Ideal TTV Target Systems¶
OpenAstro first TTV campaign — target selection and rationale. Last updated: March 2026.
1. Selection Criteria¶
For OpenAstro's first TTV campaign to produce publishable, science-grade results, each target must satisfy all of the following:
C1. Host star brightness: V < 13. Enables 0.1–0.3% photometric precision on a 0.2m–0.4m aperture telescope with a cooled CMOS or CCD camera in a single 60-second exposure, using differential photometry against comparison stars in the same field. For V > 13 targets, a 0.4m or larger aperture is needed to reach the photon-noise floor before systematics dominate.
C2. Transit depth: >0.5%. Ensures the transit is detectable (not just the TTV) from amateur apertures. Hot Jupiters typically produce 1–2% depth; Saturn-class planets ~0.5%. Below 0.5%, ingress/egress timing precision per transit degrades toward ±5 minutes, making TTV detection require an impractically large number of observed transits.
C3. Orbital period: 1–10 days. Short periods mean many transits per year, building baseline quickly. Periods <1 day are challenging to phase-cover globally (every night is a transit, but full ingress+egress coverage requires coordination). Periods >10 days require scheduling windows spanning multiple sites to catch ingress and egress and reduce the per-year transit count.
C4. TTV evidence or strong theoretical motivation. Either (a) observed TTVs already in the literature, (b) a confirmed or suspected additional companion near mean-motion resonance (MMR), which dramatically amplifies the TTV signal per Agol et al. (2005), or (c) a measured or predicted secular period drift (tidal decay, apsidal precession). Systems where ExoClock O-C residuals already show drift are highest priority.
C5. Good sky accessibility. Declination between −40° and +60° ensures observability from both Northern and Southern hemisphere OpenAstro nodes. Targets near the ecliptic plane have the longest annual observability windows. Continuous visibility (not in solar conjunction for more than ~3 months per year) is required so the observing baseline accumulates without interruption.
C6. Archival transit data baseline. At least 2 years of published transit mid-times (from ETD, ExoFOP, ExoClock, or the Kepler/TESS archive) provides the Stage 1 baseline for ephemeris fitting before OpenAstro begins contributing new observations. Longer baselines stretch the lever arm for detecting period drift.
C7. Ephemeris degradation. ExoClock III (Kokori et al. 2022, arXiv:2209.09673) found that ~40% of known exoplanet ephemerides deviate significantly from their published values due to unmodeled TTVs or simply outdated linear fits. Targets where the ExoClock ephemeris uncertainty is >5 minutes are high priority for scheduling reasons (extended observing windows needed) and scientifically (the deviation may be a real TTV signal).
C8. No severe field contamination. The target field should not contain nearby bright stars (separation < 15 arcsec, magnitude within 4 magnitudes of the target) that blend into the photometric aperture and dilute the transit depth, reducing effective timing precision.
Criterion summary table¶
| Criterion | Threshold | Notes |
|---|---|---|
| Host V magnitude | < 13 (Tier 1), < 14.5 (Tier 2) | Sets photometric precision |
| Transit depth | > 0.5% | Below this, ingress timing noise explodes |
| Orbital period | 1–10 days | 3–6 days is optimal |
| TTV signal | Detected or theoretically motivated | Detections preferred for pipeline validation |
| Declination | −40° to +60° | Global network coverage |
| Archival baseline | ≥ 2 years of published mid-times | For Stage 1 pipeline test |
| ExoClock O-C drift | > 1 min preferred | Signals active TTV |
2. Data Sources for Target Selection¶
2.1 ExoClock (https://www.exoclock.space)¶
What it is: A coordinated citizen-science programme (Kokori et al. 2022, arXiv:2209.09673) that monitors ~300 exoplanet systems for ephemeris drift. Observing priority scores are updated nightly based on ephemeris uncertainty propagation. The ExoClock III release documents 450+ updated ephemerides with quantified drift rates.
What it offers OpenAstro: - A continuously updated priority list of targets where the ephemeris needs new data — directly aligns with OpenAstro scheduling - Open data pipeline: the ExoClock Python package (https://github.com/ExoClock/ExoClockS) provides end-to-end FITS → mid-transit-time reduction with standardized output format - The O-C (Observed minus Computed) plots per system reveal whether existing drift is secular (TTV-like) or noise - Community coordination prevents multiple observers wasting time on the same transit window
Key insight for OpenAstro: ExoClock's priority score combines ephemeris uncertainty, period, and sky accessibility into a single number. OpenAstro should ingest this list directly into its scheduling algorithm rather than maintaining a parallel priority system.
2.2 Exoplanet Transit Database (ETD, http://var2.astro.cz/ETD/)¶
What it is: The oldest public amateur transit database, maintained since 2009, with >10,000 transit light curves from amateur observers. ETD computes O-C diagrams for all submitted targets.
