BLOG of an astronomer

On this webpage I will share my experiences and answers to some of the most interesting questions with you. • EN/NL

On this blog you can follow me in my search for biosignatures of the Earth as an exoplanet. Ever since the first detection of a planet outside our solar system, our hope to find life on an other planet has been increasing. However, to detect life we need high-tech instruments and systems can carry out a series of steps to know what we are looking at. Observatories are at the moment looking for planets that look similar to the Earth. ••• In my project I will start with something that we all know, our own Earth, the only planet for which we know there is life. Starting off with existing research, models and instruments I will work on an instrument that can measure (signatures of) life on Earth. The ultimate goal is to send the instrument into space and look back at the Earth to make an image of how the Earth, with its life, looks like from space.

Do you want to go back to my homepage? Click here

Posts from December 2021

Rewrite the history of the Cosmos

Leiden, the Netherlands

It takes a planet to explore the Universe.

The process of research is never linear. To keep this line of thought alive, I will also let the current blog take a little bit of a turn. While I expected to tell more about my own research projects in the past month, the launch of the James Webb Space Telescope became slightly too exciting!

After 25 years, the time had come. On Christmas morning, my family was aware of the importance of the James Webb Space Telescope for astronomers and we all sat in front of the television, watching the NASA livestream. Everyone held their breath during the last 10 seconds to 13:20, only to then see Ariane-5 disappear into the clouds in a sigh of relief. Seconds later, the mobile phone is glowing with the messages: "Merry Christmas”. After all, this Christmas, all astronomers received a wonderful Christmas present: a successful launch of the James Webb Space Telescope.

James Webb Space Telescope • Learn more about the Cosmos with the James Webb Space Telescope

The James Webb Space Telescope, which originally was known as the Next Generation Space Telescope, wrote history before it was even launched. Where the original 'simple' design began as a 500 million project taking only 11 years to complete, it has evolved into an astronomically complicated 10 billion project that required thousands of researchers from more than 300 institutes to finish and prepare for launch. What makes the telescope so extraordinary and complex?
  • Gigantic: It is the biggest space telescope ever. The entire telescope is as tall as a three-storey flat and as wide as a tennis court. Since the Ariane rocket body is 'only' 4.5 metres wide, the telescope was designed to be folded in origami-style to fit inside the rocket. Once in space, the telescope unfolds.
  • Honingraad: Honeycomb: The primary mirror is made up of 18 golden individual hexagonal mirrors, each of which is almost 1.5 metres tall. Obviously, everyone currently knows exactly how tall 1.5 metres is. Combined, the mirrors form one single large mirror that is no less than 6.5 metres tall and wide.
  • Smooth: The surface of the individual mirrors should be so smooth that, if the entire surface of the mirror were as large as Europe, there should not be any hills or valleys higher than an ankle's height.
  • Solar protection: A solar shield is meant to protect the telescope from infrared radiation coming from the Sun, the Earth and the Moon. In this way, the shield forms a barrier between the solar and shadow sides, which have a temperature of about 110 °C and about -230 °C respectively. It is like using sunscreen, only then with a sun factor of 1 million. In order to form this barrier, the sunshield consists of 5 separate layers. Each layer is made of Kapton and is as thin as a single human hair.
  • Powerfull: Webb is 100 times as powerful as the Hubble Space Telescope. Webb can capture light from objects created 50 million years after the Big Bang. These new discoveries will reveal what the primordial stars looked like 13.7 billion years ago. Until now, the record for observing the oldest and most distant object has been set by Hubble. This object was located at a distance of no less than 13.4 billion years.
  • Unreachable: After the first observations made with Hubble, it appeared that the first mirror of the telescope had an optical aberration problem. Hubble orbits the Earth at a distance of about 570 kilometres. This made it possible for astronauts to repair Hubble while it was in orbit. For Webb this will not be a possibility. Webb will be heading towards the Sun-Earth L2 point which is 1.5 million kilometres from Earth and in line with the Sun. This is almost 4 times the distance between the Earth and the Moon. While it takes Webb only 3 days to travel the distance to the Moon's orbit, it will take Webb 30 days to reach the L2 point. Consequently, the telescope cannot be reached when optical aberrations occur. At least, not with today's technology.

For many astronomers, the observations of the James Webb Space Telescope promise to be very valuable. Personally, I am mainly interested in the search for the building blocks of life elsewhere in the Universe. Webb will give us more insight into the constituents of the atmospheres of extraterrestrial planets orbiting stars other than our Sun. In these atmospheres we look for example for water, oxygen, ozone, methane, phosphine, and in particular certain combinations of these atmospheric gases. With particular mixtures of these gases, we are trying to find life as we know it on Earth today.

December 31, 2021


Posts from November 2021


Leiden, the Netherlands

Simplicity lies concealed in this chaos, and it is only for us to discover it.

During the past month, I disappeared once more into the optical laboratory of the University of Leiden. This is where, next to Treepol1 and a cosy plastic Christmas tree, you will find the instrument I am currently working on. This instrument is called the Life Signature Detection Polarimeter, also known as LSDpol. It is a spectropolarimeter which measures the polarisation of light over a spectrum (i.e. the polarisation of the specific colours of the incident light).

The special feature of LSDPol is that it will be the first spectropolarimeter to measure at one moment what percentage of the incoming light is linear and circularly polarised, without any moving part. This is quite important if we want to design an instrument that could potentially be launched into and used in space. However, the principle involves many things. Given that I have not told so much about the instrumentation side of my research up to now, I think it would be nice to change this in the coming posts.

Before I can explain more about the design of and the measurements done with LSDPol, I have to start off with a little theoretical explanation concerning light. Therefore, today I will introduce you to an important physical effect that is important to take into account when designing and building many optical instruments.

