A planet-hunting consortium is marking five successful years by releasing data on more than 50 worlds.
Astronomers designed the CARMENES instrument to find Earth-size planets in their habitable zones. And it has delivered. Since it came online in 2016, it has resulted in a flurry of exciting results.
The CARMENES consortium is now celebrating its first five years of operations with the publication of visible-light measurements for more than 350 stars in Astronomy & Astrophysics. (Infrared data is forthcoming.) In these results, the collaboration announces 33 new planets, 26 planets confirmed from previous transit surveys, and a re-analysis of 17 known planets. Four of the planets are “potentially habitable,” an optimistic shorthand meaning they are rocky planets that could host liquid water on their surfaces. All of the planets in the CARMENES sample are within 60 light-years of Earth.
CARMENES resides on the 3.5-meter telescope at the Calar Alto Observatory in Spain. It consists of two spectrographs, one collecting light in the visible range and another in the infrared, both of them used to measure changes in the radial velocity of small, cool stars, small shifts that reveal the gravitational tugs of planets.
The new data set enables the team to calculate how often stars host planets. “We found an occurrence rate of 1.4 planets per star, which has never been determined before at high precision for low-mass stars using the radial velocity technique,” explains project scientist Ignasi Ribas (Institute of Space Sciences and Institute of Space Studies of Catalonia, Spain).
The Kepler space telescope previously found a similarly high planetary occurrence rate using the transit method, by which astronomers measure the radius of a planet. Radial velocity measurements provide mass estimates instead. The fact that two different techniques obtain similar results is significant, providing good evidence that nearly all cool stars have at least one planet.
Stéphane Udry (Geneva Observatory), who is not on the CARMENES team, suggests that there is some evidence of a relationship between stellar mass and planet size. “It seems to be the case, as we have seen from different surveys as well, that smaller mass stars have a larger number of small planets,” he says.
“But CARMENES also found Jupiter-mass planets orbiting low-mass stars,” he adds, “which is a difficulty in the context of planet formation models. This brings up the whole question of the establishment of the architecture of planetary systems.”
There are a few different planet-formation scenarios. Some focus on gravitational instabilities in the disks of planetary material that surround young stars. Massive and cold disks are particularly prone to such instabilities. Other ideas focus instead on how tiny grains stick together, eventually gathering into planetesimals. None of these scenarios easily produce Jupiter-mass planets around low-mass stars. But CARMENES found 12 such systems, something that scientists will need to explain.
CARMENES also found time to follow up on some planet candidates previously found by the K2 and Transiting Exoplanet Survey Satellite (TESS) missions, adding mass data to the radii estimated from the transit detections.
CARMENES is also helping astronomers get a better handle on stars’ behavior, helping to disentangle planetary signals from starspots and other stellar activity. “A stellar activity signal will have a stronger presence in the optical wavelength and a lower presence in the near-infrared,” says Diana Kossakowski (Max Planck Institute), who works on statistical analysis for the team. “So, because we have both, if we see a signal is larger in the optical and smaller in the near-infrared, we could say it is most likely due to the star and not a planetary companion.”
Now that the first public data release has been made, it is up to the rest of the scientific community to vet their results and hopefully, make further discoveries. “It is good, robust work on a quite large sample of stars, so I think it is reliable,” sums up Céline Reylé (Besançon Observatory, France), who is not a member of the consortium. “It demonstrates that we can have important results on planets around low-mass stars from a ground-based telescope.”