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Planet 42

Exploring the diversity of exoplanets in the mass-density diagram

Principal Investigators

  • Prof. Dr. A. Hatzes
    Thüringer Landessternwarte Tautenburg, Tautenburg
  • Dr. M. Pätzold
    Rheinisches Institut für Umweltforschung an der Universität zu Köln, Köln
  • Prof. Dr. H. Rauer
    Deutsches Zentrum für Luft- und Raumfahrt e.V., Berlin


There are currently several thousand known exoplanets and these exhibit a bewildering diversity in their densities. For example, giant planets the size of Jupiter can have densitites that differ by a factor of ten or more. A Superearth planet (radii of ~ 4 REarth) can have a density  consistent with icy planets like Neptune (densities of ~ 2 gm cm-3) or greater than the density of the Earth (5- 10 gm cm-3 ). Understanding the source of this diversity can hold keys to understanding the formation and evolution of  planetary systems.
Transiting exoplanets are the best way to characterize these systems. The dip in the star’s light caused by the transit tells us the radius of the planet. Spectroscopic measurements using the Doppler effect yields the planet’s mass. From the calculated bulk density we have the first indications of the planet’s internal structure and composition. This project will analyse light curves from NASA’ Transiting Exoplanet Survey Satellite (TESS) to find transiting planets. Ground-based spectroscopic measurements will confirm these and measure the planetary mass. The key science questions to be answered are:

  • What is the nature between the boundary of icy and giant planets in the mass-density diagram?
  • Is there a boundary between Brown Dwarfs, objects considered to be between a planet and star, and giant planets?
  • What is the source of the diversity in densities found among small planets. What role does the central star play (e.g. photoevaporation of the planetary atmosphere due to heating from the host star).
  • Are there so-called “third parameters” besides the mass and density which can shed light on this diversity.

The findings of this project will help prepare the German community for PLATO mission. PLATO is a flagship mission of the European Space Agency that will use the transit method to find exoplanets including small planets in the habitable zone of sun-like stars.


Massimiliano Esposito, Kristine Lam, Sascha Grziwa, Eike Günther

Invited Guests

Michael Endl

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