---
title: 'TOI-2076: Young Multi-Planet System'
url: https://www.emergentmind.com/topics/toi-2076-system
type: topic
---

# TOI-2076: Young Multi-Planet System

Searching arXiv for recent TOI-2076 papers to ground the article in the literature.
TOI-2076 is a nearby young planetary system around the bright K-dwarf BD+40 2790 (TIC 27491137), identified by TESS as one of two comoving planet-hosting stars within 50 pc. It was first reported as a three-planet transiting system with a secure \(10.356\) d planet and two outer planets whose periods were initially ambiguous because TESS recorded only two non-consecutive transits for each outer body; subsequent photometric follow-up established a compact chain of sub-Neptunes on \(\sim 10\), 21, and 35 d orbits, and later TESS re-analysis added an inner \(1.35\,R_\oplus\) planet on a 3.02 d orbit. The system has since been used as a key laboratory for young multi-planet dynamics, spin-orbit alignment, stellar-activity-limited radial-velocity extraction, and photoevaporative atmospheric evolution [2111.01311; 2203.03194; 2212.06266; 2505.06358; 2408.10629; 2603.02550].

## 1. Discovery sequence and observational chronology

The original discovery paper reported TOI-2076 as a nearby \((41.9\ \mathrm{pc})\), young \((204\pm50\ \mathrm{Myr})\), bright \((K = 7.115\ \mathrm{in\ TIC\ v8.1})\) multi-planetary system. TESS photometry revealed three transiting planets with radii \(R_b=3.3\pm0.04\,R_\oplus\), \(R_c=4.4\pm0.05\,R_\oplus\), and \(R_d=4.1\pm0.07\,R_\oplus\), while only TOI-2076 b had a unique period, \(P_b=10.356\ \mathrm{d}\). For TOI-2076 c and d, TESS saw only two transits separated by a 2-year interval in which no data were collected, leaving a range of periods \(>17\) d consistent with the data. The same work emphasized that both TOI-2076 and its comoving companion system TOI-1807 exhibit significant, periodic variability due to star spots, characteristic of young ages [2111.01311].

This initial three-planet architecture was explicitly provisional for the outer planets. Osborn et al. used MonoTools to rank the allowed “duotransit” aliases for c and d and then obtained targeted follow-up with CHEOPS, SAINT-EX, and LCO. CHEOPS identified TOI-2076 c at \(P=21.01538^{+0.00084}_{-0.00074}\) d, while ground-based observations ruled out the remaining aliases for TOI-2076 d and confirmed \(P=35.12537\pm0.00067\) d. This resolved the principal ambiguity in the original TESS discovery and converted TOI-2076 from an incompletely timed young system into a fully phased compact multi-planet system [2203.03194].

A further observational revision arrived with the TESS Investigation -- Demographics of Young Exoplanets analysis, which reported a fourth transiting planet, TOI-2076 e, a \(1.35\,R_\oplus\) inner planet on a 3.02 d orbit. In that study, signals at 3.02 d, 10.4 d, 21.0 d, and 35.1 d were recovered from TESS light curves using a custom extraction and detrending workflow, with validation for TOI-2076 e supported by TRICERATOPS at \( \mathrm{FPP} \approx 10^{-5}\) and by the system’s multi-transit architecture [2505.06358].

