Let's first state the obvious: the universe is endlessly fascinating. When the first ever picture of a black hole was released this spring, it easily made front pages. The researchers at KU Leuven are also filled with amazement. Our university is a partner in numerous space missions of ESA, the European Space Agency. We’re highlighting three: PLATO, ARIEL and LISA. Each of them are missions that inspire big dreams: from new earths to the darkest corners of the universe, to the beginning of everything. Launching them is still a pipe dream at the moment, but the clock is ticking. In space research, the next decade means ‘tomorrow’.
PLATO: Finding planet B
Professor Conny Aerts of the Institute of Astronomy is the Belgian Principal Investigator (B-PI) of PLATO. In 2026, this space mission will start looking for a second planet Earth, to sum it up in ambitious terms.
“With PLATO, we want to find exoplanets – planets outside of our solar system – that are sufficiently similar to our earth to allow for life to exist”, says Aerts. Today, there are over 4,000 known exoplanets. Most of them, however, are giant gas planets and, thus, unfit for life as we know it. PLATO will specifically look for ‘earths’. “Habitable primarily means that the planet orbits its parent star in the habitable zone – not too hot, not too cold – and has an appropriate density – no gas, for example."
Conny Aerts: With PLATO, we want to find planets outside of our solar system that are sufficiently similar to our earth to allow for life to exist.
The PLATO satellite will build an entire catalogue, creating a bit of order in the zoo of planets that is the universe. To do this, the satellite will use a battery of 26 cameras – that KU Leuven is helping to develop – to scan a large part of the southern skies for a year, after which it will turn to the northern skies for a year. The goal is to spot ‘transits’: the moment when a planet passes between us and its parent star. “During such a transit, the planet blocks a fraction of the light of its sun. In doing so, it temporarily dims the apparent brightness of the star. We will measure these dips in brightness, and with this info, we’ll be able to categorise stars and planets based on age and size. Over the course of two years, we will monitor hundreds of thousands of stars, and we hope to spot around 5,000 exoplanets.”
“You also have to be willing to look to one side long enough. If you look for less than a year, you might miss the transit of a planet such as Earth, which has an orbital period of one year around the Sun.”
Conny Aerts has been part of the PLATO mission from the very beginning. A huge advantage for her team, she says. “We are the first people to get access to the results of PLATO. But more importantly: I was able to define what we will be researching. In other words, PLATO literally does what I want (laughs). I’m quite proud of that: there's only a 1% change that your satellite proposal is accepted.”
And what Aerts wants to research is asteroseismology: the oscillations of stars. “Each star oscillates because of its internal activity. Our Sun also has oscillations of around 5 minutes each. It's one of the few ways to discover something about the internal structure and, subsequently, the life and age of a star. With PLATO, we will apply this technique on a large scale to weigh, measure and date all visible stars with unprecedented accuracy.”
Why does this need to be done from space? Because the Earth's atmosphere puts a spanner in the works. “If you look at the stars from earth, you’ll see that they always ‘flicker’ a bit: their light is hindered by turbulence in our atmosphere. Due to this veil, we can only observe the large oscillations from here, meaning we can only discover the large planets. But with this mission we want to find the smaller planets. This requires unhindered measurements of deviations on a scale of parts per million, which is only possible in space.”
ARIEL: All aboard?
High time to talk about ARIEL, a satellite that will, in a sense, share jobs with PLATO. First, PLATO will detect a variety of planets around bright star. Two years later, in 2028, ARIEL will further examine the atmosphere of hundreds of hot planets. The goal is to better comprehend the overall picture of exoplanet atmospheres. This is necessary, because for a planet to be habitable, it not only needs a good orbit and density, the chemical composition of the atmosphere also has to be adequate. Is there enough oxygen, for instance? Or better yet: carbon dioxide, methane or ozone? These types of molecules determine the chance of life.
Bart Vandenbussche: Travelling to inhabitable planets? A lifetime wouldn’t be enough to get there. We are purely doing this to expand our knowledge.
But what are the chances of this happening exactly? Bart Vandenbussche, research coordinator at the Institute of Astronomy and the Belgian Principal Investigator (B-PI) of ARIEL, makes an educated guess. “Of the 5,000 somewhat planets that PLATO will find, we estimate that 5 to 10 will actually be habitable. Next step: going there? No, we are purely doing this to expand our knowledge. A lifetime wouldn’t be enough to get there. The stars and planets we will find with PLATO will already be a thousand times closer than what we found with the NASA Kepler telescope. But the closest star, Proxima Centauri, is still 40 trillion kilometres away: that’s a four with 13 zeroes. That’s simply too far, even if we do manage to someday travel at the speed of light.”
“Isn't it remarkable that the Institute of Astronomy is providing the Belgian principal investigators for PLATO and ARIEL”, concludes Conny Aerts. “KU Leuven is extremely good at continuously translating hard science to the ESA engineering world. That is what's so important in space research. Because what we want to know is one thing, which device to use to measure this is another.”
LISA: Back to the Big Bang
Does PLATO sound ambitious? Then what about LISA, the space mission that will measure gravitational waves from 2034 onwards. Three satellites will be launched into space and will form a giant triangle, floating 2 million kilometres apart. In the heart of each satellite will be a test mass, a golden nugget, freely floating inside. When a gravitational wave – a cosmic ripple – passes through the triangle, the distance between the golden nuggets in each satellite will slightly change. LISA is massive in every sense: the project costs around 2 billion euros, and over 1,000 scientists are working on it.