What it offers OpenAstro: - The O-C diagram for any system is generated from all submitted light curves — this is the TTV signal directly - ETD quality flags (0–5 scale) allow filtering to only use high-quality timing measurements - Historical depth: some popular systems have 10+ years of O-C data from ETD alone - Submission API: OpenAstro can submit processed transit mid-times directly to ETD, contributing to the community database and getting credit in future analyses
Limitation: ETD light curves are not uniformly calibrated. Timing uncertainties are sometimes underestimated because systematic noise is not modeled. OpenAstro's pipeline should derive independent timing uncertainties using the beta-factor (Winn et al. 2008) rather than trusting ETD error bars.
2.3 NASA Exoplanet Archive (https://exoplanetarchive.ipac.caltech.edu/)¶
What it is: The authoritative catalogue of confirmed exoplanets with orbital parameters, stellar properties, and discovery references.
What it offers OpenAstro: - Accurate system parameters (period, depth, transit duration, stellar magnitude) for all targets - Direct download of Kepler/TESS photometry and timing products for systems with archival data - The "Confirmed Planets" table filterable by V magnitude, period, transit depth, and declination — ideal for generating the initial candidate list - The TESS Follow-up Observing Program (TFOP) candidate lists, which include systems flagged for ground-based timing follow-up
For TTV specifically: The NASA Exoplanet Archive hosts the Holman et al. (2010) Kepler-9 discovery data and the Nesvorny et al. (2013) Kepler-88 TTV analysis products, which serve as the gold-standard validation datasets for the OpenAstro pipeline.
2.4 ExoFOP (https://exofop.ipac.caltech.edu/)¶
What it is: The Exoplanet Follow-up Observing Program portal, where the professional community posts requests for ground-based follow-up of TESS candidates and confirmed planets.
What it offers OpenAstro: - Target lists for systems actively requested for timing follow-up by professional astronomers — these are the highest scientific demand targets - Access to TESS sector data for each target, allowing computation of TESS-based transit times for Stage 1 pipeline validation - A submission pathway: ground-based observers can upload light curves directly to ExoFOP, where they are used by the TESS mission team — this is a direct route for OpenAstro to contribute to active professional science
2.5 Holman & Murray (2005) and Agol et al. (2005) — The Foundational Papers¶
Holman & Murray (2005), Science 307, 1288: "Use of Transit Timing to Detect Terrestrial-Mass Extrasolar Planets." This paper demonstrated analytically that a perturbing planet of Earth mass near mean-motion resonance produces timing variations of seconds to minutes in the transiting planet — detectable with then-future photometry. It established the theoretical framework that motivates all TTV science. OpenAstro's pipeline implements the inverse of Holman & Murray's forward calculation.
Agol et al. (2005), MNRAS 359, 567: "On Detecting Extrasolar Planets by Timing Their Transits." Simultaneously derived the resonant amplitude formula and showed that near-resonance systems produce dramatically larger TTVs (minutes to hours vs. seconds for off-resonance). The formula $A_\text{TTV} \approx (m_c/M_\star) \cdot P_b / (2\pi |\Delta|)$ comes from this paper and is the basis for the sensitivity calculations in TTV Sensitivity and N-Body Math.md.
What these papers offer for target selection: They define the physical parameter space where TTVs are large enough to detect. Any system with a known companion within 10% of a low-order MMR ($|\Delta| < 0.1$) is a priority target. The papers' Table 1 examples have been confirmed by Kepler; the remaining uncharacterized systems from Kepler and TESS are the current frontier.
3. The Top 10 Candidate Systems¶
Systems are ordered within each tier by observational accessibility. Tier 1 = accessible to 8"–12" (0.2m–0.3m) apertures. Tier 2 = requires 14" (0.35m) or larger.
