The Discovery
A possible “super-Earth” orbits a relatively close, Sun-like star, and could be a habitable world – but one of extreme temperature swings, from scorching heat to deep freeze.
Key Facts
The newly confirmed planet is the outermost of three detected so far around a star called HD 20794, just 20 light-years from Earth. Its 647-day orbit is comparable to Mars in our solar system. But this planet’s orbit is highly eccentric, stretched into an oval shape. That brings the planet close enough to the star to experience runaway heating for part of its year, then carries it far enough away to freeze any potential water on its surface. The planet has been bouncing between these extremes roughly every 300 days – perhaps for billions of years.
Details
The planet spends a good chunk of its year in the “habitable zone” around its star, the orbital distance that would allow liquid water to form on the surface under the right atmospheric conditions. But because of its eccentric orbit, it moves to a distance interior to the inner edge of the habitable zone when closest to the star, and outside the outer edge when farthest away. At its closest, the planet’s distance from the star is comparable to Venus’s distance from the Sun; at its farthest point, it is nearly twice the distance from Earth to the Sun. The planet is possibly rocky, like Earth, but could be a heftier version – about six times as massive as our home planet.
Star HD 20794 and its posse of possible planets have been extensively studied, but the international team of astronomers that confirmed the outer planet, led by Nicola Nari of Light Bridges S.L. and the Instituto de Astrofisica de Canarias, examined more than 20 years worth of data to pin down all three planets’ orbits and likely masses.
The scientists relied on data from two ground-based, precision instruments: HARPS, the High Accuracy Radial velocity Planet Searcher in La Silla, Chile, and ESPRESSO, the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations in Paranal, Chile. Both instruments, connected to powerful telescopes, measure tiny shifts in the light spectrum of stars, caused by the gravity of planets tugging the star back and forth as they orbit.
But such tiny shifts in the star’s spectrum also can be caused by imposters – spots, flares, or other activity on the star’s surface, carried along as the star rotates and masquerading as orbiting planets. The science team spent years painstakingly analyzing the spectrum shifts, or “radial velocity” data, for any sign of background noise or even jitters from the instruments themselves. They confirmed the reputation of HD 20794 as a fairly quiet star, not prone to outbursts that might be confused for signs of orbiting planets.
Fun Facts
The elliptically orbiting super-Earth appears to be an ideal target for future space-based telescopes designed to search for habitable worlds, seeking possible signs of life. High on the list is NASA’s Habitable Worlds Observatory, which will someday examine the atmospheres of Earth-sized planets around Sun-like stars. When launched in the decades ahead, the observatory would spread the light from such planets into a spectrum to determine which gases are present – including those that might reveal some form of life. The relative closeness of HD 20974, only 20 light-years away, its brightness, and its low level of surface activity – not to mention the third planet’s wild temperature swings – could make this system a prime candidate for scrutiny by HWO.
The Discoverers
The international science team that confirmed the eccentric super-Earth was led by researcher Nicola Nari of the Light Bridges S.L. and the Instituto de Astrofisica de Canarias, and included Dr. Michael Cretignier of the University of Oxford, who first picked up the potential planet’s signal in 2022. Their paper, “Revisiting the multi-planet system of the nearby star HD 20794,” was published online by the journal, Astronomy and Astrophysics, in January 2025.
