English (UK)
A new research led from the Centro de Astrobiología in Madrid (CAB, INTA-CSIC) has revised a previous study from NASA that had claimed the discovery, by the transit method alone, of an exoplanet orbiting the star K2-399.
Using more complete observations, including those taken from Calar Alto with CARMENES, the new research concludes that the detected transits, those mini-eclipses, are not due to a planet but to a pair of red dwarf stars orbiting K2-399, while eclipsing each other, which perfectly mimics a typical exoplanetary transit. This case reminds us that revision based on new data, and where appropriate, correction of previous results, is the basis of the scientific method.
Schematic view of the K2-399 system: a binary made of two red (cold), M-type dwarf stars orbits the main solar-like star, while eclipsing each other.
Artist’s view of an exoplanet with an atmosphere evaporating due to the radiation of its host star. Credits: NASA/JPL-Caltech.
In February 2018, the TESS space telescope launched by NASA observed that the Sun-like star K2-399 dimmed for a little more than an hour and a half before recovering its brightness. Eighteen hours later, the same phenomenon repeated: the star dimmed slightly again, only to regain its brightness shortly thereafter. All this pointed to a planetary transit: when passing in front of its star, from our point of view, the planet (a dark body with no own light) blocks part of the star light, which then appears a bit less bright. As it completes one full orbit and returns to the same position, the phenomenon repeats itself, like periodic mini-eclipses. And so on for the life of that planetary system.
The transits of K2-399 were analyzed by a group of researchers from the NASA Exoplanet Science Institute (NExSCI) at the California Institute of Technology (Caltech). In their analysis, the researchers concluded that these transits must indeed come from a planet. In their work, they ruled out other possibilities that could also have caused the star to dim... or appear to do so.
Nevertheless, since we do not see the planet itself but its indirect effects (the dimming of light as it passes in front of its star), Nature may be fooling us with other phenomena that would produce the same effect. For example, we can imagine a Sun-like star aligned almost perfectly in our sky with a pair of very faint stars eclipsing each other. In that case, and since we cannot see the stars individually, we would see a small eclipse that might appear to come from a planet around the star K2-399.
What the NExSCI researchers did was to statistically rule out any scenario other than a planet and they concluded that the probability that the signal was due to a planet was over 99.9995%. Then they considered the planet as confirmed and published the discovery in The Astronomical Journal.
But this supposed exoplanet was not just any exoplanet. Its characteristics made it extremely interesting: it was a very exotic type of planet, the one we know as a “hot Neptune”.
Indeed, we know dozens of “hot Jupiters”, planets like Jupiter (larger than Neptune) that orbit very close to their stars, with years that last only a few days. We also know of rocky planets like ours (smaller than Neptune) that also orbit this close to their stars. Still, there is a desert of planets like Saturn or Neptune very close to their parent stars. It is what we call “Neptune desert”. We do not yet know exactly it origin, but the fact that it exists gives us a lot of information about how planets form and what processes they undergo during their lifetime. In particular, how their atmospheres are volatilized (or evaporated) when they get so close to their parent stars.
However, K2-399 b was there, in the middle, wandering in the Neptune desert, undeterred by the very high radiation from its star which should have evaporated its entire atmosphere. How had K2-399b managed to retain its gas envelope? It was a real mystery and several groups of researchers requested observation time on both ground- and space-based telescopes equipped with state-of-the-art instruments.
A research team led by Jorge Lillo-Box, from Centro de Astrobiología (INTA-CSIC) in Madrid, realized that, despite being a planet for which we could measure its mass very well, no one seemed to have been able to obtain it. So, they requested observing time on the CARMENES instrument at the 3.5 meter Calar Alto telescope. The observations gave very strange results, which seemed inconsistent with the presence of a planet in the system.
So they asked other colleagues in the United States, Italy and England, who had also obtained data on K2-399 but had never published them. Putting all the data together, the reality quickly emerged: K2-399b was not a planet but a triple star system. Three stars whose configuration perfectly mimicked that of a planet.
In reality, the system was made of a Sun-like star (although somewhat hotter) around which orbit, with a very long period, two very cold stars very close to each other, with a period of a few days. These two cool stars eclipse each other, producing the transits observed by the TESS mission: they do not correspond to an exoplanet transit but to a pair of red dwarf stars eclipsing each other while orbiting with a long period (more than two years) around the main star, the sun of the triple system.
Jesús Aceituno, co-author of the study and Calar Alto Observatory director, believes that “CARMENES spectra have been fundamental to review the intriguing case of K2-399 and to show how important it is to complete the data taken by space telescopes with other data taken from ground-based observatories, such as Calar Alto, a leading figure of observational astronomy in the Old Continent”.
Jorge Lillo-Box, first author of the article to be published in the journal Astronomy & Astrophysics, emphasizes that the case of K2-399b “is not a story of failure, but of reaffirmation and transparency of Science and of the scientific method. Scientific results are not immutable. And there lies precisely the beauty and reliability of Science. There are many examples throughout history of scientific theories refuted by new observations or new evidence. And nothing bad happens, because science is made by individuals and based on the data and evidence available at the time each discovery is made.”
PUBLICATIONS:
Lillo-Box et al. 2024, accepted by Astronomy & Astrophysics
CONTACT
Centro de Astrobiología (INTA-CSIC)
Jorge Lillo-Box
Observatorio de Calar Alto
Jesús Aceituno
The Calar Alto Observatory is one of the infrastructures that belong to the national map of Unique Scientific and Technical Infrastructures (Spanish acronym: ICTS), approved on March 11th, 2022, by the Science, Technology and Innovation Policy Council (CPCTI).
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