Two temperate Earth- and Neptune-sized planets orbiting fully convective M dwarfs

TL;DR

This work formalises a broader temperate-zone definition for exoplanets and introduces TEMPOS, a SPECULOOS-driven effort to yield precise radii for temperate planets around cool M dwarfs. It reports the discovery and validation of two temperate planets, TOI-6716 b (Earth-sized) and TOI-7384 b (Neptune-sized), orbiting fully convective M dwarfs with instellations near the inner edge of the temperate zone. Through extensive follow-up photometry, high-resolution imaging, and statistical validation, the authors place these planets in the context of the current population and discuss the prospects for mass measurements and atmospheric characterization with JWST. The results highlight the synergy between TESS discoveries and ground-based facilities in building a targeted temperate-planet sample around M dwarfs, with implications for future atmospheric studies and planetary formation around fully convective stars.

Abstract

As the diversity of exoplanets continues to grow, it is important to revisit assumptions about habitability and classical HZ definitions. In this work, we introduce an expanded 'temperate' zone, defined by instellation fluxes between $0.1<S/\mathrm{S}_\oplus<5$, thus encompassing a broader range of potentially habitable worlds. We also introduce the TEMPOS survey, which aims to produce a catalogue of precise radii for temperate planets orbiting M dwarfs with $T_\mathrm{eff}\leq3400\,$ K. This work reports the discovery and characterisation of two planets in this temperate regime orbiting mid-type M dwarfs: TOI-6716\,b, a $0.98\pm0.07\,\mathrm{R}_\oplus$ planet orbiting its M4 host star ($R_\star=0.231\,\pm0.015\mathrm{R}_\odot$, $M_\star=0.223\pm0.011\,\mathrm{M}_\odot$, $T_\mathrm{eff}=3110\pm80\,\mathrm{K}$) with a period $P=4.7185898^{+0.0000054}_{-0.0000041}\,\mathrm{d}$, and TOI-7384 b, a $3.56\pm0.21\,\mathrm{R}_\oplus$ planet orbiting an M4 ($R_\star=0.319\,\pm0.018\mathrm{R}_\odot$, $M_\star=0.318\pm0.016\,\mathrm{M}_\odot$, $T_\mathrm{eff}=3185\pm75\,\mathrm{K}$) star every $P=6.2340258^{+0.0000034}_{-0.0000036}\,\mathrm{d}$. The radii of TOI-6716 b and TOI-7384 b have precisions of $6.8\%$ and $5.9\%$ respectively. We validate these planets with multi-band ground-based photometric observations, high-resolution imaging and statistical analyses. We find these planets to have instellation fluxes close to the inner (hotter) edge of the temperate zone, with $4.4\pm1.1\,\mathrm{S}_\oplus$ and $4.9\pm1.1\,\mathrm{S}_\oplus$ for TOI-6716 b and TOI-7384 b respectively. Also, with a predicted TSM similar to the TRAPPIST-1 planets, TOI-6716 b is likely to be a good rocky-world JWST target, should it have retained its atmosphere.

Figures (15)

• Figure 1: Our adopted definition of the temperate zone. In the lower panel we plot transiting planets retrieved from the NASA Exoplanet Archive in terms of equilibrium temperature and instellation. Pink points in the foreground represent the full sample, purple points have $R_{\rm p}\leq 4~\rm R_\oplus$, and brown points have $R_{\rm p}\leq 1.5~\rm R_\oplus$. The seven temperate planets of the TRAPPIST-1 system are shown as red points, and in green we highlight K2-18 b and LHS 1140 b, two canonical temperate planets. The green shaded area indicates the conservative habitable zone as defined by kopparapu2014; the temperate zone is enclosed by the green dashed lines. The two upper panes show the KDE curves for the planet samples, with the colours matching the points in the lower panel.
• Figure 2: Shane/Kast optical spectrum of TOI-6716 (top) and Magellan/LDSS-3 spectrum of TOI-7384 (bottom), compared to their best-fit M4 SDSS spectral template from 2007AJ....133..531B. Key spectral features are labelled, including regions of residual telluric absorption ($\oplus$). Inset boxes show the 6520--6770 Å region encompassing H$\alpha$ and Li I features. The gap in the Kast spectrum between 5600 Å and 5900 Å corresponds to the gap between that instrument's blue and red channels.
• Figure 3: SpeX/SXD spectrum of TOI-6716 (top) and Magellan/FIRE spectrum of TOI-7384 (bottom). The target spectra (red) are compared to M4V standard Ross 47 and the M3.5 standard GJ 273, respectively, both shown in grey and offset vertically. Strong M-dwarf spectral features and spectral regions with strong telluric absorption are indicated.
• Figure 4: Spectral energy distributions of TOI-6716 (top) and TOI-7384 (bottom). Red symbols represent the observed photometric measurements, where the horizontal bars represent the effective width of the passband. Blue symbols are the model fluxes from the best-fit NextGen atmosphere model (black). The inset axes show the absolute flux-calibrated Gaia spectrophotometry as a grey swathe overlaid on the best-fit model.
• Figure 5: TESS 2-minute cadence photometry (grey) for TOI-6716 for Sectors 7 (top), 34 (top-middle), 61 (bottom-middle), 87 and 88 (bottom). Purple shows the data binned by one hour. Transits are indicated with green triangles, although are not visually obvious in the TESS data.