The search for habitable worlds

In our hunt for Earth analogs, extrasolar planets have been searched intensively in the past decades (e.g., Borucki et al., 2010), with the main focus on detecting as many as possible, regardless of their properties, and mainly biased and limited by instrumental precision. Thanks to all these previous efforts (including ground- and space-based observations), more than 4100 planets are currently known. Their properties are extremely diverse, showing characteristics quite distant from those expected from the components of the Solar System and initially even from those envisioned by theories of planet formation. The search for habitable worlds has first focused on stellar properties similar to those of our Sun (e.g., Kepler and HARPS efforts). In the latter years, M-dwarfs have been intensively targeted (e.g., Bonfils et al., 2005; Quirrenbach et al., 2014), because their habitable zone is closer to the star, hence making the detection of habitable Earth analogs easier. Both regimes, although very relevant, suffer from important difficulties, from both the instrumentation point of view and from the physical side. M-dwarfs are very active, thus making the radial velocity analysis utterly difficult to unveil planet-like signals at the meter-per-second level. Also, the strong flares on these active stars challenge their habitability. The low stellar mass and the sharp habitable zone regime leave few dynamical room for scaled-down versions of the Solar System around these stars, which is demonstrated by the low occurrence rate of multi-planet systems around M-dwarfs against the high population in G-dwarfs. On the other side, G-dwarfs host their habitable zones at periods around one year. This creates critical difficulties on the detection of rocky planets through the radial velocity technique, requiring several-year-long stable instruments at the centimetre-per-second level. So far, only ESPRESSO is capable of doing this task, and will still be challenging. A new perspective is thus needed to face the detection of habitable worlds that can be confirmed and characterised by the radial velocity technique and provide a census of confirmed habitable rocky worlds.

The K-dwarf opportunity

K-dwarfs, and more specifically, the late K-dwarfs (K4-K9, with effective temperatures between 3800-4600 K) offer the perfect compromise between technical and physical feasibilities.

Habitability: K- vs M-dwarfs

Although the conditions for habitability on the surface of a planet are still poorly understood, there are different properties that we know might hazard sustainable life as we know it. The first of these conditions is the ability of the planet to retain liquid water on its surface. The range of distances from the star where the incident flux on the planet allows this is called the habitable zone (HZ). In the case of M-dwarfs, given their low luminosity, this region is located at periods closer than 30 days. This, although favourable to the planet detection, has some key downsides on the real habitability of the planet. At first, stellar activity on M-dwarfs is a key actor, with energetic stellar flares increasing the luminosity of the star by a relevant percentage and increasing the coronal emission. Flares can also potentially reach the location of the habitable zone threatening any kind of life on its surface. Big stellar spots may also create relevant variations in the incident flux. Also, being closer to their parent stars planets in the HZ might be tidally locked, always facing the same side to the parent star and thus decreasing the probability for life to be sustained in its surface. By contrast, K-dwarfs have their HZ located at longer periods, where planets can have their rotation and orbital periods decoupled, hence allowing the planet to have day-night cycles. Stellar activity and magnetic flaring is dramatically diminished for stars earlier than M3 and specially in the late K-type domain. Consequently, habitability is not threatened by these effects as much as it is in the HZ planets around M dwarfs. Besides, unlike M-dwarfs, we can derive precise stellar parameters and chemical abundances, relevant to properly characterise the planets and the the star-planet connection.

Planet detection in the habitable zone

The K-dwarf habitable zone ranges between 0.1-0.3 AU for the latest types (corresponding to orbital periods between 17-90 days). This corresponds to radial velocity semi-amplitudes of 2.4 - 4.2 m/s in the case of K9 stars and 1-2 m/s for K4 stars with a 10 Earth masses planet in the habitable zone. Added to this, the imprint of stellar activity and magnetic cycles on the radial velocity of K-dwarfs is smaller than in the case of M-dwarfs. For the late K-dwarf stars, the signals induced by magnetic cycles typically have amplitudes below 3 m/s and typical periods of 7 years. On the other hand, although rotation periods span between 15-45 days, K dwarfs in the colour range 1.0 < B − V < 1.3 have the lowest level of activity jitter (Isaacson & Fisher, 2010), significantly less than 1 m/s, thus becoming the perfect targets to search for habitable planets. With a demonstrated precision of around 1.3 m/s in the long-term (after corrections are applied), CARMENES is one of the few instrument in the Northern hemisphere that can reach such precision and stability over a long period of time. The 1.3 m/s precision allows the detection of planets in the HZ regime of K-dwarfs down to the rocky regime. Indeed, it allows completeness for planets with masses above 10 MEarth around late K-dwarf stars and detection limits down to 3 MEarth (see Figure 1).

The K-dwarf habitable zone desert

The strategy followed by ground-based surveys and space-based missions has missed the HZ of K-dwarfs. This is evident in Figure 2, where only a handful of validated transiting planets (i.e., no mass measurement) and two confirmed planets populate the habitable zone around late K-dwarf stars. The histogram on Figure 2 (right panel) illustrates this desert, with a large number of temperate worlds detected around G-type stars (mainly with RV surveys) and another large sample in the low stellar mass regime. This desert is even drier when we focus on planets with determined masses (red histogram). Stellar population studies, however, do not show a paucity of this type of stars in the solar neighbourhood compared to G- and M-types (e.g., Kroupa et al., 1993). Consequently, the HZ-planet desert is indeed an observational. The reasons for this paucity of planets in the habitable zone are clear: the focus on solar-like stars for similarity with the Solar System and the hunt for planets around M-dwarfs due to detectability reasons. A focused and systematic program exploring the habitable zone of K-dwarfs is thus missing. The M2K project (Apps et al., 2010) in 2010 used a small fraction of the Keck/HIRES instrument to look for planets around MK stellar types, only a handful number of systems were announced. The HARPS GTO program (PI: X. Bonfils) has also followed-up some of K-dwarfs. However, these studies were not focused on the HZ of K-dwarfs and hence the sample and cadence was insufficient to reach the rocky regime in the habitable zone. This is mainly due to the lack of telescope time. A dedicated service program with the flexibility of a moderate number of nights per semester and distributed along a sufficiently large time span is thus required to reach this regime. We propose to take the lead on this niche with the KOBE experiment.

The KOBE experiment

Goals, legacy, and expected outcome

We propose that Calar Alto and CARMENES become the leading facility in this endeavor by performing the first systematic and dedicated survey in search for habitable planets around late type K-dwarfs. The present call for Legacy Programs puts Calar Alto in a unique position to carry out a survey that would otherwise be practically impossible to be developed in any other facility (the chances of getting > 20 nights/semester in open time on any state-of-the-art instrument are extremely low if not impossible). With the KOBE experiment we propose a guided search for habitable planets (from gaseous to rocky compositions) around a minimum of 50 K-dwarfs by monitoring a carefully selected sample of K4-K9 stars. We will obtain an average of 90 data points per target spread over 5 semesters, using 35 nights per semester (including overheads) of the Calar Alto 3.5m telescope. The experiment will not exclude gaseous giants in the habitable zone since this niche is also very relevant in different aspects (e.g., future search for habitable exomoons, co-orbital worlds or atmospheric characterisation if they transit). Based on the planet occurrence rates and taking into account the guided nature of this experiment (not being a blind search but instead maximising the probability of finding planets based on empirical and theoretical studies on the particular systems), we expect a planet yield of 15-40 new planets for this low-demanding legacy program, with a relatively high percentage of them residing in the habitable zone and being in the super-Earth regime (3