A study discovers a new super-Earth in a nearby planetary system

The discovery, published in the journal ‘Astronomy and Astrophysics’, increases the number of known planets in this system to three and confirms the value of prolonged observation campaigns to detect small worlds with wide orbits, as reported by the scientific institution in a press release.

The observation campaign of HD 176986, an orange dwarf or K-type star, slightly smaller than the Sun and located about 91 light years away, would demonstrate the importance of prolonged monitoring of this type of objectives.

Specifically, this star has been known to host planets since 2018, when a scientific analysis led by IAC researcher Alejandro Suárez Mascareño, co-author of the new study, allowed the discovery of two planets that orbit around it with periods of 6.5 and 16.8 days, called HD 176986 by HD 176986 c.

“We continued observing the star for years with state-of-the-art instruments. It was very gratifying when, when all the observations were put together, the signal from the third planet appeared,” explains Nicola Nari, first author of the study recently published in Astronomy and Astrophysics and a doctoral student at the IAC.

About the Planetary System

The new planet, HD176986 d, has a minimum mass of less than seven times that of Earth. This places it between its two neighbors in the same system: the planet closest to the star, with a minimum mass of five times that of Earth, and the most distant, which reaches about ten times the mass of our planet.

HD176986 d completes one orbit around its star every 61.4 days, following a wider orbit than the inner planet. Due to its size and mass, it is classified within the category of so-called super-Earths, a type of planet more massive than Earth but considerably smaller than the gas giants.

Specifically, only a dozen planets are known with orbital periods greater than 50 days and masses less than seven times that of Earth. These types of worlds are especially difficult to detect.

The main reason is that small planets far from their star produce very weak signals, which require a large number of observations and prolonged monitoring to be reliably identified, as has happened in the case of HD 176986 d.

“Not many super-Earths have been detected around K dwarfs with orbital periods greater than 50 days; only a specific long-term study can resolve their wide orbit and low amplitude signals,” says Alejandro Suárez Mascareño, second author of the article and IAC researcher.

“We continued observing the target and, in the end, the signal appeared,” adds Jonay I. González Hernández, research coordinator at the IAC and co-author of this work.

Observations against weak signals

The IAC emphasizes that one of the “most complex” tasks to detect a new planet is to determine if the signal found in the data is planetary in nature or if it is related to stellar activity.

“We carried out different tests to rule out 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 article.

The detection was also favored by the use of innovative techniques that allow the spectra – the light data of the star – to be refined and better separate the effects of stellar activity and possible instrument imperfections. This advance was possible thanks to the YARARA analysis tool.

“YARARA corrects for noise sources that can mimic or obscure a planetary signal and invalidate the investigation of weaker signals,” says Michael Cretignier, a postdoctoral researcher at the University of Oxford, developer of YARARA and co-author of the work.

“It was exciting to see that the signal was still there after the YARARA correction, a sigh of relief,” says Xavier Dumusque, associate professor at the University of Geneva and co-author of the study.

Find

The planet was discovered with the radial velocity (RV) method, which measures the motion of the star induced by the gravitational attraction of the planets orbiting it.

Specifically, more than 350 nights of observations were collected with the HARPS, ESPRESSO and HARPS-N spectrographs. HARPS and ESPRESSO are installed in Chile, on the 3.6 m telescope of the La Silla Observatory and on the VLT telescope of the Paranal Observatory, respectively, while HARPS-N is installed on the Telescopio Nazionale Galileo of the Roque de los Muchachos Observatory, on La Palma.

“Our observation facilities on La Palma have once again demonstrated their fundamental importance for new scientific discoveries,” concludes Rafael Rebolo López, IAC researcher and co-author of the article.