TOI-1516b: Hot Jupiter with TTV and Orbital Decay

• TOI-1516b is a hot Jupiter on a 2.056-day orbit around an F-type star, confirmed via TESS photometry and extensive radial-velocity follow-up.
• Observations combined TESS full-frame imaging with ground-based telescopes, enabling precise measurements of planetary mass, radius, and transit geometry.
• Transit-timing analysis reveals both a quadratic timing trend and periodic variations, suggesting potential orbital decay or dynamical perturbations.

Searching arXiv for the specified TOI-1516b papers to ground the article in published sources. TOI-1516b is a hot Jupiter on a short-period orbit around the F-type main-sequence star TOI-1516 (TIC 376637093). It was presented as one of three new hot Jupiters discovered by the TESS space mission and confirmed through joint transit and radial-velocity analysis, with subsequent work extending the system’s characterization through a combined TESS and ground-based study of transit timing variations (TTVs) (Kabáth et al., 2022, Sonbas et al., 25 Sep 2025). The planet is characterized by an orbital period near $2.056$ days, a mass of $3.16 \pm 0.12\,M_{\mathrm{J}}$, a radius of $1.36 \pm 0.03\,R_{\mathrm{J}}$, and a highly irradiated orbit at $a = 0.0330 \pm 0.0010$ AU (Kabáth et al., 2022). Later timing analysis found that, although an orbital-decay ephemeris minimizes the Bayesian Information Criterion (BIC), the O$-$C residuals contain a periodic component with false-alarm probability $0.00014$, which the authors state “suggests a likely dynamical origin that warrants further investigation” (Sonbas et al., 25 Sep 2025).

1. Discovery, confirmation, and observational setting

TOI-1516b was confirmed by combining TESS photometry with extensive radial-velocity monitoring from a coordinated network of mid-aperture telescopes at Ondřejov (OES), Tautenburg (TCES), and McDonald Observatory (Tull) (Kabáth et al., 2022). In the discovery and characterization study, the system was observed by TESS in Sectors 17, 18, 24, and 25 using 30-minute full-frame images, and the team extracted SAP light curves and modeled them with the Transit and Light Curve Modeller (TLCM) (Kabáth et al., 2022). Because only full-frame images were available, TLCM used subexposure numerical integration with five subexposures per cadence to mitigate the long-integration effects (Kabáth et al., 2022).

Ground-based confirmation was obtained through a follow-up transit observed at CRCAO (0.6 m) in Rc band on 2020 August 10, with $205 \times 90$ s exposures, and the combined TESS+ground dataset was modeled jointly (Kabáth et al., 2022). High-resolution imaging with Gemini-North ‘Alopeke speckle imaging at 562 and 832 nm found no companions, achieving $\Delta \mathrm{mag} \approx 4$ at $0.2$ arcsec and thereby ruling out contaminating blends (Kabáth et al., 2022). The planetary interpretation was further supported by the large, coherent radial-velocity semi-amplitude, the consistency of the RV phase with the transit ephemeris, and the multi-instrument agreement after fitting instrument-specific velocity offsets (Kabáth et al., 2022).

The confirmation paper emphasized that the main instruments used for the radial-velocity follow-up of TOI-1181b, TOI-1516b, and TOI-2046b were located at Ondřejov, Tautenburg and McDonald Observatory, all on 2–3 meter aperture telescopes, illustrating the role of mid-aperture telescope networks in follow-up of gas giants discovered by TESS and, prospectively, by PLATO (Kabáth et al., 2022). This places TOI-1516b within an observational program that was not limited to discovery, but extended to coordinated photometric, imaging, and spectroscopic validation.

