[Paper Review] The HARPS search for Earth-like planets in the habitable zone: I -- Very low-mass planets around HD20794, HD85512 and HD192310

F. Pepe, C. Lovis+8 more|Archive ouverte UNIGE (University of Geneva)|Aug 17, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy42 references171 citations

TL;DR

This study presents the first results of a high-precision radial-velocity survey using the HARPS spectrograph to detect low-mass planets in the habitable zones of solar-type stars. It identifies three super-Earths around HD20794, a 3.6 M⊕ planet at the inner edge of the habitable zone around HD85512, and confirms a Neptune-mass planet with a second companion in the outer habitable zone around HD192310, demonstrating the prevalence of low-mass planets around Sun-like stars.

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%

Motivation & Objective

• To detect very-low-mass planets, including rocky super-Earths, in the habitable zones of solar-type stars.
• To assess the frequency of low-mass planets around stable, quiet, slowly-rotating stars using ultra-precise radial-velocity measurements.
• To test whether planetary systems with Earth-like planets are common, especially around stars with low intrinsic activity.
• To improve detection sensitivity by minimizing stellar noise through high-cadence, multi-epoch observations.

Proposed method

• High-cadence radial-velocity measurements were obtained using the HARPS spectrograph on the ESO 3.6 m telescope at La Silla Observatory.
• Three nightly observations per night, each with 10 minutes of integration, spaced by two hours, to mitigate granulation and pulsation noise.
• Stellar activity was monitored via the log(R′_HK) index and bisector span (BIS) to distinguish planetary signals from stellar variability.
• Orbital parameters were derived using Keplerian modeling of radial-velocity variations.
• The HARPS data-reduction pipeline was updated to enhance spectroscopic stability and measurement precision.
• Statistical validation of planetary signals was performed, with particular attention to low-amplitude signals (e.g., 0.56 m s⁻¹) indicative of super-Earths.

Experimental results

Research questions

• RQ1 What is the frequency of low-mass planets, especially super-Earths and rocky planets, in the habitable zones of solar-type stars?
• RQ2 Can ultra-precise radial-velocity measurements detect planetary signals as low as 0.56 m s⁻¹, corresponding to a 2.4 M⊕ planet?
• RQ3 How do stellar activity indicators like log(R′_HK) and BIS help in distinguishing planetary signals from stellar noise?
• RQ4 Are multiple low-mass planets commonly found in systems around quiet, stable stars?
• RQ5 Can the detection of a 3.6 M⊕ planet at the inner edge of the habitable zone of HD85512 be confirmed as a rocky world?

Key findings

• A system of three super-Earths with minimum masses of 2.4, 4.0, and 8.4 M⊕ was detected around HD20794, with orbital periods of 40, 18, and 90 days, respectively.
• A 3.6 M⊕ planet was discovered in a 58-day orbit around HD85512, placing it at the inner edge of the habitable zone of this late-K-type star.
• The radial-velocity semi-amplitude of the 40-day planet in the HD20794 system is only 0.56 m s⁻¹, the lowest planetary signal detected to date.
• A Neptune-mass planet (m sin i ≈ 14 M⊕) was confirmed around HD192310, with a second companion of similar mass on a wider orbit in the outer habitable zone.
• Stellar activity indicators (log(R′_HK) and BIS) were measured with milli-dex and sub-m s⁻¹ precision, enabling robust separation of planetary and stellar signals.
• The HARPS instrument achieved sufficient radial-velocity precision and spectroscopic stability to detect low-mass planets despite stellar noise, confirming the feasibility of such surveys.