English (UK)
Thanks to multiple observations performed at Calar Alto, a European research team, led from Centro de Astrobiología (INTA-CSIC) in Madrid, makes public the first planetary system discovered in the framework of the KOBE project.
KOBE uses data from the CARMENES instrument, mounted on the CAHA 3.5 m telescope, to monitor since 2021 about 50 orange dwarf stars, which are somewhat smaller and less hot than our Sun. Due to their characteristics, orange dwarf stars are el Dorado (sweet spot) of the search for life beyond the Solar System, and this first KOBE discovery paths the way to new ones.
The only known planet having biological activity is our own Earth: a world mostly covered with liquid water and orbiting the Sun, a G-type dwarf star. Until recently, habitable exoplanets were searched essentially around other G dwarfs and also the cooler M-type ones. However, both G- and M-type dwarf stars present difficulties that can be overcome by a type of star with intermediate properties, the so-called K-type dwarf stars. These seem to provide the ideal environment for the development of life on the surface of planets around them.
Astronomers define the habitable zone as the region around a star in which a planet can harbor liquid water on its surface. This abundant compound that we have on Earth is considered the minimum requirement for the development of life as we know it. To consider a planet to be in the habitable zone of its star, it must be neither too close to its star (as the water on the surface would evaporate) nor so far away that the water on its surface would freeze.
The range of distances at which a planet can remain at the right temperature for water to be in liquid form (the habitable zone) thus depends on the temperature of the star. In stars like the Sun, this region is found for orbital periods of several hundred days, as is the case of the Earth with its 365-day year. Detecting planets at such distances is very complex with current techniques. M-type stars are cooler, so that the habitable zone is very close, with periods of a few tens of days, facilitating the detection of planets. However, these cool dwarfs are very active, spreading energetic flares that can reach this habitable zone, threatening any life that might arise on planets within this region.
K-type stars, on the other hand, have the best properties of both types. The periods in which the habitable zone is found are accessible to current instrumentation. In addition, they are very quiet stars, without major activity events. Therefore, they are considered the sweet spot (El Dorado) of stellar habitability. The search for planets around K-type dwarf stars is thus a fundamental objective in modern exoplanetary exploration, focused on astrobiological objectives.
Illustration showing the possible appearance of the exoplanets KOBE-1b and KOBE-1c. Credits: J. Lillo-Box
The KOBE experiment is an observational program led by Centro de Astrobiología (CAB, INTA-CSIC) in collaboration with the Instituto de Astrofísica de Portugal, the Laboratoire d'Astrophysique de Marseille and the Observatoire de Genève. The KOBE observations are made with the CARMENES instrument at Calar Alto Observatory. The objective of this project is to search for planets in the habitable zone of 50 K-type stars. To this end, since 2021, the KOBE team has been monitoring, with the CARMENES spectrograph, the velocity of these 50 stars, carefully selected at the beginning of the project to maximize the probability of success.
In one of these stars, named KOBE-1, the team, led in this work by Olga Balsalobre-Ruza, PhD student at CAB, has found the signal of two orbiting planets with periods of 8.5 (KOBE-1b) and 29.7 days (KOBE-1c). Thanks to the CARMENES data, it has been possible to establish a minimum mass for these planets of 8.8 and 12 times the mass of the Earth, respectively. However, since no radius measurement is available, their composition is still unknown. Olga Balsalobre explains that “with these masses, both could correspond to super-Earth type planets (rocky but slightly larger than Earth) or sub-Neptunes (with large hydrogen and helium atmospheres being lighter than our Neptune). We hope to be able to resolve this question with the arrival of new space instrumentation in the coming decades, which will allow us to take direct images of the two planets.”
Although these new planets are not in the habitable zone, using the same data, the team has been able to rule out planets with masses greater than about 8 times the mass of Earth in this region of great astrobiological interest. This means that, if there are any planets in this range of distances to the star, they would be in the rocky regime. More data are still needed to explore this regime in detail.
Jorge Lillo-Box, a researcher at CAB, co-author of the paper and principal investigator of the KOBE experiment, highlights that “programs like KOBE are an exception in the scientific field, since they require a long observation time during several years to be able to detect these signals” and continues pointing out that “KOBE has been possible thanks to the Calar Alto Observatory's commitment to a scientifically innovative project, yet risky because of the large long-term investment, but which can provide important advances in our knowledge of the best planetary environments for the emergence and development of life beyond Earth, informing future space missions such as PLATO of the European Space Agency”.
The exoplanet KOBE-1b orbits its star in just 9 days, while KOBE-1c does so in 30 days, when its periods would need to be between 60 and 240 days to be located within the habitable zone. Credits: J. Lillo-Box.
Jesús Aceituno, Calar Alto Observatory Director, concludes that “this first discovery of the KOBE experiment shows that the CARMENES instrument, initially designed to detect planets orbiting red dwarf stars, is perfectly capable of detecting them around other types of stars. This confirms the versatility and accuracy of the instrumentation available at Calar Alto telescopes”.
The results are published in the Astronomy & Astrophysics journal on February 3rd, 2025.
Lillo-Box et al. 2025, accepted by Astronomy & Astrophysics.
CONTACT
Centro de Astrobiología (INTA-CSIC)
Jorge Lillo-Box
Observatorio de Calar Alto
Jesús Aceituno
El Observatorio de Calar Alto es una de las infraestructuras que pertenecen al Mapa Nacional de Infraestructuras Científicas y Técnicas SIngulares (ICTS), aprobado por el Consejo de Política Científica, Tecnológica y de Innovación (CPCTI) el 11 de marzo de 2022.
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