YZ Ceti d: Orbit, Minimum Mass, and What We Know

YZ Ceti d is the outermost known planet in an ultra-compact three-planet system only about 12.1 light-years from Earth. “Outermost” is relative: it circles its red-dwarf star every 4.66 days at roughly 0.0285 astronomical units, far closer than Mercury orbits the Sun.

Precision radial-velocity data indicate a minimum mass close to 1.09 Earth masses in a widely used 2020 analysis. That places YZ Ceti d in the terrestrial-mass regime, but it does not reveal the planet’s radius, density, atmosphere, or surface. The popular label “super-Earth” is therefore less informative than it sounds and, depending on the adopted mass boundary, may not even be the clearest description.

Where YZ Ceti d fits in the system

The host star YZ Ceti, also called GJ 54.1, is an M4.5 V red dwarf about 3.712 parsecs away. It has roughly 14 percent of the Sun’s mass and 16 percent of its radius. The star is accompanied by three detected planets: b at about 2.02 days, c at 3.06 days, and d at 4.66 days.

Planet d lies outside the other two, but the entire system is extraordinarily compact. Its orbit of about 0.0285 AU is only 7 percent of Mercury’s average orbital distance. If placed in our Solar System, all three known YZ Ceti planets would orbit deep inside Mercury’s path.

For an overview comparing all three worlds, read our guide to the YZ Ceti planetary system. This article concentrates on what observations specifically establish for planet d.

Discovery by a stellar wobble

YZ Ceti d was announced with planets b and c in 2017 after astronomers analyzed high-precision spectra from HARPS. The planet is not visible as a resolved dot. Its gravity pulls the star slightly toward and away from Earth, shifting stellar absorption lines through the Doppler effect.

The signal is extremely small. The star’s radial-velocity changes associated with the planets are on the order of one or two meters per second. Researchers need many observations, stable instruments, careful calibration, and statistical modeling to recover such motion.

CARMENES later contributed new data, and a 2020 reanalysis added 229 radial-velocity measurements collected after the discovery publication. The combined dataset strengthened the three-planet interpretation and resolved competing aliases for the periods.

Why the 4.66-day period was questioned

Ground-based observations are interrupted by the day-night cycle and by scheduling. Those gaps create aliases: false alternative periods that can reproduce parts of the same sampled signal. For planet d, a roughly 1.27-day alternative competed with the 4.66-day solution.

The 2020 analysis tested aliases by injecting signals at candidate frequencies into data with the actual observing schedule. Researchers compared not only the location of periodogram peaks but also their heights and phases. A 4.66-day input best reproduced the pattern associated with YZ Ceti d.

This result illustrates why published orbital values can evolve. The planet did not physically change its year between studies. Additional measurements and better sampling allowed investigators to identify which mathematical peak represented the true orbit.

Period and orbital distance

The favored period is about 4.656 days. Applying Kepler’s laws with the star’s low mass gives a semimajor axis near 0.02851 AU in the 2020 circular model. A semimajor axis describes the scale of the orbit; it is not a direct photograph of a planet-star gap.

Because the host has only about 0.142 solar masses, a short period corresponds to a very small orbit. The planet moves rapidly, and gravitational interactions with its neighboring worlds may produce subtle changes. Current radial-velocity precision does not yield a complete, unique dynamical map.

Minimum mass, not exact mass

Radial velocity measures the line-of-sight component of stellar motion. The inferred quantity is M sin i, where i is orbital inclination. For YZ Ceti d, the 2020 circular fit gives about 1.09 Earth masses, with an uncertainty around 0.12 Earth masses.

If the system is viewed nearly edge-on, the true mass is close to that value. At lower inclination, the true mass is larger because sin i is smaller. No transit fixes the geometry. Dynamical stability excludes some very low-inclination, high-mass versions of the system, yet it leaves a substantial range.

The 2017 discovery analysis reported a minimum mass around 1.14 Earth masses. The two values are compatible within differences in data and modeling. Quoting one without its study, uncertainty, and “minimum” qualifier creates false precision.

Is YZ Ceti d a super-Earth?

“Super-Earth” usually means a planet more massive than Earth but lighter than ice giants such as Neptune. Definitions vary, and the term says nothing by itself about temperature or habitability. With a minimum mass around 1.09 Earth masses, YZ Ceti d sits barely above Earth in one analysis and can reasonably be described more simply as an Earth-mass or terrestrial-mass planet.

The true mass could be higher if inclination is lower, so a super-Earth classification remains possible. But an article title should not turn a broad category into a measured composition. The planet could be rocky, iron-rich, volatile-rich, or something not represented by a Solar System twin.

Why radius and density remain unknown

A transiting planet blocks a fraction of its star’s light, revealing its size relative to the star. Combining transit radius with radial-velocity mass yields bulk density, which constrains composition. Searches in TESS photometry did not find transits from the three known YZ Ceti planets.

For planet d, the non-transiting geometry means there is no measured radius and therefore no density. Mass-radius relations can estimate a likely size if the planet is rocky, but that would assume the very composition researchers want to test.

