HD 20794 d spends its long year moving between two very different neighborhoods around its star. At one point in its orbit, it reaches a distance compared with Mars in our solar system. At another, it moves inward to a distance compared with Venus. That strange path is why a nearby planet in the right zone for liquid water may still be a difficult place to imagine as life-friendly.
An international team confirmed the planet in a study published in Astronomy & Astrophysics on January 28, 2025. The work used more than 20 years of observations of HD 20794, a nearby Sun-like star about 20 light-years from Earth. The system now has three confirmed planets, but the outer one matters most because it crosses the region astronomers search when looking for possible liquid water.
That region is called the habitable zone, but HD 20794 d shows why the label can mislead. The term describes distance from a star, not the full reality of a planet. A world also needs the right surface, atmosphere, pressure, chemistry, and climate for liquid water to matter. For HD 20794 d, even the heat it receives may change sharply during one orbit.
A Nearby Planet With an Awkward Orbit
HD 20794 is bright enough to be seen without a telescope, which makes it useful for exoplanet studies. A bright, nearby star gives astronomers a better chance of measuring tiny signals and may make the system more attractive for future telescope missions. As Scientific American reported, the star lies in the constellation Eridanus.
The three confirmed planets orbit at very different speeds. HD 20794 b circles the star in about 18.3 days, HD 20794 c takes about 89.7 days, and HD 20794 d has a 647-day orbit. The outer planet is the odd one because its orbit is highly stretched rather than nearly circular. It does not sit calmly in one part of the habitable zone. It moves through it.
That motion is the real story. When HD 20794 d is far from its star, any water on or near the surface could face colder conditions. When it swings inward, the same world could receive much stronger heating. Scientific American described the planet as a possible world of fire and ice because its orbit may expose it to large swings in stellar energy.
A Planet Found by a Barely Visible Wobble
No telescope has taken a picture of HD 20794 d. Dr. Michael Cretignier first noticed a candidate signal in 2022 while analyzing archived data from HARPS, the High Accuracy Radial Velocity Planet Searcher at La Silla Observatory in Chile. The planet was inferred from tiny shifts in the star’s light, not from a visible dot beside the star.
The method is called radial velocity. A planet’s gravity tugs on its star, making the star wobble slightly toward and away from Earth. That motion changes the star’s spectrum by a tiny amount. For HD 20794 d, the signal was less than one meter per second and repeated over nearly two Earth years, which made it easy to confuse with stellar activity or instrument noise.
To test the signal, the team combined long-term observations from HARPS and ESPRESSO, another precise spectrograph in Chile. The University of Oxford announcement republished by Astrobiology said researchers spent years analyzing the data and ruling out possible contamination before confirming the planet.
Scientific American reported that the YARARA algorithm, led by Cretignier, helped separate planet signals from stellar and instrumental noise. The final analysis supported HD 20794 d, strengthened the case for the two inner planets, and weakened the evidence for another previously proposed companion.
Super-earth Describes Mass, Not Habitability
HD 20794 d is often called a super-Earth, but that phrase can create the wrong impression. It means the planet is more massive than Earth, not that it is Earth-like. The study and related reports place its mass at about six to 6.5 times Earth’s mass. That measurement does not reveal whether the planet has continents, oceans, clouds, a thin atmosphere, or a deep envelope of gas.
Radial velocity measurements give astronomers a minimum mass, but not the planet’s radius or density. Without those details, HD 20794 d could be a large rocky world, a water-rich planet, or something closer to a mini-Neptune. Renyu Hu of NASA’s Jet Propulsion Laboratory, who was not involved in the study, told Scientific American that there is not yet proof the planet is rocky.
The eccentric orbit makes every possibility harder to judge. If the planet has water, changing distance from the star could affect whether that water freezes, remains liquid, or turns to vapor during different parts of the year. Xavier Dumusque, an astronomer at the University of Geneva and a study co-author, suggested that deep oceans could complicate that picture because pressure can affect how water behaves.
Future Telescopes May Test the Target
HD 20794 d is compelling not because it is known to be habitable, but because it is close enough and strange enough to test future tools. At about 20 light-years away, it may be reachable for missions that aim to separate faint planet light from bright starlight and examine nearby worlds more directly. Cretignier said its proximity gives researchers hope that future space missions may be able to obtain an image of it.
Those missions include the Extremely Large Telescope, NASA’s planned Habitable Worlds Observatory, and the proposed Large Interferometer For Exoplanets, known as LIFE. Their goals include studying nearby Earth-like planets, characterizing atmospheres, and searching for biosignatures, chemical signs that could point to possible life. Scientific American reported that NASA and ESA have already considered HD 20794 for possible future target lists.
That is the useful lesson of the discovery. The planet is nearby, detectable, and located where liquid water might be possible, yet almost every detail that would make it truly Earth-like remains unresolved. For now, HD 20794 d is confirmed as a nearby planet of at least six Earth masses, orbiting a Sun-like star about 20 light-years away on an eccentric path that crosses the star’s habitable zone.