## 2. Host star and age determinations

Published characterizations classify the host between K0–K2 V, K1–K2 V, and K2 V. Gaia DR2 places the star at \(d = 41.963 \pm 0.028\) pc, while the later HARPS-N-based study adopted Gaia DR3 parallax \(\pi=23.8052 \pm 0.0125\) mas. Spectroscopic analyses give closely similar atmospheric parameters but not identical solutions: SpecMatch-Emp returned \(T_{\rm eff}=5201^{+66}_{-61}\) K, \([\mathrm{Fe/H}]=+0.017^{+0.077}_{-0.056}\), and \(R_\star=0.772^{+0.015}_{-0.016}\,R_\odot\), whereas ARESv2+MOOG on ATLAS9 model atmospheres gave \(T_{\rm eff}=5200 \pm 100\) K, \(\log g = 4.52 \pm 0.05\), and \([\mathrm{Fe/H}] = -0.03 \pm 0.06\). EXOFASTv2-based solutions reported \(M_\star=0.883\pm0.017\,M_\odot\), \(R_\star=0.775\pm0.015\,R_\odot\), and \(\log g=4.608\pm0.018\) in one analysis, and \(M_\star=0.849^{+0.027}_{-0.026}\,M_\odot\), \(R_\star=0.758\pm0.014\,R_\odot\), and \(\rho_\star=2.74^{+0.15}_{-0.14}\ \mathrm{g\,cm^{-3}}\) in another [2212.06266; 2408.10629].

The stellar rotation period is consistently near \(7.3\) d across independent analyses. Eight years of KELT photometry yielded \(P_{\rm rot}=7.27\pm0.23\) d and TESS autocorrelation gave \(7.251\pm0.073\) d; CLEAN periodograms of TESS, ASAS-SN, and SuperWASP photometry returned \(P_{\rm rot}=7.31\pm0.03\) d; and the later co-moving-star age analysis measured \(P_{\rm rot}=7.36\pm0.30\) d via Lomb–Scargle periodograms. Spectral broadening and synthesis measurements of stellar rotation gave \(v\sin i_\star=5\pm1\ \mathrm{km\,s^{-1}}\) and \(5.2\pm0.4\ \mathrm{km\,s^{-1}}\), consistent with a nearly edge-on stellar spin axis [2212.06266; 2408.10629; 2505.06358].

Age estimates evolved materially as the system was recharacterized. The discovery paper quoted \(204\pm50\) Myr; EXOFASTv2 in the NEID obliquity study implied \(0.338^{+0.077}_{-0.081}\) Gyr; Osborn et al. adopted \(0.34\pm0.08\) Gyr; and the GAPS characterization treated the star as \(\sim 300\) Myr old. The most explicit age revision came from analysis of likely co-moving stars associated with TOI-2076 and TOI-1807. That work combined four independent chronometers—gyrochronology of 125 co-moving stars, lithium equivalent widths for eight members, color–magnitude diagram isochrone fitting, and Gaia-band variability ages—to obtain a weighted system age of \(210 \pm 20\) Myr. This suggests that the system is best regarded as an approximately \(200\) Myr benchmark, while retaining a literature history in which ages between \(\sim 200\) and \(\sim 340\) Myr have been used [2505.06358].

## 3. Planetary inventory and architecture

The present literature describes TOI-2076 as a four-planet transiting system comprising an inner super-Earth and three outer sub-Neptunes. The inner planet, TOI-2076 e, was reported with \(P = 3.0223445 \pm 0.0000032\) d, \(R_p = 1.355^{+0.10}_{-0.098}\,R_\oplus\), \(a=0.0385^{+0.0021}_{-0.0049}\) AU, \(\delta \approx 243\) ppm, \(T_{\rm eq}=1138^{+72}_{-27}\) K, and \(S \approx 280\,S_\oplus\). A later four-planet characterization gave closely similar values, \(P = 3.0223753\) d, \(a = 0.03863\) AU, \(R_p = 1.301 \pm 0.059\,R_\oplus\), \(M_p = 4.7 \pm 1.5\,M_\oplus\), and \(i = 89.46 \pm 0.42^\circ\), with eccentricity fixed at approximately zero [2505.06358; 2603.02550].

