HD 60779 c Exoplanet Overview

• HD 60779 c is an outer planet in the HD 60779 system, detected via precise HARPS-N radial-velocity measurements and characterized by a 104-day orbit with a minimum mass of 27.7 Earth masses.
• The orbital solution shows a nearly circular orbit (e < 0.067) with a semimajor axis of 0.44 AU, indicating strong dynamical stability in a non-resonant two-planet system.
• Despite targeted TESS and CHEOPS photometry, non-detection of transits underscores observational challenges, while the system remains promising for comparative atmospheric studies.

HD 60779 c is the outer planet in the HD 60779 planetary system, a system orbiting the bright ($V = 7.2$), nearby ($35\ \mathrm{pc}$), Sun-like star HD 60779. It was identified through precision radial-velocity measurements and is characterized by an orbital period of $104.25^{+0.30}_{-0.29}$ days, a minimum mass $m \sin i = 27.7 \pm 1.6\ M_\oplus$, and a semimajor axis of $0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$. The orbit is consistent with circularity, with a $1\sigma$ upper limit $e < 0.067$, and the available data are not sufficient to determine whether the planet transits (DiTomasso et al., 22 Aug 2025).

1. Discovery and evidentiary basis

HD 60779 c was detected in radial velocities obtained with HARPS-N on the 3.6 m TNG. The observing campaign acquired 299 spectra between May 2021 and April 2025; 12 early points and one outlier were excluded, leaving 286 radial-velocity measurements with a median internal uncertainty of $0.92\ \mathrm{m\ s^{-1}}$. The adopted final fit simultaneously modeled two Keplerians—HD 60779 b as a transiting inner planet and HD 60779 c as a non-transiting outer planet—plus a third-order polynomial radial-velocity trend, chosen as the most agnostic way to absorb a long-term signal whose origin is uncertain. The fit was performed with PyORBIT and cross-checked with SCALPELS+kima; both analyses robustly detect the $\sim 30$-day and $\sim 105$-day planets, while the $35\ \mathrm{pc}$0-day activity signal is weak (DiTomasso et al., 22 Aug 2025).

This detection context is important because HD 60779 c is not established from transit photometry but from the coherence and repeatability of the radial-velocity signal. The combination of a large dataset, sub-$35\ \mathrm{pc}$1-class internal uncertainties, and agreement between two analysis pipelines makes the planet’s identification substantially more secure than a single-method or sparsely sampled detection. The retained polynomial trend also indicates that the system may contain an additional long-timescale component or another source of low-frequency structure that remains unresolved.

2. Orbital solution and measured parameters

The fitted orbital solution for HD 60779 c yields the following reported parameters:

Parameter	Value	Note
Orbital period $35\ \mathrm{pc}$2	$35\ \mathrm{pc}$3 days	Radial-velocity fit
Time of conjunction $35\ \mathrm{pc}$4	$35\ \mathrm{pc}$5	$35\ \mathrm{pc}$6
RV semi-amplitude $35\ \mathrm{pc}$7	$35\ \mathrm{pc}$8	Radial-velocity fit
Minimum mass $35\ \mathrm{pc}$9	$104.25^{+0.30}_{-0.29}$0	Derived from RVs
Semimajor axis $104.25^{+0.30}_{-0.29}$1	$104.25^{+0.30}_{-0.29}$2	Derived from $104.25^{+0.30}_{-0.29}$3 and $104.25^{+0.30}_{-0.29}$4
Eccentricity constraint	$104.25^{+0.30}_{-0.29}$5	$104.25^{+0.30}_{-0.29}$6; orbit consistent with circular

The eccentricity parameterization was reported as $104.25^{+0.30}_{-0.29}$7 and $104.25^{+0.30}_{-0.29}$8. No significant eccentricity was detected, and the orbit is therefore treated as consistent with circularity (DiTomasso et al., 22 Aug 2025).

The semimajor axis follows from Kepler’s third law,

$104.25^{+0.30}_{-0.29}$9

with the approximation in astronomical units

$m \sin i = 27.7 \pm 1.6\ M_\oplus$0

Using $m \sin i = 27.7 \pm 1.6\ M_\oplus$1 and $m \sin i = 27.7 \pm 1.6\ M_\oplus$2 days $m \sin i = 27.7 \pm 1.6\ M_\oplus$3 yr gives $m \sin i = 27.7 \pm 1.6\ M_\oplus$4, matching the reported value. The uncertainty propagation $m \sin i = 27.7 \pm 1.6\ M_\oplus$5 gives approximately $m \sin i = 27.7 \pm 1.6\ M_\oplus$6, consistent with the quoted $m \sin i = 27.7 \pm 1.6\ M_\oplus$7.

The reported radial-velocity semi-amplitude is likewise internally consistent with the minimum mass. For $m \sin i = 27.7 \pm 1.6\ M_\oplus$8 and $m \sin i = 27.7 \pm 1.6\ M_\oplus$9,

$0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$0

or, in the simplified estimate,

$0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$1

Substituting $0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$2, $0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$3, and $0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$4 gives $0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$5, in excellent agreement with the fitted $0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$6.

3. Transit status and geometric constraints

HD 60779 c is not seen to transit, and the available data are not sufficient to determine whether it does. Existing TESS and CHEOPS photometry do not overlap the predicted transit times for c given the radial-velocity ephemeris, so current photometry cannot rule transits in or out (DiTomasso et al., 22 Aug 2025).

