PHYSICS & ASTRONOMY_Exoplanets The Search for a Second Earth To date, astronomers have discovered nearly 800 planets orbiting distant stars. So far, only three of them have been found to potentially offer life-sustaining conditions. However, there are probably many second Earths in the Milky Way. But how can traces of life be detected on exoplanets? At the Max Planck Institute for Astronomy in Heidelberg, Lisa Kaltenegger is trying to answer this question. TEXT THOMAS BÜHRKE 48 MaxPlanckResearch 3 | 12 The home of extraterrestrials? Astronomers suspect there are several billion terrestrial planets orbiting distant stars in the Milky Way alone. In the coming years, researchers want to find out whether life exists on one or another of them. ax Planck once said of versity in Graz. “It was a ten-minute cialists in extrasolar planets. This year himself that, on leaving bike ride,” she says, to explain how she she was awarded the prestigious Heinz school, he could just as coped with this pentathlon of courses. Maier-Leibnitz Prize for physics by the well have studied music German Research Foundation and the as the classics. The fact SPECTRAL FINGERPRINT German Ministry of Research. M that he opted for physics was thanks to OF OUR PLANET In 1993, American astrophysicist his math teacher and the “desire to Carl Sagan published the spectral fin- study the laws of nature in greater de- She couldn’t keep this up long term, gerprint of the Earth, which had been tail.” Lisa Kaltenegger is certainly mod- obviously, and finally she settled on recorded with the Galileo space probe. est enough not to compare herself with engineering physics and astronomy – Inhabitants of distant planets could the pioneer of quantum physics, but though a career advisor had urged also make such a spectrum of our plan- what they have in common are their against a career in the natural sciences. et and use it to deduce our existence. wide-ranging interests and the impetus It would be difficult for a woman to Conversely, it should be possible for us provided by a committed teacher. hold her own in the field, he said. to search for such traces of life on one Unlike Max Planck, Lisa actually Less than twenty years later, Lisa of the exoplanets. did start with a broad range of disci- Kaltenegger leads an Emmy Noether The discovery of the first exoplanet, plines, studying Japanese, film and me- Group at the Max Planck Institute for a celestial body orbiting a distant star, dia studies, business studies, engineer- Astronomy in Heidelberg, and is simul- created not only a new, fast-expanding ing physics and astronomy in order to taneously a Research Associate at the branch of astronomy in 1995, but also find out what fascinated her most. This renowned Harvard-Smithsonian Cen- electrified Lisa Kaltenegger in her final meant she was constantly commuting ter for Astrophysics, where she spends year at school. This enthusiasm was to and fro between the University of three months of the year. She is one of encouraged by a good physics teacher Photo: ESO Technology and the Karl-Franzens Uni- the most creative and competent spe- who also offered courses in astronomy. > 3 | 12 MaxPlanckResearch 49 It must be said that her home town of edge of both engineering and astrono- Her work also focuses on Earth’s evolu- Kuchl, near Salzburg, Austria, is less my was clearly an asset in this position. tion in order to learn about habitable known for astronomy than for its wood Though Darwin was put on ice, as planets. The idea is simple in principle: processing and agriculture. But her par- was a similar NASA mission, this didn’t “When we are able to investigate a dis- ents ensured lively discussion on a wide mean that the search for a second Earth tant rocky planet spectroscopically, scope of topics at the dinner table. was abandoned – especially not by Lisa what could be the indicators of life? Kaltenegger. NASA’s Kepler mission in We can’t assume that any possible life PALEONTOLOGISTS AND particular excited her, because its fasci- there is at the same stage of evolution BIOLOGISTS ALSO ON BOARD nating discoveries greatly increased the as we are now.” So the first thing she number of potential terrestrial planets, did was to investigate how Earth’s at- “At the end of the 1990s, nobody in which made missions such as Darwin mosphere had evolved since the forma- Austria was working on characterizing seem more realistic again. “Discovering tion of our planet, which involved exoplanets,” says Lisa. She would have traces of life on another planet would close collaboration with biologists and to leave the country. Research visits to be one of the truly great steps in the ex- paleontologists. the Instituto de Astrofísica de Canari- ploration of the universe,” says Lisa. In