TOI-451: Young Multi-Planet System

• TOI-451 planetary system is a young, benchmark system featuring a G8 V star, three transiting exoplanets, a debris disk, and a wide binary companion.
• Detailed transit and radial velocity analyses from TESS, Spitzer, and ESPRESSO have refined orbital parameters and masses, enabling studies of atmospheric properties and dynamical interactions.
• Its well-constrained age (~125 Myr), observable photoevaporation signatures, and measurable transit-timing variations offer a unique window into early planetary formation and evolution.

The TOI-451 planetary system is a benchmark young multi-planet system composed of a G8 V primary (TOI 451) and three validated transiting planets, with a co-moving wide binary M-dwarf companion and a debris disk. The primary star is a confirmed member of the 120–125 Myr-old Pisces–Eridanus (Psc–Eri) stream, making the system a key laboratory for probing planetary formation and atmospheric evolution processes at an early epoch. Initial discovery was accomplished through TESS photometry, with confirmation via Spitzer, ground-based facilities, and ESPRESSO radial velocities, using multidimensional Gaussian Process (GP) frameworks that leverage contemporaneous photometric and spectroscopic activity indicators (Newton et al., 2021, Barragán et al., 14 Jan 2026, Barragán et al., 2021).

1. Stellar Properties and Age

TOI 451 (TIC 257605131) is characterized as a G8 V star with $T_{\mathrm{eff}} = 5490 \pm 115$ K, [Fe/H]$= +0.02 \pm 0.06$, $M_{\star} = 0.93 \pm 0.04 \, M_{\odot}$, and $R_{\star} = 0.85 \pm 0.03 \, R_{\odot}$, placing it slightly below solar mass and radius (Barragán et al., 14 Jan 2026). The system’s age is kinematically and chemically constrained at $125 \pm 8$ Myr, consistent with the Psc–Eri stream and with lithium abundance $A({\rm Li}) = 2.80\pm0.10$ dex, UV excess, rapid rotation ($P_{\rm rot} = 5.1 \pm 0.1$ d, $v\sin i = 8.7 \pm 0.96$ km s⁻¹), and chromospheric activity (Newton et al., 2021). The star exhibits photometric amplitudes of 1–2%, and its kinematics match the stream with $$(U,V,W) = (-10.92, -4.18, -18.81) \pm (0.05, 0.08, 0.09)$$ km s⁻¹ (Newton et al., 2021). Membership probability in Psc–Eri is estimated at $\sim$97%.

2. Planetary Architecture and Orbital Parameters

The system contains three transiting planets, TOI-451 b, c, and d. Their orbital and transit parameters, as refined by simultaneous modeling of multiple TESS campaigns and ground-based photometry, are summarized below.

Parameter	TOI-451 b	TOI-451 c	TOI-451 d
Period $P$ (d)	$1.8586273\pm0.0000012$	$9.1926242\pm0.0000044$	$16.364881\pm0.0000085$
$R_p$ ($R_\oplus$)	$1.71\pm0.08$	$2.96\pm0.12$	$3.92\pm0.12$
$a$ (AU)	$0.0272\pm0.0006$	$0.0745\pm0.0010$	$0.1056\pm0.0015$
Inclination (deg)	$87.14\pm0.29$	$89.06\pm0.17$	$88.34\pm0.14$
Impact param. $b$	$0.64\pm0.04$	$0.26\pm0.04$	$0.43\pm0.06$

Ephemerides have been refined using joint pyaneti modeling and all available TESS data, yielding mid-transit times with $\sigma_{T_0} \lesssim 3$ minutes; the periods are stable to $<10^{-5}$ d (Barragán et al., 2021, Barragán et al., 14 Jan 2026).

3. Mass Determinations and Bulk Compositions

Radial velocities from ESPRESSO, modeled with a two-dimensional “multi-GP” approach that incorporates contemporaneous NGTS/LCO photometry as an activity indicator, yield Doppler semi-amplitudes:

• $K_b = 2.8\pm1.1$ m s⁻¹
• $K_c = 1.4^{+1.2}_{-0.9}$ m s⁻¹
• $K_d = 2.9\pm1.4$ m s⁻¹

Inferred planet masses and $2\sigma$ upper limits:

Planet	$M_p (M_\oplus)$	$2\sigma$ Upper Limit ($M_\oplus$)
b	$5.0\pm1.9$	$<9.7$
c	$3.1\pm2.7$	$<11.5$
d	$9.7\pm3.6$	$<20.7$

