The light from distant worlds

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Exoplanets tend to be shy and retiring. When identified in the neighborhood of distant stars – either because they briefly block the light from their home stars as they pass in front of them on their orbits, or because they exert such a pull on the stars that the starlight seems to pulsate – the alien worlds do not reveal all too much about themselves. Astronomers can merely estimate the size, mass, and thus the density of the exoplanets from such observational data, but that is all.

There is a trick, however, for teasing further details out of the unforthcoming planets: If the alien worlds, of which researchers have been able to identify over 5,600 and counting, possess a gas envelope, a tiny portion of the starlight penetrates this atmosphere on its way to Earth. Necessarily, the molecules present in the envelope deposit their chemical signatures in the starlight. Although these traces are faint, today’s astronomers are able to detect them with suitably powerful telescopes.

“If we want to know more than the size, mass, and density, if we want to know something about the chemistry or biology on exoplanets, then the atmosphere is our window on that,” says Kevin Heng. Although ‘Chair Professor of Theoretical Astrophysics’ is what it says on his business card, exoplanet atmosphere researcher would be a more accurate designation. Heng, who was born in Singapore and has been teaching and researching at LMU since August 2022, seeks to learn more about exoplanets through the signatures from the gas envelope: How did they form? What conditions exist there? Are there signs of geological activity, or even of some form of biology? And, right at the top of the list: Are we alone in the cosmos? “The study of exoplanetary atmospheres is perhaps our best chance to discover alien life in the universe,” says Heng.

Light is the key: molecules leave their traces in the atmosphere

It was around twenty years ago when researchers first discovered the atmosphere of an exoplanet, in which they identified traces of sodium – a rather uninteresting element for biology and geology. Two decades later, the scientific landscape has been transformed. In particular, the launch of the James Webb Space Telescope in December 2021, which peers deeper and with greater resolution into the universe than all predecessors, has given a fillip to the study of exoplanetary atmospheres. Finding evidence of water, oxygen, methane, and carbon dioxide has become routine. And light is the key to these discoveries.

Spectroscopy is the name of the technique that allows scientists to investigate planetary atmospheres many light years away: Each molecule of the planetary gas envelope absorbs the light of the home star at very specific wavelengths. If this starlight is captured by terrestrial telescopes and split into its constituent colors like a rainbow – spectrum is the term physicists use – all these dark absorption lines become visible. “Each molecule can produce a plethora of such lines,” says Kevin Heng. “If the observational data are good enough, this is like a unique fingerprint.”

So much for technique. The art or – in Heng’s words – the scientific challenge consists in taking this jumble of lines and drawing the correct conclusions about the geological, chemical, and biological conditions on the exoplanets.This is far from easy, as many traces, many fingerprints, can be misleading. Oxygen, for example, is a clear indicator of life when found on Earth. On a water-rich exoplanet exposed to the harsh ultraviolet radiation of its home star, however, other dynamics could be at play: First, the UV lights splits the water into hydrogen and oxygen. The gravitational pull of the planet is not strong enough to retain the lightweight hydrogen, which escapes into space. This leaves behind a whole lot of oxygen, the fingerprint of which is clearly recognizable in the spectrum. And yet this has got nothing to do with organic compounds, never mind life. The same goes for carbon dioxide: On Earth, the gas usually has a biological origin; on an exoplanet, it might just be the product of volcanic activity.

This is far from easy, as many traces, many fingerprints, can be misleading. Oxygen, for example, is a clear indicator of life when found on Earth. On a water-rich exoplanet exposed to the harsh ultraviolet radiation of its home star, however, other dynamics could be at play: First, the UV lights splits the water into hydrogen and oxygen. The gravitational pull of the planet is not strong enough to retain the lightweight hydrogen, which escapes into space. This leaves behind a whole lot of oxygen, the fingerprint of which is clearly recognizable in the spectrum. And yet this has got nothing to do with organic compounds, never mind life. The same goes for carbon dioxide: On Earth, the gas usually has a biological origin; on an exoplanet, it might just be the product of volcanic activity.

The researcher as tracker

It is precisely such false leads that Kevin Heng wants to reveal through his work. And he wants to know which patterns, which molecular traces need to be present in starlight in order to understand exoplanetary geochemistry or to be able to speak of biological origins with any certainty. “As research currently stands in our field, it’s important to measure as many molecules as possible,” says the 45-year-old. “It’s a bit like a treasure hunt where you find out later how valuable the treasure is and what it reveals.”

