Characterizing other Earths

Name of applicant

Alexander Rathcke


Postdoctoral Fellow


Harvard & Smithsonian | Center for Astrophysics


DKK 1,230,000



Type of grant

Reintegration Fellowships


The study of extrasolar planets (exoplanets) is progressing at a remarkable pace. Specialized space- and ground-based telescope surveys are discovering new planets on a daily basis and we are now in an age where population statistics is made possible with more than 5000 planets discovered to date. Simoultanestly, the biggest and best telescopes have been used to observe the atmospheres of several large gas planets, shedding light on the physical and chemical processes governing these distant worlds. However, observations of the atmosphere of Earth-sized planets have not yet been possible and their true nature is, therefore, still largely unconstrained. This project will perform first-of-its-kind observations of the atmospheres of Earth-like planets orbiting M-dwarfs (the smallest of stars) by using the recently launched James Webb Space Telescope (JWST) and will ultimately help put the Solar System in a broader context and help us to understand the diversity and habitability of terrestrial planets similar to our own Earth.


Remarkably, we now live in a time where the answer to whether life exists elsewhere in the Cosmos is within reach. The most prominent pathway toward finding an answer is by locating exoplanets similar to the Earth and searching their atmospheres for the presence of biosignatures (i.e., signs of life). With both, NASA and ESA recently recognizing this endeavor to be their top scientific priority, it is now time to commence the first observations of the atmospheres of Earth-sized planets. Because the signal-to-noise in such observations scales with the planet-to-star radius ratio, even when using the powerhouse JWST, such observations will for the foreseeable future be limited to Earth-sized planets orbiting M-dwarfs. However, approximately 2/3 of all stars are M-dwarfs, and learning what Earth-sized planets are like in these different stellar environments will provide paramount information on the total number of planets with habitable conditions throughout the Universe.


The reflected or emitted light from exoplanets, and in particular Earth-sized exoplanets, are way too dim and are completely drowned out in the overwhelming amount of light from their host star to be observed when we point our telescopes at these distant systems. Instead, we make use of indirect methods to measure their atmospheres. Coupled with the extraordinary observing capabilities of JWST, this project will use the technique of transmission spectroscopy to tease out the atmospheric spectrum of these small planets. Transmission spectroscopy requires observations to be carried out during planet primary transit (when the planet passes in front of its host star, as seen from our line of sight) where starlight will filter through the planetary atmosphere on its way to the telescope detector. Light traveling through the atmosphere will interact with atoms and molecules present in the atmosphere, causing photons of specific energies to be absorbed or scattered. Effectively, this means that the atmospheric spectrum is imprinted in the stellar spectrum. We can then compare the obtained atmospheric spectrum with millions of atmospheric models to measure the chemical composition and temperature structure of these atmospheres which will provide us with crucial information in understanding the climate and habitability of these alien worlds.

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