Tier 1 — First-campaign targets (0.2m apertures)¶
System 1: WASP-18 b¶
Host star: WASP-18, V = 9.3, spectral type F6V — the brightest accessible TTV host Planet period: 0.9415 days Transit depth: ~1.0% Transit duration: ~2.1 hours Declination: −45° 40' — southern-favoured; northern nodes contribute March–September Known TTV: Secular period decay from tidal dissipation. Shporer et al. (2019, AJ 157, 178; arXiv:1901.05621) measured the orbital decay consistent with $dP/dt \approx -3.5 \times 10^{-10}$ days/cycle. Cumulative timing offset from a 5-year-old linear ephemeris is now ~30–40 seconds. TTV amplitude: ~30–40 s cumulative secular drift. Not a sinusoidal gravitational TTV, but a monotonically growing residual. Scientific interest: WASP-18 b orbits at only ~3 stellar radii. The tidal dissipation rate, encoded in the stellar tidal quality factor $Q'_\star$, constrains stellar interior physics. The measured $dP/dt$ is at the high end of tidal theory predictions — suggesting either efficient tidal dissipation or an unrecognized contribution (e.g., an unseen outer companion causing Rømer delay). Distinguishing these requires a long timing baseline. Minimum aperture: 0.15m (any telescope with a cooled sensor) Key papers: Hellier et al. (2009), Nature 460, 1098 (discovery); Shporer et al. (2019), arXiv:1901.05621
System 2: HAT-P-13 b¶
Host star: HAT-P-13, V = 10.6, spectral type G4V Planet period: 2.9160 days Transit depth: ~0.7% Transit duration: ~2.5 hours Declination: +47° 21' — excellent northern target; observable April–October from northern nodes Known TTV: Indirect — HAT-P-13 c is a confirmed outer companion on a 428-day eccentric orbit ($e_c \approx 0.69$). Bakos et al. (2009, ApJ 707, 446) discovered the system. Batygin, Bodenheimer & Laughlin (2009, ApJL 704, L49; arXiv:0907.3027) showed that HAT-P-13 b's orbit precesses at a rate set by HAT-P-13 c, and that this precession rate encodes the Love number $k_2$ of HAT-P-13 b — a direct measure of the planet's internal mass concentration. TTV amplitude: Apsidal precession period ~100+ years. Year-to-year timing drift ~10–30 seconds. Measurable over a 5+ year baseline at OpenAstro's precision level. Additionally, the eccentricity of planet c induces short-period TTV "chopping" at $P_c = 428$ d — potentially detectable with ~3 years of timing data. Scientific interest: The only hot Jupiter where its internal structure (core mass fraction) is potentially measurable from transit timing alone. A detection of the precession rate would constrain $k_2$ to ~20%, distinguishing rocky-core from coreless models. This is genuinely novel science not accessible through other methods. Minimum aperture: 0.2m Key papers: Bakos et al. (2009), ApJ 707, 446; Batygin et al. (2009), ApJL 704, L49; Winn et al. (2010), ApJL 718, L145 (updated ephemeris)
System 3: GJ 436 b¶
Host star: GJ 436, V = 10.6, spectral type M2.5V Planet period: 2.6439 days Transit depth: ~0.69% Transit duration: ~1.0 hour (M-dwarf: small star makes transit shallow in time) Declination: +26° 42' — optimal for northern networks; observable nearly year-round Known TTV: No confirmed detection, but a long-standing open question. GJ 436 b has a measurable orbital eccentricity ($e \approx 0.15$) that should have been tidally circularized on Gyr timescales given the planet's proximity. A resonant perturber is the standard explanation for sustained eccentricity ("eccentricity pumping"). Multiple companion candidates (GJ 436 c) have been proposed and disputed since 2007. Trifonov et al. (2018, arXiv:1807.08742) placed upper limits on a companion but did not rule out the resonant scenario. Maciejewski et al. (2014) set TTV limits of ~30 seconds — just at OpenAstro's achievable precision. TTV amplitude: If a resonant perturber exists, expected amplitude ~1–5 minutes. If non-resonant, ~10–60 seconds. Scientific interest: Resolving what maintains GJ 436 b's eccentricity is a 20-year-old open question in exoplanet dynamics. A TTV detection here would be an ApJL paper. Even a non-detection (deeper upper limits than Maciejewski et al.) constrains the perturber parameter space usefully. Minimum aperture: 0.2m Key papers: Butler et al. (2004), ApJ 617, 580 (discovery); Maciejewski et al. (2014, AcA 64, 323); Trifonov et al. (2018), arXiv:1807.08742
System 4: WASP-12 b¶
