Bulgarian astronomers have identified a new super-Earth exoplanet orbiting the star GJ 1137, with their findings published in the respected journal Astronomy & Astrophysics. The discovery was led by the EXO-RESTART project team at the Department of Astronomy, Sofia University "St. Kliment Ohridski," with doctoral student Denitsa Stoeva serving as lead author.

The newly discovered planet, named GJ 1137c, lies approximately 91 light-years from Earth and revolves around the star, also known as Macondo, in just 9.4 days. Macondo, smaller and cooler than the Sun, resides in the southern constellation Pompeii and was named after the fictional town in Gabriel García Márquez’s One Hundred Years of Solitude. With a mass exceeding five times that of Earth, GJ 1137c is classified as a super-Earth - a planet larger than our planet but notably smaller than Neptune, with no analogues in the Solar System.

This discovery relied on a comprehensive analysis of long-term observations collected between 2004 and 2017 using the HARPS spectrograph at the European Southern Observatory in La Silla, Chile. Over 140 precise measurements of the star’s radial velocities were examined to identify the planet’s signal. In parallel, the team refined parameters for the previously known GJ 1137b, a "warm Saturn," adjusting its mass from 0.37 to 0.45 Jupiter masses - around 20% higher than earlier estimates.

The study highlights the importance of accounting for stellar activity in exoplanet detection. Long-period magnetic cycles, similar to those observed in the Sun, can complicate planet detection. By carefully modeling the star’s radial velocities alongside indicators of stellar activity, the team was able to differentiate between the star’s intrinsic variability and the planetary signals. "The extensive dataset spanning 13 years allowed us to separate the ‘pulse’ of the star from the exoplanetary signals," explained Stoeva. Atanas Stefanov, a co-author, added that this approach prevented misinterpreting stellar activity as a Jupiter-like planet.

The discovery of GJ 1137c underscores the crucial role of sustained observations in exoplanetary science and demonstrates Bulgaria’s growing contribution to this rapidly advancing field. The EXO-RESTART project, coordinated by Trifon Trifonov and supported by the Scientific Research Fund under the national program VIHREN-2021, represents a significant step forward in understanding planetary systems beyond the Solar System and refining the characterization of known exoplanets.