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The GAPS Programme at the TNG: LXX. TOI-5734b: A hot sub-Neptune orbiting a relatively young K dwarf with an Earth-like density

Using TESS photometry and HARPS-N radial velocity data, the GAPS programme confirmed TOI-5734b as a young, hot sub-Neptune with an Earth-like density and a depleted primary envelope, validating its potential as a target for future atmospheric studies.

Original authors: S. Filomeno, T. Trifonov, M. Damasso, M. Baratella, S. Benatti, K. Biazzo, K. A. Collins, R. Cosentino, S. Desidera, C. Di Maio, D. Locci, A. Maggio, L. Mancini, S. Messina, L. Naponiello, D. Nardiell

Published 2026-04-01

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Original authors: S. Filomeno, T. Trifonov, M. Damasso, M. Baratella, S. Benatti, K. Biazzo, K. A. Collins, R. Cosentino, S. Desidera, C. Di Maio, D. Locci, A. Maggio, L. Mancini, S. Messina, L. Naponiello, D. Nardiello, K. G. Stassun, F. Amadori, S. Antoniucci, F. Biassoni, A. S. Bonomo, L. Cabona, C. A. Clark, M. Gonzalez, A. F. Lanza, D. W. Latham, V. Lorenzi, L. Malavolta, G. Mantovan, M. Pedani, G. Scandariato, A. Shporer, A. Sozzetti, T. Zingales, D. Ciardi, M. Everett, M. B. Lund, H. Osborn, I. A. Strakhov, J. Villaseñor, D. Watanabe, A. Youngblood, R. Zambelli

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Here is the story of TOI-5734b, told in simple terms with a few creative analogies.

The Cast of Characters

Imagine a young, energetic star named TOI-5734. Think of it as a "teenager" in the universe. It's only about 500 million years old (which is very young for a star that will live for billions of years). Because it's young, it's still very active, spinning fast and throwing out magnetic storms, much like a restless teenager who can't sit still.

Orbiting this star is a planet named TOI-5734b. This planet is a "hot sub-Neptune."

Hot: It's very close to its star, baking in intense heat.
Sub-Neptune: It's bigger than Earth but smaller than Neptune.
The Mystery: Astronomers wanted to know: Is this planet a fluffy ball of gas (like a mini-Neptune), or is it a dense, rocky world (like a super-Earth)?

The Detective Work: Finding the Planet

Finding a planet around a young, active star is like trying to hear a whisper at a loud rock concert. The star's "noise" (sunspots, flares, and magnetic activity) is so loud that it often drowns out the tiny signal of a planet tugging on the star.

To solve this, the astronomers (the "detectives") used two main tools:

TESS (The Camera): A space telescope that watched the star and saw it dim slightly every 6.18 days. This meant something was passing in front of it (a transit).
HARPS-N (The Scale): A super-precise instrument on a telescope in the Canary Islands that measured how much the star was wobbling back and forth due to the planet's gravity.

The Challenge: The star's "rock concert" noise was messing up the measurements. To fix this, the team used a sophisticated mathematical tool called Gaussian Processes.

Analogy: Imagine trying to hear a friend's voice in a noisy room. You don't just turn up the volume; you use noise-canceling headphones that specifically filter out the background chatter. The astronomers used these "mathematical headphones" to filter out the star's activity, revealing the planet's true voice underneath.

The Discovery: What is TOI-5734b?

Once they filtered out the noise, the picture became clear:

Size: It is about 2.1 times wider than Earth.
Mass: It is about 9 times heavier than Earth.
Density: This is the big reveal. It has a density very similar to Earth's.

The Verdict: TOI-5734b is likely a rocky world that has lost almost all of its original atmosphere.

Analogy: Imagine a giant, fluffy marshmallow (a gas planet) sitting in a hot oven. Over time, the heat blows away all the fluffy air, leaving behind just the dense, hard marshmallow center. TOI-5734b is that "naked" core. It used to have a thick layer of gas, but the young, fiery star stripped it away, leaving a dense, rocky planet behind.