No transit also prevents conventional transmission spectroscopy, in which starlight passes through an atmosphere during a crossing. Claims about oxygen, water vapor, carbon dioxide, clouds, or surface pressure would be speculative.

How much energy does the planet receive?

Although YZ Ceti is faint, planet d orbits close enough to receive an estimated 2.70 times the stellar energy Earth receives from the Sun. The 2020 paper calculated an equilibrium temperature around 357 kelvin under assumptions about energy redistribution and reflectivity.

Equilibrium temperature is not a weather forecast. It excludes a measured greenhouse effect, atmospheric circulation, clouds, surface heat capacity, internal heating, and local day-night differences. A bare dark rock and a reflective cloud-covered planet at the same orbit would not share one surface temperature.

The estimated irradiation places YZ Ceti d inside the conventional habitable zone rather than comfortably within it. Some optimistic climate boundaries may be framed differently, but no atmospheric evidence supports calling the planet habitable.

Red-dwarf weather matters

YZ Ceti produces flares and magnetic activity. Close-in planets can encounter ultraviolet and X-ray radiation, stellar-wind particles, and changing magnetic conditions. These processes may heat upper atmospheres and drive escape over long timescales.

Planet d receives less flux than b and c, but it remains close to the star. Its ability to retain an atmosphere would depend on initial volatile inventory, gravity, atmospheric chemistry, stellar history, magnetic environment, and replenishment through geology or impacts.

A magnetic field can redirect charged particles, yet it is not a universal shield. Field geometry may channel energy toward polar regions, and extreme ultraviolet photons are not blocked by magnetism. No field has been measured for YZ Ceti d.

Could the planet be tidally locked?

Tidal forces grow rapidly at small orbital distance. Over time they can synchronize a planet’s rotation with its orbit, leaving the same side generally facing the star. For a mature planet on a 4.66-day orbit, synchronous or near-synchronous rotation is a plausible expectation.

It is not directly observed. Eccentricity, internal structure, neighboring planets, and spin-orbit resonances influence the final state. Even a synchronously rotating planet would not necessarily have an airless frozen nightside: a sufficiently dense atmosphere or ocean could move heat. For YZ Ceti d, those heat-transport ingredients are unknown.

Orbital stability and possible resonance

The planets’ periods form a compact chain. Numerical integrations tested solutions over one million orbits of planet b and found that many unconstrained fits were unstable. Stability filtering narrowed acceptable inclinations, masses, and eccentricities.

Some stable configurations showed a three-body Laplace angle oscillating rather than circulating, a possible signature of resonant structure. Such relationships can preserve clues about migration through the original gas disk. The precise resonant state is not fully settled, and stronger interaction measurements would be valuable.

Planet d helps anchor the outer end of that chain. Small changes to its mass or eccentricity influence the stability of all three worlds, which is why a planet-specific orbit contributes to understanding the whole system.

Why direct imaging is unrealistic today

At 0.0285 AU and 3.712 parsecs distance, the maximum angular separation is only a few thousandths of an arcsecond. The planet is also enormously fainter than the star. Current direct-imaging surveys generally target young, self-luminous giant planets at much wider separations.

Future interferometers or space concepts may eventually probe close terrestrial planets around nearby stars, but no existing image shows YZ Ceti d. Illustrations of the planet are artist interpretations based on orbital and stellar data, not photographs.

What observations would improve the picture?

• More radial-velocity measurements could refine the period, minimum mass, and eccentricity.
• Activity indicators collected simultaneously can reduce confusion with the star’s magnetic behavior.
• Long-baseline dynamical analysis could test resonant configurations and mutual interactions.
• High-precision astrometry might eventually constrain system inclination, although the expected signal is minute.
• Deep photometric monitoring can continue to check for rare or previously missed crossing geometries, while current data do not support a transit.
• Radio and spectropolarimetric observations can map the star’s environment, even if they do not directly detect planet d.

Frequently asked questions

How far away is YZ Ceti d?

The system is about 3.712 parsecs, or 12.1 light-years, from the Sun.

How long is a year on YZ Ceti d?

About 4.66 Earth days.

Is it the same size as Earth?

Unknown. Its minimum mass is close to Earth’s, but it does not transit and has no measured radius.

Could humans live there?

There is no evidence for a suitable surface or atmosphere. High irradiation, close-orbit tidal effects, and red-dwarf activity make it a poor target for confident habitability claims.

Is YZ Ceti d the coldest planet in the system?

It receives the least stellar energy of the three known planets, so standard models give it the lowest equilibrium temperature. Its actual climate is unknown.

Bottom line

YZ Ceti d is a well-supported radial-velocity planet with a 4.66-day orbit and an Earth-scale minimum mass. It is the outermost known member of a compact nearby system, but still receives roughly 2.7 times Earth’s stellar flux. Without a transit, astronomers cannot measure its radius, density, or atmosphere. Its value lies in what precision spectroscopy and dynamics can reveal—not in unsupported claims that it is an Earth twin.

Sources

• Astronomy & Astrophysics (2020): CARMENES characterization
• Astronomy & Astrophysics (2017): Original planet discovery
• NASA Exoplanet Archive: YZ Ceti overview

Evidence reviewed: October 16, 2026.