HD 60779 b: Long-Period Sub-Neptune

• HD 60779 b is a transiting sub-Neptune exoplanet orbiting a nearby bright G1 V star, characterized by a ~30-day period, 14.7 M⊕ mass, and 3.25 R⊕ radius.
• It was characterized by combining TESS and CHEOPS transit photometry with HARPS‐N radial velocities, revealing a near-circular orbit and grazing transit geometry along with an outer companion.
• The host star's high systemic radial velocity shifts its Lyα emission, making the system exceptionally favorable for atmospheric escape studies and detailed spectroscopic follow-up.

Searching arXiv for the specified paper to ground the article in the cited source. HD 60779 b is a transiting sub-Neptune orbiting the bright, nearby G1 V star HD 60779, also known as GJ 9237 and TIC 65672998. The host star lies about $35.393^{+0.020}_{-0.022}$ pc away, has $M_\star = 1.050 \pm 0.044\,M_\odot$, $R_\star = 1.129 \pm 0.013\,R_\odot$, and $V \approx 7.2$. In the final joint fit to HARPS-N radial velocities together with TESS and CHEOPS transit photometry, HD 60779 b has an orbital period of $29.986175^{+0.000030}_{-0.000033}$ d, a radius of $3.250^{+0.100}_{-0.098}\,R_\oplus$, and a mass of $14.7^{+1.1}_{-1.0}\,M_\oplus$, making it a long-period sub-Neptune with well-constrained bulk properties. The system is especially notable because the star’s unusually high systemic radial velocity, $129.75 \pm 0.12$ km s$^{-1}$, makes the planet a particularly favorable target for Ly$\alpha$ transit spectroscopy and atmospheric escape studies (DiTomasso et al., 22 Aug 2025).

1. Discovery and observational identification

HD 60779 b first came to attention through a single TESS transit near the end of Sector 34, centered at BJD $M_\star = 1.050 \pm 0.044\,M_\odot$0. The transit depth implied a sub-Neptune-sized object, and the fact that only a single event was initially seen already required a period longer than about 25 days. A second TESS transit was later captured in Sector 88, and CHEOPS independently observed a transit as well, providing the follow-up needed to pin down the ephemeris and refine the system parameters (DiTomasso et al., 22 Aug 2025).

The host star’s brightness and proximity are central to the planet’s detectability and follow-up value. The system was further characterized with 286 HARPS-N radial velocity measurements, which both measured the mass of the transiting planet and revealed the presence of an additional outer companion. This observational sequence—initial sparse transit identification, photometric recovery, and dense radial-velocity follow-up—was decisive because the original single-transit detection did not by itself determine the orbital period uniquely.

2. Host star and system architecture

HD 60779 is described as a bright, nearby, Sun-like star. The paper identifies it as a G1 V star and gives a distance of about 35.393 pc, a stellar mass of $M_\star = 1.050 \pm 0.044\,M_\odot$1, and a stellar radius of $M_\star = 1.050 \pm 0.044\,M_\odot$2. The brightness, $M_\star = 1.050 \pm 0.044\,M_\odot$3, places the system among the more accessible nearby hosts of long-period small planets (DiTomasso et al., 22 Aug 2025).

The planetary system contains at least two planets. HD 60779 b is the transiting inner planet. The outer companion, HD 60779 c, has an orbital period of $M_\star = 1.050 \pm 0.044\,M_\odot$4 days and a minimum mass $M_\star = 1.050 \pm 0.044\,M_\odot$5. The data are not sufficient to determine whether planet c transits. This architecture is important for interpretation of the inner planet because the dynamical state of the full system informs the discussion of eccentricity and long-term orbital evolution.

3. Orbital solution and transit geometry

In the adopted joint solution, HD 60779 b has $M_\star = 1.050 \pm 0.044\,M_\odot$6 d and a transit midpoint $M_\star = 1.050 \pm 0.044\,M_\odot$7 (BJD$M_\star = 1.050 \pm 0.044\,M_\odot$8) of $M_\star = 1.050 \pm 0.044\,M_\odot$9. The semimajor axis is $R_\star = 1.129 \pm 0.013\,R_\odot$0 AU. The transit solution yields $R_\star = 1.129 \pm 0.013\,R_\odot$1, inclination $R_\star = 1.129 \pm 0.013\,R_\odot$2 deg, impact parameter $R_\star = 1.129 \pm 0.013\,R_\odot$3, and transit duration $R_\star = 1.129 \pm 0.013\,R_\odot$4 hr (DiTomasso et al., 22 Aug 2025).

