Target Stars and Systems 2025 (TSS25)

• TSS25 is a prioritized catalog that identifies nearby stars ideal for direct-imaging of Earth-like planets in habitable zones.
• The catalog employs yield simulations using frameworks like AYO and EXOSIMS to rank targets based on their potential exoEarth yield.
• TSS25 drives precursor science by integrating multi-technique observations to refine stellar properties critical for biosignature interpretation.

Target Stars and Systems 2025 (TSS25) is a community-developed, prioritized catalog of potential stellar targets for the Habitable Worlds Observatory (HWO) ExoEarth survey. It was designed to bridge HWO mission design studies and precursor science by identifying the nearby stars most likely to matter for directly imaging Earth-sized planets in the habitable zone, while remaining robust against uncertainty in the observatory’s final aperture, coronagraph design, inner working angle, contrast floor, detectors, and mission lifetime (Tuchow et al., 24 Sep 2025).

1. Origins, scope, and scientific role

TSS25 emerged from the HWO Target Stars and Systems sub-working group as a practical interface between architecture studies and pre-launch stellar characterization. Its immediate scientific context is HWO’s Decadal Survey goal of directly imaging and spectrally characterizing at least 25 Earth-sized planets in the habitable zones of nearby Sun-like stars. In that setting, TSS25 is not merely a star list; it is a prioritization framework for where limited precursor resources should be concentrated before launch (Tuchow et al., 24 Sep 2025).

The catalog is built on the premise that target selection for a direct-imaging flagship cannot be reduced to distance or brightness alone. The Living Worlds Working Group under NASA-DARES framed target selection around three linked goals: maximizing the probability of finding potentially habitable, rocky planets; maximizing the probability of detecting and correctly interpreting biosignatures in UV–Vis–NIR spectra; and ensuring that targets are observationally feasible for a direct-imaging flagship in the presence of contrast, inner working angle, signal-to-noise ratio, and integration-time constraints (Parenteau et al., 10 Jan 2026).

This framing gives TSS25 a dual character. On one side, it is an HWO survey input catalog. On the other, it is a precursor-science agenda: a statement that nearby stars likely to matter for HWO should be characterized in advance in stellar multiplicity, activity, elemental abundances, ages, ultraviolet and X-ray environments, and planetary-system architecture. In practical terms, the TSS25 concept is therefore embedded in a broader cross-divisional astrobiology strategy that connects astrophysics, planetary science, heliophysics, and Earth science (Parenteau et al., 10 Jan 2026).

2. Construction and yield-based methodology

The starting point for TSS25 is the HWO Preliminary Input Catalog (HPIC), an input catalog for yield simulations constructed from a union of the TESS Input Catalog and Gaia DR3, with cuts of distance $d \leq 50$ pc and brightness $G \leq 12$ and $T \leq 12$. HPIC contains 12,944 stars and supplies positions, proper motions, parallaxes, distances, luminosities, radii, multiplicity flags, and related basic properties, with typical uncertainties of about $10$–$20\%$, sufficient for yield trade studies (Tuchow et al., 24 Sep 2025).

TSS25 then combines HPIC with the NASA Exoplanet Exploration Program Mission Star List, which contributes 164 high-priority stars selected for a 6-m coronagraphic mission on the basis of habitable-zone angular separation, habitable-zone planet–star contrast, brightness, distance, and screening for multiplicity and circumstellar material. The catalog construction is explicitly yield-driven rather than purely geometric. Two independent frameworks were used: AYO (Altruistic Yield Optimization) and EXOSIMS. The AYO calculations included an 8-m LUVOIR-B scenario and a 6-m “Scenario F,” while the EXOSIMS ensemble spanned 354 design scenarios with $D = 6\text{–}9$ m, coronagraph throughput $\mathcal{T} = 0.1\text{–}0.6$, inner working angle $\mathrm{IWA} = 20\text{–}70$ mas, and contrast floor $\sim 10^{-9}\text{–}10^{-11}$ (Tuchow et al., 24 Sep 2025).

The underlying logic is the usual direct-imaging completeness formalism. Very schematically,

$Y = \sum_i P_{\mathrm{det},i},$

with per-star completeness

$G \leq 12$0

and yield

$G \leq 12$1

The accessible habitable-zone geometry is governed by

$G \leq 12$2

while the reflected-light contrast for an Earth analog is approximated by

$G \leq 12$3

These relations encode the central TSS25 trade: the most valuable stars are not simply the nearest, but those whose habitable zones are both resolvable and observable at plausible contrast and exposure time (Tuchow et al., 24 Sep 2025).

Tier 2 was derived by trimming the yield outputs rather than by simple catalog cuts. In the AYO runs, each scenario had 996 Monte Carlo iterations, and stars were retained if their selection rate exceeded $G \leq 12$4. In the EXOSIMS ensemble, stars were ranked by a panchromatic yield metric defined as the average exoEarth yield over UV, visible, and IR, and the top $G \leq 12$5 by cumulative panchromatic yield were retained. The union of those trimmed sets, after removing Tier 1 stars, defines Tier 2 (Tuchow et al., 24 Sep 2025).

