MANGOS II: Five new giant planets orbiting low-mass stars

TL;DR

The paper addresses the puzzle of giant planets on short orbits around low-mass stars by presenting the MANGOS program, which combines TESS detections with a comprehensive suite of ground-based photometry and spectroscopy to validate and characterise giant companions to M dwarfs. Using a multi-instrument, multi-wavelength approach and robust Bayesian modelling, the study reports five new MANGOS planets with radii near $1\,R_{\rm Jup}$ and short periods, including a planet with a marginal eccentricity and a potential metallicity–density trend. The findings largely support core-accretion formation for most hosts, while highlighting cases that push the limits of standard disc-mass expectations and opening avenues for atmospheric and obliquity studies. The work adds to the growing sample of M-dwarf planetary systems, providing valuable targets for future transmission spectroscopy and dynamical analyses to illuminate giant-planet formation in the low-mass regime.

Abstract

Giant planets orbiting low-mass stars on short orbits present a conundrum, as in the most extreme cases their existence cannot be reconciled with current models of core accretion. Therefore, surveys dedicated to finding these rare planets have a key role to play by growing the sample to overcome small number statistics. In this work we present MANGOS, a programme dedicated to the search for giant objects (planets, brown dwarfs, and low-mass stars) orbiting M dwarfs. We report on the discovery of five new giant planets (TOI-3288 Ab, TOI-4666 b, TOI-5007 b, TOI-5292 Ab, TOI-5916 b) first detected by TESS, and confirmed using ground-based photometry and spectroscopy. The five planets have radii in the range 0.99-1.12 $\mathrm{R_{Jup}}$, masses between 0.49--1.69~$\mathrm{M_{Jup}}$, and orbital periods between 1.43 and 2.91 days. We reveal that TOI-3288 and TOI-5292 are wide binaries, and in the case of TOI-5292 we are able to characterise both stellar components. We demonstrate that the planets presented are suitable for further characterisation of their obliquities and atmospheres. We detect a small but significant eccentricity for TOI-5007 b, although for this to be more robust, more observations are needed to fully sample the orbit. Finally, we reveal a correlation between stellar metallicity and planet bulk density for giant planets orbiting low-mass stars.

Figures (16)

• Figure 1: TESS 120 second cadence data of TOI-5007 (top), TOI-5292 A (middle) and TOI-5916 (bottom). In this cadence, TOI-5007 was observed in Sector 65, TOI-5292 A in Sectors 70 and 92, and TOI-5916 in Sector 82. Light grey points show the 120 second exposures, and the coloured lines show the flux in one-hour bins. The transit events of each planet candidate are indicated by coloured arrows.
• Figure 2: SpeX SXD spectra of TOI-5292 A (top) and TOI-5916 (bottom). The target spectra (red) are shown along with the closest spectral standard (grey). Spectral features of late-type stars are highlighted, and regions of strong tellurics are shaded.
• Figure 3: TripleSpec4.1 spectra of TOI-3288 A (top), TOI-4666 (middle), and TOI-5007 (bottom). The figure elements are the same as those of Fig. \ref{['fig:spex']}. Note that the TripleSpec4.1 spectra of the targets (red) have higher spectral resolving power than the SpeX spectra of the standards ($R{\sim}3500$ vs. $R{\sim}2000$).
• Figure 4: Optical spectra of (from bottom to top) TOI-4666 (MagE), TOI-5916 (Kast), TOI-5007 (Goodman), TOI-3288 A (MagE), and TOI-5292 A and B (Kast; black lines) compared to their best-fit spectral standards from 2007AJ....133..531B. All spectra are normalized at 7400 Å and offset by constants (dashed lines). Key spectral features are labeled, including uncorrected telluric absorption ($\oplus$) in the MagE and Goodman spectra.
• Figure 5: Distribution of ages for matched stars in the GALAH DR3 catalogue for each star in our sample in their respective colours. In the background in green we show the full GALAH sample for comparison.