TIC-65910228 b / NGTS-38 b, a 180 day transiting warm super-Jupiter

TL;DR

The paper reports the discovery and precise characterization of TIC-65910228 b / NGTS-38 b, a transiting warm super-Jupiter with $R_p = 1.081 \pm 0.047\,R_J$ and $M_p = 4.78^{+0.40}_{-0.37}\,M_J$ on a $P = 180.52791 \pm 0.00038$ d, $e = 0.308 \pm 0.011$ orbit around a bright, metal-rich host with $T_ ext{eff} \approx 6310$ K and $[Fe/H] = 0.33 \pm 0.09$. The discovery leveraged a TESS monotransit, followed by NGTS photometry and extensive CORALIE/HARPS RV monitoring to confirm the period and mass, culminating in a joint allesfitter analysis that also yields planetary interior properties. With a relatively cool $T_{eq} = 458 \pm 11$ K and a wide orbit ($a \approx 0.695$ au), NGTS-38 b provides a valuable data point in the population of long-period transiting giants, including insights into tidal evolution and formation channels. The paper further discusses atmospheric observability via emission spectroscopy (ESM ~ 22.5) vs transmission spectroscopy (TSM ~ 2.1–2.9), potential RM measurements, and the system’s suitability for exomoon/exoring studies in the PLATO era, along with constraints from an archival comparative analysis. The interior modelling suggests a heavy-element content of $M_Z \approx 110^{+90}_{-80}\,M_\oplus$ and a planetary enrichment of $Z_p/Z_\star \approx 2.4^{+2.0}_{-1.7}$, contributing to the discourse on giant-planet composition at high metallicity.

Abstract

We present the discovery of TIC-65910228 b / NGTS-38 b, a giant exoplanet with a radius of $1.081\pm0.047$ R$_\text{J}$ and a mass of $4.78_{-0.37}^{+0.40}$ M$_\text{J}$ on a long-period ($180.52791\pm0.00038$ day), moderately eccentric ($e=0.308\pm0.011$) orbit transiting a bright (V=$10.230\pm0.020$ mag) metal rich ([Fe/H]=$0.33\pm0.09$, 'dex') F6V-F7V type host star. The planet was initially detected from a single transit in TESS Sector 33. A photometric monitoring campaign of 228 nights with NGTS detected a transit egress of the planet, which together with spectroscopic radial velocity monitoring with CORALIE and HARPS identified an orbital period of ~180.5,d. These radial velocity measurements also showed the mass of the companion to be planetary. Additional transit observations coordinated by the TESS follow-up observing program allowed further confirmation and refinement of this period. With its relatively cool equilibrium temperature of $458\pm11$ K, NGTS-38 b joins a small but growing population of well characterised transiting warm-Jupiters and has one of the longest periods of any discovered to date. The target is situated in the LOPS2 field of the upcoming PLATO mission which will allow for greater refinement of the system parameters and potential for the discovery of additional companions too small and/or too long-period to be seen by TESS or NGTS. NGTS-38 b's bright host star and wide orbital separation make it an attractive target for further study, including potential measurement of its spin-orbit alignment or targeted exomoon/ring searches.

Figures (13)

• Figure 1: TESS target pixel file for NGTS-38 in sector 33. Each red circle represents a Gaia source down to a magnitude difference of 6 from the target. The circles are also sized according to the difference in magnitude from the target, which is marked with a white x and numbered 1.
• Figure 2: Discovery photometry for NGTS-38 b. All TESS lightcurves have been binned to 30 minute cadence for ease of comparison. NGTS data has been binned to nightly cadence and outliers with flux $<0.998$ or flux errors $>0.001$ removed. Red vertical dashes along the bottom x-axis show the expected transit times. Similar green dashes are shown on the inside of the top x axis to show transits actually covered by the data. The transit predicted at BJD-2457000 $\approx3292$ does fall within the time limits of the first run of NGTS data but falls within a data gap.
• Figure 3: Discovery transit lightcurves of NGTS-38 b. Each panel shows the median transit model (see Section \ref{['section:orbit-fit']}) as a solid red line. The transit data plotted is shown as black square markers with errorbars. For NGTS the data are binned to 10-minute cadence to match TESS and the unbinned data are shown as gray dots. The top panel of each subfigure shows the data and model while the lower panel of each shows the residuals after the model has been subtracted from the observed data. The TESS data shows a systematic 'bump' in flux post transit, this is probably an artefact of the PDCSAP_FLUX flattening of the lightcurve. Our own spline fit (see Section \ref{['section:orbit-fit']}) failed to remove this but the impact on any fitted parameters is likely to be negligible.
• Figure 4: Simultaneous transit lightcurves of NGTS-38 b from the night beginning 2025 December 4 UTC (T$_{10}$=2461014.5077 BJD) Each panel shows the median transit model (see Section \ref{['section:orbit-fit']}) as a solid red line. The top panel of each subfigure shows the data and model while the lower panel of each shows the residuals after the model has been subtracted from the observed data. Unbinned data is shown as gray dots while the black squares with error bars represent the data binned to 10 minute cadence. The LCOGT data for the same diameter telescopes from CTIO and SSO have been stitched together to save space but these data were fitted separately.
• Figure 5: Radial velocity measurements of NGTS-38 with the fitted radial velocity baseline offset values subtracted (see Section \ref{['section:orbit-fit']} and Table \ref{['tab:ns_table']}). Data points from CORALIE are denoted with black circles with those from the first epoch unfilled and those from the second epoch filled. HARPS data points are shown as blue squares. The median fitted RV model is also overplotted with a red solid while the best fitting model to the 361.0 day period alias is shown with an orange dashed line in the top panel. Subfigure (a) shows the data plotted versus time while (b) shows the same data as a function of orbital phase. For each subfigure, the top panel shows the data and model, while the bottom panel shows the residuals with the model subtracted.