Hot, rocky and warm, puffy super-Earths orbiting TOI-402 (HD 15337)

Abstract

Context. The Transiting Exoplanet Survey Satellite (TESS) is revolutionising the search for planets orbiting bright and nearby stars. In sectors 3 and 4, TESS observed TOI-402 (TIC-120896927), a bright V=9.1 K1 dwarf also known as HD 15337, and found two transiting signals with periods of 4.76 and 17.18 days and radii of 1.90 and 2.21 R ⊕ , respectively. This star was observed prior to the TESS detection as part of the radial-velocity (RV) search for planets using the HARPS spectrometer, and 85 precise RV measurements were obtained before the launch of TESS over a period of 14 years. Aims. In this paper, we analyse the HARPS RV measurements in hand to confirm the planetary nature of these two signals.Methods. HD 15337 happens to present a stellar activity level similar to the Sun, with a magnetic cycle of similar amplitude and RV measurements that are affected by stellar activity. By modelling this stellar activity in the HARPS radial velocities using a linear dependence with the calcium activity index log(R HK ), we are able, with a periodogram approach, to confirm the periods and the planetary nature of TOI-402.01 and TOI-402.02. We then derive robust estimates from the HARPS RVs for the orbital parameters of these two planets by modelling stellar activity with a Gaussian process and using the marginalised posterior probability density functions obtained from our analysis of TESS photometry for the orbital period and time of transit. Results. By modelling TESS photometry and the stellar host characteristics, we find that TOI-402.01 and TOI-402.02 have periods of 4.75642±0.00021 and 17.1784±0.0016 days and radii of 1.70±0.06 and 2.52±0.11 R ⊕ (precision 3.6 and 4.2%), respectively. By analysing the HARPS RV measurements, we find that those planets are both super-Earths with masses of 7.20±0.81 and 8.79±1.68 M ⊕ (precision 11.3 and 19.1%), and small eccentricities compatible with zero at 2σ. Conclusions. Although having rather similar masses, the radii of these two planets are very different, putting them on different sides of the radius gap. By studying the temporal evolution under X-ray and UV (XUV) driven atmospheric escape of the TOI-402 planetary system, we confirm, under the given assumptions, that photo-evaporation is a plausible explanation for this radius difference. Those two planets, being in the same system and therefore being in the same irradiation environment are therefore extremely useful for comparative exoplanetology across the evaporation valley and thus bring constraints on the mechanisms responsible for the radius gap.

A Pair of TESS Planets Spanning the Radius Valley around the Nearby Mid-M Dwarf LTT 3780

“…Based on the TSM, LTT 3780c is among the best warm mini-Neptunes (Pä [10,40] days, r p ä[2, 3] R ⊕ ) for atmospheric characterization via transmission spectroscopy observations. To date, the best such planets are the TESS-discovered planets TOI-700c (Gilbert et al 2020;Rodriguez et al 2020), TOI-270d (Günther et al 2019), and LTT 3780c, whose TSM values are all within 17% of each other and at minimum 17% greater than that of the next best potential target: HD 15337c (Dumusque et al 2019). The TSM values of favorable warm mini-Neptunes are reported in Table 5 and compared in Figure 8.…”

Public HARPS radial velocity database corrected for systematic errors

“…With its unprecedented precision, HARPS is the southern hemisphere backbone Doppler validation instrument for the Transiting Exoplanet Survey Satellite (TESS; Ricker et al 2015), which is uncovering hundreds of small rocky transiting exoplanet candidates around nearby stars. HARPS also offers a large publicly available spectral archive, which has already allowed the post-detection Doppler validation of TESS candidates, for example GJ 143, HD 23472 (Trifonov et al 2019a), HD 15337 (Gandolfi et al 2019;Dumusque et al 2019) and GJ 357 (Luque et al 2019).…”

An unusually low density ultra-short period super-Earth and three mini-Neptunes around the old star TOI-561

“…Since the beginning of its observations in 2018, TESS has discovered more than 66 exoplanets, including about a dozen multi-planet systems (e.g. Dragomir et al 2019;Dumusque et al 2019;Günther et al 2019). Multi-planet systems, orbiting the same star and having formed from the same protoplanetary disc, offer a unique opportunity for comparative planetology.…”