Host star: WASP-12, V = 11.7, spectral type F Planet period: 1.0914 days Transit depth: ~1.4% Transit duration: ~3.0 hours Declination: +29° 40' — observable from northern nodes October–April; southern nodes year-round Known TTV: Yes — period decay confirmed. Maciejewski et al. (2016, A&A 585, A114; DOI:10.1051/0004-6361/201526741) detected the shrinking period. The rate $dP/dt \approx -10$ ms/year corresponds to orbital decay timescale ~3 Myr. Cumulative O-C offset from the 2008 discovery ephemeris exceeds ~3 minutes by 2026. TTV amplitude: Secular drift currently accumulating at ~1 second/day. The total accumulated offset over 5 years is ~3–4 minutes — easily detectable from any node. Scientific interest: WASP-12 b is being tidally disrupted in real time. The decay rate constrains the tidal quality factor of the F-type host star, which is poorly understood. Additionally, Bouma et al. (2020, AJ 159, 89; arXiv:1901.02711) proposed that the timing anomaly might partly be explained by stellar proper motion (Shklovskii effect) or a wide binary companion producing a Rømer delay — competing hypotheses directly testable with continued precise timing. Minimum aperture: 0.2m Key papers: Hebb et al. (2009), ApJ 693, 1920 (discovery); Maciejewski et al. (2016), A&A 585, A114; Bouma et al. (2020), arXiv:1901.02711
System 5: TrES-2 b (Kepler-1 b)¶
Host star: TrES-2 / Kepler-1, V = 11.4, spectral type G0V Planet period: 2.4706 days Transit depth: ~1.6% Transit duration: ~1.7 hours Declination: +49° 19' — northern target; excellent July–November window Known TTV: No confirmed dynamical TTV. The Kepler mission observed 1,400+ transits of TrES-2 b from 2009–2013 with ~15 second timing precision per transit, establishing an unmatched archival baseline. Raetz et al. (2019, MNRAS 483, 824) combined Kepler with ground-based timing and found no significant TTV signal, placing upper limits of ~60 seconds on any sinusoidal component. However, the Kepler baseline ends in 2013 — and any long-period perturber with $P > 4$ years would have a TTV super-period longer than the Kepler baseline, making it invisible to Kepler but potentially detectable with a new 5-year ground-based baseline starting in 2026. TTV amplitude: Upper limit ~60 seconds (Raetz et al. 2019). Anything above that would have been seen by Kepler. Scientific interest: The world's best-characterized null TTV system. Any new deviation from the Kepler ephemeris would be immediately significant. The 1.6% transit depth makes this the most photometrically comfortable target on the list. TrES-2 b is also notable for being anomalously dark (albedo < 0.01 — darker than charcoal), and the long-baseline timing can probe whether any secondary eclipse timing varies (an eccentricity probe). Minimum aperture: 0.2m Key papers: O'Donovan et al. (2006), ApJL 651, L61 (discovery); Holman et al. (2007), ApJ 664, 1185; Raetz et al. (2019), MNRAS 483, 824. Kepler archive: MAST/KIC 11446443
System 6: WASP-4 b¶
Host star: WASP-4, V = 12.5, spectral type G7V Planet period: 1.3382 days Transit depth: ~2.2% — the largest depth on this list Transit duration: ~1.7 hours Declination: −42° 04' — southern target; accessible from northern nodes May–August Known TTV: Yes — debated. Bouma et al. (2019, AJ 157, 217; arXiv:1903.05115) detected a period shortening of $dP/dt \approx -12.6$ ms/year from TESS data, consistent with tidal decay. However, the signal can also be partly explained by a Rømer delay from an undetected wide companion. Southworth et al. (2019; arXiv:1904.09876) found marginal evidence for both effects. The current situation requires a longer timing baseline to separate the two interpretations, since tidal decay produces a parabolic O-C curve while Rømer delay produces a sinusoidal one. TTV amplitude: Timing residuals from a linear ephemeris reach ~20–80 seconds depending on the dataset — well above OpenAstro's achievable ~20–30 second precision per single transit, and better with multi-node averaging. Scientific interest: The 2.2% transit depth is the highest of any Tier 1 target — accessible even to 8" (0.2m) telescopes under sub-optimal seeing. The physical interpretation of the O-C signal is unsettled. Resolving tidal decay vs. Rømer delay requires either a long timing baseline (different functional forms) or an independent RV-based Rømer detection. OpenAstro's multi-year timing baseline can independently contribute to this. Minimum aperture: 0.2m (deepest transit on the list; accessible even through moderate cloud) Key papers: Wilson et al. (2008), ApJ 675, L113 (discovery); Bouma et al. (2019), arXiv:1903.05115; Baluev et al. (2019), arXiv:1901.08900
System 7: WASP-47 b¶