Fresnel diffraction • The propagation of lightwaves behind an aperture

In 1801, Young's two-slit experiment showed that light sometimes behaves like a wave, which was completely contrary to Newton's particle theory. To this day, the two-slit experiment is a classic hands-on experiment for physics students, including myself. In the experiment you shine a light source through a single slit. The light from the light source must be spatially coherent. With coherent light one can think of:
  1. laser light: light that consists of one wavelength, i.e. has "one colour", or
  2. sunlight: light that consists of several wavelengths in which the so-called phase of the light for all the different wavelengths is similar.
Young used sunlight in his experiment. The sunlight was shining through the single slit on to two slits that together broke up the sunlight. This caused interference bands to appear at the places where the light beams overlapped. Unfortunately, as Newton's influence was so strong at the time, it took some time before the scientific community accepted the theory of "light waves". accepteerde.

Based on a theory developed by Christiaan Huygens, Augustin Fresnel proposed a mathematical model that could describe the diffraction of light rays. In 1808, Fresnel was one of the participants in a scientific competition that sought to find a mathematical description for a diffraction phenomena. The phenomena was observed in the shadow of a circular screen when it is placed in a beam of light. One of the judges, Siméon Poisson, predicted that, according to Fresnel's theory, at the centre of the circular shadow a bright spot would appear with the same luminosity as the light rays hitting the screen. Shortly afterwards, François Arago, one of the other judges, carried out this experiment which showed that a bright spot appeared on the screen. This confirmed Fresnel's theory. This bright dot is now called the Poisson, Arago or Fresnel spot.

Well, why am I telling this story about Fresnel and Fresnel's theory? Apart from the fact that I admire Fresnel for the theoretical and experimental study of the behaviour of light (basically what I also do with my instruments), I recently found out that in order to understand my instrument, I also need to learn much more about the Fresnel theory. The reason why this theory is so important will be explained in the coming posts. Stay tuned!

1 TreePol is one of the instruments that I use in my search for signs of life on Earth. The design of TreePol is made with the aim to measure the percentage of a light beam that is circularly polarised as accurately as possible. The molecules in vegetation produce this circular polarisation signal when sunlight reflects off the surface of the leafs.

29 november, 2021


Passing on knowledge is a true art

Leiden, the Netherlands

If you can't explain something simply, you don't understand it well enough. A. Einstein.

Ready, stage, go! The past few weeks I was very happy to be able to give four separate talks on my big hobby astronomy and on "life in the Universe ". Full of enthusiasm (and also a bit stage fright), I stood in front of four different groups and told them about my research. Two of them took place in a classroom, one on a pop stage and the last one even in a pub! But what does all of this have to do with Albert Einstein's quote in the title of this blog?

The quote describes very well what my approach is when I want to explain something, how I prepare for different talks and how I decide on what exactly I am going to talk about. I am always sure to be able to answer the following questions: "What am I excited about at the moment?", "How long do I have to talk about this subject?", "What does my audience know about this subject?", but mainly: "What will be my take-home message?".

"Everything should be made as simple as possible, but not simpler" said A. Einstein.1 Sometimes I am listening to a presentation and all the information goes into one ear and out the other one. At the end of the talk, I then ask a question and get an answer that is even more confusing than the presentation. For me personally, the art of transferring information is always to make things as simple as possible without losing or affecting their essence. At the end of my talk, I want to have passed on some of my knowledge to the audience. As a scientist, I dare to say that a great fear of some scientists is that when a difficult theory is simplified too much, whatever you are telling is no longer exactly accurate. There is also the fear that you will come across as 'less clever' than you actually are, because everything you say is 'simple'. For example, after a presentation, a student came up to me and told me that he/she would never study astronomy because it was too easy for him/her. This taught me that it is very important to have the perfect balance where the audience understands your presentation however, the presentation is not too simple. This concept is crucial.

In the last two weeks, while preparing my talks, I came across a splendid theory, which made me think about Einstein's quote again. This theory is also known as the "Iceberg Theory".
Draw211102.1 The power of elimination - Iceberg Theory • The density of ice is about 10% smaller than the density of water. This is why icebergs (partly) float on the water. The largest part of the iceberg lies below the water. Therefore, we only see a small part of the ice. The American writer Ernest Hemingway described his writing with this principle of an iceberg.

Ernest says that although as a reader you can only see the tip of the iceberg, you also know that there is still a lot of ice (knowledge) supporting the tip. He mentions that when a writer lowers the tip of the iceberg into the water a tiny bit deeper, the iceberg gets an even firmer support. Only when a writer leaves out some information because he doesn't have the knowledge, there is a gap in the foundation which doesn't help the story.

I dare to compare my way of presenting with this iceberg theory. When I give a presentation to an audience, I also only show the tip of the iceberg. For me it is important that when I talk about a subject I can also expose the ice beneath the water (the knowledge) when I get questions on the topic. So in other words, I have my own variation on Ernest's iceberg theory.

Important lesson 5: Before making a presentation, think about how big "the iceberg" is that you want to present. Only when you have a clear picture of the whole iceberg can you start thinking about what the tip of the iceberg is going to be like.

1 This quote by Albert Einstein has been discussed several times. Scientists are not sure whether he said it exactly in this way. It is believed that it was an interpreted summary of one of the statements in his 1933 lecture. There he mentioned that "it can hardly be denied that the highest aim of all theory is to make the irreducible basic elements as simple and as few as possible without having to give up the adequate representation of a single fact of experience". Quite a mouthful, isn't it!

