---
title: HD 60779 c Exoplanet Overview
url: https://www.emergentmind.com/topics/hd-60779-c
type: topic
---

# HD 60779 c Exoplanet Overview

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 [2508.16805].

## 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 \(\sim 20\)-day activity signal is weak [2508.16805].

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-\(\mathrm{m\ s^{-1}}\)-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 \(P_c\) | \(104.25^{+0.30}_{-0.29}\) days | Radial-velocity fit |
| Time of conjunction \(T_c\) | \(9272.6 \pm 2.4\) | \(\mathrm{BJD} - 2{,}450{,}000\) |
| RV semi-amplitude \(K_c\) | \(3.66 \pm 0.19\ \mathrm{m\ s^{-1}}\) | Radial-velocity fit |
| Minimum mass \(m_c \sin i\) | \(27.7 \pm 1.6\ M_\oplus\) | Derived from RVs |
| Semimajor axis \(a_c\) | \(0.4406^{+0.0061}_{-0.0063}\ \mathrm{AU}\) | Derived from \(P_c\) and \(M_*\) |
| Eccentricity constraint | \(e_c < 0.067\) | \(1\sigma\); orbit consistent with circular |

The eccentricity parameterization was reported as \(\sqrt{e}\sin\omega = 0.11^{+0.14}_{-0.17}\) and \(\sqrt{e}\cos\omega = -0.05^{+0.16}_{-0.15}\). No significant eccentricity was detected, and the orbit is therefore treated as consistent with circularity [2508.16805].

The semimajor axis follows from Kepler’s third law,
$$
a = \left(\frac{G M_* P^2}{4\pi^2}\right)^{1/3},
$$
with the approximation in astronomical units
$$
\frac{a}{\mathrm{AU}} \approx \left(\frac{M_*}{M_\odot}\right)^{1/3}\left(\frac{P}{1\,\mathrm{yr}}\right)^{2/3}.
$$
Using \(M_* = 1.050 \pm 0.044\ M_\odot\) and \(P_c = 104.25^{+0.30}_{-0.29}\) days \(= 0.2856\) yr gives \(a/\mathrm{AU} \approx 1.016 \times 0.434 \approx 0.441\), matching the reported value. The uncertainty propagation \(\delta a/a \approx (1/3)\delta M/M + (2/3)\delta P/P\) gives approximately \(1.6\%\), consistent with the quoted \(\pm 0.006\ \mathrm{AU}\).

The reported radial-velocity semi-amplitude is likewise internally consistent with the minimum mass. For \(e \approx 0\) and \(m_p \ll M_*\),
$$
K \approx \left(\frac{2\pi G}{P}\right)^{1/3}\frac{m_p \sin i}{(M_*+m_p)^{2/3}}\frac{1}{\sqrt{1-e^2}},
$$
or, in the simplified estimate,
$$
K \approx 28.4\,\mathrm{m\,s^{-1}}
\left(\frac{m_p \sin i}{M_{\mathrm{Jup}}}\right)
\left(\frac{P}{1\,\mathrm{yr}}\right)^{-1/3}
\left(\frac{M_*}{M_\odot}\right)^{-2/3}.
$$
Substituting \(m_c \sin i = 27.7\ M_\oplus = 0.0872\ M_{\mathrm{Jup}}\), \(P/yr = 0.2856\), and \(M_*/M_\odot = 1.050\) gives \(K \approx 3.6\ \mathrm{m\ s^{-1}}\), in excellent agreement with the fitted \(K_c = 3.66 \pm 0.19\ \mathrm{m\ s^{-1}}\).

