, let’s first have a look at the 2 main conditions for life, and we’ll take it from there.

The Goldilocks Zone

The first condition I’m going to look at is the distance from a planet to its parent star. If the planet is too close to its star, say like Mercury, then the surface temperature is too hot for life as we know it to survive. On the other hand, if the planet is too far away, such as Uranus or any of the gas giants, it’s too cold for life. The region in between, where the surface temperature of planets should be around 22 degrees Celsius, is called the Goldilocks Zone (not too cold, not too hot, but just right). The image below shows this habitable region for the Sun.

The habitable zone for the Sun lies in between the 2 green circles

The habitable zone for the Sun lies in between the 2 green circles

Of course, this will change depending on how hot the star is. For hotter stars, the habitable zone lies further out, while for cooler stars, it lies closer to the surface of the star. So, we can conclude that the goldilocks zone depends on the temperature (and hence, also size) of the star.

Presence of Water

As we understand it, water is essential for life to prosper. So what if I were to tell you that when the Earth formed, it had pretty much no water on its surface? “But wait, how did we evolve?” I hear you scream. Well, basically, we got lucky. The Earth went through a period known as the “Late Heavy Bombardment” where the inner planets of the solar system were constantly struck by large meteors and asteroids. During this period, the Earth has been described as a “living hell”, as the surface would have been incredibly hot, volcanoes would have erupted constantly across the globe, and no life could have survived. The result? After the surface of Earth had cooled down, the comets and asteroids had left behind the building blocks for water. And as Earth cooled, water (and lots of it) formed. Lucky us!

Are we unique?

So it looks like Earth is pretty unique – not only does our planet live in the habitable zone, but it also got lucky enough to get covered in water during its formation. The next logical question is: are we unique? The problem with trying to answer that questions is to be absolutely sure, we’d need to know every planet that orbits every star in the Universe. Firstly, we need to know the amount of stars in the Universe. Our Galaxy has an estimated 200-400 billion stars. So how many galaxies are there in the Universe? Look below for an idea.

A Deep Field image taken by the Hubble Space Telescope

A Deep Field image taken by the Hubble Space Telescope

The above image is a tiny portion of the sky, taken by the Hubble Space Telescope back in 1996. And it shows a plethora of galaxies, in a tiny region of the sky. So immediately we can say that the amount of stars in the Universe is essentially uncountable. And even if we could count them, detecting a planet orbiting a star is no easy task. If it’s a large planet (like a super-sized Jupiter) then we can see the planet pass in front of the star if we’re lucky, or we can detect the star wobble.

To try and get through this mess of uncountable stars and undetectable planets, scientists invented the Drake Equation. Rather than explain it, I’ll let you explore it for yourself through this link. The quote the 2 extremes of the calculations, it tells us that

  1. Pessimistic View – There is 1 civilisation in our Galaxy that can communicate with other civilisations (and it’s us)!  However, there are about 78 billion in the entire Universe.
  2. Optimistic View – There are 72,800 civilisations in our Galaxy that can communicate with other civilisations  However, there are about 10,920,000 billion in the entire Universe.

Either view shows that we shouldn’t be alone in the Universe (however, the Drake Equation isn’t the most serious of equations, so don’t take its results to heart).

Earth v2.0

A big question in the Drake Equation is how many earth-like planets are out there? Below is a lovely image summarizing all of the data on extra-solar planets we currently have.

Here, the R with the circle and cross through it stands for Earth-Radius. Hence, 1 R is the same size as the Earth, while 5 R is 5 times larger than the Earth.

Straight off, there are quite a few Earth like planets out there. So why aren’t we hearing anyone?

The answer to why we aren’t hearing anything has many different forms. Other civilisations might be communicating with us on frequencies we haven’t tried yet, or their messages just haven’t reached us. Maybe on the other planets in the Universe, the extinction of the dinosaurs never happened, and the planets are ruled by T-Rex’s with tiny arms. Or maybe, the Drake equation is completely wrong, and we truly are alone. Now, all this talk of planets orbiting other stars in different galaxies is all very good in theory, I mean it’s not like we’ll ever meet or have conversations with these civilisations (if they exist, and at least not right now). So now, let’s shorten our range. Infact, what if I told you life might exist within our own solar system?

Eureka, it’s Europa!

Below is an image of Jupiter and several of it’s moons. Of interest are the 2 largest in this image, 2 of the 4 Galilean moons, Io and Europa. Io is another example of a living hell – its about the same size as our moon, has an  atmosphere of mostly sulfur dioxide, and has over 400 active volcanoes on its surface.

Universe Sandbox - 20130225-122941 - 4063

Europa is on the extreme opposite side of the spectrum – it has a surface of ice, and an atmosphere of oxygen. But what’s underneath is a lot more interesting. Scientists have good reason to believe that underneath the ice, there are oceans of salt water! And since the water isn’t frozen, its temperature has to be high enough that life might blossom. Which means we mightn’t have to look further than Jupiter to find evidence of other life in the Universe!

So what to believe

We’ve found roughly 351 Earth sized planets. Some of them exist within the Goldilocks region, while some don’t. If they do, we don’t know if they have water on them (due to similar periods of Heavy Bombardment or other  means). In the end, the question to ask is what does this mean to us? Well, if there isn’t other life out there, it makes the Universe a very lonely place indeed. But if there is life out there, then we aren’t as unique as we’d like to believe. Which means the search for an answer to this question probably has more philosophical meaning than a physical one.

So what do you believe?


Thursdays post is going to be all about Dark Energy. If you have any contributions, or any comments, please post them below!



About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

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