November 2, 2021


Posts from October 2021

Corona virus vs. Ice Crystals

Leiden, the Netherlands

You can not escape from the aerosols

The COVID-19 aerosols are slowly disappearing from the air and with that social activities are starting to pick up again. Especially now, it is important for me to maintain the wonderful habits we picked up during the quarantine. So, it's time for a brand new (science) blog!

Since posting the June blog, I have certainly not been inactive. In July, I managed to write a proceedings paper for an instrumentation conference that was held in San Diego, California. This SPIE conference, Society of Photographic Instrumentation Engineers, focused on polarisation instruments and remote sensing. I will tell you more about this topic later!

At the moment, as the drawing of this blog illustrates, I have started an investigation into (the scattering of light by) ice crystals. But why ice, and how does this connect to my search for "biosignatures” and my models of an Earth-like exoplanet outside our solar system?

Well... Not only is looking at beautiful ice crystals a wonderful occupation, but apart from the fact that you can stare endlessly at the infinite different possible shapes and structures, they are very important in, for instance, climate studies. The effects of the presence of ices in the atmosphere are still very unclear though could potentially cause major changes in the temperature of a planet, including our Earth. I do think that the climate is very important, but what I am looking for is how light is scattered by ice crystals. The scattering properties of ice strongly depend on the shape and size of the individual crystals. For example, many small crystals will reflect more sunlight than less large ones. In many models, researchers assume that the ice crystals look like a hexagon. This shape makes it easier to calculate how light is scattered, but we can actually say that the assumption is not entirely correct. In fact, if all the ice crystals in the atmosphere had a hexagonal shape, we would witness many more halos.

Important lesson 4: Do not try to google "ice halo" (ijs halo) in Dutch. You will only find pictures of (very tasty) ice cream in cups.

Ice halos are beautiful optical effects that are created by the refraction of sunlight. There are various types of halos that can be created. The most commonly known ice halo is the 22° halo. It gets its name from the fact that the radius of the circle is 22° with the light source located in the centre of the circle.

Okay, I am deviating from the subject, because why are ice crystals so important for my research? Our Earth's atmosphere contains many thin ice clouds that scatter sunlight in a different way than the spherical (liquid) water clouds. If I want to investigate whether I can measure the signals of life on Earth through the clouds, I also need to understand the effects of ice clouds above the Earth's surface. That is what I am working on at the moment. So, in the coming months there will be updates about my new passion: ice crystals.

Oktober 10, 2021


Posts from June 2021

Do you know liquid crystals? Well, you look probably look at them right now!

Leiden, the Netherlands

Liquid crystals in your television, as well as in my optical instruments

The first time I heard the term: 'liquid crystal', I thought of a kind of pan on a very high heat in which crystals were melted. A liquid is a liquid, right? Well, technically not entirely. Molecules in liquid crystals are capable of flowing, but at the same time they are also ordered and oriented with respect to each other. This means that if we look at the crystal face forwards, it has different properties than if we look at it from the sides. We can modify the ordered direction of the molecules by passing an electric current through the crystal or by exposing it to light or heat. This enables us to change the optical properties of the crystal. One important property, for example, is the refractive index1, which depends on the polarisation properties of the crystal.
Draw210627.1 Birefringence • Some materials influence light by an optical property that we call birefringence. Birefringence, also referred to as double refraction, is a property in which the refractive index of a transparent material depends on its polarisation direction. A non-polarised light beam incident on the material is split into two polarised light beams that have their polarization directions perpendicular to each other. For instance, horizontal and vertical. Christiaan Huygens, a famous Dutch astronomer from the 17th century, described this phenomenon while exploring the double refraction of light in a crystal called calcite.

Why am I telling you about liquid crystals and this birefringent property? Well, a nice fact is that you look at these liquid crystals (almost) every day! The abbreviation LCD stands for liquid crystal display. The crystals in the pixels of the screen make sure that we can easily switch the pixels off and on with a tiny electrical current.

In addition, I work with liquid crystals as part of my research. Not only do I spend a lot of time behind a computer screen, but they are also incorporated into components of the instruments that I use to do my research: Treepol and LSDpol. In their liquid crystal components, we can use birefringence to change incoming polarisation signals. For example, we can transform linear polarised light to elliptical polarised light. Soon I will tell more about how we can also use these crystals without using electricity. This is essential for optical instruments that we want to send into space.

1 The refractive index of a material indicates how much light is deflected when it travels through the interface of two different materials. The index of refraction depends on the wavelength (also called 'colour') of the incoming light. This wavelength dependency helps us to understand why we can use a prism to split white sunlight into different colours.

June 27, 2021


Posts from May 2021

Did you try to turn it off and on again?

Leiden, the Netherlands

Do not panic! Trust me, I am an engineer, or... am I?

Last week, I spent quite some time working in the lab again. In the near future, I would love to do some outdoor measurements, to start searching for life. However, there are still a few technical issues that are holding me back from going outside.

One of the instruments I work with is called TreePol and measures the circular polarisation of light. This state of polarisation is generated, among other things, as a result of the homochiral property of chlorophyll (in green leaves).The instrument is a spectropolarimeter. It measures polarisation over a spectrum, just as the word spectropolarimeter implies. Looking at the polarisation signal over a wider wavelength range is important. Chlorophyll molecules found in leaves react most strongly to light when it has a wavelength in the red region of 680 nm. This causes part of the sunlight hitting the molecules is circularly polarised. This circular signal is very small (~ 1/10 of a percent!) but big enough to tell whether a leaf is alive or dying.

But how can TreePol measure this (circular) polarisation state of lightwaves? In optics, we have optical elements that slow down the electrical direction of light, so-called retarders. With these, we can turn circular polarisation into linear, and vice versa. You can find more information below!

