Stellar noise and planet detection. I. Oscillations, granulation and sun-like spots

TL;DR

This paper discusses the impact of stellar noise from oscillations, granulation, and activity on high-precision radial velocity measurements for planet detection, emphasizing the need for improved observational strategies.

Contribution

It analyzes different stellar noise sources affecting RV measurements and proposes the necessity for new strategies to mitigate these effects for detecting Earth-like planets.

Findings

Stellar noise induces m/s RV variations on different time scales.

Current observational strategies average out oscillation noise but not granulation or activity.

Detecting Earth-mass planets requires strategies that mitigate all stellar noise sources.

Abstract

Spectrographs like HARPS can now reach a sub-m/s precision in radial-velocity (RV) (Pepe & Lovis 2008). At this level of accuracy, we start to be confronted with stellar noise produced by 3 different physical phenomena: oscillations, granulation phenomena (granulation, meso- and super-granulation) and activity. On solar type stars, these 3 types of perturbation can induce m/s RV variation, but on different time scales: 3 to 15 minutes for oscillations, 15 minutes to 1.5 days for granulation phenomena and 10 to 50 days for activity. The high precision observational strategy used on HARPS, 1 measure per night of 15 minutes, on 10 consecutive days each month, is optimized, due to a long exposure time, to average out the noise coming from oscillations (Dumusque et al. 2010) but not to reduce the noise coming from granulation and activity. The smallest planets found with this strategy (Mayor et al. 2009) seems to be at the limit of the actual observational strategy and not at the limit of the instrumental precision. To be able to find Earth mass planets in the habitable zone of solar-type stars (200 days for a K0 dwarf), new observational strategies, averaging out simultaneously all type of stellar noise, are required.