## 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 [2508.16805].

For a circular orbit, the geometric transit probability is approximated by
$$
p_{\mathrm{tr}} \approx \frac{R_*}{a}.
$$
Using \(R_* = 1.129 \pm 0.013\ R_\odot\), \(a_c = 0.4406\ \mathrm{AU}\), and \(R_\odot/\mathrm{AU} \approx 0.004650\),
$$
p_{\mathrm{tr}} \approx \frac{1.129 \times 0.004650}{0.4406} \approx 0.0119 \approx 1.2\%.
$$
The uncertainty is small, at the few \(\times 10^{-4}\) level, and is dominated by \(R_*\) and \(a\).

The inclination geometry relative to the inner planet is especially relevant. If HD 60779 c shared the inclination of HD 60779 b, \(i_b = 88.566^\circ\), then c would not transit because b itself has a high impact parameter, \(b \approx 0.923\). 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 \(m_c \sin i = 27.7 \pm 1.6\ M_\oplus\) places HD 60779 c in the Neptune to super-Neptune mass range, with Neptune being approximately \(17\ M_\oplus\) [2508.16805]. 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 \(3\)–\(5\ R_\oplus\), depending on composition and envelope fraction.

The equilibrium temperature estimate uses the stellar effective temperature \(T_* = 6081 \pm 60\ \mathrm{K}\) and the relation
$$
T_{\mathrm{eq}} = T_* \left(\frac{R_*}{2a}\right)^{1/2}(1-A_B)^{1/4},
$$
for Bond albedo \(A_B\) and a circular orbit. With \(R_*/a = (1.129\ R_\odot)/(0.4406\ \mathrm{AU}) = 0.0119\), one obtains \(\sqrt{R_*/(2a)} \approx 0.0771\), giving
$$
T_{\mathrm{eq}}(A_B=0) \approx 6081 \times 0.0771 \approx 469\ \mathrm{K}.
$$
For a representative Neptune-like \(A_B \approx 0.3\), the estimate becomes
$$
T_{\mathrm{eq}} \approx 469 \times (0.7)^{1/4} \approx 429\ \mathrm{K}.
$$
More generally,
$$
T_{\mathrm{eq}} \approx 469\ \mathrm{K}\times (1-A_B)^{1/4}.
$$
The uncertainty is at the \(\sim 1\)–\(2\%\) level from \(R_*/a\) and \(T_*\).

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 \(29.986175^{+0.000030}_{-0.000033}\)-day orbit, and the outer planet HD 60779 c on a \(104.25^{+0.30}_{-0.29}\)-day orbit. Their period ratio is
$$
\frac{P_c}{P_b} = \frac{104.25}{29.986175} \approx 3.48.
$$
This is near but not commensurate with a low-order resonance, close to \(7{:}2 \approx 3.5\), and no resonant behavior is reported [2508.16805].

Both planets have orbits consistent with being circular, with \(1\sigma\) upper limits \(e_b < 0.053\) and \(e_c < 0.067\). 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,
$$
R_{\mathrm{H,mut}}=
\left(\frac{m_b+m_c}{3M_*}\right)^{1/3}\frac{a_b+a_c}{2},
\qquad
\Delta = \frac{a_c-a_b}{R_{\mathrm{H,mut}}}.
$$
With \(m_b = 14.7\ M_\oplus\), \(m_c \sin i = 27.7\ M_\oplus\), \(M_* = 1.050\ M_\odot\), \(a_b = 0.1920\ \mathrm{AU}\), and \(a_c = 0.4406\ \mathrm{AU}\), the reported numerical evaluation gives
\(R_{\mathrm{H,mut}} \approx 0.0109\ \mathrm{AU}\) and \(\Delta \approx 22.8\).
The interpretation attached to this result is explicit: \(\Delta \gg 10\) 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 \(K_c\) and \(e_c\), 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 [2508.16805].

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 \(\pm 2.4\) days on \(T_c\) and \(\pm 0.30/-0.29\) 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, \(129.75 \pm 0.12\ \mathrm{km\ s^{-1}}\), which allows its Lyman-\(\alpha\) 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.

Source: https://www.emergentmind.com/topics/hd-60779-c