Visit to Laura Kreidberg
For two years now, the Max Planck Institute for Astronomy in Heidelberg has had a new department where researchers study the atmospheres of extrasolar planets. Its young director Laura Kreidberg has made a name for herself with sensational observations and is one of the lucky ones who will be observing with the new James Webb Space Telescope.
Text: Thomas Bührke
Being appointed as a new director at a renowned Max Planck Institute is a great honour. But if it happens in times of corona lockdown, such a move can prove to be an unexpected challenge. This is what happened to Laura Kreidberg when she wanted to come to Heidelberg in June 2020. "What was most difficult was the preparation," says the young American, "but fortunately we received a lot of help from the Institute during this time."
The head of administration visited flats for them, the executive director wrote a formal document, among other things, justifying Kreidberg's arrival with one of the few transatlantic flights. "It was exhausting, but we made it!" By "we", the scientist also means her husband, who quickly found a job at a start-up company in Germany thanks to his field of work in data science. Although in Berlin, home office has become standard since the pandemic. At the time of her appointment, Laura Kreidberg was just thirty years old, making her one of the youngest directors in the history of the Max Planck Society. With her research field "Atmospheric Physics of Exoplanets", she seamlessly follows on from the field of "Planet and Star Formation", in which a focus on the discovery and study of extrasolar planets has long since emerged. Since the spectacular discovery of the first planet in a distant star in 1995, hardly any other field in astronomical research has developed as rapidly as this one. To date, around 5000 exoplanets are known.
The diversity of these sometimes very exotic bodies has surprised the experts. There are gas planets with a temperature of over 1000 degrees, some of which are so close to their central sun that they virtually evaporate. Others are made of rock and may be similar to Earth. Exactly what these distant worlds are like, what temperature they are at or whether they have an atmosphere and perhaps allow life to develop – these are the questions that fascinate Laura Kreidberg and led her to study astronomy in the first place. Because unlike many a colleague, she didn't start out with a telescope with which she observed the starry sky with fascination. "In fact, until today I have only very rarely looked through a telescope," she confesses, and she is not very familiar with the constellations either. "When someone asks me where in the sky this or that planet is that I have studied, I can only shrug my shoulders."
The researcher grew up in the medium-sized city of Reno in the US state of Nevada, which is best known for its casinos. However, the proximity to the Sierra Nevada and Lake Tahoe invites hiking – a passion that Kreidberg still indulges in today. In high school, she became interested in physics because in this subject she "didn't have to learn as many details as in biology, for example". Now and then she took part in Science Bowls, a kind of scientific competition.
There was no tradition of science in her family, but popular science books on astronomy by her father inspired her as a child to ask the really big questions. "I remember asking my mother where the edge of the universe was," she says. Books by Stephen Hawking and Brian Greene also fascinated her immensely, as they opened up a view of a universe that was as mysterious as it was immeasurable. Classes strongly oriented towards classical physics soon bored and frustrated her, when another coincidence steered her towards astronomy: She became aware of the work of Nate Silver – a journalist who used statistical methods to analyse the results of baseball games and later applied this technique to presidential elections. "I found that you could also use these methods to address certain questions in astronomy," Kreidberg says. "I then did that in my bachelor's thesis, which was about the masses of black holes."
"We've even seen planets orbiting around two stars, like Tatooine in Star Wars."
During this time at Yale University, she was looking for a topic for her doctoral thesis and became aware of exoplanets. These were very popular at the time, but the spark did not initially catch. Kreidberg was not sure whether this line of research was not a short-lived trend. New bodies were constantly being discovered, but little more was known about them than their masses and the distances to their central stars. Then she heard that someone had studied the atmosphere of an exoplanet for the first time. That was her personal epiphany. "Studying the composition of the atmosphere and the climate on a distant planet places the highest demands on observational technology, which fascinated me greatly," she recalls. "It was clear to me that this task had the potential to captivate me all the way throughout my career."
As part of her doctoral thesis at the University of Chicago, Laura Kreidberg jumped on the topic and used the Hubble Space Telescope to observe a planet orbiting a star 48 light years away called Gliese 1214. This body, called Gliese 1214 b, is a so-called super-Earth – a type of planet that does not exist in our solar system. Gliese 1214 b is seven times heavier and almost three times larger than Earth, but smaller than Neptune.
Astronomers had already tried to analyse the atmosphere of Gliese 1214 b before – without clear results. One assumption was that the gas envelope consists mainly of water vapour, which is why the planet could also be covered to a large extent or completely by water – in other words, an ocean planet. However, the new observational data from Kreidberg and colleagues largely ruled out this scenario. The atmosphere must be covered by dense clouds, they concluded.
Observations of other exoplanets followed, which made their diversity clear time and again. In our solar system there are the inner, terrestrial rocky planets and beyond the asteroid belt the gas giants. Most other solar systems do not look like this. Many planets exist that orbit so closely around their central stars that their atmospheres vaporise or the rock melts into lava. In the case of an extremely hot gaseous planet, clouds could be detected that consist mainly of metals such as iron, magnesium, chromium and vanadium. "We've even seen planets orbiting two stars, like Tatooine in Star Wars," Kreidberg says. The exoplanet zoo is rich in exotic members.
She made a name for herself with her doctoral thesis and other publications, and also won several prizes. No wonder, then, that she was soon able to choose from several attractive job offers, for example, from the renowned Harvard University. Her decision to become director of the Max Planck Institute for Astronomy in Heidelberg ultimately had several reasons. Here she has the opportunity to build up her own research group with fixed long-term funds. Seven of a total of fifteen approved positions have been filled, and she has already acquired more through scholarships. The advantage: experts from all over the world and different disciplines work under one roof, observers as well as theorists who can calculate complex atmospheric models. "Research on exoplanets is interdisciplinary," says Laura Kreidberg.