3. Tier structure and catalog content

TSS25 assigns every HPIC star to one of three tiers. The tier system is central because it encodes both expected HWO utility and urgency of precursor characterization.

Tier	Stars	Role
Tier 1	164	Most accessible targets for exoEarth direct imaging
Tier 2	495	Targets that could plausibly be observed by proposed HWO designs
Tier 3	12,285	Additional nearby, bright objects that are potential HWO targets

Tier 1 consists of the 164 stars adopted directly from the ExEP Mission Star List. These are mostly nearby FGK dwarfs, with average $G \leq 12$6-band magnitude $G \leq 12$7 and distances typically $G \leq 12$8 pc. Tier 2 contains 495 stars that extend to somewhat larger distances and lower brightness than Tier 1, while remaining plausible contributors to exoEarth yield across the HWO trade space. Tier 3 is the remainder of HPIC: 12,285 stars that are useful for future trade analyses but are not central to current yield expectations. The full catalog therefore contains 12,944 stars (Tuchow et al., 24 Sep 2025).

The table schema is designed for immediate cross-matching with precursor surveys. For each star, TSS25 includes identifiers such as star_name, TSS_tier, TIC, Gaia DR3, Hipparcos, HD, 2MASS, Gliese, and SIMBAD IDs; astrometric parameters such as RA, Dec, proper motions, parallax, and distance; and at least Johnson $G \leq 12$9 magnitude and $T \leq 12$0. The catalog is publicly available via Zenodo and is planned for hosting on CDS VizieR, reflecting its role as a shared data product rather than an internal mission worksheet (Tuchow et al., 24 Sep 2025).

A recurrent misunderstanding is that TSS25 is simply HPIC with a ranking overlay. In fact, HPIC is the broad input catalog; TSS25 is the prioritized derivative built by combining HPIC with ExEP selections and yield simulations. Another misunderstanding is that Tier 3 represents a third-priority precursor campaign sample. The TSS25 paper states that coarse stellar parameter estimates are sufficient for most Tier 3 stars and that detailed precursor campaigns are not warranted for the bulk of that tier (Tuchow et al., 24 Sep 2025).

4. Characterization layers required to make TSS25 scientifically actionable

The scientific usefulness of TSS25 depends on whether the stars in Tiers 1 and 2 can be placed in a sufficiently rich stellar and planetary context. The Living Worlds white paper identifies four stellar data products as especially critical in advance of HWO: stellar elemental abundances to about $T \leq 12$1 precision, stellar multiplicity, NUV/FUV fluxes, and stellar ages with precision ideally $T \leq 12$2 Gyr. Those quantities determine, respectively, the likely bulk composition of rocky planets, the dynamical stability of habitable-zone architectures, the host-driven photochemistry that controls false positives and false negatives in biosignature interpretation, and the evolutionary context of planetary atmospheres (Parenteau et al., 10 Jan 2026).

Subsequent TSS-linked catalogs show that this contextual layer is only partially in place. A survey of archival UV and X-ray data for 98 Tier 1 stars found that only $T \leq 12$3 have usable data in all four high-energy bands considered—X-ray, EUV, FUV, and NUV—and only $T \leq 12$4 have usable spectroscopy in all four. If EUV is dropped, an additional $T \leq 12$5 have good X-ray, FUV, and NUV spectroscopic coverage, but truly comprehensive high-energy spectral energy distributions remain rare (Peacock et al., 10 Sep 2025). This is directly relevant because the same study concludes that most datasets are single-epoch, limiting assessment of variability and flares, which are key inputs to atmospheric escape and photochemistry models (Peacock et al., 10 Sep 2025).

The activity-and-rotation layer is substantially better, but still incomplete. The ARC catalog for potential HWO targets reports that stellar activity properties such as the S-index and $T \leq 12$6, and rotation diagnostics such as $T \leq 12$7 and $T \leq 12$8, have been measured for at least $T \leq 12$9 of systems that are currently of high interest as potential HWO atmospheric characterization targets. However, stellar activity cycles are measured for fewer than $10$0 of those high-interest stars, even though cycle knowledge is critical for anticipating epochs of elevated magnetic activity during HWO observations (Fetherolf et al., 21 May 2026).

The age layer is markedly sparse. The HAges catalog, which covers all 659 Tier 1 and Tier 2 stars, finds that only $10$1 of the sample have asteroseismic ages and $10$2 have gyrochronal ages, with only $10$3 having constraints from both methods. The same study emphasizes that $10$4 Gyr, or $10$5 at solar age, is the required precision scale for distinguishing evolutionary trends in atmospheric biosignatures of terrestrial habitable-zone planets (Ware et al., 12 May 2026).