Draw210520.1 The principle of Treepol • measuring by modulating

By shining non-polarised light through (transmission) or on (reflection) an ivy leaf, we activate this characteristic feature of chlorophyll. We aim TreePol at the leaf and the scattered light falls on the instrument.

  1. At the front there is a half-wave plate that we rotate very fast with a very fast motor. Only circular polarised light can travel through this, bringing this signal into our instrument at high-speed (like a train).
  2. Thereafter the light passes through a Fresnel romb. This is also a retarder that delays the light twice, like a kind of bridge, by one-eighth of a wavelength. In total it delays the wave by a quarter of a wavelength, changing the circular signal into a linear one.
  3. A ferroelectric liquid crystal functions as a kind of traffic light that we can control with electricity. When it is red, it changes the incoming signal in exactly the opposite way to when the traffic light is green. As a result either a vertically or horizontally linearly polarised signal comes out.
  4. Finally, we have a polarising beam splitter which, like a train switch, sends the vertical and horizontal in two different directions.
At both 'track ends' we place a spectrometer that measures the vertical and horizontal signal. Afterwards, we can use mathematical calculations to find out how much circular polarisation entered our instrument! Clever, isn't it?

According to the Cambridge Dictionary, the definition of an engineer is a person whose work consists of repairing or controlling machines, motors or electrical equipment. The verb engineer means to design and build something according to scientific principles. On the Internet I also found some terms that are applicable to me. For example, I am a person who is very good at solving problems that I didn't know I had.

1 See my blog post on March 21 to find more information regarding homochirality.

May 20, 2021


The Beetles

Leiden, the Netherlands

"When the night is cloudy there is still a light that they will see"

Yes, I know that this sentence is not to be found in "Let it be", but wow, it really fits my thoughts perfectly. Last week, I had the pleasure of giving two presentations to tell an audience and colleagues about my research. And I confess: while preparing these talks, I always get super excited again about my research!

For example, take the case of the theory of homochirality: the left- or right-handedness of molecules that is characteristic of life. In my blog of 21 March, I explain that we are looking for unambiguous signs of life, of which the concept of homochirality is central to my research.

During my talks, I explained the principle of homochirality with a shiny green beetle. Green beetles are 100% homochiral. When sunlight shines on their skin, the reflected light is 100% left circularly polarised. This causes us to see a green beetle with one eye and a pitch black beetle with the other, when we look at the beetle through 3D glasses! Isn't that cool!

Besides this phenomenon, non-green beetles can also be linked to astronomy. African dung beetles study the sky at night. They navigate themselves with the Moon and, when the sky is cloudy, with the polarisation direction of the Moonlight. Moonlight is reflected sunlight, so during daytime they do the same thing but then with sunlight! In this way, they always know how to find their direction of travel at all times.

"When the night is cloudy there is still a light that they will see", the Beetles.

May 10, 2021


Science: more than just solving a puzzle

Leiden, the Netherlands

More than half of the process is spent on actually identifying and organising the puzzle pieces

Everyone who has ever solved a puzzle knows the following technique. Your puzzle is square-shaped. First you identify the four corners. They are easy to recognise and it gives you a rough idea of the orientation of the puzzle right in front of you. Then you take care of the foundation, the sides. The sides tell you something about the size of the task at hand. Then you start sorting out the colours. This is the toughest and least interesting process. But you know that after you have spent all this time on this preparatory work, you will spend less time collecting the pieces. All of a sudden, you will be able to lay many more pieces in one go.

If you did not recognise the following with puzzles, then perhaps you could do so when building with LEGO or even when putting together an IKEA kit. You do not want to find out that when you get to the last few parts, a piece of the puzzle is missing.

At the moment, my model of the Earth is also a giant puzzle. I am looking at 287 unique combinations of surfaces including clouds over a range of 209 wavelengths. If I include the four different phase angles as well, I will be looking at up to 240 thousand pieces1! That is a massive puzzle. Now you can probably imagine that until yesterday I did not realise that I was still missing about 40 thousand pieces. Fortunately, I found them all today! Now it's time for the final steps. First, write a program so that my computer can solve this puzzle properly, and then do the most important thing for a scientist: understanding exactly what is depicted on the puzzle.

Oh, I forgot.. Did I already tell you that I still have two more of these puzzles to solve?

1 One puzzle requires about 3.5 TB of data storage.

May 3, 2021


Posts from April 2021

Programming is not about what you already know, but what you can find out

Leiden, the Netherlands

Murphy's Law: Anything that can go wrong will go wrong.

Three months. Ninety days. Twenty-one hundred and sixty-one hours. One hundred twenty-nine thousand six hundred minutes. Aka 7,776,000 seconds of continuous code execution before I found out that there was one (1) error(s) in my code. Unfortunately, I can re-execute the entire code once again. "But", I tell myself smiling with crooked toes, "I will never forget the importance of using the correct data types again!"

Programming is based on a simple concept: The computer does exactly what you tell it to do. However, it is not as easy as just asking Siri, Google or Alexa a simple question. But if we cannot talk, how can we get a computer to do something for us?

As programmers, we write a finite set of instructions to go from an initial state towards a final goal. These instructions are referred to as an algorithm or program and are written in a simple text editor. A general text editor that many use is Microsoft Word. However, this is not recommended for writing programs! I personally use the PyCharm and the BBEdit text editor. Unfortunately, just writing a simple instruction is not enough. To be able to communicate with a computer, we have to write it in a specific language. A computer understands as much about a proper English text as I understand the Hebrew language, which is nothing. Just as we have many different languages in this world, there are also several languages available for computers. Common and widely used languages are Python, Java(Script), C(++) and R. I typically write programmes in Python programming language and in a somewhat older language, called Fortran.