The "Radius Valley" Connection

In the universe, there is a strange gap in the sizes of planets. There are lots of small, rocky planets (Super-Earths) and lots of big, puffy gas planets (Sub-Neptunes), but very few planets in the middle. This gap is called the "Radius Valley."

TOI-5734b is sitting right on the edge of this valley. It's the perfect example of a planet that was once a "Sub-Neptune" but is currently in the process of becoming a "Super-Earth" by losing its gas. It's a cosmic snapshot of a planet in transition.

The Future: A Rocky Fate

The astronomers ran simulations to see what will happen to this planet in the future.

The Prediction: The star is still very young and energetic. It will continue to blast the planet with high-energy radiation.
The Outcome: In about 300 million years, the planet will lose its last remaining trace of atmosphere. It will become a "Chthonian planet"—a term for a world that is just a bare, scorched rock with no air left.

Why Does This Matter?

This discovery is like finding a time capsule. Because the star is young, we are seeing a planet at a stage of its life that most other planets have already passed.

For the Future: This planet is a prime target for future telescopes like JWST and the upcoming Ariel mission. Scientists hope to look at the thin atmosphere that remains to understand exactly how stars strip away the air from their planets.

In Summary:
TOI-5734b is a young, hot, rocky planet that is currently being "stripped naked" by its energetic star. It's a perfect example of how planets evolve from gas-rich worlds into dense, rocky ones, helping us understand the history of our own solar system and the many others out there.

1. Problem Statement

The study addresses the challenges in characterizing exoplanets orbiting young, magnetically active stars. Young stars exhibit significant stellar activity (spots, faculae, flares) that induces noise in radial velocity (RV) measurements, often masking the subtle signals of low-mass planets. Furthermore, understanding the formation and evolutionary pathways of "sub-Neptunes" (planets with radii between 1.8 and 4.0 Earth radii) requires precise mass and radius measurements for young systems to test atmospheric loss theories (photo-evaporation) and the "radius valley" hypothesis. TOI-5734, a young K-dwarf with a transiting candidate, presented a difficult target due to its high activity levels, requiring advanced statistical techniques to disentangle planetary signals from stellar noise.

2. Methodology

The authors employed a multi-faceted approach combining photometric and spectroscopic data with advanced statistical modeling:

Observations:
- Photometry: Data from three TESS sectors (20, 47, and 60) were processed using the PATHOS pipeline. Light curves were detrended and corrected for dilution from nearby stars using Gaia DR3 data.
- Spectroscopy: 97 high-resolution RV observations were obtained using the HARPS-N spectrograph at the Telescopio Nazionale Galileo (TNG) between 2022 and 2024.
- Stellar Characterization: High-resolution spectroscopy was used to determine fundamental stellar parameters ( $T_{\text{eff}}$ , $\log g$ , [Fe/H], $v \sin i_*$ ) via equivalent width analysis of Fe and Ti lines. Lithium abundance ( $A_{\text{Li}}$ ) was measured to constrain age.
Stellar Activity Analysis:
- Activity indicators (BIS span, FWHM, Contrast, $\log R'_{HK}$ ) were analyzed using Generalized Lomb-Scargle (GLS) periodograms.
- A strong correlation was found between RVs and the BIS span, confirming stellar activity as the dominant noise source.
Joint Modeling (GP Regression):
- The core of the analysis utilized the exo-striker toolbox to jointly model the TESS light curves and HARPS-N RVs.
- Gaussian Processes (GPs): To separate the planetary signal from stellar activity, the authors employed a Double Stochastically Driven, Damped Simple Harmonic Oscillator (dSHO) kernel within the celerite package. This kernel was chosen over a standard quasi-periodic kernel because it could simultaneously model the stellar rotation period ( $P_{\text{rot}}$ ) and its first harmonic (at $P_{\text{rot}}/2$ ), which is a signature of flux variations caused by spots/faculae.
- Priors: Informative priors were derived from the light curve analysis to guide the GP hyperparameters, particularly for the rotation period.
Atmospheric Evolution Modeling:
- A hydrodynamic mass-loss model (ATES code) coupled with core-envelope structure models (Fortney et al.; Lopez & Fortney) was used to simulate the planet's atmospheric evolution from 10 Myr to 5 Gyr, accounting for X-ray/EUV irradiation history.