Host star: WASP-47, V = 11.9, spectral type G9V Planet period: 4.1591 days (WASP-47 b) Transit depth: ~1.3% (WASP-47 b) Transit duration: ~3.6 hours Declination: −12° 01' — accessible from both hemispheres; excellent multi-longitude target Known TTV: Yes — confirmed multi-planet gravitational TTV. WASP-47 b sits in a compact multi-planet architecture (WASP-47 e at 0.790 days, WASP-47 b at 4.159 days, WASP-47 d at 9.031 days, and an outer giant WASP-47 c at 572 days). Becker et al. (2015, ApJL 812, L18; arXiv:1508.02411) detected this system with K2. The TTVs of WASP-47 b are induced primarily by WASP-47 e (the inner super-Earth) with amplitude ~2–5 minutes peak-to-peak. TTV amplitude: ~2–5 minutes peak-to-peak for WASP-47 b Scientific interest: WASP-47 b is one of a tiny number of hot Jupiters with confirmed nearby companions — a rare architecture that challenges formation models. Measuring WASP-47 b's TTVs provides an independent (non-RV) mass measurement of the super-Earth WASP-47 e. Each new transit timing datum constrains the dynamical model. The system is also a target for Ariel (ESA) atmospheric characterisation, making the orbital architecture especially valuable to constrain. Minimum aperture: 0.25m Key papers: Hellier et al. (2012), MNRAS 426, 739 (discovery); Becker et al. (2015), ApJL 812, L18; Weiss et al. (2017), AJ 153, 265 (radial velocity confirmation of architecture)
Tier 2 — Requires 0.35m+ aperture¶
System 8: Kepler-88 b¶
Host star: Kepler-88 (KOI-142), V = 13.6 Planet period: 10.916 days (Kepler-88 b); Kepler-88 c at 22.34 days (near 2:1 resonance) Transit depth: ~0.5% Transit duration: ~4.1 hours Declination: +40° 07' — northern Known TTV: Yes — one of the best-characterized TTV systems from Kepler. Nesvorny et al. (2013, ApJ 777, 3; arXiv:1306.3561) performed a full TTV inversion to derive the mass of Kepler-88 c ($m_c \approx 0.7 M_J$) from TTVs alone, without RV. The TTV amplitude is up to 10 hours peak-to-peak — the largest known for any confirmed system. This extreme amplitude arises because Kepler-88 b and c are offset from 2:1 resonance by $|\Delta| \approx 0.02$, placing them near the resonant divergence. TTV amplitude: Up to 10 hours peak-to-peak — detectable even if timing precision is only ±5 minutes Scientific interest: The canonical TTV inversion target. Nesvorny et al. (2013) demonstrated that TTV-derived masses agree with RV-derived masses to within 10% — validating the whole TTV-as-dynamical-probe approach. New transit times from OpenAstro extend the baseline beyond Kepler (which ended in 2013) and can detect any long-term drift in the resonant configuration. A third planet (Kepler-88 d) was detected at 1,403 days (Nesvorny et al. 2013); its influence on Kepler-88 b's TTVs is subtle and would require a 5-year extended baseline to characterize. Minimum aperture: 0.4m with cooled detector Key papers: Nesvorny et al. (2013), ApJ 777, 3; Mazeh et al. (2013), ApJ 768, 153
System 9: Kepler-9 b¶
Host star: Kepler-9, V = 13.9 Planet periods: Kepler-9 b (19.24 days), Kepler-9 c (38.91 days) — near 2:1 MMR Transit depth: ~0.7% for b; ~0.3% for c Transit duration: ~3.4 hours (b) Declination: +38° 24' — northern Known TTV: Yes — the first confirmed multi-planet system detected via TTVs. Holman et al. (2010, Science 330, 51) demonstrated that the two Saturn-class planets produce TTVs of 39 minutes (planet b) and 15 minutes (planet c) — the largest known at time of discovery. The TTV super-period of this pair is ~7 months, fully sampled within a single Kepler year. TTV amplitude: ~39 minutes peak-to-peak for Kepler-9 b — detectable with timing precision up to ±5 minutes per transit Scientific interest: The historical anchor of the TTV method. Kepler-9 d (a super-Earth at 1.59 days) was also detected photometrically. A third planet in the system complicates the dynamical model, creating an open question about whether the current TTV model fully accounts for all bodies. Long-baseline monitoring extends the resonant libration coverage and may reveal the full three-body dynamical evolution. Minimum aperture: 0.4m (V=13.9 is challenging; requires dark skies and careful calibration) Key papers: Holman et al. (2010), Science 330, 51; Torres et al. (2011), ApJ 727, 24 (follow-up)
System 10: HAT-P-11 b¶
Host star: HAT-P-11, V = 9.5, spectral type K4 Planet period: 4.8878 days Transit depth: ~0.35% — shallow (Neptune-radius planet around a K-dwarf) Transit duration: ~2.0 hours Declination: +48° 05' — excellent northern target Known TTV: HAT-P-11 b has a measurable orbital eccentricity ($e \approx 0.218$) — anomalous for a close-in planet that should have circularized. Bakos et al. (2010, ApJ 710, 1724) noted the eccentricity puzzle. Yee et al. (2018, AJ 155, 255) detected a long-period outer companion (HAT-P-11 c, $P \approx 9.3$ years) via RV, providing the eccentricity pumping source. The key question is whether HAT-P-11 b's transit times show TTVs consistent with the RV-measured outer companion — this is a direct test of the dynamical model. TTV amplitude: Expected chopping signal from the long-period companion: ~30–90 seconds. Requires sustained high-precision monitoring. Scientific interest: HAT-P-11 b is a warm Neptune — a class of planets with mysterious eccentricities and compositional puzzles (observed by Hubble/WFC3 to have a clear, water-bearing atmosphere). Connecting the orbital architecture (eccentricity pumped by HAT-P-11 c) to the atmospheric state requires knowing the eccentricity history, which timing can constrain. The brightness (V=9.5) means this is accessible to small apertures despite the shallow depth — scintillation noise rather than photon noise dominates, making multi-telescope simultaneous coverage highly effective. Note on depth: 0.35% is below the C2 criterion. This is a conditional inclusion — feasible for well-equipped nodes (0.3m+ with good seeing and high-cadence readout) but not for the minimum hardware tier. Listed here because the host brightness partially compensates. Minimum aperture: 0.3m, high cadence (60s or faster), dark skies Key papers: Bakos et al. (2010), ApJ 710, 1724 (discovery); Yee et al. (2018), AJ 155, 255 (outer companion)