Another important reason was easier access to the large European observatories, especially the Very Large Telescope of the European Southern Observatory ESO in Chile. In addition, the largest telescope on Earth, the Extremely Large Telescope of the Eso, will be added by the end of this decade. It will have a collecting mirror with a diameter of 39 metres, setting new standards in observational astronomy. The Max Planck Institute in Heidelberg is involved in the construction of an instrument that will take images as well as spectra in the infrared range. "This will enable the Max Planck Society to allow me to observe with the world's best instruments over a very long period of time," says Kreidberg.
But before that happens, the astronomer will observe with the new superstar of the scene, the James Webb Space Telescope. After years of delays and cost increases, the ten-billion-dollar instrument finally made it into space in December 2021 and reached its destination four weeks later – 1.5 million kilometres from Earth. There, in deepest darkness, it is expected to surpass the Hubble telescope's capabilities many times over. "It will be 10 000 times better," Kreidberg enthuses. The mirror, the coverage of wavelengths and the spectral resolving power are each better by a factor of ten, he says. "By observing in infrared light, we can get to much cooler and thus potentially more habitable planets than before, and we can also detect certain molecules in the atmospheres of exoplanets more easily."
However, the James Webb is an all-round instrument that is just as suitable for studying distant galaxies, black holes or faint comets. The demand from researchers for observation time is correspondingly high. A total of 1172 applications from scientists from 44 countries were received for the 6000 hours available in the first observation cycle. Of the 266 proposals finally selected, one third came from member states of the European Space Agency (Esa), which is involved in the new super telescope.
Laura Kreidberg emerged from this competition particularly successful: the international committee approved two of her applications at once. "I even wrote four applications, which was really a hard time," she says. In the applications, all the details had to be worked out down to the last detail. "For a month I sat at home with my laptop and filled out the applications while my husband supplied me with ice cream." Work and ice cream costs paid off. Until now, atmospheres could only be detected on hot planets the size of Jupiter, which is why these celestial bodies are called hot Jupiters. The James Webb Space Telescope is now expected to make a breakthrough and also make atmospheres of smaller rocky planets accessible. The problem in all cases is that the star is millions of times brighter than the nearby planet and outshines it.
This is also the case with an exoplanet called Trappist-1 c, which Kreidberg selected for her observations with the space observatory. This rocky planet, 40 light years away, is only slightly larger than Earth, but orbits its star at such a close distance that it has a temperature similar to that on Venus. "This planet is the coolest rocky world we can detect with James Webb," says the astronomer. "We want to find out if it has an atmosphere."
In a first step, this is done in an indirect way, because the star and planet themselves cannot be observed separately with James Webb. In the infrared range, where the planet emits thermal radiation, the brightness of the star and planet are measured together. Then a second time, when the planet passes behind the star and its radiation is blocked by the star. From the change in the total brightness, the brightness of the planet can be determined, and from this, its temperature. This depends strongly on whether there is an atmosphere or not. Without an atmosphere, all the starlight hits a hemisphere and heats it up very strongly. But if the planet is surrounded by a thick atmosphere, the gas envelope transports the heat from the hot day side to the cooler night side and thus provides a balance. With an atmosphere, the day side is not as hot as without. Such a measurement therefore provides an indirect indication of an atmosphere.
Should this observation indicate the existence of an atmosphere, Kreidberg would request a supplementary measurement for the next round with James Webb. This time with a spectrograph. As the planet passes in front of the star, some of the light passes through the planet's atmosphere, whose molecules leave their fingerprints in a spectrum. "In this way, we can determine whether the atmosphere contains water, methane or carbon dioxide, for example," she says.
"We want to look for traces of sulphur dioxide as a result of volcanic outgassing."
Since recording a spectrum is time-consuming, it is only done when the previous brightness measurement has confirmed that an atmosphere exists at all. The astronomer only learned a curious detail in Heidelberg. The colour filter through which Trappist-1 c is photographed is part of a filter wheel whose mechanics were developed and built by the Max Planck Institute. "Yet another reason why I came here," says Kreidberg with a smile.
The second rocky planet, LHS 3844 b, is also slightly larger than Earth, but around 770 degrees hot and probably has no atmosphere. In this case, the aim is to understand the geological conditions of a terrestrial planet for the first time. With James Webb, Laura Kreidberg wants to search for volcanic basalts, solidified magma surfaces and granite – indicators of crustal reprocessing and tectonics. "We also want to look for traces of sulphur dioxide as a result of volcanic outgassing."
The big goal of this research is to detect biosignatures in the atmosphere. Most commonly mentioned here is the simultaneous presence of oxygen and methane. Normally, these gases react quickly with each other to form carbon dioxide and water. "So if we see the two gases at the same time, it means that they are constantly being produced and replenished by something, and that something is life on Earth," says the Max Planck director. However, even James Webb will probably fail at this task. Oxygen in particular leaves only a very weak spectroscopic signature. Moreover, clouds in the atmosphere make such observations difficult.
The question of what other biomarkers there might be is currently highly topical. We know that the Earth's atmosphere contained hardly any oxygen until about 600 million years ago, but that life did exist, even if it was primitive. The atmosphere at that time therefore had different biomarkers than today's atmosphere. Perhaps these will be detectable on other planets in the coming years.
In any case, the scientist is convinced that life exists somewhere out there. Here, too – as with exoplanets – there could be a much greater diversity than we imagine. "For the search for life features in the atmospheres of distant planets, we need even better telescopes – and patience," the astronomer sums up. At Nasa and Esa, the search for biomarkers has top priority. Plans are already underway for the next generation of space telescopes, which could perhaps be launched in the 2040s. "That would still be in the course of my career," Laura Kreidberg says and laughs.