Heng is not the kind of theoretical physicist you might find in cliché-ridden textbooks. He is not somebody who sits at a plain wooden desk with pen and paper for weeks on end, solving one equation after another. Nor is he someone who spends all day staring at the tiled back wall of his 1970s-era office at University Observatory Munich, mulling over problems. Heng is much too active and engaged for that. And his field of research, which is part of the ORIGINS Excellence Cluster in Munich for good reason, is much too broad and interdisciplinary for such grand isolation.

Can our terrestrial cycles be applied to distant planets?

Naturally, theoretical work in the classical sense is also part of Heng’s everyday practice. There is the key question, for example, as to whether and how theories that were developed for the Earth and our solar system, can be applied to distant exoplanets. The terrestrial carbon cycle, say, determines how the carbon dioxide content in the Earth’s atmosphere develops over long timescales. Does this sort of cycle happen on exoplanets as well? “This is still far from clear,” says Heng. “And yet the question regarding the extent to which fundamental physical and chemical principles can be universally applied is crucial for our understanding of exoplanets.”

Heng has specifically sought out geochemists to work with in the laboratory. One of the key questions is: Which gases are released when pieces of rock of various compositions melt? And how does this fit in with the fingerprints of rocky exoplanets that can be captured with telescopes? They have designed experiments to provide more precise information and plan to incorporate their findings into theories and simulations.

When Heng started to research exoplanets ten years ago, while still at the University of Bern, one of his first approaches involved applying climate simulations which had been developed for Earth to distant planets. And carrying out such calculations on the computer remains an important aspect of Heng’s work today. For example, he and his team at his chair in Munich work on ways of better adapting atmospheric and climate models to the latest measurements.

Working with real datasets

In their investigations, the researchers also use real observational data. A dozen datasets from the James Webb Space Telescope, for instance, have already arrived at Heng’s chair in Munich, where they are analyzed, spectral fingerprints are taken, and the results are compared against theoretical models.

Heng dreamed of being an astronaut when he was a boy, and so it is fitting that he is involved in space missions. Not as an instrument maker – he is a theorist after all – but as a provider of ideas. For the European space telescope Cheops, which was designed to search for exoplanets and has its scientific headquarters in Bern, Heng helped develop the observation program. And when Europe launches its next exoplanet hunter, Ariel, at the end of the decade, he will be involved again. It was on his initiative that LMU joined the scientific consortium for the space mission. Heng says: “We want to make LMU the place in Germany where the specialists evaluating the Ariel data will be based.”

Astronomers recording the spectra of exoplanets; data scientists analyzing the fingerprints; geologists, climate researchers, and chemists creating and refining models – the work at the chair has long gone far beyond classical astrophysics. For Heng, whose doctoral thesis about the remnants of exploding stars was still very much in a traditional mold, this interdisciplinarity has become second nature. When he started his study of exoplanets, he recalls, he had to read up on climate and atmospheric research first. And in Munich, he hired a geochemist right away. The two of them are now teaching each other the subtleties of their respective subjects – and the corresponding language.

Being open to knowledge from other fields

“You can’t just be a physicist if you want to understand the atmospheres of exoplanets,” says Heng. “You have to be open to knowledge from different fields, and you have to find a way to talk with people from these disciplines. I needed years to build up these contacts.” He was helped by his previous job as director of the Center for Space and Habitability in Bern, an interdisciplinary research center devoted to the search for life in the universe.

Heng’s most important realization was that it is not enough to just assemble top researchers from different disciplines in a room, as wonderful as their CVs might be. “What happens then, in my experience, is pretty much nothing.” What you need instead, Heng observes, is the right people with the right personalities: open-minded, communicative, capable of having discussions beyond the limits of their own specialist field. “You can’t discover these things in someone’s CV, alas; it always takes some trial and error.”

All this – the interdisciplinarity and mutual understanding – will become all the more important if the search for life on exoplanets intensifies one day, bringing biology into play. After all, we still have no definite idea what life on other worlds could look like. Like life on Earth? Or completely different? And in the latter case, what fingerprints should we be actually hunting for?

“The search for life is of course the ultimate goal,” says Heng, while cautioning against unreasonable expectations. “It’ll be the next generation of telescopes and people that can answer this question. What’s important is that we create the foundations now for reaching this goal.”

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Prof. Dr. Kevin Heng is Chair Professor of Theoretical Astrophysics of Extrasolar Planets at LMU and a member of the ORIGINS Excellence Cluster. Born in Singapore in 1978, he studied astrophysics in Colorado before moving to the legendary Institute for Advanced Study at Princeton University. After that, he was director of the Center for Space and Habitability at the University of Bern until he came to LMU in 2022.

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