4. Summary Table¶
| # | System | Host V | Period (d) | Transit Depth | TTV Status | Min Aperture | Tier |
|---|---|---|---|---|---|---|---|
| 1 | WASP-18 b | 9.3 | 0.94 | 1.0% | Tidal decay confirmed | 0.15m | 1 |
| 2 | HAT-P-13 b | 10.6 | 2.92 | 0.7% | Apsidal precession predicted | 0.2m | 1 |
| 3 | GJ 436 b | 10.6 | 2.64 | 0.69% | Perturber suspected (open question) | 0.2m | 1 |
| 4 | WASP-12 b | 11.7 | 1.09 | 1.4% | Tidal decay confirmed | 0.2m | 1 |
| 5 | TrES-2 b | 11.4 | 2.47 | 1.6% | Long-baseline null; perturber search | 0.2m | 1 |
| 6 | WASP-4 b | 12.5 | 1.34 | 2.2% | Decay + possible Rømer delay | 0.2m | 1 |
| 7 | WASP-47 b | 11.9 | 4.16 | 1.3% | Confirmed multi-planet gravitational TTV | 0.25m | 1 |
| 8 | Kepler-88 b | 13.6 | 10.92 | 0.5% | Confirmed 10-hr amplitude | 0.4m | 2 |
| 9 | Kepler-9 b | 13.9 | 19.24 | 0.7% | Confirmed 39-min amplitude | 0.4m | 2 |
| 10 | HAT-P-11 b | 9.5 | 4.89 | 0.35% | Perturber detected via RV; TTV expected | 0.3m | 1/2 |
5. The "First Paper" Target: Recommended Choice¶
Recommendation: WASP-4 b¶
Rationale — why WASP-4 b over all others:
WASP-4 b satisfies the single most important criterion for a first publication: it has an active, scientifically unresolved TTV signal that (a) is already above the noise floor achievable by OpenAstro's minimum hardware tier, and (b) has a genuinely open physical interpretation. This means OpenAstro's data will matter regardless of what it finds.
The specific case for WASP-4 b:
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Depth: 2.2% — the largest transit depth in the Tier 1 list. A 0.2m telescope in 2 mmag photometric conditions achieves $\sigma_{t_m} \approx 25$ seconds per transit (calculated from the Carter et al. 2008 formula with $A_\text{depth} = 0.022$, $\tau_\text{ing} = 20$ min, cadence = 2 min). This is excellent precision for a first-campaign target.
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The scientific question is live and decided by data: The Bouma et al. (2019) tidal decay interpretation and the Rømer delay interpretation predict different O-C curve shapes: parabolic vs. sinusoidal. OpenAstro's timing baseline, combined with existing ETD data going back to 2007, can distinguish these by 2028–2029.
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First paper template: The result takes the form: "We present N new transit mid-times of WASP-4 b from the OpenAstro distributed network, spanning 2026–2028. Combined with archival ETD data, the O-C diagram now spans 21 years. We fit parabolic and sinusoidal models and find [Model X] preferred at Bayesian evidence [Y]. If parabolic (tidal decay), $Q'_\star = Z$. If sinusoidal (Rømer delay), the implied companion has minimum mass $m \sin i = W M_J$ at period $P = V$ days." This is a complete, publishable result from a single season of systematic monitoring.
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Southern target provides immediate differentiation: WASP-4 b is at Dec −42°. Most amateur TTV monitoring comes from the northern hemisphere (ETD contributors skew European and North American). A sustained, organized southern-hemisphere monitoring programme through OpenAstro's network generates new, non-redundant data from day one.
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Pipeline validation: The existing ETD record (2007–present) gives an anchor ephemeris. The ExoClock pipeline handles WASP-4 b and provides FITS-to-mid-time reduction in a standard format. OpenAstro Stage 1 (archival reanalysis) and Stage 2 (live network) work in exactly the sequence described in the TTV inference document.
Runner-up: WASP-47 b¶
WASP-47 b is the scientifically richer target (confirmed gravitational TTV, multi-planet architecture, direct perturber mass measurement possible) but it requires the full network to be operating with good sky coverage from multiple longitudes to catch the 3.6-hour transit fully. As a first paper target, the logistics are harder. As a second paper target — once the network is operational and coordinated — WASP-47 b is the highest-impact system on the list.