For a circular orbit, the geometric transit probability is approximated by

$0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$7

Using $0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$8, $0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}$9, and $1\sigma$0,

$1\sigma$1

The uncertainty is small, at the few $1\sigma$2 level, and is dominated by $1\sigma$3 and $1\sigma$4.

The inclination geometry relative to the inner planet is especially relevant. If HD 60779 c shared the inclination of HD 60779 b, $1\sigma$5, then c would not transit because b itself has a high impact parameter, $1\sigma$6. However, a modest inclination difference could make c transit. This suggests that non-detection in the current data is not equivalent to a strong geometric exclusion; rather, the principal limitation is that the extant photometric coverage misses the predicted transit windows.

4. Physical characterization and thermal environment

The measured $1\sigma$7 places HD 60779 c in the Neptune to super-Neptune mass range, with Neptune being approximately $1\sigma$8 (DiTomasso et al., 22 Aug 2025). Because the planet is not confirmed to transit, its true radius and mean density remain unconstrained. The reported physical interpretation therefore remains necessarily non-unique: a super-Neptune minimum mass is consistent with a volatile-rich planet, specifically an H/He envelope atop a rocky/icy core, or with a water-rich world. Without a measured radius, plausible radii span approximately $1\sigma$9–$e < 0.067$0, depending on composition and envelope fraction.

The equilibrium temperature estimate uses the stellar effective temperature $e < 0.067$1 and the relation

$e < 0.067$2

for Bond albedo $e < 0.067$3 and a circular orbit. With $e < 0.067$4, one obtains $e < 0.067$5, giving

$e < 0.067$6

For a representative Neptune-like $e < 0.067$7, the estimate becomes

$e < 0.067$8

More generally,

$e < 0.067$9

The uncertainty is at the $0.92\ \mathrm{m\ s^{-1}}$0–$0.92\ \mathrm{m\ s^{-1}}$1 level from $0.92\ \mathrm{m\ s^{-1}}$2 and $0.92\ \mathrm{m\ s^{-1}}$3.

These values locate HD 60779 c in a moderate-irradiation regime compared with short-period sub-Neptunes. A plausible implication is that its atmospheric properties, if eventually constrained by a transit detection or other means, would provide a useful contrast to the inner transiting planet b, which is the obvious target for atmospheric escape studies in the same system.

5. System architecture and dynamical state

The HD 60779 system contains an inner transiting sub-Neptune, HD 60779 b, on a $0.92\ \mathrm{m\ s^{-1}}$4-day orbit, and the outer planet HD 60779 c on a $0.92\ \mathrm{m\ s^{-1}}$5-day orbit. Their period ratio is

$0.92\ \mathrm{m\ s^{-1}}$6

This is near but not commensurate with a low-order resonance, close to $0.92\ \mathrm{m\ s^{-1}}$7, and no resonant behavior is reported (DiTomasso et al., 22 Aug 2025).

Both planets have orbits consistent with being circular, with $0.92\ \mathrm{m\ s^{-1}}$8 upper limits $0.92\ \mathrm{m\ s^{-1}}$9 and $\sim 30$0. The system is therefore interpreted as likely having experienced a dynamically quiet history, consistent with the very long tidal circularization timescales reported for the system.

The separation can be quantified through the mutual Hill radius,

$\sim 30$1

With $\sim 30$2, $\sim 30$3, $\sim 30$4, $\sim 30$5, and $\sim 30$6, the reported numerical evaluation gives $\sim 30$7 and $\sim 30$8. The interpretation attached to this result is explicit: $\sim 30$9 indicates strong long-term dynamical stability and very weak mutual perturbations.

These dynamical indicators together support a picture of a non-resonant, widely spaced, and nearly circular two-planet architecture. This suggests that any secular interactions are likely to be weak at the presently inferred masses and separations.

6. Observational prospects and broader significance

Continued HARPS-N monitoring, together with complementary facilities, can further tighten the constraints on $\sim 105$0 and $\sim 105$1, test for subtle eccentricity or apsidal precession, and clarify the origin of the long-term radial-velocity trend, whether that trend arises from an outer planet, stellar activity, or instrumental offsets. Gaia DR4 astrometry may help assess the possibility of an additional distant companion (DiTomasso et al., 22 Aug 2025).

Transit searches remain observationally attractive despite the low geometric probability. Targeted photometric campaigns, including CHEOPS or ground-based facilities, can be scheduled around the radial-velocity-predicted conjunction windows. The required observing window is set by the present ephemeris uncertainty, namely $\sim 105$2 days on $\sim 105$3 and $\sim 105$4 days on the period. Because existing TESS and CHEOPS photometry do not overlap the predicted transit times, the null result to date is a coverage limitation rather than a decisive nondetection.

The system’s broader observational value arises partly from the host star rather than from c alone. HD 60779 has an unusually high systemic radial velocity, $\sim 105$5, which allows its Lyman-$\sim 105$6 emission to avoid absorption by the interstellar medium. This makes the system a prime candidate for probing atmospheric escape from the inner transiting planet b. Although HD 60779 c itself has unknown transit status, the brightness and kinematics of the host make the system a valuable laboratory for comparative atmospheric and dynamical studies if additional transits or planets are found.

In that sense, HD 60779 c occupies a specific role within the system: it is a well-separated outer companion with a securely measured minimum mass and orbit, but with radius, density, and true inclination still unresolved. The present data establish its dynamical existence and basic architecture; future observations determine whether it can also become a directly characterized world.