[Paper Review] TESS's first planet: a super-Earth transiting the naked-eye star $π$ Mensae

D. Gandolfi, Oscar Barragán+30 more|Sep 20, 2018

Stellar, planetary, and galactic studiesPhysics and Astronomy57 references40 citations

TL;DR

The paper confirms and characterizes π Men c, TESS's first transiting planet, a 6.27-day super-Earth around the bright G0V star π Mensae, with mass 4.52±0.81 M⊕ and radius 2.06±0.03 R⊕, combining TESS photometry with Gaia and radial velocities from UCLES/HARPS.

ABSTRACT

We report on the confirmation and mass determination of Pi Men c, the first transiting planet discovered by NASA's TESS space mission. Pi Men is a naked-eye (V=5.65 mag), quiet G0 V star that was previously known to host a sub-stellar companion (Pi Men b) on a long-period (Porb = 2091 days), eccentric (e = 0.64) orbit. Using TESS time-series photometry, combined with Gaia data, published UCLES@AAT Doppler measurements, and archival HARPS@ESO-3.6m radial velocities, we found that Pi Men c is a close-in planet with an orbital period of Porb = 6.27 days, a mass of Mc = 4.52 +/- 0.81 MEarth, and a radius of Rc = 2.06 +/- 0.03 REarth. Based on the planet's orbital period and size, Pi Men c is a super-Earth located at, or close to, the radius gap, while its mass and bulk density suggest it may have held on to a significant atmosphere. Because of the brightness of the host star, this system is highly suitable for a wide range of further studies to characterize the planetary atmosphere and dynamical properties. We also performed an asteroseismic analysis of the TESS data and detected a hint of power excess consistent with the seismic values expected for this star, although this result depends on the photometric aperture used to extract the light curve. This marginal detection is expected from pre-launch simulations hinting at the asteroseismic potential of the TESS mission for longer, multi-sector observations and/or for more evolved bright stars.

Motivation & Objective

• Confirm the transiting signal detected by TESS as a bona fide planet around π Mensae.
• Measure the mass of the transiting planet through archival and new radial velocity data.
• Characterize the host star and place the planet on the mass-radius diagram to infer its composition and atmosphere potential.

Proposed method

• Analyze TESS Sector 1 short-cadence photometry to detect and validate the transit signal.
• Combine Gaia photometry, UCLES, and HARPS RV data to derive planetary mass and orbital parameters.
• Perform joint transit-RV modeling with priors informed by stellar parameters and Keplerian dynamics.
• Use asteroseismic analysis of TESS data to constrain stellar properties (where possible).
• Assess photometric contamination and verify the transit source via Gaia DR2 and archival imaging.

Experimental results

Research questions

• RQ1 Is the transit signal observed by TESS due to a bona fide planet around π Mensae or a false positive?
• RQ2 What is the mass and radius of π Men c, and what does this imply about its composition and atmosphere?
• RQ3 How does the presence of the outer companion π Men b influence the system's dynamical stability and evolution?

Key findings

• π Men c has an orbital period of 6.27 days, a mass of 4.52±0.81 M⊕, and a radius of 2.06±0.03 R⊕.
• The mean density of π Men c is 2.82±0.53 g cm−3, suggesting a composition consistent with Mg silicates and water, or a rocky core with a gaseous envelope.
• The host star π Mensae is bright (V=5.65 mag), enabling future atmospheric characterization of π Men c via transmission spectroscopy.
• π Men b is a long-period, eccentric companion with P ≈ 2091 days and e ≈ 0.64, indicating a dynamically interesting architecture with both a close-in super-Earth and a distant giant.
• The joint transit-RV analysis confirms the transiting planet and provides robust mass and orbital parameters for π Men c, placing it near the radius gap for short-period planets.
• Dynamical stability analysis shows the system remains stable over 100,000 years with only small variations in orbital elements.