3. Key Contributions

Validation of TOI-5734b: The study confirms the planetary nature of the candidate using Bayesian validation tools (TRICERATOPS) and centroid analysis, ruling out false positives from background eclipsing binaries or blended sources.
Advanced Activity Mitigation: The successful application of the dSHO kernel to model both the rotation period and its harmonic in a young, active K-dwarf system, allowing for a precise mass determination that would otherwise be impossible.
Characterization of a Young Sub-Neptune: Provides one of the few precise mass and radius measurements for a sub-Neptune orbiting a star with an age of $\sim$ 500 Myr, a critical epoch for understanding planetary atmospheric loss.

4. Key Results

Stellar Parameters (TOI-5734):

Type: K3–K4 V dwarf.
Age: $500^{+300}_{-150}$ Myr, derived from gyrochronology ( $P_{\text{rot}} \approx 11.09$ d), chromospheric activity ( $\log R'_{HK}$ ), X-ray luminosity, and Lithium abundance.
Rotation Period: $P_{\text{rot}} = 11.09^{+0.07}_{-0.08}$ d.
Mass/Radius: $M_* = 0.724 \pm 0.009 M_\odot$ , $R_* = 0.639 \pm 0.034 R_\odot$ .

Planetary Parameters (TOI-5734b):

Orbital Period: $P_b = 6.1841876^{+0.0000080}_{-0.0000094}$ d.
Radius: $R_b = 2.10 \pm 0.12 R_\oplus$ .
Mass: $M_b = 9.1^{+2.6}_{-2.6} M_\oplus$ .
Density: $\rho_b = 0.98^{+0.36}_{-0.30} \rho_\oplus$ ( $\approx 5.4^{+2.0}_{-1.7}$ g/cm $^3$ ).
Equilibrium Temperature: $T_{\text{eq}} \approx 688$ K.
Composition: The density is consistent with an Earth-like composition (rocky core with a thin H/He envelope) or a "water world" (rocky core with a significant water layer). The data favors a rocky composition with an almost completely depleted primary envelope.

Atmospheric Evolution:

Modeling suggests the planet likely formed with a larger radius ( $\sim 3.41 R_\oplus$ ) and a more massive H/He envelope.
Due to intense high-energy irradiation from the young host, the planet has lost most of its primordial atmosphere.
Future Fate: The planet is predicted to lose its remaining primary atmosphere within $\sim 300$ Myr, evolving into a "Chthonian planet" (a bare rocky core).

5. Significance

The Radius Valley: TOI-5734b is located at the upper edge of the "radius valley" (the gap in the distribution of planet radii between super-Earths and sub-Neptunes). Its existence in a young system provides a snapshot of a planet currently transitioning from a sub-Neptune to a super-Earth via photo-evaporation.
Atmospheric Studies: Despite the predicted loss of its atmosphere, the planet remains a prime target for future atmospheric characterization. The authors calculate that the Ariel space mission (Tier 1 observation) could detect a primary atmosphere in just 19 transits, which would validate the evolutionary models.
Methodological Benchmark: The paper demonstrates the efficacy of using flexible GP kernels (dSHO) to recover planetary signals in the RV data of active young stars, a crucial step for the demographic study of young planetary systems.

In conclusion, TOI-5734b represents a critical case study in the evolution of close-in sub-Neptunes, offering observational evidence for the photo-evaporation process and highlighting the potential of young systems to refine our understanding of planetary formation and atmospheric retention.