For the previously known outer planets, Osborn et al. improved the radii to \(2.518\pm0.036\), \(3.497\pm0.043\), and \(3.232\pm0.063\,R_\oplus\) for b, c, and d, respectively. Their periods were measured as \(P_b = 10.35509 \pm 0.00014\) d, \(P_c = 21.01538^{+0.00084}_{-0.00074}\) d, and \(P_d = 35.12537 \pm 0.00067\) d. The GAPS transit analysis obtained \(P_b = 10.35523 \pm 0.00001\) d, \(P_c = 21.01549 \pm 0.00003\) d, \(P_d = 35.12551 \pm 0.00007\) d; \(a_b = 0.0880 \pm 0.0009\) AU, \(a_c = 0.1411 \pm 0.0015\) AU, \(a_d = 0.1988 \pm 0.0021\) AU; and radii \(R_b = 2.54 \pm 0.04\,R_\oplus\), \(R_c = 3.35 \pm 0.05\,R_\oplus\), and \(R_d = 3.29 \pm 0.06\,R_\oplus\). The 2026 four-planet solution reported \(R_b = 2.59 \pm 0.10\,R_\oplus\), \(R_c = 3.54 \pm 0.14\,R_\oplus\), and \(R_d = 3.27 \pm 0.13\,R_\oplus\), with masses \(6.8 \pm 1.8\), \(7.2 \pm 1.4\), and \(7.3 \pm 2.7\,M_\oplus\), and inclinations \(89.75 \pm 0.18^\circ\), \(89.76 \pm 0.12^\circ\), and \(89.16 \pm 0.014^\circ\) [2203.03194; 2408.10629; 2603.02550].

The architecture is exceptionally flat. The later four-planet analysis stated that mutual inclinations are all \(\lesssim 0.6^\circ\), and the GAPS stability study found mutual inclinations consistent with transiting geometry, \(\Delta i \lesssim 1^\circ\). The compactness of the chain, together with the progression from a \(1.3\)–\(1.4\,R_\oplus\) inner planet to \(2.5\)–\(3.5\,R_\oplus\) outer planets, underpins its use for comparative studies of radius evolution, atmospheric retention, and intra-system uniformity at young ages [2408.10629; 2505.06358; 2603.02550].

## 4. Period recovery, transit timing variations, and near-resonant dynamics

The key technical obstacle after discovery was the alias structure of the two outer planets. Osborn et al. assigned probabilities to the allowed aliases using a geometric plus temporal window-function prior \(p_{\rm geom}\propto 1/P\), an “orbital-velocity” prior derived numerically from the observed eccentricity distribution of multi-planet systems, and a Hill-stability filter on candidate multi-planet configurations. CHEOPS then tested the highest-probability windows, decisively detecting TOI-2076 c at \(\sim 21.0154\) d and ruling out three likely aliases for d, while LCO, SAINT-EX, and MuSCAT-3 discriminated between the remaining \(25.1\) d and \(35.1\) d solutions by obtaining a transit ingress at the \(35.1\) d window and a non-detection at the \(25.1\) d window [2203.03194].

Transit timing variations are central to the system’s dynamical interpretation. Osborn et al. reported a clear anti-correlated TTV signal between planets b and c with amplitude \(\sim \pm 0.04\) d, corresponding to early/late deviations of \(\sim \pm 50\)–\(60\) min from a linear ephemeris, and a TTV super-period
\[
P_{\rm super}=\left|\frac{1}{P_b}-\frac{2}{P_c}\right|^{-1}\simeq 713\pm3\ \mathrm{d}.
\]
The same study emphasized that the TTV-derived masses and eccentricities remained highly prior-dependent and degenerate. In the NEID obliquity study, a global fit allowing each transit epoch its own midtime yielded \(\sim 10\)–\(15\) min TTVs among b, c, and d, with b–c near \(2{:}1\) commensurability and c–d near \(5{:}3\), again pointing to strong mutual interactions and dynamical coupling [2203.03194; 2212.06266].

The period ratios place the outer chain just wide of exact mean-motion resonance. Osborn et al. gave \(P_c/P_b \simeq 2.0296\), implying an offset from exact \(2{:}1\) of \(\Delta_{2:1}\simeq0.0296\), and \(P_d/P_c \simeq 1.6713\), implying an offset from exact \(5{:}3\) of \(\Delta_{5:3}\simeq0.0046\). The later Hamiltonian analysis formalized this near-resonant structure by defining
\[
\Delta=\frac{(P_{i+1}/P_i)}{j/(j-1)}-1.
\]
Using that framework, the b–c pair was found to have \(\Delta_{bc}=+1.5\%\) relative to \(2{:}1\), and the c–d pair \(\Delta_{cd}=+0.3\%\) relative to \(5{:}3\). Both pairs lie outside the true resonant libration zones: for \(2{:}1\), the proximity parameter was reported as \(\delta\approx-7.0\pm0.8\), well below the \(+0.95\) value needed to bifurcate into libration, and for \(5{:}3\), \(J^\ast<0\), likewise implying circulation. The system was therefore characterized as “near-resonant” but non-librating and dynamically fragile [2203.03194; 2603.02550].