Well, and then what? We have the instructions, but how do we give those to the computer? When programming in Python this is extremely simple. Python code is interpreted directly by a Python interpreter that we use to run the program. Other languages require a compiler. A compiler is a programme that converts the programming language into machine code, which can then be executed directly by the computer's processor. Since there is no extra interpreter between the code and its execution, working with a compiled code is much faster.

So why do we not all work with compiled codes. Well, writing this compiled code is a bit more complicated. For example, at the beginning of each program, you have to specify the type of each individual variable (string, integer, float) and you have to specify the amount of memory you are going to use for storing these variables. In addition, we can not actually read compiled code: Only the code that the compiler compiles is human readable.

"Every advantage has its disadvantage", I think that a knowledge of several languages will always come in handy. Just like in this world, you never know what you can expect and where you will end up but you always want to be a little bit ahead of the game.

April 24, 2021


The polarisation of light: more than just an intensity

Leiden, the Netherlands

What is polarisation of light and what do sunglasses have to do with it?

The past couple of weeks, I have been working on the calibration of a polarimeter. As the name suggests, the instrument measures the polarisation of light. So what exactly is polarisation?

We describe light in terms of three variables: its brightness, its colour and its polarisation. The brightness is a measure of the amplitude of light. We also call this intensity. Intensity is what we measure with cameras. The camera expresses the intensity in a unit of counts which we then convert into another unit: for example, flux. The colour of light is determined by its wavelength1. This wavelength cannot be directly measured. What I mean by this is that we can not infer the wavelength of light by only using a detector but instead (in the case of wavelength measurements) we look at diffraction patterns. Finally we have the polarisation, which is the direction of vibration of the light. This direction is always perpendicular to the direction of propagation. Thus, measuring polarisation is measuring this direction of the vibrations. Or more accurately, measuring the percentage of waves that have this same vibration direction.

But how can you measure this direction of vibration? I just said that detectors in cameras can only measure intensities. To explain this concept, I will give an example of Polaroid sunglasses and 3D glasses below.

Draw210410.1 Polaroid sunglasses vs. 3D glasses • The modulation of light.

Sunlight consists of bundles of light rays that all have different directions of vibration. The absence of a preferred direction causes this light to be referred to as unpolarised light. Light beams from light bulbs are unpolarised as well. In contrary, laser light is often polarised. This means that the direction of vibration of laser light has a preferred vibration direction. This preferential direction can result in linear, circular and elliptically polarized light. Usually, the light from lasers is linear polarised. In nature, linear polarised light is also the most common type of polarised light.

But wait a minute... We just said that sunlight is unpolarised? How does (sun)light get polarised? When light from the sun enters the atmosphere, its waves still have a random direction of vibration. In the atmosphere the light can be scattered (reflected) by colliding with molecules in the air, water and small dust particles (aerosols). This scattering is also called Rayleight scattering. Rayleight scattering causes parts of the light to become polarised. When the sun's rays hit flat surfaces, like water, snow and ice on the earth, part of the light becomes horizontally (linearly) polarised. We see this with our eyes as a blinding glare. Polaroid sunglasses have a filter for this specific horizontal linear polarisation. This makes the sunglasses improve vision and prevent eye strain on sunny days.

But then what do 3D spectacles have to do with polarisation? 3D spectacles use polarising lenses to create the illusion of a 3D image. Each of the glasses contains a different polarising filter. With each filter, only light that is polarised in one direction will pass through. Light that is polarised in the opposite direction is blocked. This causes both eyes to see a slightly different image. This causes us to see the image with our two eyes from two apparent positions. As these positions differ, the illusion of a 3D image is produced.

Keep an eye on the blog! Soon I will tell more about how I plan to measure polarisation! 1 See my post from January 19 for a more detailed explanation regarding sunlight and its wavelengths.

April 10, 2021


Posts from March 2021

Meeting fellow colleagues during the pandemic

Leiden, the Netherlands

Working together on projects at the intersection of Earth sciences and astronomy

As a scientist, it is incredibly important to keep working together with other scientists in your field. When multiple scientists work together towards an overall goal, this is sometimes referred to as a scientific network. My project is part of a specific network: the PEPSci programme.

PEPSci • Pepsi (cola) is known in the Netherlands as a tasty soft drink. This is very similar to the name: PEPSci, a science programme that has a completely different meaning in Dutch astronomy: "Planetary and Exoplanetary Science". The initiative of the interdisciplinary PEPSci programme started in 2013. It is a partnership where scientists combine their strengths and knowledge. They operate at the intersection of earth sciences, planetary geology, astronomy and chemistry. Our projects are roughly divided in two themes: "Building blocks of life: from disks to exoplanets", and "Earth-like planets: from colliding pebbles to dynamic exoplanets".

Last Friday I met Christiaan, Elina, Orr, Tara and Vivian. Soon I will also meet Alexandra and Mark online. (Actually, I happen to know Mark already from my Astronomy degree). Together, we are an enthusiastic group of eight PhD candidates who, in the next 3.5 years, will be looking at how planets (were) formed, how we can observe them, and (importantly for me) what the signs are of possible life on these planets. I look forward to it!

We are currently working on setting up a website and also contacting our predecessors!

March 28, 2021


The riddle of life's single-handedness

Leiden, the Netherlands

Searching for unambiguous signs of life

Despite the fact that we are in the middle of the COVID-19 pandemic, every week a world is opening up to me. I discover more and more how many disciplines are helping each other to answer important research questions. Especially when we are talking about the definition of life there are many descriptions to be found throughout the various disciplines. All together, these hopefully provide an answer to the question: "What is life? This week, I went digging deeper into the biology and chemistry of 'life'.

