The HARPS search for Earth-like planets in the habitable zone: I -- Very low-mass planets around HD20794, HD85512 and HD192310 (1108.3447v3)

Abstract: In 2009 we started an intense radial-velocity monitoring of a few nearby, slowly-rotating and quiet solar-type stars within the dedicated HARPS-Upgrade GTO program. The goal of this campaign is to gather very-precise radial-velocity data with high cadence and continuity to detect tiny signatures of very-low-mass stars that are potentially present in the habitable zone of their parent stars. Ten stars were selected among the most stable stars of the original HARPS high-precision program that are uniformly spread in hour angle, such that three to four of them are observable at any time of the year. For each star we recorded 50 data points spread over the observing season. The data points consist of three nightly observations with a total integration time of 10 minutes each and are separated by two hours. This is an observational strategy adopted to minimize stellar pulsation and granulation noise. We present the first results of this ambitious program. The radial-velocity data and the orbital parameters of five new and one confirmed low-mass planets around the stars HD20794, HD85512, and HD192310 are reported and discussed, among which is a system of three super-Earths and one that harbors a 3.6 Earth-mass planet at the inner edge of the habitable zone. This result already confirms previous indications that low-mass planets seem to be very frequent around solar-type stars and that this may occur with a frequency higher than 30%

• The paper identifies subtle radial velocity signals revealing super-Earths with masses as low as 2.4 Earth masses.
• It employs high-cadence, 50-point observation strategies to minimize stellar noise and isolate planetary signals.
• The findings imply that over 30% of solar-type stars may host low-mass, rocky planets, supporting theoretical planet formation models.

The HARPS Search for Earth-like Planets in the Habitable Zone

The paper titled "The HARPS Search for Earth-like Planets in the Habitable Zone" investigates the potential of very low-mass planets to exist in the habitable zones of nearby solar-type stars, utilizing the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on the La Silla Observatory's 3.6 m telescope. The research aims to detect subtle radial velocity (RV) signatures indicative of low-mass planets, a task necessitating high precision and long-term observation strategies to distinguish planetary signals from stellar noise.

Overview of Observational Strategy

The team selected a carefully curated sample of ten nearby, chromospherically inactive, and slowly-rotating stars likely to host low-mass planets. The selection was partly based on prior HARPS data, focusing on stars with minimal RV dispersion. Observation strategies implemented included taking multiple measurements per night, minimizing stellar noise from pulsations and granulation by high cadence and spreading 50 data points across the observation season.

Key Findings

Among the targets, evidence for low-mass planets was found around three stars: HD 20794, HD 85512, and HD 192310. Notably, the paper highlights:

1. Planetary System of HD 20794: A system of three super-Earths was identified, with minimum masses of 2.7, 2.4, and 4.8 times that of Earth and orbital periods of 18, 40, and 90 days, respectively. The semi-amplitudes of the RV signals were exceptionally low, below 1 m/s, marking a significant detection threshold accomplishment.
2. Planet HD 85512 b: Around HD 85512, a single super-Earth with a minimum mass of 3.6 M⊕ was detected on a 58-day orbit, placing it at the inner edge of the habitable zone of the K5V star. The semi-amplitude of its RV signature was 0.77 m/s.
3. System of HD 192310: Confirming a previously announced Neptune-mass planet, the paper detected a second Neptune-mass planet on a longer orbit of 526 days. The semi-amplitudes of these detections were robust, correlating with significant RV variations.

Implications and Future Directions

The empirical findings underscore that low-mass planets, including potentially rocky ones like super-Earths, could be common around solar-type stars, with an occurrence rate potentially exceeding 30%. The paper corroborates theoretical predictions regarding planet formation, emphasizing that many solar-type stars might host low-mass planets rather than gas giants.

Practically, these detections hold promise for comprehensive follow-up investigations, potentially involving astrometry or photometry, to further explore the atmospheric composition and surface conditions of these candidates. The research specifies an ongoing effort within the HARPS-Upgrade GTO to monitor for RV precision improvements and signals correlated with stellar activity, posing new diagnostic opportunities.

Conclusion

This paper illustrates the significance of minute RV measurement precision in exoplanet discovery near habitable zones. It exemplifies the effectiveness of HARPS in expanding our understanding of planet formation frequencies and characteristics, paving the way for future missions dedicated to examining planetary atmospheres and habitability in detail. The continuation of such research endeavors will play a pivotal role in refining theoretical models and potentially guiding initiatives for seeking habitability elsewhere in our galaxy.