## 5. Spin-orbit geometry and the Rossiter–McLaughlin measurement of TOI-2076 b

TOI-2076 b is one of the few young planets in a multi-transiting system with a measured obliquity. Using NEID on the WIYN 3.5 m Telescope, investigators modeled the Rossiter–McLaughlin effect during a transit of the planet. In the simplified description adopted in that study, the RM semi-amplitude is
\[
\Delta v \simeq \left(v\sin i_\star\right)\frac{R_p^2}{R_\star^2},
\]
while the full rmfit model depends on \(v\sin i_\star\), the sky-projected obliquity \(\lambda\), quadratic limb darkening \((u_1,u_2)\), and the transit geometry \((R_p/R_\star,\ a/R_\star,\ i_p,\ P,\ T_C)\). The in-transit velocities were extracted with the SERVAL pipeline, modeled with the analytic Hirano et al. formula, and fit simultaneously with a linear RV slope attributed to stellar activity while holding the Keplerian semi-amplitude at \(K=0\) during the \(\sim 5.5\) h window [2212.06266].

The resulting sky-projected obliquity was \(\lambda=-3^{+16}_{-15}\) deg. Combining this with the stellar size, rotation period, and \(v\sin i_\star\) yielded an estimate of the true obliquity via
\[
\cos\psi=\sin i_\star\,\sin i_p\,\cos\lambda+\cos i_\star\,\cos i_p,
\]
giving \(\psi=18^{+10}_{-9}\) deg and a 95% upper limit \(\psi<34^\circ\). Simultaneous diffuser-assisted \(i^\prime\)-band photometry from ARCTIC on the ARC 3.5 m telescope showed the expected \(0.08\%\) transit depth and no flares or rapid variability during the RM sequence, ruling out flare-induced RV distortions. On that basis, TOI-2076 b was classified as consistent with an aligned orbit [2212.06266].

The interpretation advanced in the same work linked the low obliquity with the TTV-rich compact architecture. Because TOI-2076 b has large \(a/R_\star \simeq 25\), making tidal realignment inefficient, the low obliquity was argued to disfavor high-amplitude scattering or secular misalignment. The preferred scenario was convergent Type I/II disk migration in an initially well-aligned disk, with the planets trapped into near-resonant orbits and later observed in a compact but still orderly configuration [2212.06266].

## 6. Radial velocities, atmospheric escape, and evolutionary interpretation

Long-baseline RV characterization of TOI-2076 has been difficult because the host is young and active. The GAPS Programme collected more than 300 high-resolution spectra over \(\sim 3\) yr—specifically 294 HARPS-N spectra, plus NEID and CARMENES-VIS data—and found activity-induced RV scatter larger than \(30\ \mathrm{m\,s^{-1}}\). Three RV extraction strategies were tested: the standard DRS CCF K5 mask, SERVAL template matching, and a line-by-line algorithm. Activity filtering used Gaussian-process regression with a quasi-periodic kernel,
\[
k(\Delta t)=A^2\exp\!\left[-\frac{\Delta t^2}{2\lambda^2}-\frac{\sin^2(\pi\Delta t/\theta)}{2w^2}\right],
\]
with \(\theta\sim P_{\rm rot}\), and some fits simultaneously modeled RVs and BIS or FWHM in multidimensional GP form. Example best-fit semi-amplitudes from the BIS-trained MGP model were \(K_b = 1.1 \pm 0.6\ \mathrm{m\,s^{-1}}\), \(K_c = 1.2 \pm 0.6\ \mathrm{m\,s^{-1}}\), and \(K_d = 1.6 \pm 0.6\ \mathrm{m\,s^{-1}}\), but none reached a \(\geq 3\sigma\) mass measurement. Instead, the study reported model-averaged \(3\sigma\) upper limits of \(m_b < 11\)–\(12\,M_\oplus\), \(m_c < 12\)–\(13.5\,M_\oplus\), and \(m_d < 14\)–\(19\,M_\oplus\), with a tentative \(m_d \simeq 7\)–\(8\,M_\oplus\) signal at \(2.3\)–\(2.5\sigma\) [2408.10629].