Bulk densities are $4.8\pm2.2$ g cm⁻³ (b), $1.2\pm1.0$ g cm⁻³ (c), $1.25\pm0.58$ g cm⁻³ (d), indicating that c and d require substantial H/He envelopes ($\sim$5–8% by mass if Earth-like core), while b may be a rocky planet retaining a tenuous H/He layer or a water-dominated body (Barragán et al., 14 Jan 2026). The orbital spacing and period ratios do not strongly suggest resonant behavior, but transit-timing variations (TTVs) at the $\sim0.5$–2 min level are predicted (between c and d over $\sim$75 d timescales) (Newton et al., 2021).

4. Photometric and Radial Velocity Methodologies

Light curves from TESS (Cycles 1 and 3), NGTS, and LCO were detrended for stellar variability using a quasi-periodic Gaussian Process kernel:

$$k(t, t') = A^2 \exp\left[-\frac{(t-t')^2}{2\lambda_e^2} - \frac{\sin^2[\pi (t-t')/P_{\mathrm{GP}}]}{2\lambda_p^2}\right]$$

Photometric time series provide direct constraints on stellar rotation ($P_{\mathrm{GP}}\sim5.13$–5.18 d) and the active-region evolution timescale ($\lambda_e$), as well as priors on the quasi-periodic activity kernel used for RV joint modeling (Barragán et al., 14 Jan 2026, Barragán et al., 2021). This methodology robustly disentangles planetary signals from chromospheric activity, which is essential in young, rapidly rotating stars where RV jitter is typically significant.

5. Stellar Environment: Debris Disk and Wide Binary Companion

TOI 451 exhibits infrared excesses in WISE W3 (26 ± 7%) and W4 (70 ± 30%) bands, statistically significant at 3.7σ and 2.3σ, indicative of a debris disk with inferred temperatures $<$ 300 K (Newton et al., 2021). The co-moving wide companion TOI 451 B (TIC 257605132) has $T_{\mathrm{eff}}\sim3500$ K (M3V), lies at a projected separation of 37.8″ ($\sim4700$ AU), and is itself likely an unresolved near-equal mass binary (Gaia RUWE = 1.24, $M_{B,\rm total}\sim0.45\,M_{\odot}$). The wide binary architecture and presence of a debris disk are consistent with early dynamical stability and ongoing planetary system dynamical processes.

6. Atmospheric Characterization and Evolutionary Implications

Atmospheric transmission signals are predicted to be $\Delta D \approx$ 260 (b), 460 (c), and 760 (d) ppm; scale heights are $H \sim$ 4000–5500 km, assuming $\mu=2.3m_p$ and measured $T_{\mathrm{eq}}$ values (Newton et al., 2021). Transmission Spectroscopy Metric (TSM) values place c and d among the top $\sim5\%$ of small planets for atmospheric follow-up with JWST. All three planets offer prospects for measuring atmospheric loss via helium 10 830 Å absorption, especially in the context of post-peak high-energy irradiation for $\tau\sim125$ Myr (Barragán et al., 14 Jan 2026). Both photoevaporation and core-powered mass-loss processes may be ongoing; TOI-451 planets provide a rare window onto processes sculpting the radius valley at early evolutionary times.

7. Prospects and Future Directions

The TOI-451 system supports high-cadence photometry (e.g., PLATO), intensive RV campaigns ($>125$ ESPRESSO epochs across five months recommended), and transmission and emission spectroscopy campaigns with JWST and HST. Precise monitoring of stellar activity remains critical for RV disentanglement; the system's rapidly evolving spot coverage and active-region lifetimes were directly observed to change over two-year timescales. The system's well-determined age, non-resonant compact multi-planet architecture, and the presence of a debris disk and wide binary companion provide essential boundary conditions for testing planet formation, migration, tidal evolution, and mass-loss modeling (Barragán et al., 2021, Barragán et al., 14 Jan 2026).

A plausible implication is that TOI-451 serves as a reference point for comparative studies of sub-Neptune/super-Earth evolution and photoevaporation in young systems, informing demographic and atmospheric studies as observational precision improves. This makes TOI-451 an archetype for dynamical and atmospheric studies at the boundary of the radius valley in a post-disk, but still strongly irradiated, stellar environment.