Introducing Astrophysicist Lisa Kaltenegger - Simulating Worlds Stranger than Fiction

bridges vol. 20, December 2008 / News From the Network: Austrian Researchers Abroad

By Daniela Klammer



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Dr. Lisa Kaltenegger

"My favorite planet? So far, it's Gliese 581 d," discloses Lisa Kaltenegger, an associated lecturer at Harvard University and a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Kaltenegger, who earns her living by hunting extraterrestrial planets, explains why this planet - unknown to most of us - made it on the top of her list: Gliese 581 d is a planet outside of our solar system that might hold the potential of being habitable.


{access view=guest}Access to the full article is free, but requires you to register. Registration is simple and quick – all we need is your name and a valid e-mail address. We appreciate your interest in bridges.{/access} {access view=!guest} How to become a planet hunter

Exoplanets
or extraterrestrial planets  are planets orbiting stars be- yond our Solar System. 
 

Kaltenegger, chosen by Smithsonian Magazine in 2007 as one of America's Young Innovators holds a Ph.D. in astrophysics from the University of Graz, Austria. Her Ph.D. was awarded sub auspiciis praesidentis rei publicae by the Austrian president. This honor goes to the most outstanding graduates with an A average from high school through the defense of their doctoral dissertation. She originally began studying five different major subjects at university - engineering, astronomy, chemistry, translation studies, and film & media studies - to figure out which one she liked best. Kaltenegger finally decided to continue her engineering and astronomy studies up to the master's level. After successfully completing her master's theses on physical applications in biomedicine (M.Eng.) and extraterrestrial planet search (M.Sc.), respectively, Kaltenegger had to make a decision as to which subject she would focus on for her doctoral studies.  Kaltenegger remembers, "The question was whether I wanted to cure mankind or whether I wanted to go and find other planets. Not to say I could have cured mankind, but you want to have some high hopes. For some time it was a close call. In the end, the planet search won the race, since I consider finding the first habitable planet other than our Earth simply to be one of the most tempting discoveries in our century."

Creating new worlds as a daily routine

A planet’s ID: the spectral fingerprint
To get the fingerprint of a planet, as a first step, its light is broken up in a spectrograph. In the visible range, it will be split up across the spectrum from red to blue light, just like in school experiments with prisms. If each wavelength appears with the same intensity, the planet does not have an atmosphere. However, if certain wavelengths of that spectrum are missing, scientists are able to tell which chemicals are in the atmosphere. Each chemical has its own lines in the spectrum. This characteristic pattern is the so-called spectral fingerprint of a planet.
 
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Being a postdoctoral researcher of the Origin of Life Initiative at the Harvard-Smithsonian Center for Astrophysics, Kaltenegger mainly focuses on modeling different worlds on her computer: "What I am doing on a daily basis is creating spectral fingerprints [see info box at left] for different planets that could be habitable and to figure out how we could detect that. You come to work and you have to imagine a completely different world and how this other world might work. It's a lot of fun. You even break out of this very small margin that's set by science fiction or most of the books - one certainly has to think outside of the box."

The research group in which Kaltenegger works started by putting the Earth back through time. In their simulations, they went back to the time when Earth was about 2 billion years old to see what its spectral fingerprint looked like in those days. Why? Because once a planet similar to Earth is found, it is quite unlikely to be at the same evolutionary stage as we are now, so one needs to understand how the atmosphere was composed back then.

The next step: analyzing light collected from exoplanets for traces of life


Once light coming from exoplanets is collected by space missions like Darwin (see info box) or by the Terrestrial Planet Finder - a proposed NASA mission - astrophysicists like Kaltenegger will be ready to analyze the data.

But for now, Kaltenegger and her collaborators are still trying to understand how different atmospheres could work for exoplanets - and how they affect the probability that life could develop on those planets.

Kaltenegger's spectral fingerprint simulations make sure that one knows in which regions the instruments of a space mission will have to be sensitive enough to detect life on exoplanets. In addition, researchers also get a map of all the planets that have the potential to be habitable - as Gliese 581 d could be. This in return will make the data analysis easier once a mission like Darwin is up and running.

Biomarkers - the imprint of life

About Darwin
Darwin is a proposed space mission by the European Space Agency that will take photos of exoplanets. It is especially designed to search for habitable planets and investigate their atmosphere for biosignatures, the gas products known to be produced by the carbon macromolecule chemistry on which our life is based. Darwin has the potential to find life out there, and a launch has been envisioned as early as 2013.

