After three decades since the first exoplanet was confirmed, we have now identified over 7,000 of these celestial bodies within our Galaxy. Yet, countless more await discovery! Simultaneously, researchers focused on exoplanets have started exploring their features with the objective of identifying signs of life in the cosmos. This context highlights the recent finding of the super-Earth known as HD 20794 d by an international collaboration including the University of Geneva (UNIGE) and the NCCR PlanetS. This new planet follows an unusual orbit, leading it in and out of its star’s habitable zone. This significant discovery results from 20 years of meticulous observations using some of the world’s most advanced telescopes. The findings have been published today in the journal Astronomy & Astrophysics.
”Are we alone in the Universe?” This profound question was largely philosophical for millennia, but recent scientific advances are beginning to offer concrete hypotheses and evidence. Despite this progress, astronomers continue to move slowly in their research. Each new finding, whether theoretical or observational, expands our understanding and pushes the limits of what we know. A landmark moment occurred in 1995, with the discovery of the first planet outside our solar system, a breakthrough that earned UNIGE scientists Michel Mayor and Didier Queloz the Nobel Prize in Physics in 2019.
Nearly 30 years later, astronomers have made significant strides, leading to the detection of over 7,000 exoplanets. The prevailing view among scientists is that nearly every star in our galaxy is likely to have its own planetary system. Now, researchers are targeting exoplanets that offer unique features or are simpler to study, allowing them to validate their theories and deepen their understanding. This represents the case of HD 20794 d, recently identified by a team that includes members from the UNIGE Astronomy Department.
Located in the habitable zone of its star
This intriguing planet is classified as a super-Earth, which is a rocky planet larger than our own. It belongs to a planetary system that has two other planets. HD 20794 d orbits a G-type star, similar to the Sun, located just 19.7 light-years away, which within cosmic distances, makes it quite near to Earth. This proximity allows for improved observational studies since its light signals are clearer and stronger. ”The star HD 20794, with which HD 20794 d is associated, is not a common star,” remarks Xavier Dumusque, a Senior Lecturer and researcher in the UNIGE Astronomy Department and co-author of the research. “Its brightness and nearness make it an excellent candidate for future telescopes designed to directly observe the atmospheres of exoplanets.”
HD 20794 d’s significance arises from its position within the habitable zone of its star, where conditions may allow for the presence of liquid water, a key ingredient for life as we understand it. This habitable zone varies based on several factors, primarily the characteristics of the star. For stars like the Sun or HD 20794, this range can span from 0.7 to 1.5 astronomical units (AU), which includes not just Earth’s orbit but also Mars’s in our solar system. The exoplanet HD 20794 d completes an orbit around its star in 647 days, roughly 40 days shorter than Mars’s orbital period.
Instead of maintaining a mostly circular orbit like Earth or Mars, HD 20794 d follows an elliptical path that leads to considerable variation in its distance from its star throughout its year. Consequently, the planet oscillates between the inner edge of its star’s habitable zone (0.75 AU) and beyond it (2 AU). This orbital pattern is particularly fascinating for astronomers, allowing them to refine their theoretical models and assess their understanding of planetary habitability. Water on HD 20794 d could shift from ice to liquid during its orbit, creating conditions favorable for the emergence of life.
Years of extensive observations
Identifying this super-Earth was a challenging and iterative process. The research team analyzed over two decades’ worth of data collected by advanced instruments like ESPRESSO and HARPS. With HARPS, they utilized YARARA, a recently-developed data reduction algorithm at UNIGE. For years, signals indicating planetary presence had been obscured by noise, complicating the confirmation of their existence. ”After years of careful analysis, we worked tirelessly to filter out sources of contamination,” explains Michael Cretignier, a post-doctoral researcher at Oxford University, co-author of the study, and the creator of YARARA during his PhD at UNIGE.
The discovery of HD 20794 d offers a valuable opportunity for scientists to model and evaluate new hypotheses in their quest for extraterrestrial life. The close proximity of this planetary system to its luminous star positions it as a key target for next-generation instruments, such as the ANDES spectrograph for ESO’s Extremely Large Telescope (ELT). Determining whether this planet supports life will necessitate reaching several scientific goals and adopting a multidisciplinary approach, with ongoing studies at the newly established Centre for Life in the Universe (CVU) at UNIGE’s Faculty of Science examining its habitability conditions.