The orbit is consistent with being circular. In the paper’s parameterization, $R_\star = 1.129 \pm 0.013\,R_\odot$5 and $R_\star = 1.129 \pm 0.013\,R_\odot$6, summarized as $R_\star = 1.129 \pm 0.013\,R_\odot$7 at $R_\star = 1.129 \pm 0.013\,R_\odot$8. This is an important distinction: the result does not establish a strictly zero eccentricity orbit, but rather shows that the data are compatible with a low-eccentricity configuration. The large impact parameter indicates a grazing-adjacent transit geometry, which helps explain why multiple well-timed photometric datasets were valuable for refining the ephemeris and transit shape.

4. Mass, radius, density, and internal composition

The measured RV semi-amplitude is $R_\star = 1.129 \pm 0.013\,R_\odot$9 m s$V \approx 7.2$0, corresponding to a planetary mass of $V \approx 7.2$1. Combined with the transit-derived radius of $V \approx 7.2$2, this places HD 60779 b securely in the sub-Neptune regime. The derived bulk density is $V \approx 7.2$3 g cm$V \approx 7.2$4, and the equilibrium temperature is $V \approx 7.2$5 K (DiTomasso et al., 22 Aug 2025).

The authors note that the density is consistent with either a water-rich world or a rocky core with a small H$V \approx 7.2$6 envelope. More specifically, composition models from Zeng et al. are described as compatible with either a 100% H$V \approx 7.2$7O composition or a rocky core plus a $V \approx 7.2$82% H$V \approx 7.2$9 envelope. This is a compositional degeneracy rather than a definitive interior determination. A plausible implication is that the measured bulk properties constrain the planet to a relatively restricted region of mass–radius space while still permitting materially different internal structures.

5. Dynamical state and evolutionary interpretation

Both planets’ orbits are reported as consistent with being circular. For HD 60779 b, the eccentricity constraint is $29.986175^{+0.000030}_{-0.000033}$0 at $29.986175^{+0.000030}_{-0.000033}$1; the outer planet is similarly described as having low eccentricity. The paper interprets this jointly as evidence for a dynamically quiet history and very long circularization timescales (DiTomasso et al., 22 Aug 2025).

This interpretation should be read as an inference from the fitted orbital architecture rather than as a direct observation of past evolution. The available measurements indicate an absence of strong present-day dynamical excitation, but they do not uniquely determine the full formation pathway. What is firmly established is that the currently observed configuration does not require substantial eccentricity pumping or recent disruptive dynamical interactions.

6. Spectroscopic accessibility and atmospheric follow-up

HD 60779 b is especially notable for follow-up prospects. The host star’s systemic radial velocity of $29.986175^{+0.000030}_{-0.000033}$2 km s$29.986175^{+0.000030}_{-0.000033}$3 Doppler-shifts stellar Ly$29.986175^{+0.000030}_{-0.000033}$4 emission out of much of the interstellar absorption band, making the system a prime target for Ly$29.986175^{+0.000030}_{-0.000033}$5 transit spectroscopy and atmospheric escape studies. This property is unusual and is one of the principal reasons the planet stands out among long-period sub-Neptunes with measured masses and radii (DiTomasso et al., 22 Aug 2025).

The paper quotes a transmission spectroscopy metric of $29.986175^{+0.000030}_{-0.000033}$6 and an emission spectroscopy metric of 19, both described as unusually favorable for a long-period sub-Neptune. The authors also estimate a secondary-eclipse probability of at least 97% and note that the planet is a promising Rossiter–McLaughlin target, with an expected amplitude of about 1.3 m s$29.986175^{+0.000030}_{-0.000033}$7 if aligned. These values collectively indicate that the system is unusually tractable for atmospheric and spin-orbit characterization despite the planet’s relatively long orbital period.

7. Comparative significance among long-period sub-Neptunes

The authors emphasize two closely related comparative statements. They describe HD 60779 as the brightest known host of a $29.986175^{+0.000030}_{-0.000033}$8 d sub-Neptune with measured mass and radius, and they also state that HD 60779 b is the third-brightest host of a long-period sub-Neptune with measured bulk properties (DiTomasso et al., 22 Aug 2025).

These rankings matter because long-period sub-Neptunes with both measured mass and radius remain observationally demanding: long periods reduce transit opportunities, while low RV amplitudes complicate mass determination. In that context, HD 60779 b occupies a distinctive position as a bright-host, long-period sub-Neptune with a nearly circular orbit, a measured density, and unusually favorable leverage for transmission, emission, Ly$29.986175^{+0.000030}_{-0.000033}$9, and Rossiter–McLaughlin follow-up. The combination of $3.250^{+0.100}_{-0.098}\,R_\oplus$0 d, $3.250^{+0.100}_{-0.098}\,R_\oplus$1, and $3.250^{+0.100}_{-0.098}\,R_\oplus$2 makes it a comparatively well-characterized example of this observationally sparse class.