TOI-2076: Young Multi-Planet System

• TOI-2076 is a nearby, young K-dwarf system hosting a compact chain of four transiting exoplanets with near-resonant orbits.
• Observations from TESS, CHEOPS, and ground-based telescopes have precisely measured orbital periods, radii, and transit timing variations, establishing the system as a benchmark for studying multi-planet dynamics.
• Recent analyses, including Rossiter–McLaughlin measurements and RV challenges, highlight the system’s low obliquity and ongoing photoevaporative atmospheric evolution.

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 (Hedges et al., 2021, Osborn et al., 2022, Frazier et al., 2022, Barber et al., 9 May 2025, Damasso et al., 2024, Wang et al., 3 Mar 2026).

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 $\sim 10$0 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 (Hedges et al., 2021).

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 $\sim 10$1 d, while ground-based observations ruled out the remaining aliases for TOI-2076 d and confirmed $\sim 10$2 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 (Osborn et al., 2022).

A further observational revision arrived with the TESS Investigation -- Demographics of Young Exoplanets analysis, which reported a fourth transiting planet, TOI-2076 e, a $\sim 10$3 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 $\sim 10$4 and by the system’s multi-transit architecture (Barber et al., 9 May 2025).

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 $\sim 10$5 pc, while the later HARPS-N-based study adopted Gaia DR3 parallax $\sim 10$6 mas. Spectroscopic analyses give closely similar atmospheric parameters but not identical solutions: SpecMatch-Emp returned $\sim 10$7 K, $\sim 10$8, and $\sim 10$9, whereas ARESv2+MOOG on ATLAS9 model atmospheres gave $1.35\,R_\oplus$0 K, $1.35\,R_\oplus$1, and $1.35\,R_\oplus$2. EXOFASTv2-based solutions reported $1.35\,R_\oplus$3, $1.35\,R_\oplus$4, and $1.35\,R_\oplus$5 in one analysis, and $1.35\,R_\oplus$6, $1.35\,R_\oplus$7, and $1.35\,R_\oplus$8 in another (Frazier et al., 2022, Damasso et al., 2024).

The stellar rotation period is consistently near $1.35\,R_\oplus$9 d across independent analyses. Eight years of KELT photometry yielded $(41.9\ \mathrm{pc})$0 d and TESS autocorrelation gave $(41.9\ \mathrm{pc})$1 d; CLEAN periodograms of TESS, ASAS-SN, and SuperWASP photometry returned $(41.9\ \mathrm{pc})$2 d; and the later co-moving-star age analysis measured $(41.9\ \mathrm{pc})$3 d via Lomb–Scargle periodograms. Spectral broadening and synthesis measurements of stellar rotation gave $(41.9\ \mathrm{pc})$4 and $(41.9\ \mathrm{pc})$5, consistent with a nearly edge-on stellar spin axis (Frazier et al., 2022, Damasso et al., 2024, Barber et al., 9 May 2025).

Age estimates evolved materially as the system was recharacterized. The discovery paper quoted $(41.9\ \mathrm{pc})$6 Myr; EXOFASTv2 in the NEID obliquity study implied $(41.9\ \mathrm{pc})$7 Gyr; Osborn et al. adopted $(41.9\ \mathrm{pc})$8 Gyr; and the GAPS characterization treated the star as $(41.9\ \mathrm{pc})$9 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 $(204\pm50\ \mathrm{Myr})$0 Myr. This suggests that the system is best regarded as an approximately $(204\pm50\ \mathrm{Myr})$1 Myr benchmark, while retaining a literature history in which ages between $(204\pm50\ \mathrm{Myr})$2 and $(204\pm50\ \mathrm{Myr})$3 Myr have been used (Barber et al., 9 May 2025).

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 $(204\pm50\ \mathrm{Myr})$4 d, $(204\pm50\ \mathrm{Myr})$5, $(204\pm50\ \mathrm{Myr})$6 AU, $(204\pm50\ \mathrm{Myr})$7 ppm, $(204\pm50\ \mathrm{Myr})$8 K, and $(204\pm50\ \mathrm{Myr})$9. A later four-planet characterization gave closely similar values, $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$0 d, $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$1 AU, $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$2, $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$3, and $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$4, with eccentricity fixed at approximately zero (Barber et al., 9 May 2025, Wang et al., 3 Mar 2026).

