The KOBE experiment: K-dwarfs Orbited By habitable Exoplanets

Abstract

The detection of habitable worlds is one of humanity's greatest endeavors. Thus far, astrobiological studies have shown that one of the most critical components for allowing life to develop is liquid water. Its chemical properties and its capacity to dissolve and, hence, transport other substances makes this constituent a key piece in this regard. As a consequence, looking for life as we know it is directly related to the search for liquid water. For a remote detection of life in distant planetary systems, this essentially means looking for planets in the so-called habitable zone. In this sense, K-dwarf stars are the perfect hosts to search for planets in this range of distances. Contrary to G-dwarfs, the habitable zone is closer, thus making planet detection easier using transit or radial velocity techniques. Contrary to M-dwarfs, stellar activity is on a much smaller scale, hence, it has a smaller impact in terms of both the detectability and the true habitability of the planet. Also, K-dwarfs are the quietest in terms of oscillations, and granulation noise. In spite of this, there is a dearth of planets in the habitable zone of K-dwarfs due to a lack of observing programs devoted to this parameter space. In response to a call for legacy programs of the Calar Alto observatory, we have initiated the first dedicated and systematic search for habitable planets around these stars: K-dwarfs Orbited By habitable Exoplanets (KOBE). This survey is monitoring the radial velocity of 50 carefully pre-selected K-dwarfs with the CARMENES instrument over five semesters, with an average of 90 data points per target. Based on planet occurrence rates convolved with our detectability limits, we expect to find 1.68 ± 0.25 planets per star in the KOBE sample. Furthermore, in half of the sample, we expect to find one of those planets within the habitable zone. Here, we describe the motivations, goals, and target selection for the project as well as the preliminary stellar characterization.

A Blind Search for Transit Depth Variability with TESS

“…Referring back to our speculation based on the results of Morales et al (2008), one might expect larger transit depth variability for planets orbiting cooler hosts. However, there does not seem to be a trend of higher nested ratios for cooler stars; in fact, if we ignore the error bars, there seems to be an absence of high nested ratios (N > 1) for planets around stars below ∼5000 K, thus suggesting that nested ratios are lower for planets around stars below ∼5000 K. We note that 5000 K corresponds to K dwarfs, which have been found to exhibit less stellar variability and are good host stars in the search for extraterrestrial life (Cuntz & Guinan 2016;Lillo-Box et al 2022).…”

An unusually low-density super-Earth transiting the bright early-type M-dwarf GJ 1018 (TOI-244)

“…It is of special interest to constrain the presence of planets in the habitable zone (HZ) of GJ 1018, given that late K-dwarfs and early M-dwarfs represent an ideal trade-off between detectability and true habitability of their planets (see the KOBE experiment; Lillo-Box et al 2022). For the HZ of GJ 1018, which is located in the period range 24-102 days (Kopparapu et al 2013), the current data allows us to discard planets with masses above 20 M ⊕ .…”

The Near-infrared Ca ii Triplet as a Stellar Activity Indicator: A Library and Comparative Study

“…Studies of stellar activity are essential for improving our understanding of stellar dynamo models and related studies such as stellar age and rotation or activity relation, and stellar flares and the stellar-activity cycle. On the other hand, stellar activity is important for exoplanet studies, since magnetic activity, especially flares, will have an impact on planetary habitability (Shields et al 2016;Howard et al 2018;Lillo-Box et al 2022). Also, jitters in both photometry and radial velocity measurements caused by stellar magnetic activity will hinder the detection of Earthlike exoplanets (Wright 2005).…”