2. Stellar host and system parameters

The host star TOI-1516 is classified as an F8 V star from template fitting of iodine-free Tull spectra degraded to $R \approx 5500$ using the Indo-US library (Kabáth et al., 2022). Its adopted atmospheric parameters in the discovery modeling are $3.16 \pm 0.12\,M_{\mathrm{J}}$0 K, $3.16 \pm 0.12\,M_{\mathrm{J}}$1, and $3.16 \pm 0.12\,M_{\mathrm{J}}$2, with projected rotation $3.16 \pm 0.12\,M_{\mathrm{J}}$3 km s$3.16 \pm 0.12\,M_{\mathrm{J}}$4 (Kabáth et al., 2022). The derived stellar mass and radius are $3.16 \pm 0.12\,M_{\mathrm{J}}$5 and $3.16 \pm 0.12\,M_{\mathrm{J}}$6, consistent with ARIADNE SED-fit results of $3.16 \pm 0.12\,M_{\mathrm{J}}$7 and $3.16 \pm 0.12\,M_{\mathrm{J}}$8 (Kabáth et al., 2022).

The star has stellar density $3.16 \pm 0.12\,M_{\mathrm{J}}$9 kg m$1.36 \pm 0.03\,R_{\mathrm{J}}$0 and an age of $1.36 \pm 0.03\,R_{\mathrm{J}}$1 Gyr from ARIADNE isochrones using Gaia EDR3 parallax (Kabáth et al., 2022). The Gaia parallax is $1.36 \pm 0.03\,R_{\mathrm{J}}$2 mas, corresponding to a distance of approximately $1.36 \pm 0.03\,R_{\mathrm{J}}$3 pc; the catalog photometry includes $1.36 \pm 0.03\,R_{\mathrm{J}}$4 and $1.36 \pm 0.03\,R_{\mathrm{J}}$5 (Kabáth et al., 2022). A periodogram shows a dominant peak at $1.36 \pm 0.03\,R_{\mathrm{J}}$6 c/d, corresponding to approximately $1.36 \pm 0.03\,R_{\mathrm{J}}$7 d, together with low-frequency peaks, but the detection of the rotational frequency expected from $1.36 \pm 0.03\,R_{\mathrm{J}}$8 and $1.36 \pm 0.03\,R_{\mathrm{J}}$9 was reported as inconclusive in subsets (Kabáth et al., 2022).

Within this stellar context, TOI-1516b orbits an F main-sequence host star rather than an evolved subgiant, distinguishing it from TOI-1181b in the same discovery paper (Kabáth et al., 2022). The host-star characterization is important because the later TTV analysis uses priors from Kabath et al. (2022) and Fox & Wiegert (2022) in EXOFASTv2 global light-curve and RV fits, and because the tidal-quality-factor estimate derived in the timing work depends explicitly on stellar and orbital parameters (Sonbas et al., 25 Sep 2025).

3. Planetary orbit, transit geometry, and bulk properties

The discovery solution reported an orbital period of $a = 0.0330 \pm 0.0010$0 days and a reference mid-transit epoch $a = 0.0330 \pm 0.0010$1 (Kabáth et al., 2022). The transit geometry is described by impact parameter $a = 0.0330 \pm 0.0010$2, inclination $a = 0.0330 \pm 0.0010$3, scaled semi-major axis $a = 0.0330 \pm 0.0010$4, and radius ratio $a = 0.0330 \pm 0.0010$5, implying a transit depth $a = 0.0330 \pm 0.0010$6, or approximately $a = 0.0330 \pm 0.0010$7 (Kabáth et al., 2022). The transit duration is $a = 0.0330 \pm 0.0010$8 hours (Kabáth et al., 2022).

The planet has mass $a = 0.0330 \pm 0.0010$9 and radius $-$0 (Kabáth et al., 2022). The radial-velocity semi-amplitude is $-$1 m s$-$2, and the orbit was tested with pyaneti, which corroborated TLCM and favored a circular orbit; eccentricity was fixed to zero in the final solution (Kabáth et al., 2022). The semi-major axis is reported as $-$3 AU, consistent with the relation

$-$4

The planet’s mean density is $-$5 g cm$-$6 and its surface gravity is $-$7 m s$-$8 (Kabáth et al., 2022).

