I study extrasolar planets using the technique of astrometry. I am particularly interested in the transition region between planets and brown dwarfs. Those have overlapping mass ranges and understanding their formation and evolution requires comprehensive observational characterisation and the study of a large number of objects.

Planets and brown dwarfs orbiting Sun-like and red stars

I combined very precise radial-velocity measurements and Hipparcos astrometry to investigate the mass distribution of giant planets around Sun-like stars. I showed that the high-mass tail of the planet distribution intersects with the mass distribution of substellar companions in binary stars in the range of ∼30 – 40 Jupiter masses (Sahlmann et al. 2011). Thus, both planet and binary formation scenarios have to incorporate a small fraction (≃0.6 %) of 30 Jupiter-mass companions, and mass alone is insufficient to trace back the formation paths of such objects.

In Sahlmann et al. 2016 I reported the astrometric mass measurement of the giant planet GJ676A b, previously discovered using radial velocities. It is the first time that such work was accomplished with ground-based measurements. The planet has a mass of about seven Jupiter masses and I found that it is a candidate for reflected light observations with the WFIRST coronagraph and for thermal imaging with extremely large telescopes. This result demonstrates the synergy of radial-velocity and astrometric surveys that is necessary to identify the best targets for future direct imaging of mature exoplanets.
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Orbital motion of GJ 676A as a function of time. The orbital signature due to its planet is shown in Right Ascension (left) and Declination (right).

Planets orbiting brown dwarfs

To explore the unknown occurrence of planets around the lowest mass stars and brown dwarfs, I am leading an astrometric planet search around 20 M/L-transition dwarfs using the FORS2 camera at the ESO VLT (Sahlmann et al. 2014). These are faint optical sources and therefore difficult to study with high-precision. We discovered two brown dwarf binary systems at 20 pc distance, for which we could constrain the masses and/or ages of the components. Systems like this are rare and important to further our understanding of brown dwarfs in a mass range shared with the most massive planets. I used the unprecedented accuracy (~100 micro-arcsecond residual dispersion) of our survey to show that super-Jupiter planets are rare around M/L-dwarfs and I am now following up on several planet candidates with enough sensitivity to discover Neptune-mass planets.


Astrometric motion of an L2+L6 ultracool binary discovered in our survey. Panel a shows proper and parallactic motion in the sky plane. Panel b is a close-up of the photocentric orbit of the binary. Observations with uncertainties and the best-fit model are shown as black circles and grey curve. From Sahlmann et al. 2015.

Gaia’s exoplanet yield

Thanks to its all-sky survey with extreme astrometric precision, the Gaia mission will discover thousands of giant planets around single stars. This prediction is solid because we have reasonable knowledge of the occurrence rates of those planets. In contrast, the abundance and properties of planets orbiting binary stars – circumbinary planets – are largely unknown because they are difficult to detect with currently available techniques. Results from the Kepler satellite and other studies indicate a minimum occurrence rate of circumbinary giant planets of  about 10 per cent, yet only a handful are presently known.

I simulated the Gaia survey of nearby binary stars and showed that Gaia may discover hundreds of giant planets around binaries with FGK-dwarf primaries and that Gaia is critically sensitive to the properties of giant circumbinary planets, including their mutual inclinations and occurrence as a function of stellar mass and evolutionary state (Sahlmann, Triaud & Martin 2015).