Theoretical physicist Thomas Hertog is chair of the Fundamental Physics Working Group of LISA. His office is that of a true theorist: books with formulas, stacks of paper with formulas, a blackboard with even more formulas. What is Hertog looking for in space? “There has never been a theory that was formulated without the input of observations. I don’t want to lose sight of reality”, says Hertog.
“Very specifically, I hope to test the limits of Einstein's theory of relativity with LISA. Einstein already predicted the existence of gravitational waves in 1916. But we also know that his theory cannot be applied inside black holes and for the Big Bang. And we’re convinced that gravitational waves from those extreme corners of our universe can tell us something about where and how Einstein's theory fails.”
Tip of the iceberg
Explaining what these gravitational waves are exactly is no easy feat. “They’re ripples in the fabric of space time that are generated when large masses start to move. If two black holes circle each other and merge, for instance, the fabric of space begins to vibrate, just as waves on water. These ripples spread out into the universe at the speed of light. What they’re made of? They’re pure geometry, no particles whatsoever are involved.”
We have known about the waves for a while now: they were first detected in observatories on Earth in 2015, and these researchers won the Nobel Prize in Physics for their discovery in 2017. So why do we still need to go to space? “Because these first observations from Earth are merely the tip of the iceberg. Thousands of gravitational waves constantly pass through our planet”, says Hertog. “And Earth itself is too small to detect the low frequency waves: there's no room for an experiment of 2 million kilometres.”
Thomas Hertog: Over the past 400 years, we’ve mapped the history of the ‘illuminated’ universe. Now, we’ll write the parallel history of the dark side of the universe.
We're entering a new phase with LISA, says Hertog. “Over the past 400 years, we’ve mapped the history of the ‘illuminated’ universe on the basis of light waves. Now, we’ll write the parallel history of the dark side of the universe. By observing gravitational waves, we can peer deeper into the universe than ever before: from areas impenetrable by light to the first galaxies, and even the young universe right after the Big Bang. I expect surprises. Every time mankind looked at the universe with new eyes we have been surprised by what we found. That's what I expect from LISA as well.”
A waiting game
The launches of PLATO and ARIEL are planned for 2026 and 2028, LISA will have to wait until 2034. How difficult is it as a scientist to wait this long? “LISA cannot be launched today. We’re simply not ready yet”, says Hertog. “We don’t just want to observe gravitational waves, we also want to interpret them. Each wave that passes through the LISA triangle is a symphony of a thousand sources. ‘Reading’ this score, understanding the physics behind the sources that generate the waves, requires an entire catalogue of templates, which is what we’re compiling right now.”
“But we’re starting to feel that we’re entering the final phase of preparations. LISA has been with me my entire career. I first heard of it in 1996, as a student of Professor Christoffel Waelkens. And in 1999, when I worked for Stephen Hawking, he predicted that LISA would challenge Einstein. I’m already very happy that I won’t be retired yet when the results finally come in (laughs).”
Failure is not an option
Conny Aerts is also pleased that she’ll be able to actively work with the results of ‘her’ PLATO in 2028. “Space science is a waiting game. But failure is not an option: the PLATO satellite costs 500 million euros; it’s not something you build twice.” As such, it requires a lot of patience, and a decent risk analysis, from the smallest component to the assembled unit. “You want a reliability of 99.99% for each component”, confirms Bart Vandenbussche. “It can still go sideways even then, but at least we kept the chances of this happening marginally small.”
The result of this meticulous preparation is the slightly absurd situation that the material that is launched into space is always relatively ‘old’. “We’re currently testing the CCD sensors for PLATO – the light sensitive chips that you also find in a digital camera. Of course, these are the chips we have today, and not the state-of-the-art chips we’ll have in seven years’ time. And that’s the case for everything on board the satellite. The computer that will pilot the PLATO satellite is less powerful than the one in your smartphone, for instance. But that's completely fine. What's more: it needs to be able to withstand cosmic radiation, which is something versions of these older processors are more resistant to. These things are inevitable. I wouldn't be surprised if we had to look on eBay for replacement parts for the computer of the International Space Station (laughs).”
Finally, the million dollar question: does space travel deserve this much time and effort in this day and age? What about the urgent issues on Earth? Conny Aerts does not see a contradiction. “On the contrary: we’re actually arousing the curiosity of the people. And this is exactly what drives all those STEM people, who we desperately need to fix the problems on Earth. Who isn't fascinated by the universe? Who never gazes at the stars in the sky? Recently, fifteen-year olds around the world were asked what they considered the most fascinating question. Right at the top of the list for boys and girls was the same question: ‘Does extraterrestrial life exist?’. Curiosity and wonder are so incredibly crucial; they form the basis of innovation.”
Conny Aerts: More urgent problems? We arouse the curiosity of the people. And this is exactly what drives all those STEM people, who we desperately need to fix the problems on earth.
“To me, this fundamental curiosity is still the essence”, says Hertog. “Thinking about what the cosmos can teach us, and what our place is. With LISA, we will try to find our deepest origin. Isn't that amazing? And yes, you’re developing new technologies at the same time. There's a high-tech ecosystem surrounding a space mission such as LISA, from cutting edge data science to the development of new sensors and reading equipment. You need colliding black holes in the universe to move forward the innovation train here in Flanders.”