The discovery of Earth-like exoplanets around main sequence stars are among the most exciting challenges in astronomy today.  

More specifically, an Earth-mass planet at 1AU orbital around the Sun can produce ~9.6cm/s amplitude for Doppler radial velocity (RV) measurement. How to achieve this ultimate precision is still an open question. 

Researchers from National Astronomical Observatories (NAOC) of Chinese Academy of Sciences proposed effective methods and new models to improve the RV precision to achieve cm/s level by using the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph. The study was published in Astronomy & Astrophysics. 

"We apply an Astro-Comb on HARPS to investigate the RV precision," said Dr. ZHAO Fei from NAOC, lead author of the study. Astro-Comb is the ideal wavelength calibrator that can produce thousands of uniformly spaced lines within the wavelength domain covered by a modern cross-dispersed astronomical spectrograph. The line frequencies can be locked to an atomic clock, from which they inherit their stability at long time scales.   

"Based on the new models of RV noise we build for the first time, we have improved the wavelength calibration precision for two to six times better than before," said Dr. ZHAO Fei. 

This improvement can enhance the HARPS' RV precision at cm/s level especially for long-term detection, which is required for detecting exoplanets in the habitable zone. 

The exoplanets distribution and the Doppler measurement precision. The green area is the habitable zone around the host stars. The four dashed lines are different RV precision level depending on various planet masses. (Image by ZHAO Fei)  

The main uncertainty of RV precision lies in errors in two main aspects: the wavelength calibration of the spectra and the measurement of the accurate shape for spectral line profile.  

By analyzing the Astro-Comb data on HARPS, the researchers demonstrated the new models of RV precision based on the function of the line position and line intensities. These new models can correct and re-calibrate the spectra and mitigate the noisy effects and RV uncertainties.  

"The future of exoplanet science is depended on the precision that can ultimately be achieved with Doppler measurements. With Astro-Comb as the calibration source, more science cases can be studied in the future such as confirming and characterizing exoplanets from transit missions and providing mass measurements for space-based missions," said Prof. ZHAO Gang from NAOC, a co-author of the study.