Assuming Bond albedo $-$9 and full heat redistribution, the equilibrium temperature is

$0.00014$0

which yields $0.00014$1 K using $0.00014$2 K, $0.00014$3, and $0.00014$4 AU (Kabáth et al., 2022). The estimated insolation is $0.00014$5, so TOI-1516b receives about two thousand times Earth’s insolation (Kabáth et al., 2022). The discovery paper placed the planet among highly irradiated hot Jupiters and described it as modestly inflated relative to Jupiter, with density indicating only moderate inflation compared to more extremely inflated systems (Kabáth et al., 2022). No detections of a secondary eclipse or phase curve were reported, and spin–orbit alignment was not measured for TOI-1516b (Kabáth et al., 2022).

4. Transit-timing dataset and timing-extraction methodology

A later study expanded the temporal baseline for TOI-1516b by combining ground-based and space-based transit measurements from 2020 to 2024 (Sonbas et al., 25 Sep 2025). Ground-based observations comprised 16 transits obtained with the 0.6 m telescope ADYU60 between September 2020 and December 2024 (Sonbas et al., 25 Sep 2025). Space-based timing information came from TESS short-cadence 2-minute observations in four sectors, 57, 58, 77, and 78; the study also used previously published mid-transit times from the Exoplanet Transit Database (ETD) and one NEOSSat mid-time from Fox & Wiegert (2022) (Sonbas et al., 25 Sep 2025). The O$0.00014$6C dataset therefore contained 1 mid-time from NEOSSat, 48 from ETD, 42 from TESS, and 16 from the new work (Sonbas et al., 25 Sep 2025).

The photometric reduction and modeling pipeline was heterogeneous but explicitly specified. AstroImageJ (AIJ) was used for calibration with bias and flat frames, differential photometry through Multi-Aperture, and extraction of detrend parameters including airmass, time, sky background, FWHM, comparison-star counts, and $0.00014$7–$0.00014$8 centroid (Sonbas et al., 25 Sep 2025). Detrending in the subsequent light-curve modeling used an additive scheme in EXOFASTv2 with the same set of detrend parameters per source (Sonbas et al., 25 Sep 2025). For TESS, the 2-minute TIC cadence light curves were extracted from target pixel files using aperture/background separation, quality-flag filtering, and iterative sigma-clipping (Sonbas et al., 25 Sep 2025).

Time stamps were converted from JD to BJD through AIJ and Eastman applets, and BJD TDB was reported in the O$0.00014$9C tables (Sonbas et al., 25 Sep 2025). Mid-transit times for TOI-1516 from ADYU60 and TESS runs were derived with EXOFASTv2; EXOTIC was used for T100 data in general, but T100 was not used for this system (Sonbas et al., 25 Sep 2025). Global light-curve plus radial-velocity fits employed EXOFASTv2 with NOMIST and TORRES options, priors from Kabath et al. (2022) and Fox & Wiegert (2022), and convergence criteria of Gelman–Rubin $205 \times 90$0 and chain length $205 \times 90$1 (Sonbas et al., 25 Sep 2025). For TOI-1516, adopted or interpolated quadratic limb-darkening values included example results $205 \times 90$2 for the TESS fit and $205 \times 90$3 for the ADYU60 fit, with $205 \times 90$4 for TESS and $205 \times 90$5 for ADYU60 (Sonbas et al., 25 Sep 2025).

The timing precision reported for TOI-1516 reflects the differing photometric quality of the data sources. For the global ADYU60 fit, the binned residual RMS is $205 \times 90$6 mmag, whereas for the TESS fit it is $205 \times 90$7 mmag (Sonbas et al., 25 Sep 2025). The study also notes typical per-transit RMS of 1.2–4.2 mmag across all systems for ADYU60 observations (Sonbas et al., 25 Sep 2025). Sigma-clipping was applied to the O$205 \times 90$8C sequences, and points removed in this process were marked as red crosses in the TOI-1516 O$205 \times 90$9C figure (Sonbas et al., 25 Sep 2025).