For the previously known outer planets, Osborn et al. improved the radii to $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$5, $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$6, and $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$7 for b, c, and d, respectively. Their periods were measured as $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$8 d, $(K = 7.115\ \mathrm{in\ TIC\ v8.1})$9 d, and $R_b=3.3\pm0.04\,R_\oplus$0 d. The GAPS transit analysis obtained $R_b=3.3\pm0.04\,R_\oplus$1 d, $R_b=3.3\pm0.04\,R_\oplus$2 d, $R_b=3.3\pm0.04\,R_\oplus$3 d; $R_b=3.3\pm0.04\,R_\oplus$4 AU, $R_b=3.3\pm0.04\,R_\oplus$5 AU, $R_b=3.3\pm0.04\,R_\oplus$6 AU; and radii $R_b=3.3\pm0.04\,R_\oplus$7, $R_b=3.3\pm0.04\,R_\oplus$8, and $R_b=3.3\pm0.04\,R_\oplus$9. The 2026 four-planet solution reported $R_c=4.4\pm0.05\,R_\oplus$0, $R_c=4.4\pm0.05\,R_\oplus$1, and $R_c=4.4\pm0.05\,R_\oplus$2, with masses $R_c=4.4\pm0.05\,R_\oplus$3, $R_c=4.4\pm0.05\,R_\oplus$4, and $R_c=4.4\pm0.05\,R_\oplus$5, and inclinations $R_c=4.4\pm0.05\,R_\oplus$6, $R_c=4.4\pm0.05\,R_\oplus$7, and $R_c=4.4\pm0.05\,R_\oplus$8 (Osborn et al., 2022, Damasso et al., 2024, Wang et al., 3 Mar 2026).

The architecture is exceptionally flat. The later four-planet analysis stated that mutual inclinations are all $R_c=4.4\pm0.05\,R_\oplus$9, and the GAPS stability study found mutual inclinations consistent with transiting geometry, $R_d=4.1\pm0.07\,R_\oplus$0. The compactness of the chain, together with the progression from a $R_d=4.1\pm0.07\,R_\oplus$1–$R_d=4.1\pm0.07\,R_\oplus$2 inner planet to $R_d=4.1\pm0.07\,R_\oplus$3–$R_d=4.1\pm0.07\,R_\oplus$4 outer planets, underpins its use for comparative studies of radius evolution, atmospheric retention, and intra-system uniformity at young ages (Damasso et al., 2024, Barber et al., 9 May 2025, Wang et al., 3 Mar 2026).

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 $R_d=4.1\pm0.07\,R_\oplus$5, 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 $R_d=4.1\pm0.07\,R_\oplus$6 d and ruling out three likely aliases for d, while LCO, SAINT-EX, and MuSCAT-3 discriminated between the remaining $R_d=4.1\pm0.07\,R_\oplus$7 d and $R_d=4.1\pm0.07\,R_\oplus$8 d solutions by obtaining a transit ingress at the $R_d=4.1\pm0.07\,R_\oplus$9 d window and a non-detection at the $P_b=10.356\ \mathrm{d}$0 d window (Osborn et al., 2022).

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 $P_b=10.356\ \mathrm{d}$1 d, corresponding to early/late deviations of $P_b=10.356\ \mathrm{d}$2–$P_b=10.356\ \mathrm{d}$3 min from a linear ephemeris, and a TTV super-period

$P_b=10.356\ \mathrm{d}$4

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 $P_b=10.356\ \mathrm{d}$5–$P_b=10.356\ \mathrm{d}$6 min TTVs among b, c, and d, with b–c near $P_b=10.356\ \mathrm{d}$7 commensurability and c–d near $P_b=10.356\ \mathrm{d}$8, again pointing to strong mutual interactions and dynamical coupling (Osborn et al., 2022, Frazier et al., 2022).