Multiplicity is another essential layer because both target feasibility and planetary-system interpretation depend on close companions. Optical speckle observations of stars in the provisional HWO star list have so far covered 80 of the 164 Tier 1 stars. Simulations of companions down to $10$6 indicate that $10$7–$10$8 would be detected in the existing speckle images; the remainder are either too faint or too close in and will require long-term spectroscopy or space-based techniques (Hartman et al., 8 Jan 2026). This implies that TSS25 must be read as a dynamically evolving target set whose multiplicity status is still being refined.

5. Use of TSS25 in HWO survey design, precursor science, and biosignature interpretation

In HWO planning, TSS25 functions as the baseline from which realistic observing programs, precursor campaigns, and yield analyses are built. Designers can assume that credible exoEarth target sets will be drawn predominantly from Tiers 1 and 2, and community precursor programs can therefore focus on those 659 stars rather than on the full 12,944-star HPIC. This is why the TSS25 paper links the tier system directly to a tier-dependent precursor strategy: intensive characterization for Tier 1, substantial precursor work for Tier 2, and only basic parameters for most Tier 3 stars (Tuchow et al., 24 Sep 2025).

The required precursor observations are explicitly multi-technique. Radial-velocity campaigns are needed to detect or rule out massive planets and constrain planetary architectures; high-resolution imaging is needed to identify stellar companions; precise luminosities, radii, masses, and effective temperatures are needed because planet properties inferred from reflected light are relative to host-star parameters; activity and rotation monitoring constrains stellar noise; high-energy emission in X-ray and UV sets the planetary photochemical environment; and astrometry identifies long-period planets and wide binaries (Tuchow et al., 24 Sep 2025). A plausible implication is that TSS25 is less a static target list than a coordination layer across Gaia, TESS, PLATO, Roman, RV and astrometry programs, UV and X-ray facilities, and ground-based spectroscopic surveys, exactly as envisaged in the NASA-DARES Living Worlds synthesis (Parenteau et al., 10 Jan 2026).

TSS25 also anchors the interpretation framework for biosignatures. The Living Worlds white paper repeatedly argues that biosignatures must be interpreted in the context of the whole system, including host-star NUV/FUV fluxes, multiplicity, stellar abundances, age, and planetary-system architecture. In that sense, TSS25 formalizes the precondition for a false positive/false negative framework: the observatory can only interpret O$10$9, O$20\%$0, CH$20\%$1, hazes, or surface features rigorously if the host star and system were characterized beforehand (Parenteau et al., 10 Jan 2026).

A common misconception is that precursor science can be restricted to Tier 1 because Tier 1 contains the “most accessible” stars. The TSS groups explicitly state that limiting precursor campaigns to only the 164 Tier 1 stars is risky, because HWO will likely detect exoEarths around Tier 2 stars as well; neglecting Tier 2 would delay or degrade interpretation of discoveries outside the narrowest core sample (Tuchow et al., 24 Sep 2025).

6. Architecture dependence, limitations, and the “living catalog” model

TSS25 was designed to be robust against uncertainty in the final HWO implementation, but it is not architecture-independent in an absolute sense. The catalog paper states explicitly that detailed target lists for a specific observatory will still depend on final values of aperture $20\%$2, inner working angle, contrast, throughput, bandpass, and mission lifetime. Tier 1 is therefore “very robust,” Tier 2 is “broader but still constrained,” and Tier 3 remains primarily a reservoir for future trade studies (Tuchow et al., 24 Sep 2025).

This architecture dependence interacts with incomplete stellar characterization. HPIC-level uncertainties of about $20\%$3–$20\%$4 are adequate for yield studies, but not for the most precise interpretation of reflected-light spectra. High-energy coverage is sparse for many Tier 1 stars, cycle measurements are rare, and precise ages are scarce. These deficits are not peripheral; they define where TSS25 is still immature as an operational precursor framework. The associated TSS products—high-energy surveys, activity catalogs, age compilations, and multiplicity campaigns—are therefore best understood as extensions of TSS25 rather than as separate efforts (Peacock et al., 10 Sep 2025).

The catalog’s response to this incompleteness is procedural rather than eliminative. TSS25 is explicitly a living document: it will be updated as new Gaia releases improve distances and multiplicity, as new exoplanets are discovered, and as stellar parameter estimates are refined. Tier assignments can change when new information about target suitability becomes available, including unseen companions, disruptive giant planets, or problematic exozodiacal dust (Tuchow et al., 24 Sep 2025).

In encyclopedia terms, TSS25 is best regarded as the prioritized stellar backbone of the HWO exoEarth program. It converts a broad nearby-star inventory into a mission-relevant target hierarchy, but it also exposes the remaining research program required to make those targets interpretable. Its enduring significance lies in that combined role: it is simultaneously a target list, a precursor-science agenda, and a continuously updated interface between exoplanet yield theory and the astrophysical reality of nearby stellar systems (Tuchow et al., 24 Sep 2025).