[Paper Review] Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions

Teruyuki Hirano, John H. Livingston+59 more|arXiv (Cornell University)|Mar 23, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy145 references31 citations

TL;DR

This study confirms two ultra-short-period (USP) super-Earths, TOI-1634b and TOI-1685b, orbiting bright M dwarfs using TESS photometry and ground-based radial velocity (RV) measurements. The planets have Earth-like compositions, with masses of 10.14 ± 0.95 M⊕ and 3.43 ± 0.93 M⊕, radii of 1.749 ± 0.079 R⊕ and 1.459 ± 0.065 R⊕, and orbital periods of 0.9893436 ± 0.0000020 days and 0.6691416 ± 0.0000019 days, respectively, placing them near the radius valley and making them key targets for studying rocky planet formation and atmospheric evolution.

ABSTRACT

We present observations of two bright M dwarfs (TOI-1634 and TOI-1685: $J=9.5-9.6$) hosting ultra-short period (USP) planets, identified by the TESS mission. The two stars are similar in temperature, mass, and radius ($T_\mathrm{eff}\,\approx\,3500$ K, $M_\star\,\approx\,0.45-0.46\,M_\odot$, and $R_\star\approx 0.45-0.46\,R_\odot$), and the planets are both super-Earth-sized ($1.25\,R_\oplus<R_p<2.0\,R_\oplus$). For both systems, light curves from the ground-based photometry exhibit planetary transits, whose depths are consistent with those by the TESS photometry. We also refine the transit ephemerides based on the ground-based photometry, finding the orbital periods of $P=0.9893436\pm0.0000020$ day and $P=0.6691416\pm0.0000019$ day for TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV) observations using IRD on the Subaru 8.2m telescope, we confirm the planetary nature of the TOIs, and measure their masses: $10.14\pm0.95\,M_\oplus$ and $3.43\pm0.93\,M_\oplus$ for TOI-1634b and TOI-1685b, respectively, when the observed RVs are fitted with a single-planet circular-orbit model. Combining those with the planet radii of $R_p=1.749\pm 0.079\,R_\oplus$ (TOI-1634b) and $1.459\pm0.065\,R_\oplus$ (TOI-1685b), we find that both USP planets have mean densities consistent with an Earth-like internal composition, which is typical for small USP planets. TOI-1634b is currently the most massive USP planet in this category, and it resides near the radius valley, which makes it a benchmark planet in the context of discussing the size limit of rocky planet cores as well as testing the formation scenarios for USP planets. Excess scatter in the RV residuals for TOI-1685 suggests the presence of a possible secondary planet or unknown activity/instrumental noise in the RV data, but further observations are required to check those possibilities.

Motivation & Objective

• To confirm the planetary nature of two ultra-short-period (USP) planet candidates identified by TESS around bright M dwarfs.
• To measure the masses and radii of the planets to assess their internal compositions.
• To refine orbital ephemerides using ground-based photometry and radial velocity data.
• To evaluate the potential presence of additional planets or stellar activity through RV residual analysis.
• To identify these systems as high-priority targets for future atmospheric characterization, especially with JWST.

Proposed method

• Utilized TESS space-based photometry to detect planetary transits in the light curves of two bright M dwarfs (TOI-1634 and TOI-1685).
• Conducted ground-based photometry to validate and refine transit depths and ephemerides, confirming consistency with TESS data.
• Performed intensive radial velocity (RV) measurements using the IRD spectrograph on the Subaru 8.2m telescope to confirm planetary signals and determine masses.
• Fitted RV data with a single-planet circular-orbit model to derive planet masses and radii, enabling density and composition analysis.
• Analyzed RV residuals using periodogram analysis and Bayesian information criterion (BIC) to assess the likelihood of additional planets or stellar activity.
• Combined mass and radius measurements to compute mean densities and compare with theoretical Earth-like composition models.

Experimental results

Research questions

• RQ1 Do the two USP planet candidates around bright M dwarfs have planetary masses consistent with rocky, Earth-like compositions?
• RQ2 What are the precise orbital periods and transit ephemerides of TOI-1634b and TOI-1685b, as refined by ground-based photometry?
• RQ3 Is there evidence for additional planetary companions or stellar activity in the radial velocity residuals of these systems?
• RQ4 How do the masses and radii of these planets compare to theoretical models of rocky planet interiors?
• RQ5 Are these systems suitable targets for future atmospheric characterization using emission spectroscopy?

Key findings

• TOI-1634b has a mass of 10.14 ± 0.95 M⊕ and a radius of 1.749 ± 0.079 R⊕, yielding a mean density consistent with an Earth-like composition.
• TOI-1685b has a mass of 3.43 ± 0.93 M⊕ and a radius of 1.459 ± 0.065 R⊕, also consistent with an Earth-like internal structure.
• The orbital periods were refined to 0.9893436 ± 0.0000020 days for TOI-1634b and 0.6691416 ± 0.0000019 days for TOI-1685b.
• Both planets lie near the theoretical radius valley and are among the most massive USP planets with Earth-like compositions.
• Excess scatter in the RV residuals of TOI-1685 suggests a possible additional planet or stellar activity, though further observations are needed to confirm.
• The systems are among the brightest known hosts of USP super-Earths, making them prime targets for future atmospheric studies with JWST and other large-aperture telescopes.