5. Ephemerides and model comparison

The timing analysis considered linear, orbital-decay, and apsidal-precession timing models (Sonbas et al., 25 Sep 2025). The linear ephemeris adopted the timing law

$\Delta \mathrm{mag} \approx 4$0

For TOI-1516b, the best-fit parameters from the ExoPdot constant-period model are $\Delta \mathrm{mag} \approx 4$1 BJD TDB and $\Delta \mathrm{mag} \approx 4$2 days, with $\Delta \mathrm{mag} \approx 4$3, $\Delta \mathrm{mag} \approx 4$4, and $\Delta \mathrm{mag} \approx 4$5; under the Kass and Raftery interpretation adopted in the paper, this model is “Very Strongly Rejected” relative to the best model (Sonbas et al., 25 Sep 2025). The study notes that Kabath et al. (2022) gave $\Delta \mathrm{mag} \approx 4$6 d, and that the updated linear fit finds $\Delta \mathrm{mag} \approx 4$7 smaller by approximately $\Delta \mathrm{mag} \approx 4$8 d, or about $\Delta \mathrm{mag} \approx 4$9 ms, consistent within uncertainties (Sonbas et al., 25 Sep 2025).

The quadratic orbital-decay model used

$0.2$0

with $0.2$1 in units of days epoch$0.2$2 and negative $0.2$3 indicating decay (Sonbas et al., 25 Sep 2025). The best-fit TOI-1516b parameters are $0.2$4 BJD TDB, $0.2$5 days, and $0.2$6 days epoch$0.2$7, with reported “Transit timing shift” $0.2$8 s, $0.2$9, $R \approx 5500$0, and $R \approx 5500$1 (Sonbas et al., 25 Sep 2025). This is the statistically preferred model in the paper’s BIC comparison.

The apsidal-precession model followed Patra et al. (2017):

$R \approx 5500$2

$R \approx 5500$3

For TOI-1516b, the fit returned $R \approx 5500$4 BJD TDB, $R \approx 5500$5 days, $R \approx 5500$6, $R \approx 5500$7 rad, and $R \approx 5500$8 rad epoch$R \approx 5500$9, with $3.16 \pm 0.12\,M_{\mathrm{J}}$00, $3.16 \pm 0.12\,M_{\mathrm{J}}$01, and $3.16 \pm 0.12\,M_{\mathrm{J}}$02, hence “Strongly Rejected” (Sonbas et al., 25 Sep 2025).

The BIC itself was defined as

$3.16 \pm 0.12\,M_{\mathrm{J}}$03

with $3.16 \pm 0.12\,M_{\mathrm{J}}$04 the number of free parameters and $3.16 \pm 0.12\,M_{\mathrm{J}}$05 the number of mid-times (Sonbas et al., 25 Sep 2025). The interpretation adopted in the study classified $3.16 \pm 0.12\,M_{\mathrm{J}}$06 as weak, $3.16 \pm 0.12\,M_{\mathrm{J}}$07–$3.16 \pm 0.12\,M_{\mathrm{J}}$08 as positive, $3.16 \pm 0.12\,M_{\mathrm{J}}$09–$3.16 \pm 0.12\,M_{\mathrm{J}}$10 as strong, and $3.16 \pm 0.12\,M_{\mathrm{J}}$11 as very strong evidence against the higher-BIC model (Sonbas et al., 25 Sep 2025). For TOI-1516b, this framework favors orbital decay over both constant period and apsidal precession (Sonbas et al., 25 Sep 2025).

Model	Key parameters	Statistical result
Constant period	$3.16 \pm 0.12\,M_{\mathrm{J}}$12, $3.16 \pm 0.12\,M_{\mathrm{J}}$13 d	$3.16 \pm 0.12\,M_{\mathrm{J}}$14, BIC = 1482.7, $3.16 \pm 0.12\,M_{\mathrm{J}}$15BIC = 15.2
Orbital decay	$3.16 \pm 0.12\,M_{\mathrm{J}}$16, $3.16 \pm 0.12\,M_{\mathrm{J}}$17 d, $3.16 \pm 0.12\,M_{\mathrm{J}}$18 d epoch$3.16 \pm 0.12\,M_{\mathrm{J}}$19	$3.16 \pm 0.12\,M_{\mathrm{J}}$20, BIC = 1467.5, $3.16 \pm 0.12\,M_{\mathrm{J}}$21BIC = 0.0
Apsidal precession	$3.16 \pm 0.12\,M_{\mathrm{J}}$22, $3.16 \pm 0.12\,M_{\mathrm{J}}$23 rad, $3.16 \pm 0.12\,M_{\mathrm{J}}$24 rad epoch$3.16 \pm 0.12\,M_{\mathrm{J}}$25	$3.16 \pm 0.12\,M_{\mathrm{J}}$26, BIC = 1476.8, $3.16 \pm 0.12\,M_{\mathrm{J}}$27BIC = 9.3

