Every exoplanet discovery is an opportunity to refine models of planet formation, solar system architecture, habitable zones, and habitability itself. Each new planet injects more data into the scientific endeavour to understand what’s going on and how things got this way. However, some planets have such unusual characteristics that they invite a deeper focus and intense follow-up observations.
That’s the case for one new exoplanet. It’s a super-Earth on an unusual orbit that’s giving astronomers an opportunity to test the ideas of habitability and optimistic and pessimistic habitable zones.
The planet is named HD 20794 d, and it orbits a Sun-like star about 20 light-years away. Its eccentric orbit takes it from 0.7 to 1.5 AU from its host star. It spends half of its time beyond the putative habitable zone before travelling back into the zone and slightly inside of it.
Could life somehow survive on a planet like this?
In stellar terms, the exoplanet is right next door, and since the star is bright, the planet is in a great location for observation and study. The discovery of the planet was first reported in 2023, and in new research, a team of astronomers confirms its existence and points out how it’s in a prime location for further study.
The new research is “Revisiting the multi-planetary system of the nearby star HD 20794,” and it’s published in the journal Astronomy and Astrophysics. The lead author is N. Nari from the Instituto de Astrofísica de Canarias in Spain.
Though HD 20784 d was discovered a couple of years ago, it remained a candidate until this new research confirmed it. The planet was known as the 640 d planet because it appeared to have an approximately 640-day orbit around its star. This new study adds more observational details to the planet, including how it’s a great candidate for follow-up atmospheric study. Because it moves in and out of its star’s habitable zone, it’s an opportunity to learn more about habitability and to test and refine scientific models.
“HD 20794, around which HD 20794 d orbits, is not an ordinary star,” explains Xavier Dumusque, Senior Lecturer and researcher in the Department of Astronomy at the University of Geneva and co-author of the study. “Its luminosity and proximity make it an ideal candidate for future telescopes whose mission will be to observe the atmospheres of exoplanets directly.”
Since it’s so close and bright, it’s already been the target of observations. Exoplanetologists have 20 years of data from facilities like HARPS and ESPRESSO to work with. HARPS is the High Accuracy Radial Velocity Planet Searcher, and ESPRESSO is the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations.
“HD 20794 has been part of long radial velocity (RV) surveys dedicated to the search for low-amplitude long-period signals around solar-type stars, with hundreds of nights of observations with HARPS and ESPRESSO spanning more than 20 years available,” the paper states.
Detecting the super-Earth was difficult. Twenty years of data helped, but it took the development of a new algorithm to find the planet in the data. It’s called YARARA, and it’s a data reduction algorithm recently developed at the University of Geneva (UNIGE). Planets are often obscured by noise in the data, and YARARA can sift through the data and filter out the noise.
“We analyzed the data for years, carefully eliminating sources of contamination,” explained Michael Cretignier, a post-doctoral researcher at Oxford University, co-author of the study and developer of YARARA. Cretignier is also the lead author of the 2023 paper that reported the initial detection of HD 20794 d.
The discovery of HD 20794 d has made astronomers want to monitor the star more closely. “The low stellar activity level and the brightness of HD 20794 has made this target one of the most well-suited candidates for this purpose,” the authors explain.
The system hosts three planets, and this research concludes that all three of them are super-Earths, though there’s some possibility that HD 20794 d could be a mini-Neptune with a non-negligible H/He atmosphere. It follows an elliptical orbit with an eccentricity of 0.45 and has about 5.8 Earth masses.
The planet’s most interesting feature is its orbit. “The orbital period of HD 20794 d resides both in the optimistic and conservative HZ,” the authors write in their research. “This is an interesting result because we do not have many examples of planets with M < 10 Earth masses with mass measurement from RVs in the HZ of Sun-like stars.”
The planet travels between the inner edge of its star’s HZ (0.75 AU) and outside of it (2 AU) as it follows its eccentric orbit. If the planet hosts water, it would shift from its frozen state to its liquid state and back again repeatedly.
This is an exciting planet. Not only does it follow an unusual orbit, but it’s close to and orbits a bright star. “The closeness of the planetary system, summed with the distance of the star and the planet and the planet-to-star contrast ratio, makes this planet a good candidate for future atmospheric characterization through direct imaging facilities,” the authors write.
One of those facilities is the ANDES instrument on the European Southern Observatory’s Very Large Telescope (VLT). ANDES stands for ArmazoNes high Dispersion Echelle Spectrograph. AndES is a high-resolution instrument that can search for signs of life on Earth-like planets. The detection of biosignatures from exoplanet atmospheres is listed as one of the instrument’s top science cases.
Signs of biosignatures on HD 20794 d won’t jump out at scientists. It’ll take a lot of work among multiple scientific disciplines. Some of that work has begun.
Researchers at the Centre for Life in the Universe (CVU) at the UNIGE’s Faculty of Science are already studying the conditions for the planet’s habitability.
Press Release: A super-Earth laboratory for searching life elsewhere in the Universe
Paper: Revisiting the multi-planetary system of the nearby star HD 20794