the beginning, carbon dioxide as on Tenerife, Johns Hopkins Univer- “This great discovery would have (CO2), nitrogen and water dominated sity in Baltimore and the European social, religious and philosophical con- the primeval atmosphere. When the Space Agency in the Netherlands fol- sequences, of course. From the scientif- first organisms appeared about 3.5 bil- lowed. Here, she was part of a three- ic point of view, it would also provide lion years ago, they produced meth- person design team working on the the opportunity to learn something ane, which enriched rapidly in the air, Darwin project, an ambitious plan to about the evolution of our own planet while the CO2 content decreased fur- use several telescopes in space to find and to take a purely statistical look into ther. About 2.4 billion years ago, the Earth-like planets around other stars the future of terrestrial planets,” ex- first organisms began to produce oxy- and to characterize them. Her knowl- plains the Max Planck scientist. gen in abundance; its concentration in O2 CO2 H2O H2O H2O H2O CH4 CH4 CH4 O2 H2OH2O H2O O3 H2O O3 H2O H2O H2O H2OH2O CH4/H2O H2O H2OH2O H2O 3.9 billion years ago 2.4 billion years ago 800 million years ago 300 million years ago 0 10 20 30 40 30 20 0 10 Proportion light (%) of reflected 0.6 0.8 10.6 0.8 1 0.6 0.8 1 0.6 0.8 1 Wavelength (μm) Those who want to roam far from home must first be familiar with their local surroundings. This explains why Lisa Kaltenegger is studying the evolution of the terrestrial atmosphere before she transfers these findings to other, as yet unknown planets. She is particularly interested in which fingerprints would be observable in the spectra from different eras. For example, 3.9 billion years ago, in the early history of our planet, water (H2O) and carbon dioxide (CO2) predominated. Then the first organisms produced methane (CH4), and later, oxygen. Its quantity grew and the increase in molecular oxygen (O2) was accompanied by the proliferation of ozone (O3) in the atmosphere. The concentration of oxygen, 21 at percent, has remained almost unchanged for around 300 million years. Photo: ESA, NASA and Frédéric Pont (Geneva University Observatory); graphics: MPI for Astronomy – Lisa Kaltenegger 50 MaxPlanckResearch 3 | 12 PHYSICS & ASTRONOMY_Exoplanets Stellar eclipse: There are several ways to detect exoplanets. Brightness One of them uses the transit: Seen from Earth, the planet transits in front of its parent sun – as Venus transited in front of the Sun on June 5/6, 2012. Astronomers deduce the characteristics of the exoplanet from the decreasing brightness and the light curve during the passage; under the most favorable conditions, the method also allows spectral observations of the planet’s atmosphere. Time [hours] the atmosphere increased slowly at how the chemical composition of planet and its atmosphere. It is what first, with fluctuations, until around Earth’s atmosphere varied during geo- allows a feedback mechanism that re- 300 million years ago, when it reached logical evolution. The resulting spectral cycles gases like CO2. about 21 percent, which has remained fingerprint of our planet through geo- The lava descending into Earth’s in- almost unchanged since. The increase logical time can indicate life. The sur- terior can bind carbon dioxide and re- in molecular oxygen, O2, was accom- prisingly positive result was that, “For move it from the atmosphere. Volca- panied by the increase of ozone (O3). about half of Earth’s history to date, noes, in contrast, introduce CO2 into At the same time, the proportions of extraterrestrials could have detected the atmosphere again. Tectonics thus carbon dioxide and methane changed. traces of life in our atmosphere as a acts like a carbon dioxide buffer. If a For a long time, astronomers had combination of oxygen or ozone with planet doesn’t have this compensation assumed that high concentrations of methane and water.” mechanism, it can become too hot oxygen and ozone were, in themselves, even with small increases in external certain indicators of life. But as Lisa NO GAS RECYCLING WITHOUT influences, such as an increase in the Kaltenegger explains: “The crucial TECTONICS luminosity of its sun over time. On the things are combinations – of molecu- other hand, Earth, for its part, would lar oxygen or ozone with a reducing Something similar could then also have been completely frozen when it gas such as methane, for example.” If hold true for other terrestrial exoplan- was young and the Sun less luminous. present in larger quantities, these gas- ets. Of course, all of these consider- Conditions for life as we know it es in combination are the better biosig- ations require that life there function can be detected in the atmosphere of nature.