6. TTV signal, physical interpretation, and outstanding issues

Although the orbital-decay ephemeris minimizes the BIC, the residual timing structure is not exhausted by a secular quadratic trend (Sonbas et al., 25 Sep 2025). A Generalized Lomb–Scargle periodogram with analytic false-alarm probability assessment found, for TOI-1516b, a maximum power of $3.16 \pm 0.12\,M_{\mathrm{J}}$28, frequency $3.16 \pm 0.12\,M_{\mathrm{J}}$29 (reported in the text with unit “Hz”), and a period of $3.16 \pm 0.12\,M_{\mathrm{J}}$30 days (Sonbas et al., 25 Sep 2025). The fitted sine amplitude is $3.16 \pm 0.12\,M_{\mathrm{J}}$31 days, corresponding to $3.16 \pm 0.12\,M_{\mathrm{J}}$32 minutes, with false-alarm probability $3.16 \pm 0.12\,M_{\mathrm{J}}$33, rounded to $3.16 \pm 0.12\,M_{\mathrm{J}}$34 in the abstract and explicitly described as well below the $3.16 \pm 0.12\,M_{\mathrm{J}}$35 threshold (Sonbas et al., 25 Sep 2025).

The timing study therefore presents a tension between the preferred secular fit and the recovered periodicity. On the one hand, the decay parameter $3.16 \pm 0.12\,M_{\mathrm{J}}$36 days epoch$3.16 \pm 0.12\,M_{\mathrm{J}}$37 implies a cumulative $3.16 \pm 0.12\,M_{\mathrm{J}}$38 timing shift of $3.16 \pm 0.12\,M_{\mathrm{J}}$39 s over the fitted baseline (Sonbas et al., 25 Sep 2025). Using

$3.16 \pm 0.12\,M_{\mathrm{J}}$40

with $3.16 \pm 0.12\,M_{\mathrm{J}}$41 and $3.16 \pm 0.12\,M_{\mathrm{J}}$42, the authors estimate a host-star tidal quality factor $3.16 \pm 0.12\,M_{\mathrm{J}}$43 for TOI-1516b (Sonbas et al., 25 Sep 2025). On the other hand, the same study emphasizes that the highly significant periodic signal in the O$3.16 \pm 0.12\,M_{\mathrm{J}}$44C residuals “suggests a likely dynamical origin that warrants further investigation,” while providing no explicit constraints on the mass or period of a putative perturber (Sonbas et al., 25 Sep 2025).

This does not amount to a settled detection of orbital decay in an unqualified sense. The paper notes caveats including variable ground-based photometric precision, exclusion of partial transits by quality cuts, possible red noise or systematics, and model degeneracies between secular effects and dynamical TTVs (Sonbas et al., 25 Sep 2025). A plausible implication is that the decay-like quadratic term may partly absorb structure generated by an additional body, but the study does not claim such a decomposition explicitly. The phase of the GLS sinusoid is not tabulated, and the apsidal-precession phase parameter $3.16 \pm 0.12\,M_{\mathrm{J}}$45 rad belongs to a model that is statistically disfavored for this system (Sonbas et al., 25 Sep 2025).

The recommended path forward is continued high-precision photometric monitoring and RV follow-up to determine whether the periodic TTVs arise from an additional body or can be reconciled with secular processes, and to refine both the decay parameter and the associated tidal constraints (Sonbas et al., 25 Sep 2025). In that sense, TOI-1516b occupies a dual role in the current literature: it is both a well-characterized, fairly massive hot Jupiter around an F8 main-sequence star (Kabáth et al., 2022) and a timing system in which the statistically preferred ephemeris and the most conspicuous residual periodicity point toward different physical interpretations (Sonbas et al., 25 Sep 2025).