Existence Of Exoplanet HD 20794 d In A Habitable Zone Confirmed
An international team has confirmed the discovery of a super-Earth orbiting in the habitable zone of a nearby Sun-like star. The planet was originally detected two years ago by Oxford University scientist Dr Michael Cretignier. This result, drawing on over two decades of observations, opens a window to future studies of Earth-like exoplanets that may have conditions suitable for life.
The new planet, named HD 20794 d, has a mass six times that of Earth and orbits a star similar to our Sun, located just 20 light years away. Its orbit places it within the habitable zone of the system, meaning it is at the right distance from its star to sustain liquid water on its surface, a key ingredient for life as we know it.
Dr Michael Cretignier first identified a candidate exoplanet signal in 2022, while analysing archived data recorded by the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph at the La Silla Observatory in Chile. This data analyses the light absorbed and emitted by objects. Dr Cretignier spotted distinct, periodic shifts in the spectrum of light emitted by the host star, which could have been caused by the gravitational pull of a nearby planet. Due to the faintness of the signal, however, it was not clear whether this was caused by a planet, induced by the star itself, or due to an instrumental error.
To verify the signal, an international team analysed highly-precise measurements recorded over two decades by HARPS and its successor ESPRESSO, also in Chile. These instruments are among the most advanced in the world for measuring tiny variations in light spectra.
‘We worked on data analysis for years, gradually analysing and eliminating all possible sources of contamination,’ added Dr Cretignier. Advanced processing methods and meticulous analyses were needed to distinguish the planetary signal from background noise and subtle instrumental effects. By combining the results from the two instruments, the discovery was finally confirmed.
‘For me, it was naturally a huge joy when we could confirm the planet’s existence,’ Dr Cretignier said. ‘It was also a relief, since the original signal was at the edge of the spectrograph’s detection limit, so it was hard to be completely convinced at that time if the signal was real or not. Excitingly, its proximity to us (only 20 light-years) means there is hope for future space missions to obtain an image of it.’
Although the planet is located in the system’s habitable zone, it is too early to say whether it could host life. Unlike most planets, HD 20794 d’s orbit is not circular but elliptical. Its distance from its star changes significantly, causing the planet to move from the outer edge of the habitable zone to the inner edge throughout its year.
In any case, HD 20794 d will provide an invaluable test case for upcoming space projects designed to detect signs of life outside our solar system. These include the Extremely Large Telescope, Habitable Worlds Observatory, and Large Interferometer For Exoplanets (LIFE). These instruments will observe the atmosphere of nearby Earth-like planets in the habitable zone for tell-tale ‘biosignatures’ indicative of life.
Dr Cretignier added: ‘With its location in a habitable zone and relatively close proximity to Earth, this planet could play a pivotal role in future missions that will characterise the atmospheres of exoplanets to search for biosignatures indicating potential life.
‘While my job mainly consists of finding these unknown worlds, I’m now very enthusiastic to hear what other scientists can tell us about this newly discovered planet, particularly since it is among the closest Earth-analogues we know about and given its peculiar orbit.’
Revisiting the multi-planetary system of the nearby star HD 20794. Confirmation of a low-mass planet in the habitable zone of a nearby G-dwarf, N Nari et al, Astronomy & Astrophysics, 28 January 2025 (open access).
Astrobiology