Once the spectral fingerprint of a potentially habitable planet is available, one can search the atmosphere for possible biomarkers, which are chemicals that are features of biological activities. In the spectrum of Earth, for example, you find ozone, water, methane, and carbon dioxide. These molecules are basically the imprint of life in the atmosphere. Water and carbon dioxide are not necessarily biomarkers that tell you there is life on a planet. But if you could see, for instance, a lot of methane and oxygen in the spectral fingerprint of an exoplanet, that would imply a strong likelihood of biological activity. These two gases react in the atmosphere with each other, so they would not be there if it were not for something producing them. The only processes we know for doing that in huge amounts are of biological origin. Another biomarker would be nitrogen oxide, of which 99 percent on Earth is produced by bacteria. Again, no production mechanisms are known other than biological ones.

Exoplanets in a nutshell - a new challenge in astronomy
 

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The first photo of an exoplanet: Fomalhaut b. Click to enlarge photo. 

In 1995 a new era in astronomy began when Michel Mayor and Didier Queloz published their article in Nature on the first detection of an exoplanet orbiting the star 51 Pegasi. Since then 326 exoplanets have been detected, and only a month ago, on November 13, 2008, the first direct images of exoplanets were seen around the world. What started back in the 1990s as a tiny field of underdogs has become the fastest-growing field in astronomy. "However," Kaltenegger points out, "fortunately you don't need to be a senior scientist to make a big contribution within the community, because it is still such a young vibrant field with so many important open questions."

Finding exoplanets is a difficult business because the star of an extrasolar system is so much brighter than its orbiting planets. This makes it extremely difficult to photograph the planet. Think of taking a shot into the light with your camera, where you can see nothing but the sun. When it comes to the precision you need, Kaltenegger gives an impressive analogy: What you try to do compares to the situation of being in L.A. and trying to descry a penny in Washington, DC. Not until the late 1990s was astronomical technology advanced enough to master these difficulties.

During the last 13 years tremendous progress has been made, but also many new questions have popped up - which is the "fun part," as Kaltenegger puts it. When Kaltenegger studied astronomy in Graz, the predominant opinion was that "we know how a planetary system forms." However, in 1995, with the newly realized ability to actually measure extrasolar systems, the well established theory of planet formation got into serious trouble. It was developed from a sample of one - our own Solar System - and the first detected extrasolar giant planet - 51Peg - did not fit into this theory. It was found that 51Peg and other quite massive planets were closer to their stars than predicted. Later it turned out that these massive planets were gas planets, now known as "Hot Jupiters." According to standard planet formation theory, gas planets can only form at a certain distance from a star.  Within this space it is simply too hot for gas to form a planet. Today it is believed that these planets form in the outer regions of the extrasolar systems, but then migrate inwards. Exactly how this happens and, most of all, how the planet stops migrating still remain a riddle that scientists like Kaltenegger try to solve. The detection of the first exoplanet was the start of an exciting new field of research and the first step toward understanding how planets form and evolve and what it really takes for a planet to be habitable. So far more than 300 extrasolar planets were found and scientists are just reaching the threshold where planets that just might be a ‘small blue dot' themselves can be detected.

When the medieval philosopher Giordano Bruno (1548-1600 AD) imagined extraterrestrial worlds in his opus De l'infinito, universo e mondi (On the infinite, the universe and the worlds), he had to die for his visions during the Inquisition. Astronomy has come a long way since then. Lisa Kaltenegger is confident that in 10 or 20 years we finally might know whether Bruno was right and whether life on Earth is unique. We should be prepared to adjust our place in the universe once more.


 

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The author, Daniela Klammer, is a Ph.D. candidate in the Working Group of Mathematical Physics at the University of Vienna.



References:

New York Times article on the first direct images of exoplanets:
http://www.nytimes.com/2008/11/14/science/space/14planet.html

Science publication on the first direct images of exoplanets:
http://www.sciencemag.org/cgi/content/short/322/5906/1335

Dr. Kalteneggers' webpage:
http://www.cfa.harvard.edu/~lkaltenegger/

Information about Darwin and the search for exoplanets:
http://arxiv.org/abs/0805.1873

America's Young Innovators 2007 by Smithsonian Magazine: Lisa Kaltenegger:
http://www.smithsonianmag.com/specialsections/innovators/kaltenegger.html

Nature publication by Michel Mayor and Didier Queloz on the first evidence for exoplanets:
http://www.nature.com/nature/journal/v378/n6555/abs/378355a0.html
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