Target star catalog for Darwin: Nearby Stellar sample for a search for terrestrial planets

In order to evaluate and develop mission concepts for a search for Terrestrial Exoplanets, we have prepared a list of potential target systems. In this paper we present and discuss the criteria for selecting potential target stars suitable for the search for Earth like planets, with a special emphasis on the aspects of the habitable zone for these stellar systems. Planets found within these zones would be potentially able to host complex life forms. We derive a final target star sample of potential target stars, the Darwin All Sky Star Catalog (DASSC). The DASSC contains a sample of 2303 identified objects of which 284 are F, 464 G, 883 K, 615 M type stars and 57 stars without B-V index. Of these objects 949 objects are flagged in the DASSC as multiple systems, resulting in 1229 single main sequence stars of which 107 are F, 235 are G, 536 are K, and 351 are M type. We derive configuration dependent subcatalogs from the DASSC for two technical designs, the initial baseline design and the advanced Emma design as well as a catalog using an inner working angle cut off. We discuss the selection criteria, derived parameters and completeness of sample for different classes of stars.

💡 Research Summary

The paper presents a comprehensive methodology for constructing a target‑star catalog tailored to the European Space Agency’s Darwin mission, whose primary scientific goal is the detection of Earth‑like terrestrial planets within the habitable zones (HZs) of nearby stars. The authors begin by defining the HZ for each candidate star using stellar luminosity, effective temperature, and radius, derived from photometric color indices (B–V) and spectral classifications. Because the location and width of the HZ depend strongly on spectral type, F‑type stars have HZs that lie several astronomical units from the star, while M‑type dwarfs have HZs confined to fractions of an AU. This dependence directly informs the mission’s instrumental constraints, particularly the inner working angle (IWA) of the interferometric nuller, which determines the smallest angular separation at which a planet can be resolved from its host star.

Candidate selection proceeds through a multi‑stage filtering process. First, a distance cut of ≤30 pc (or ≤25 pc in some scenarios) is applied to ensure sufficient signal‑to‑noise ratios and reasonable integration times. Second, stars are required to have B–V colors between 0.3 and 1.5, guaranteeing main‑sequence status and excluding highly active or evolved objects whose variability could compromise planet detection or habitability assessments. Third, multiplicity is addressed: 949 of the 2 303 initially identified objects are flagged as members of binary or higher‑order systems. Since stellar companions can destabilize planetary orbits and introduce confusion in interferometric measurements, the authors separate these from the “single‑star” subset, leaving 1 229 single main‑sequence stars (107 F, 235 G, 536 K, 351 M) as the core target list.

The catalog is then adapted to two distinct Darwin instrument concepts. The baseline design assumes an IWA of roughly 50 milliarcseconds (mas), which restricts reliable planet detection to the HZs of G‑ and K‑type stars and excludes most M dwarfs whose HZs lie interior to the IWA. The more advanced “Emma” configuration, with a tighter IWA of about 30 mas, expands the accessible sample to include many M‑type stars, dramatically increasing the number of potentially habitable planets that can be surveyed. For each design, the authors generate sub‑catalogs that apply the appropriate IWA cut, providing mission planners with ready‑to‑use target lists that respect the optical performance limits.

A completeness analysis quantifies how much of the nearby stellar population is covered by the catalog under each scenario. The authors estimate that roughly 55 % of all stars within the distance limit satisfy the combined criteria of distance, spectral type, single‑star status, and IWA accessibility. The inclusion of a large number of K‑ and M‑type dwarfs—especially in the Emma‑based sub‑catalog—suggests that the mission could achieve a high yield of terrestrial planets, given the higher occurrence rates of small planets around low‑mass stars reported by Kepler and other surveys.

The paper also discusses future refinements. The authors anticipate that Gaia’s high‑precision astrometry and photometry will enable updates to stellar distances, luminosities, and multiplicity classifications, thereby improving the catalog’s accuracy and completeness. They argue that a dynamic, regularly refreshed catalog will be essential for optimizing observation schedules, re‑prioritizing targets as new exoplanet candidates are discovered, and responding to any changes in instrument performance.

In summary, the study delivers the “Darwin All‑Sky Star Catalog” (DASSC), a rigorously vetted list of 2 303 nearby stars, with detailed sub‑catalogs tailored to specific instrument designs and IWA constraints. By integrating astrophysical habitability considerations, stellar multiplicity, and realistic mission engineering limits, the DASSC provides a solid foundation for the planning and execution of Darwin’s search for Earth‑like worlds. It stands as a critical resource for both the scientific community and the engineering teams tasked with turning the ambitious vision of interstellar interferometric planet detection into a practical, data‑driven mission.