New Super-Earth Planet Discovered Orbiting Nearby Star HD 176986
An international team of astronomers led by the Instituto de Astrofísica de Canarias (IAC) has discovered a new super-Earth, designated HD176986 d, orbiting the K-type dwarf star HD 176986, located approximately 91 light-years from Earth. The finding, published in in the journal Astronomy and Astrophysics, brings the total number of confirmed planets in this system to three and underscores the value of long-term observation campaigns in identifying smaller, more distant worlds.
HD 176986, an orange dwarf star slightly smaller than our Sun, was already known to host two exoplanets: HD 176986 b and HD 176986 c, discovered in by a team led by IAC researcher Alejandro Suárez Mascareño. These planets orbit the star with periods of 6.5 and 16.8 days, respectively. The discovery of HD176986 d was made possible by continuing observations over several years using advanced spectroscopic instruments.
“We continued to observe the star for years with cutting-edge instruments, and it was rewarding when the signal of a third planet came out once we joined together all the observations,” explains Nicola Nari, first author of the study and a PhD student at the IAC. The team utilized the HARPS, ESPRESSO, and HARPS-N spectrographs, located in Chile and La Palma, to gather the data necessary to detect the subtle gravitational wobble caused by the new planet.
A Super-Earth in a Multi-Planet System
HD176986 d has a minimum mass of approximately 6.8 times that of Earth, placing it firmly within the “super-Earth” category – exoplanets with masses greater than Earth but smaller than ice giants like Neptune. This mass falls between the masses of the other two planets in the system: HD 176986 b, with a minimum mass five times that of Earth, and HD 176986 c, which has a mass of about ten times Earth’s mass.
The newly discovered planet completes one orbit around its star every 61.4 days, following a wider orbit than the inner planets. This longer orbital period, combined with the planet’s relatively small size, makes it a particularly challenging object to detect. The team notes that only a dozen planets with orbital periods exceeding 50 days and masses below seven Earth masses are currently known, highlighting the difficulty of finding such worlds.
Detecting these faint signals requires extensive observation and careful data analysis. The primary challenge lies in distinguishing the subtle effects of a small, distant planet from the inherent “noise” of stellar activity. “One of the most complex tasks when detecting a new planet is determining whether the signal found in the data has a planetary nature or is related to stellar activity,” the IAC stated.
To address this, the team employed rigorous testing to rule out any correlation with the star’s activity. “We performed different tests to discard an origin related to stellar activity. The planet passed all of them,” says Atanas K. Stefanov, a doctoral student at the IAC and co-author of the study.
Innovative Data Analysis Techniques
The detection was also aided by the use of innovative data processing techniques, specifically the YARARA tool. YARARA is designed to remove sources of noise that can mimic or obscure planetary signals, and to validate the detection of weaker signals. “YARARA corrects the sources of noise that can imitate or hide a planetary signal and invalidate the investigation of the weakest signals,” explains Michael Cretignier, a postdoctoral researcher at the University of Oxford and developer of YARARA.
The team used the radial velocity (RV) method to discover the planet, which measures the wobble of the star caused by the gravitational pull of orbiting planets. More than 350 nights of observations were collected using the HARPS, ESPRESSO, and HARPS-N spectrographs. HARPS and ESPRESSO are located in Chile, while HARPS-N is installed at the Roque de los Muchachos Observatory in La Palma.
“Our facilities for observations in La Palma have once again demonstrated their fundamental importance for new scientific discoveries,” concludes Rafael Rebolo López, a researcher at the IAC and co-author of the article. The discovery of HD176986 d demonstrates the power of persistent observation and advanced data analysis in uncovering hidden worlds around nearby stars. The IAC’s continued monitoring of HD 176986 may yet reveal further surprises within this intriguing planetary system.