A later \(\sim 200\) Myr characterization combined photodynamical and RV information and reported masses for all four planets: \(4.7 \pm 1.5\,M_\oplus\) for e, \(6.8 \pm 1.8\,M_\oplus\) for b, \(7.2 \pm 1.4\,M_\oplus\) for c, and \(7.3 \pm 2.7\,M_\oplus\) for d. That same work described the four planets as having comparable core masses but a monotonic increase in hydrogen and helium envelope mass fractions with decreasing insolation, summarized as stripped-\(1\%\)-\(5\%\)-\(5\%\) for e, b, c, and d. This suggests a system observed near the end of photoevaporation, in which the innermost planet has been stripped, the next planet retains only a thin envelope, and the outer two retain moderate envelopes [2603.02550].

Photoevaporative evolution has been modeled in two complementary ways in the literature. The GAPS study used ATES hydrodynamics coupled to Lopez & Fortney core-envelope evolution and quoted the approximate analytic mass-loss rate
\[
\dot M=\epsilon\,\frac{\pi\,F_{\rm XUV}\,R_p\,R_{\rm XUV}^2}{G\,M_p\,K_{\rm tide}},
\]
with \(\epsilon\sim0.1\)–\(0.2\). It concluded that TOI-2076 b is currently losing its H-He gaseous envelope and would lose it completely by an age within \(0.5\)–\(3\) Gyr if its current mass is lower than \(12\,M_\oplus\); TOI-2076 c could retain its atmosphere up to an age of \(5\) Gyr; and TOI-2076 d should experience almost negligible evolution of mass and radius induced by photo-evaporation. The later four-planet study adopted the canonical energy-limited form
\[
\dot{M}=\frac{\epsilon\,\pi\,F_{\rm XUV}\,R_{p}^3}{G\,M_{p}\,K_{\rm tide}},
\]
and estimated \(F_{\rm XUV}\) at age \(\approx0.21\) Gyr as \(1.4\times10^5\), \(2.6\times10^4\), \(1.0\times10^4\), and \(5.2\times10^3\ \mathrm{erg\,s^{-1}\,cm^{-2}}\) for e, b, c, and d, respectively [2408.10629; 2603.02550].

Two points in the secondary literature are especially important for interpretation. First, TOI-2076 is not a system with uncontested RV-only planet masses; the 2024 RV analysis remained upper-limit dominated because stellar activity overwhelms the expected Doppler amplitudes. Second, the planets are not treated as pure water worlds in the 2026 synthesis, because previous detections of metastable He I \(10830\ \AA\) outflows from b, c, and d are cited as ruling out a pure water-world scenario. These considerations reinforce the system’s role as a young comparative laboratory for atmospheric loss rather than as a purely dynamical or purely compositional case study [2408.10629; 2603.02550].

TOI-2076 is consequently significant at the intersection of several exoplanet subfields. Its youth, flat four-planet architecture, near-but-not-librating commensurabilities, low obliquity, activity-challenged RVs, and predicted differential envelope loss make it unusually informative for testing models of convergent disk migration, resonant-chain disruption, and early atmospheric sculpting. The outer three planets were described as excellent candidates for future comparative transmission spectroscopy with JWST, and the 2025 update noted that TOI-2076 b, c, and d are JWST cycle 3 targets [2203.03194; 2505.06358].

Source: https://www.emergentmind.com/topics/toi-2076-system