Why NOT the Kepler systems (88/9) for the first paper¶
Kepler-88 b and Kepler-9 b are the most famous TTV systems, but: - Both require 0.4m+ apertures (limiting network participation to a small fraction of nodes) - Both are northern-only targets (Dec +38° to +40°), losing the geographic distribution advantage - The existing Kepler + ground-based datasets are already extensive; a new contribution from OpenAstro would need to be sustained for 3+ years to add significantly to the published record - They are better as Stage 2 targets when the network is fully operational
6. Campaign Design Notes¶
Timing precision target: Each OpenAstro node should aim for ±30 seconds timing precision per transit. This requires: - Full transit coverage: ingress + flat baseline + egress + 30-min pre/post out-of-transit baseline - Photometric scatter <0.2% per data point in 90-second bins (2 mmag) - Differential photometry against ≥3 stable comparison stars in the same field with similar colour to the target - GPS-disciplined timestamps. NTP jitter can exceed 1 second over hours; GPS-synchronized clocks achieve <10 ms. For ±30 second timing, NTP is marginally acceptable but GPS is strongly preferred. A dedicated GPS timestamp module costs ~$30 and can be added to any observatory control computer.
Multi-node combination: When 3+ OpenAstro nodes observe the same transit simultaneously, the combined light curve achieves timing precision of ±10–15 seconds even from 0.25m apertures. Photon noise and scintillation noise are uncorrelated between sites. The combined dataset reduces to ~$1/\sqrt{N}$ noise. This is OpenAstro's core quantitative advantage over single-site amateur monitoring.
Ephemeris coordination: All targets should be checked against ExoClock (https://www.exoclock.space) before scheduling. If ExoClock ephemeris uncertainty is <1 minute, the transit prediction is reliable enough to plan standard coverage (begin 30 min before predicted ingress). If >5 minutes, schedule conservatively with an extended 30+5 minute pre-ingress window. The ExoClock scheduling tool automatically propagates ephemeris uncertainty to a current-date transit prediction time window.
Data submission pipeline: 1. Raw FITS → astrometry.net plate solve → aperture photometry (AstroImageJ or ExoClock pipeline) 2. Differential photometry light curve → Mandel-Agol or trapezoidal transit fit → mid-transit time + uncertainty 3. Submit timing to: ETD (http://var2.astro.cz/ETD/), ExoFOP (https://exofop.ipac.caltech.edu/), and OpenAstro internal database 4. Aggregate timing dataset → TTVFast forward model → MCMC posterior on perturber parameters
Red noise mitigation: Systematic errors from airmass trends, comparison star colour differences, and thin cloud are the dominant noise source for timing (not photon noise) for V < 12 targets. Use ≥3 comparison stars; check that the derived mid-transit time is stable when individual comparison stars are removed; apply a Gaussian process (GP) detrending model to remove correlated atmospheric systematics before fitting. allesfitter and juliet both implement GP detrending natively.
Reference pipeline: The ExoClock pipeline (Python, open-source, https://github.com/ExoClock/ExoClockS) provides end-to-end FITS-to-timing workflow with standardized output. OpenAstro Stage 1 implementation should fork this pipeline and validate it against published ETD timing for the chosen target before reporting any new measurements.
7. Systems Considered and Excluded¶
Kepler-51 system (V = 14.7): Three super-puff planets with confirmed TTVs and the lowest measured planetary densities in the known planet census. Scientifically extraordinary but V=14.7 requires 0.6m+ aperture. Excluded from Tier 1; listed for future consideration when the network has larger nodes.
TRAPPIST-1 (V = 12.4, all seven planets with confirmed TTVs): Confirmed mutual TTVs (Grimm et al. 2018, A&A 613, A68; Agol et al. 2021, PSJ 2, 1) are groundbreaking science. However, individual transit depths are 0.06–0.38% — below the C2 threshold for small apertures. Achieving the photometric precision needed for TRAPPIST-1 timing (< 1 mmag per datum) requires either a large (>1m) telescope or space photometry. Not a first-campaign target. Add as a Stage 2/3 aspirational target with the 0.4m+ node tier.
HAT-P-7 b (Kepler-2 b, V = 10.5, period 2.2 days, depth 0.6%): Heavily studied by Kepler. No confirmed TTV. The known retrograde orbit (spin-orbit misalignment detected via Rossiter-McLaughlin) makes it dynamically interesting but does not produce TTVs unless a perturber is present. Upper limits from Kepler are already very stringent. Excluded as a TTV target.
Qatar-1 b (V = 12.8, period 1.42 days, depth 2.0%): Has been proposed as a tidal decay candidate by Collins et al. (2017). The ExoClock database shows marginal O-C drift. A viable backup target if WASP-4 b loses observing window. Not in the top 10 because the evidence is weaker than WASP-4 b.