The period ratios place the outer chain just wide of exact mean-motion resonance. Osborn et al. gave $P_b=10.356\ \mathrm{d}$9, implying an offset from exact $\sim 10$00 of $\sim 10$01, and $\sim 10$02, implying an offset from exact $\sim 10$03 of $\sim 10$04. The later Hamiltonian analysis formalized this near-resonant structure by defining

$\sim 10$05

Using that framework, the b–c pair was found to have $\sim 10$06 relative to $\sim 10$07, and the c–d pair $\sim 10$08 relative to $\sim 10$09. Both pairs lie outside the true resonant libration zones: for $\sim 10$10, the proximity parameter was reported as $\sim 10$11, well below the $\sim 10$12 value needed to bifurcate into libration, and for $\sim 10$13, $\sim 10$14, likewise implying circulation. The system was therefore characterized as “near-resonant” but non-librating and dynamically fragile (Osborn et al., 2022, Wang et al., 3 Mar 2026).

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

$\sim 10$15

while the full rmfit model depends on $\sim 10$16, the sky-projected obliquity $\sim 10$17, quadratic limb darkening $\sim 10$18, and the transit geometry $\sim 10$19. 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 $\sim 10$20 during the $\sim 10$21 h window (Frazier et al., 2022).

The resulting sky-projected obliquity was $\sim 10$22 deg. Combining this with the stellar size, rotation period, and $\sim 10$23 yielded an estimate of the true obliquity via

$\sim 10$24

giving $\sim 10$25 deg and a 95% upper limit $\sim 10$26. Simultaneous diffuser-assisted $\sim 10$27-band photometry from ARCTIC on the ARC 3.5 m telescope showed the expected $\sim 10$28 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 (Frazier et al., 2022).

The interpretation advanced in the same work linked the low obliquity with the TTV-rich compact architecture. Because TOI-2076 b has large $\sim 10$29, 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 (Frazier et al., 2022).

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 10$30 yr—specifically 294 HARPS-N spectra, plus NEID and CARMENES-VIS data—and found activity-induced RV scatter larger than $\sim 10$31. 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,

$\sim 10$32

with $\sim 10$33, 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 $\sim 10$34, $\sim 10$35, and $\sim 10$36, but none reached a $\sim 10$37 mass measurement. Instead, the study reported model-averaged $\sim 10$38 upper limits of $\sim 10$39–$\sim 10$40, $\sim 10$41–$\sim 10$42, and $\sim 10$43–$\sim 10$44, with a tentative $\sim 10$45–$\sim 10$46 signal at $\sim 10$47–$\sim 10$48 (Damasso et al., 2024).

A later $\sim 10$49 Myr characterization combined photodynamical and RV information and reported masses for all four planets: $\sim 10$50 for e, $\sim 10$51 for b, $\sim 10$52 for c, and $\sim 10$53 for d. That same work described the four planets as having comparable core masses but a monotonic increase in hydrogen and helium envelope mass fractions with decreasing insolation, summarized as stripped-$\sim 10$54-$\sim 10$55-$\sim 10$56 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 (Wang et al., 3 Mar 2026).

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

$\sim 10$57

with $\sim 10$58–$\sim 10$59. It concluded that TOI-2076 b is currently losing its H-He gaseous envelope and would lose it completely by an age within $\sim 10$60–$\sim 10$61 Gyr if its current mass is lower than $\sim 10$62; TOI-2076 c could retain its atmosphere up to an age of $\sim 10$63 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

$\sim 10$64

and estimated $\sim 10$65 at age $\sim 10$66 Gyr as $\sim 10$67, $\sim 10$68, $\sim 10$69, and $\sim 10$70 for e, b, c, and d, respectively (Damasso et al., 2024, Wang et al., 3 Mar 2026).

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 $\sim 10$71 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 (Damasso et al., 2024, Wang et al., 3 Mar 2026).

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 (Osborn et al., 2022, Barber et al., 9 May 2025).