Last week, I pointed out some of the signals of life that we can identify when we look at the Earth from outer space. With the help of satellites we can find gases in the atmosphere, water and vegetation on the Earth's surface and over a longer period of time we can recognise the effects of seasons and even changes in the climate.

We are looking for planets that exhibit these same (bio-)signs of life. However, one important and unambiguous1 sign of life is still missing from this list: HOMOCHIRALITY.


HOMOCHIRALITY • the left- or right-handedness of molecules that are characteristic for life.
When you look at your hands, you find that your left hand is the mirror image of your right hand. This makes your hands look the same. But when you put your two hands on top of each other, you can see that they are totally different. You cannot put the two mirror images on top of each other to get two left hands or two right hands. ( I do often say that I have two left hands, because I can be very un-HANDY). Objects that cannot be placed on their mirror image are called Chiral. This is also known as asymmetry. However, there may be a preference for one of the two mirror images. Looking at humans, almost 88% use their righthand as their writing hand. Initially, there is just as much 'chance' that they would have used the left hand. This specific example is dependent on many small genetic and other influences.

Om asymmetrische, chirale moleculen te onderscheiden zeggen we ook dat ze links of rechts-handig zijn. Vanuit de theorie zou de kans op links en rechts-handige moleculen even groot moeten zijn. Alleen blijkt uit onderzoek dat bijna alle chirale moleculen in levende organismen in slechts één vorm gevonden worden. Uitsluitend links of uitsluitend rechts-handig. Zo zijn de suikers die we vinden allemaal rechtshandig, aminozuren en proteïnen linkshandig en DNA draait altijd rond in rechtshandige helices.

The drawing above shows an example of the glucose molecule, which is more commonly known as sugar. In the lab we can produce sugar with the help of chemical processes. According to probability calculations these sugars will consist of 50% left-handed (L-glucose) and 50% right-handed (D-glucose, also known as dextrose). Although L-glucose and D-glucose are simply each other's mirror image, they are quite different. Our body can not do anything with the left-handed sugars. L-glucose tastes as sweet as D-glucose, but we can not obtain any calories from it. This molecule can be harmful to your liver, but it can also be kept in your intestines, where it causes fermentation. That is not so good. But have no fear: life on Earth takes good care of us! ALL the SUGAR we find in nature is pure D-glucose. This sugar is converted into energy in our bodies. Every cell in the human body needs this energy to perform the metabolic functions that support life. Therefore, the existence of only D-glucose in nature is considered to be one of the signs of life.

1 Unambiguous means that if we find this, there is no other explanation than that there is life!

March 21, 2021


My research: looking for signals of life

Leiden, the Netherlands

My first moment of evaluation - my first presentation

Important lesson 3: Do not fear moments of evaluation in your life. A time for reflection can give you a better understanding of what you want to achieve and what you need to do in order to get there.

Whouw! Everybody has experienced those last 5 minutes before starting a presentation... Did I prepare the presentation well?" "Do I understand the subject matter well enough?" "What questions will they be asking?" All these questions and thoughts that dance around in your head just before you speak your very first word out load.

Last Monday, I gave a presentation to the promotion committee. I told them about my progress over the past six months, the knowledge I have gained and my plans for the coming 3.5 years of being a PhD student. This was my first evaluation moment since I started here in Leiden. That does make you a little nervous. Fortunately, everything went well! I can continue the research that I started with.

With this post, I would like to share with you what the ultimate goal of (continuing) my research: "To detect signals that could give us evidence of life beyond our Earth". However, just observing signals is not enough. We need scientific evidence. Below, I give some examples of signals of life that we know of, that can be recognised when we look at the Earth from space.

Draw8.1 Bio-signatures of life on Earth • "Are there other planets besides the Earth that contain life?" This is one of the questions that astronomers are exploring. Throughout the universe we look for what we call 'bio-signatures'. This is a collective term for elements, isotopes, molecules or phenomena that are considered scientific proof of life. In the drawing above, a selection of these evidences are categorised in gasses, surfaces and seasons.

  1. Gasses in the air - The Earth's atmosphere consists of a mixture of different gases. The two most common gases are nitrogen (78%) and oxygen (21%). Oxygen gas, O2 is produced by photosyntheses (plants) and ensures that we -people & animals- can live. Unfortunately, finding oxygen in an exo-atmosphere alone is not hard evidence for life. Research has shown that oxygen has to be present in combination with a number of other gases before we can call it a proof of life. One example concerns the presence of ozone gas (O3). Ozone is made from oxygen in the stratosphere. The ultraviolet radiation from the Sun is so powerful that when its photons (light particles) collide with an oxygen molecule (O2) it splits into two loose oxygen atoms (O1). Together with one other oxygen molecule, this single atom combines to form an ozone molecule (O3).
  2. Reflection of surfaces - Different Earth surfaces reflect sunlight in different ways. One reflection effect that we recognise from Earth observations is the so-called "red edge" effect. Plants absorb the green part of sunlight and reflect the red part. This causes a strong increase in reflectance in the red part of the visible spectrum1: around 700-750 nm. However, we only see this effect when plants are alive. As soon as plants die, this sudden increase reduces its intensity.
  3. The effect of seasons - The position of the Earth in relation to the Sun defines the seasons. Because the Earth revolves around the Sun once a year and the Earth a little tilted, all seasons pass by in one year as well. The temperature differences between the seasons make that the Dutch winters are colder than their summers. This causes trees to fall into a kind of 'hibernation' and to lose all their leaves. In summer, the blooming trees & plants are able to convert carbon dioxide (CO2) into oxygen well (see point 1.). In winter, this happens less efficiently. This is why we can see variations in the concentration of CO2 in the atmosphere during one year. This is an indirect consequence of 'life' on Earth.