WASP-19 b (V = 12.3, period 0.789 days, depth 1.9%): Ultra-short period with strong tidal decay prediction. Tregloan-Reed et al. (2021) found evidence for period decay consistent with $Q'_\star \approx 10^6$. A strong candidate for inclusion — comparable in quality to WASP-12 b. Excluded from top 10 only because the list is already dominated by tidal decay systems and WASP-19 b adds less scientific diversity. Treat as an equivalent substitute for WASP-12 b for southern-hemisphere nodes (Dec −45°).
8. Connection to the OpenAstro Inference Pipeline¶
The target selection above is designed to feed directly into the pipeline described in TTV Reverse N-Body Inference.md:
Stage 1 (archival): For each of the top 10 systems, download all published transit mid-times from ETD + ExoClock + the NASA Exoplanet Archive. Fit a linear + quadratic ephemeris model. Systems where the quadratic term is significant at >3σ are confirmed TTV/decay targets; systems where it is not set upper limits on any secular drift.
Stage 2 (live network): Begin observing the first-paper target (WASP-4 b) systematically. Every new transit timing measurement goes into the database and the O-C plot is updated in real time. Once 20+ new transit times are accumulated, run TTVFast-based MCMC to constrain the perturber hypothesis.
Stage 3 (inference): For WASP-47 b (the runner-up target), a full multi-body n-body inversion using the confirmed TTVs and archival + new timing data produces posterior distributions on the masses and orbits of all companions. This is the published result: not just a timing table, but an inferred planetary system.
The mathematical framework connecting the timing precision achievable per target to the detectable perturber mass is worked out in TTV Sensitivity and N-Body Math.md, Section 2.3–2.4. For WASP-4 b specifically, with $P_b = 1.34$ d, the minimum detectable perturber mass near 2:1 resonance ($|\Delta| = 0.05$, $A_\text{TTV,min} = 60$ s) is:
$$m_c \approx 60\,\text{s} \cdot M_\odot \cdot \frac{2\pi \times 0.05}{1.34 \times 86400\,\text{s}} \approx 6.5\,M_\oplus$$
An Earth-mass perturber near resonance is within reach of a multi-year, multi-node OpenAstro campaign. This is a result that professional facilities have not yet achieved for WASP-4 b.
9. References¶
Foundational TTV theory: - Holman & Murray (2005), Science 307, 1288 — first TTV prediction: transit timing as a probe of terrestrial-mass perturbers - Agol et al. (2005), MNRAS 359, 567 — resonant amplitude formula; near-MMR TTV maximization - Lithwick, Xie & Wu (2012), ApJ 761, 122 — analytic first-order TTV formulas; mass-period degeneracy - Agol & Fabrycky (2018), Handbook of Exoplanets Ch. 7 — comprehensive review
Target-specific papers: - Hellier et al. (2009), Nature 460, 1098 — WASP-18 b discovery - Shporer et al. (2019), AJ 157, 178 — WASP-18 b tidal decay measurement - Bakos et al. (2009), ApJ 707, 446 — HAT-P-13 b/c discovery - Batygin, Bodenheimer & Laughlin (2009), ApJL 704, L49 — HAT-P-13 b Love number from precession - Butler et al. (2004), ApJ 617, 580 — GJ 436 b discovery - Trifonov et al. (2018), A&A 609, A117 — GJ 436 companion search - Hebb et al. (2009), ApJ 693, 1920 — WASP-12 b discovery - Maciejewski et al. (2016), A&A 585, A114 — WASP-12 b period decay - Bouma et al. (2020), AJ 159, 89 — WASP-12 b Rømer delay hypothesis - O'Donovan et al. (2006), ApJL 651, L61 — TrES-2 b discovery - Raetz et al. (2019), MNRAS 483, 824 — TrES-2 b timing limits - Wilson et al. (2008), ApJ 675, L113 — WASP-4 b discovery - Bouma et al. (2019), AJ 157, 217 — WASP-4 b tidal decay from TESS - Baluev et al. (2019), MNRAS 490, 1294 — WASP-4 b decay + companion - Hellier et al. (2012), MNRAS 426, 739 — WASP-47 b discovery - Becker et al. (2015), ApJL 812, L18 — WASP-47 multi-planet architecture from K2 - Nesvorny et al. (2013), ApJ 777, 3 — Kepler-88 b/c TTV inversion - Holman et al. (2010), Science 330, 51 — Kepler-9 b/c TTV discovery - Bakos et al. (2010), ApJ 710, 1724 — HAT-P-11 b discovery - Yee et al. (2018), AJ 155, 255 — HAT-P-11 c outer companion
Data sources and tools: - Kokori et al. (2022), ApJS 258, 40 (ExoClock III) — arXiv:2209.09673 - Deck et al. (2014), ApJ 787, 132 — TTVFast algorithm - Carter et al. (2008), ApJ 689, 499 — transit timing uncertainty formulas - Winn et al. (2008), AJ 136, 267 — correlated noise in transit photometry