Of the more than 5,000 exoplanets now in astronomers’ catalogs, the most intriguing yet might be one confirmed last week, a potentially habitable world of fire and ice twirling around a nearby sunlike star.
Dubbed HD 20794 d (because HD 20794 is the name of its star and it is accompanied by two previously announced planetary siblings, HD 20794 b and HD 20794 c), this planet is at least as massive as 6.5 Earths. This could make it a relatively Earth-like world of mostly rock with a thin atmosphere—a so-called super-Earth—although it could instead be more of a mini-Neptune with thick layers of gas or a deep global ocean surmounting a solid core. Besides its uncertain nature, HD 20794 d’s most “intriguing” aspect is its distinctly noncircular 647-day orbit, which at one end reaches frigidly farther out from its star than Mars does from the sun and, at its other end, is as scorchingly close as Venus. This eccentric orbital path traverses the star’s habitable zone, the region in which liquid water might persist and allow life to arise, although the planet’s cycles of hot and cold could at turns boil water into steam or freeze it as ice.
And this strange new world is tantalizingly close: At scarcely 20 light-years from Earth, it’s within reach of deeper, more direct scrutiny by future space telescopes, beckoning astronomers to look closer. Someday, in order to pin down its true planetary form, researchers could scour snapshots of the world to look for evidence of a volatile climate from its peculiar orbit and clarify whether it’s actually habitable—or even inhabited.
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Located in the constellation Eridanus, HD 20794 shines bright enough in Earth’s skies to be visible to the unaided eye—a relative rarity among known exoplanet-hosting stars. That brightness has placed it high on planet-hunters’ priority lists, leading multiple teams to closely monitor the star, albeit with mixed results. Planetary candidates have come and gone around the star, with initially promising possibilities ultimately proving to be false alarms.
That’s because the hunt has focused on precisely measuring HD 20794’s motions in the sky in order to look for periodic wobbles induced by unseen worlds tugging the star to and fro as they orbit. The smaller the planet and the wider its orbit, the more subtle its associated wobble will be—HD 20794 d, for instance, pulls the star askew by less than a meter per second, in a wobble that takes nearly two Earth years to recur. Observing such slight effects—let alone confirming that they come from planets, rather than the star’s tempests and eruptions or glitchy equipment—can be treacherous, decades-spanning work.
Published in the journal Astronomy & Astrophysics, this latest study is based on more than 20 years of data, primarily from two planet-hunting instruments on the European Southern Observatory’s telescopes in Chile: HARPS (High-Accuracy Radial Velocity Planet Searcher) and ESPRESSO (Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations). The key breakthrough wasn’t really an increased amount of data, however, so much as it was a better analytic technique to discern planetary signals from stellar and instrumental noise: a new data-reduction algorithm, YARARA, spearheaded by study co-author Michaël Cretignier, a postdoctoral researcher at the University of Oxford. And the work does more than confirm the presence of HD 20794 d, which Cretignier first suggested in 2022. It also further validates the existence of the world’s two already-known planetary kin—while deflating the case for another companion that had been claimed to exist in 2011.
“So it was really a new type of analysis that allowed us to reach this this conclusion—we are really sure that [HD 20794 d] exists,” says Xavier Dumusque, an astronomer at the University of Geneva and a co-author of the study. But beyond its mere existence, this world’s most fundamentally alluring aspects remain shrouded in mystery.
Something as simple as HD 20794 d’s actual mass is still unknown, notes Renyu Hu, an exoplanet scientist at NASA’s Jet Propulsion Laboratory, who was not involved in the study. Stellar wobbles, indirect as they are, can only provide a planet’s minimum mass. “If this planet’s true mass is really about 6.5 Earth masses, it could be an Earth—a large rocky planet,” he says. But it could just as well be a rather unearthly orb, he adds, with a rocky core smothered beneath massive layers of water or hydrogen gas. “I don’t think we have proof [yet] that this planet is rocky,” he concludes.
The eccentric orbit of planet HD 20794 d sends this world swooping through a skewed swath of its star’s habitable zone.
(Dumusque et al. 2025) Gabriel Pérez Díaz, SMM (IAC)
Jessie Christiansen, chief scientist of the NASA Exoplanet Science Institute at the California Institute of Technology, has similar reservations. Astronomers have managed to measure the densities—and thus the bulk compositions—for many exoplanets, she notes, specifically some of those that, unlike HD 20794 d, cast their planetary silhouette toward Earth as they transit the face of their star. “If you look at the [distribution of] densities, most of [those] planets that are this massive aren’t rocky,” she says. “But some of them are; it’s not rare…. And a one-in-three chance is worth spending some more time investigating, right?”
The unsettled question of this planet’s composition cuts close to the heart of its habitability—or, at least, whether it could really be considered Earth-like at all. “This is actually really important for chemistry,” explains Lauren Weiss, an astrophysicist at the University of Notre Dame, who was not involved in the study. “If you think life has any sort of reliance on chemistry, that chemistry happens way faster on a two-dimensional surface than just sort of floating in space. So rocky surfaces are a huge thing [astronomers] are interested in looking for.”
The planet’s composition, too, is key to understanding what conditions prevail from the extreme ebb and flow of starlight caused by its eccentric orbit. Although a world of fire and ice might seem inhospitable, Dumusque doesn’t consider that a showstopper—especially, in fact, if HD 20794 d is more “planet ocean” than a supersized planet Earth. “If you have some deep oceans like on Earth, for example, then you have pressure that comes into play,” he says. “So then we could still have a surface that goes from ice to liquid water, and we know on Earth, life appeared deep in the ocean.”
Such speculations are part of a vibrant recurring theme in the quest for some exoplanetary “Earth 2.0.” Time and time again, astronomers have been surprised by discoveries that show “this wider diversity of ways to form habitable planets,” Hu says, “which is awesome because it really pushes our theories, our understanding of how planets could evolve.”
Fortunately, astronomers shouldn’t be stuck merely theorizing forever. Upcoming or proposed missionslike NASA’s Habitable Worlds Observatory (HWO) and ESA’s Large Interferometer For Exoplanets (LIFE) are meant to directly image, or take pictures of, potentially habitable worlds in order to measure their composition, map their surface or cloud tops and “sniff” their atmosphere for conspicuous chemical indicators of life. But finding suitable candidates to point these telescopes toward has proved a Herculean task, according to Weiss—and that could be HD 20794 d’s moment of stardom.
“We’ve discovered over 5,000 exoplanets, but a lot of those are really, really far away. We don’t know of a whole lot of nearby exoplanets that might be habitable,” she says—especially ones as curious and close as HD 20794 d.
In fact, both NASA and ESA already have HD 20794 on their radar for potential inclusion on a targeting short list for HWO and LIFE, according to Dumusque and Hu.
“We’re definitely not at the stage to select the targets for [HWO], but there’s been an effort to list out all the potential targets,” Hu says. Now that we know there likely is a planet orbiting HD 20794 that will “spend a good fraction of its time in the habitable zone and is potentially rocky—that’d certainly make the system more attractive for future observations.”
In any case, Weiss says, the search for Earth 2.0 is, in essence, the quest to learn more about Earth 1.0—our planet. “As we start discovering other exoplanets and measuring their properties and their basic environments, [that] gives us a really powerful lens for examining our own origins and appreciating whatever the natural circumstances are that led to the rise of the solar system—to maybe just help us appreciate where and when we are in space a bit more.”