1 See my post from January 19 for a more detailed explanation regarding sunlight and its wavelengths.

March 14, 2021


Posts from February 2021

Top-Hat of Down-Hat? Remote teaching!

Leiden, the Netherlands

To set priorities: an important lesson

Important lesson 2: As a researcher, you never know what will come across your path. Every week is another week where we have to look again at what has top priority at that particular moment.

I took this lesson quite seriously. On the first Monday of February, I broke my arm and I knew I had to work with one hand for at least another three weeks. The conclusion: no instrumentation in the month of February, working less efficiently and asking more help from others. Fortunately, as a researcher, there are always things you can do (with a little help)! An important example is: teaching!

Over the past four weeks I have been standing in front of a classroom for the first time. Or rather... More... Sitting in front of my laptop? It is so fascinating and exciting to be on the other side of the classroom for the first time in my career. Suddenly, you are the teacher explaining and not the student asking the questions. I was nervous, but in the end it turned out not to be necessary.

What does my schedule look like now I am also helping with teaching? On Mondays from 08:00 to 13:00 I start with correcting the students' homework. I then take until 13:30 to put all the marks with their feedback online. After a break from 13:30 to 14:00, the lesson begins and then from 16:00 to 18:00 students can ask for help with their homework. In addition to these Mondays, I spend about an additional day per week preparing and answering questions.

The course: High Contrast Imaging • How can we observe a small planet that is next to a super large and bright star? We can do this by blocking the light from a star. This month we looked at how we can do this using the computer and at the telescope. Using the computer, we can, for example, reduce the star's signal with a kind of filter: a 'Top-Hat/Down-Hat Kernel'. At the telescope it is possible to use a coronagraph to reduce the signal coming from a star during an observation.

28 februari, 2021


Posts from January 2021


Leiden, the Netherlands

Why is the sky blue and not black like the rest of the universe?

This week I was studying very small particles (that make up gases) in our air. We call these particles molecules. Many molecules make up the gases in our atmosphere. Last week I talked about the fact that light waves can collide with these molecules. Every molecule, every gas, can block/absorb very specific parts of light. So actually we can say that gases absorb certain colours of light better than other colours. This causes so-called absorption lines.

In astronomy we use these absorption lines to look at the ingredients of stars and gas clouds. Stars, after all, are super-sized balls of hot gas. Just like on the Path a Like any star, the Sun is made up of different gases. When we use a prism to break up a thin ray of sunlight into all the different colours of the rainbow, we see a number of black lines. These are the absorption lines from gasses of the sun. By looking at the location of these black lines in the light spectrum, we can see which molecules cause these lines. This is how Pierre Janssen discovered that the Sun is made up of the gas Helium. Because each type of gas causes a different dark line, we can link a star to a unique combination of black lines, a kind of barcode. Because this barcode is unique, I also like to call it a fingerprint.

So... I want to calculate how much light is reflected on the surface of the Earth. To do this, I first need to know how light travels through all the layers of the atmosphere. The air in the atmosphere consists of many gases (oxygen, water, ozone) that block light from the Sun. By calculating how much gas there is in a layer, we can make a unique fingerprint of our own atmosphere. Using this fingerprint I can calculate how much light I expect on the Earth's surface.
But how does the atmosphere ensure that we have a beautiful blue sky? Read more about that below!

Draw5.1 Why is the sky blue? • If we compare red light with blue light, red light has a longer wavelength than blue light. Therefore, the chance that blue light collides with molecules in the air is greater than for red light to collide with the molecules. The collided blue light is reflected in all directions, which we see on the Earth as a beautiful bright-blue sky!

Now for an explanation with a little madness: I like to think in pictures, as a sort of mnemonic device. This week I came up with the idea of explaining the principle of the blue sky with the help of the game Angry Birds. As a player, you can shoot coloured birds at little pigs. Each bird has its own characteristics. The red ones are big and heavy and the blue ones are small and light. Therefore, I see the red ones as long big waves and the blue ones as short small waves. If we both aim the red and blue birds (photons) at the piglets (air particles), we have a greater chance of hitting the piglets with a smaller wave. As soon as the blue birds hit a piglet, the blue birds are multiplied in all directions. In this way everywhere in the sky, and everywhere on earth, we will see more blue birds than other birds. Like a blue sky.

January 30, 2021


What is important when you make a computer model of the atmosphere?

Leiden, Nederland

One model for each layer in the atmosphere

In my previous blog, I shared a bit of background information about a project I am working on: creating a computer model of our Earth. I am investigating how light from the Sun is reflected on the Earth's surface and sent back into space. By 'measuring' this reflected sunlight, I can simulate taking a special photograph from outer space. For example from the point of view of a satellite.

The amount of light reflected from the surface of the Earth depends on several factors:

  • surface albedo. This indicates the 'whiteness' of the surface. White surfaces such as ice, snow and salt plains bounce off much more light than dark surfaces do. Black is the darkest colour and reflects no light at all. Examples of black surfaces are asphalt and lava rock. Not only black but also other colours like green (forests and grass) and dark blue (oceans) do not reflect all the sunlight.
  • clouds. In the Netherlands we know very well that there can be plenty of clouds in the sky and that they block the light coming from the Sun. The KNMI provides a very nice description of many different types of clouds at their website. We describe the amount of light that can still shine through a cloud by a number that we refer to as the optical thickness. This depends on the type of cloud particles (mini water droplets or ice crystals) and the depth of the cloud. A cloud can both absorb and reflect light.
  • air molecules. The air we inhale every day is made up of very small particles, called molecules. These molecules can also block parts of the sun's rays. As in the case of clouds, we can calculate for these molecules how much light will be absorbed or reflected.
The amount of cloud and air particles depends on the altitude of the atmosphere. Think of walking in the mountains. The higher you go, the less oxygen there is. This is not because there are more other particles. Throughout the entire atmosphere we can say that about 21% of the air consists of oxygen. Only when we are high on a mountain (and therefore higher in the atmosphere) the air pressure gets lower and the temperature colder. This means that when you count the number of particles in a block here in the Netherlands and another block on Mount Everest, you will count less particles on the mountain.

