The CARMENES search for exoplanets around M dwarfs -- LP 714-47b (TOI 442.01): Populating the Neptune desert (2011.01716v1)

Abstract: We report the discovery of a Neptune-like planet (LP 714-47 b, P = 4.05204 d, m_b = 30.8 +/- 1.5 M_earth , R_b = 4.7 +/- 0.3 R_earth ) located in the 'hot Neptune desert'. Confirmation of the TESS Object of Interest (TOI 442.01) was achieved with radial-velocity follow-up using CARMENES, ESPRESSO, HIRES, iSHELL, and PFS, as well as from photometric data using TESS, Spitzer, and ground-based photometry from MuSCAT2, TRAPPIST- South, MONET-South, the George Mason University telescope, the Las Cumbres Observatory Global Telescope network, the El Sauce telescope, the TUBITAK National Observatory, the University of Louisville Manner Telescope, and WASP-South. We also present high-spatial resolution adaptive optics imaging with the Gemini Near-Infrared Imager. The low uncertainties in the mass and radius determination place LP 714-47 b among physically well-characterised planets, allowing for a meaningful comparison with planet structure models. The host star LP 714-47 is a slowly rotating early M dwarf (T_eff = 3950 +/- 51 K) with a mass of 0.59 +/- 0.02 M_sun and a radius of 0.58 +/- 0.02 R_sun. From long-term photometric monitoring and spectroscopic activity indicators, we determine a stellar rotation period of about 33 d. The stellar activity is also manifested as correlated noise in the radial-velocity data. In the power spectrum of the radial-velocity data, we detect a second signal with a period of 16 days in addition to the four-day signal of the planet. This could be shown to be a harmonic of the stellar rotation period or the signal of a second planet. It may be possible to tell the difference once more TESS data and radial-velocity data are obtained.

• The paper confirms LP 714-47b’s existence by combining high-precision radial velocity and transit photometry from CARMENES, TESS, Spitzer, and other instruments.
• It characterizes the planet as Neptune-like, with a mass of 30.8 ± 1.5 Earth masses and a radius of 4.7 ± 0.3 Earth radii, placing it within the rare hot Neptune desert.
• The study discusses formation mechanisms such as photo-evaporation and migration, offering key insights into the processes that sculpt the Neptune desert’s boundaries.

The CARMENES Search for Exoplanets around M Dwarfs: LP 714-47 b and the Neptune Desert

The paper presents an analysis of the exoplanet LP 714-47 b, a Neptune-like planet located in the 'hot Neptune desert.' Detected as a TESS Object of Interest (TOI 442.01), the planet's existence was confirmed through comprehensive follow-up using multiple high-precision instruments such as CARMENES, ESPRESSO, HIRES, iSHELL, and PFS, as well as photometry from TESS, Spitzer, and several ground-based telescopes. The paper provides detailed measurements of the exoplanet's characteristics, revealing it as a promising subject to paper in relation to the lack of such planets in short-period orbits.

Observational Approach

The confirmation of TOI 442.01 leveraged both radial velocity measurements and a large suite of photometric data, encompassing datasets from TESS, Spitzer, and ground-based observations. Optical and infrared high-resolution spectra were utilized, notably from the CARMENES spectrograph covering a broad wavelength range which adds reliability to the planetary mass and orbit determinations. The planet's precise radius determination benefits from data from transit events observed by TESS and complemented by Spitzer and ground-based campaigns including MuSCAT2 and others.

Results and Planetary Characterization

The paper finds LP 714-47 b to be a Neptune-like exoplanet with a mass of $30.8 \pm 1.5 \, M_\oplus$ and a radius of $4.7 \pm 0.3 \, R_\oplus$. The host star LP 714-47 is identified as a slowly rotating M0 star with $T_{\rm eff}=3950 \pm 51$ K. The detailed characterization of the star provided by CARMENES spectra is crucial as it directly impacts the derived planetary parameters. The low uncertainties in these measurements position LP 714-47 b among the better-characterized exoplanets.

Discussion on the Neptune Desert

LP 714-47 b resides in a region of parameter space known as the 'hot Neptune desert,' characterized by a scarcity of Neptune-like planets with masses around 0.1 $M_{\rm Jup}$ and periods under approximately 4 days. This research explores plausible causes for this desert, emphasizing scenarios like photo-evaporation, type-I migration halting gas accretion, and high-eccentricity migration with tidal circularization.

Though LP 714-47 b, with its dense atmosphere and specified orbital parameters, does add mass to the minimal population within this desert, its existence provides constraints on the competing theories explaining the desert's formation. The planet's relatively high density suggests it has retained a significant atmosphere despite potential evaporation.

Implications and Future Prospects

With robust data supporting the existence of LP 714-47 b, further research into this system could yield insights into the nuanced processes governing planetary formation and evolution around M dwarfs. Moreover, understanding the nature of LP 714-47 b could inform models designed to explain the observed distribution of exoplanets across the period-radius mass plane, refining predictions about the outcomes of planetary evolution scenarios such as thermal escape and core accretion pathways. Future TESS observations, revisiting the sky regions previously examined, can provide additional data to further refine the understanding of LP 714-47 system dynamics, including possible additional planets hinted by the apparent 16-day signal, if found to be stable over longer monitoring periods.