Precise Measurements of Nearest Earth-Sized Exoplanet

Breakthrough measurements of exoplanet Proxima B were made with radial velocity measurements of unprecedented precision using ESPRESSO, the Swiss-manufactured spectrograph – the most accurate currently in operation – which is installed on the Very Large Telescope in Chile. Proxima b was first detected four years ago by means of an older spectrograph, HARPS – also developed by the Geneva-based team – which measured a low disturbance in the star’s speed, suggesting the presence of a companion.

Above – This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. © ESO/M. Kornmesser

The ESPRESSO spectrograph has performed radial velocity measurements on the star Proxima Centauri, which is only 4.2 light-years from the Sun, with an accuracy of 30 centimetres a second (cm/s) or about three times more precise than that obtained with HARPS, the same type of instrument but from the previous generation.

ESPRESSO has made it possible to measure the mass of the planet with a precision of over one-tenth of the mass of Earth.

Proxima b is about 20 times closer to its star than the Earth is to the Sun, it receives comparable energy, so that its surface temperature could mean that water (if there is any) is in liquid form in places and might, therefore, harbor life.

Having said that, although Proxima b is an ideal candidate for biomarker research, there is still a long way to go before we can suggest that life has been able to develop on its surface. In fact, the Proxima star is an active red dwarf that bombards its planet with X rays, receiving about 400 times more than the Earth.

Researchers confirmed the presence of the Earth-sized exoplanet Proxima b using independent measurements obtained with the new ESPRESSO spectrograph, and refined the planetary parameters taking advantage of its improved precision.

The ESPRESSO data on its own shows Proxima b at a period of 11.218 ± 0.029 days, with a minimum mass of 1.29 ± 0.13 M⊕. In the combined dataset we measure a period of 11.18427 ± 0.00070 days with a minimum mass of 1.173 ± 0.086 M⊕. We get a clear measurement of the stellar rotation period (87 ± 12 d) and its induced RV signal, but no evidence of stellar activity as a potential cause for the 11.2 days signal. We find some evidence for the presence of a second short-period signal, at 5.15 days with a semi-amplitude of only 40 cm·s
−1. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 ± 0.08 M⊕

The extended spectral range of ESPRESSO with respect to HARPS, combined with the collecting power of the VLT, allows us to split the spectrum into different wavelength bins to create independent RV series, while maintaining a good photon noise level in each bin. We find that we can measure the decline of a low-amplitude activity signal towards redder wavelengths, as would be expected for spot-induced variations. The planetary signal on the other hand shows a constant velocity amplitude across the full wavelength range, as is also expected for Keplerian signals. We define a chromatic RV, based on the difference between the red and blue velocities, which seems to efficiently track the activity variations of Proxima. Using the time series of the FWHM of the CCF and its gradient, we are able to model the stellar activity in a similar way to the F/F’ method (Aigrain et al. 2012), obtaining good results when detrending the data from activity to recover the planetary signal.

SOURCES- Arxiv, Université de Genève.
Written By Brian Wang,