Because I am working on a model of the entire atmosphere, I need to understand the different layers of the atmosphere. In this way, I can compute how a ray of sunlight behaves when it travels through the atmosphere and how much Sunlight is being reflected.

January 23, 2021


Project ExoEarth: A computer model of the Earth

Leiden, the Netherlands

How to start in the absence of a lab? Well just.. with scientist in America

I started my research last September. The most famous observatories from all over the world are closed due to COVID-19. Here in Leiden, the university is partially closed with the laboratories, so I am working from home. I haven't met all my colleagues yet, however fortunately being an astronomer there are enough questions to keep me busy. For example, I started working on a project together with my supervisor and scientists from America. We are making a computer model of the Earth while pretending it to be an exoplanet.

Let's start with the question: "How is it possible that we can see planets?". They do not produce light themselves like stars (like the Sun). Yet on 21 December 2020, Jupiter and Saturn were clearly visible during their 'cosmic kiss'. This is because their surface reflects the sunlight. We can then see and measure this reflected light from Earth. Now think about the Moon. The Moon has craters, mountains and seas that we can see with the naked eye as little specks. The amount of light that a surface can reflect is also known as the albedo, which means 'whiteness'. A pure black moon (or any other object with a surface, like planets) would have an albedo of 0.0 and being white it would have an albedo of 1.0.

At the moment, I am wondering, and hence investigating, how light from the Sun is passing through the atmosphere and reflected on the surface of the Earth. To investigate this, I look at all parts of the sunlight. What do I mean by that? Well, I break the light down like a prism into visible light, ultra-violet and infra-red. In this way I can look at the light as if it were a sort of rainbow. Would you like to know more about these different types of light? Read on below!

Draw3.1 Working with the spectrum of light •I looked up the word spectrum in the famous Dutch dictionary called Van Dale and found the following description: "range of colours produced by the decomposition of light, e.g. through a prism". Light consists of many tiny electromagnetic waves that we can see with our eyes. The length from one peak to another peak in an electromagnetic wave is called a wavelength. We do not describe this length in metres or centimetres (cm), but in nanometres (nm). One cm is exactly 10,000,000 nm. Light with a wavelength of 420 nm is known as violet light. Light with a wavelength of 780 nm can be seen as red light. All wavelengths between 420 and 780 nm are therefore called visible light.

There is more to the universe than just visible light. As soon as we can no longer see electromagnetic waves as light, we call it radiation. Waves with a wavelength greater than 780 nm (red light) are called infrared radiation and waves with a wavelength smaller than 420 nm (violet light) are called ultraviolet radiation. If the waves become even smaller we call them X-rays. Where do these different types of radiation come from? Relatively cold stars (with a temperature of 4000 °C) and planets emit infrared radiation. Visible light comes from stars as big and hot as our Sun. Hot stars (with a temperature of 10,000°C) can be 'seen' by looking at ultraviolet radiation and finally hot gas (with a temperature of 10,000,000°C) produces X-rays.

These are just a few examples of the many different sources in the universe that produce different types of radiation. However, I hope to have given you an idea of where radiation (and thus visible light) comes from.

January 19, 2021


Brief history of Willeke: Who am I?

Leiden, the Netherlands

Sort description about me

As a scientist I have learned that we often forget to tell about ourselves. What do I mean by that? Well, just imagine going to a gathering (aka. conference). You were asked to present your research to fellow participants. You are so happy to know that others want to hear more about your research. Full enthusiasm you start your presentation. After putting your full year of research in a talk of 10 minutes, the first question you get from the public: "Nice presentation, but what was your name and where do you come from?".

Important Lesson 1: Never forget to introduce yourself! Especially as people would maybe like to work together with you on a next project.

SO: Lets introduce myself. My name is Willeke. My absolute favourite colour is blue! That's why the girl I drew has blue hair. I have, as friends say, orange hair. I'm addicted to apples and love to bake! Ever since I was a child I was curious about natural phenomena. In addition, I was intrigued to figure out how THINGS work. Have you ever tried to take apart a pencil and put it back together? Well, do not try this with a toaster.. I can assure, this is way more difficult to put back together! I love physics and mathematics. With computers, I have this hate/love relationship.

January 16, 2021


What? How? Why?

Leiden, the Netherlands

Three questions that are important every day in the life of a scientist

As an astronomer, I am very lucky to have the universe as my laboratory. But what are astronomers actually doing? It always sounds like we don't keep our feet on the ground but only think about the universe. Many astronomers look at stars, or many stars together, like galaxies, but I am different. I don't look at stars, but at planets. Actually I truly love all planets in our solar system, especially the Earth. But during my normal days, I am actually looking for planets that contain life, just like the Earth. But how do we know if there is life on other planets? Why is the sky blue? How do rainbows form? During my research I will come across these and many more questions by looking at the Earth. Do you like to know some of the answers, or just want to know how the life of a researcher looks like? I will share my experiences and answers to some of the most interesting questions with you.

January 13, 2021



January 23
January 19
January 16
January 13