Barnard's star is the closest single star to the Sun and second only to the Alpha Centauri triple stellar system.
At only six light-years from us, Barnard's star appears to move across Earth's night sky faster than any other star. This red-dwarf star, smaller and older than our Sun is among the least active red dwarfs known and represents an ideal target to search for exoplanets with various methods.
Since 1997, several instruments have been gathering a large amount of measurements on the star's subtle back-and-forth wobble. An analysis of the data collected up to 2015, including observations from HIRES/Keck, and ESO's HARPS and UVES spectrometers, suggested the wobble could be caused by a planet with an orbital period of about 230 days. To confirm this, however, more measurements were deemed necessary.
Trying to see if the result could be confirmed, astronomers regularly monitored Barnard's star with high precision spectrograph such as the CARMENES (Calar Alto Observatory in Spain), and also HARPS and HARPS-N in an international effort called the Red Dots collaboration. This technique consists on using the Doppler effect on the starlight to measure how the velocity of an object along our line of sight changes with time.
"For the analysis we used observations from seven different instruments, spanning 20 years, making this one of the largest and most extensive datasets ever used for precise radial velocity studies. The combination of all data led to a total of 771 measurements", explains Ignasi Ribas.
A clear signal at a period of 233 days arose again in the re-analysis of all the measurements combined. This signal implies that Barnard's star is approaching and moving away from us at about 1.2 m/s -approximately the walking speed of a person- and it is best explained by a planet orbiting it.
"After a very careful analysis, we are over 99% confident that the planet is there, since this is the model that best fits our observations", assures Ignasi Ribas. "However, we must remain cautious and collect more data to nail the case in the future, because natural variations of the stellar brightness resulting from starspots can produce similar effects to the ones detected." Follow-up observations are already happening at different observatories.
The planet candidate, named Barnard's star b (or GJ 699 b), is a super-Earth with a minimum of 3.2 Earth masses. It orbits its cool red parent star every 233 days near the snow-line, a distance where water would be frozen. In the absence of an atmosphere, its temperature is likely to be about -150 ºC, which makes it unlikely that the planet can sustain liquid water on its surface. However, its characteristics make it an excellent target for direct imaging using the next generation of instruments such as NASA's Wide Field InfraRed Survey Telescope (WFIRST, [3]), and maybe with observations from the ESA mission Gaia.
Exoplanets so small and so far away from their parent star have not been discovered before using the Doppler technique. This means that astronomers are getting better at finding and exploring a relatively new kind of planets outside our Solar System. With the next generation of instruments, these capabilities can only expand.
"We all have worked very hard on this result", said Guillem Anglada-Escude from Queen Mary University of London and co-leader of this work. "This is the result of a large collaboration organised in the context of the Red Dots project, which is why it has contributions from teams all over the world including semi-professional astronomers coordinated by the AAVSO."
Cristina Rodríguez-López, researcher at the Instituto de Astrofísica de Andalucía (IAA, CSIC) and co-author of the paper, comments on the significance of this finding. "This discovery means a boost to continue on searching for exoplanets around our closest stellar neighbours, in the hope that eventually we will come upon one that has the right conditions to host life".
Reference:
I.Ribas et al. 2018. A super-Earth planet candidate orbiting at the snow-line of Barnard's star. Nature. DOI: 10.1038/s41586-018-0677-y
