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Using radial velocity measurements, an international team of astronomers has identified a second planet orbiting a nearby M-dwarf star known as Gliese 536. The newfound alien world turns out to be at least a few times more massive than Earth. The findings are in the upcoming issue of Astronomy & Astrophysics and currently on the arXiv preprint server.

The radial velocity (RV) method of detecting an exoplanet is based on the identification of variations in the velocity of the host star, due to the changing direction of the gravitational pull from an unseen exoplanet as it orbits the star. Using this technique, astronomers have detected more than 600 extrasolar worlds so far.

Located some 34 light years away, Gliese 536 (GJ 536) is an M dwarf about half the size and mass of the sun. The star has an effective temperature of 3,641 K and a metallicity at a level of -0.08 dex. The age of GJ 536 is estimated to be 4.2 billion years.

In 2017, an exoplanet was detected on an 8.7-day orbit around GJ 536. The planet is assumed to be a super-Earth, given that its minimum mass was measured to be 6.37 Earth masses. The planet encircles its host at a distance of some 0.067 AU from it and its equilibrium temperature is approximately 451 K.

Now, a group of astronomers led by Alejandro Suárez Mascareño of the University of La Laguna, Spain, reports the detection of another exoplanet orbiting GJ 536. The finding is a result of analyzing archival observations from the High Accuracy Radial velocity Planet Searcher (HARPS), HARPS-N, the Calar Alto high Resolution search for M-dwarfs with Exoearths with Nearinfrared and optical Échelle Spectrographs (CARMENES) and the High Resolution Echelle Spectrometer (HIRES).

"Based on an analysis of the radial velocity (RV) time series obtained from the available data of the spectrographs HARPS, HARPS-N, CARMENES and HIRES, we announce the discovery of a second low-mass planet orbiting the star," the researchers write in the paper.

According to the study, the newfound planet GJ 536 c has a minimum mass of approximately 5.89 Earth masses. It orbits the parent star every 32.76 days, at a distance of some 0.16 AU from it. The planet's equilibrium temperature is estimated to be 290.5 K.

Therefore, GJ 536 c is the second super-Earth in the system. The astronomers noted that the planet receives an insolation slightly lower than that received by Venus. The properties of GJ 536 c make it one of the low-mass exoplanets amenable to atmospheric characterization using its reflected light.

The study also found that the star GJ 536 rotates every 43.63 days. This, together with the observed variations in radial velocity, suggest that the star has a spot-dominated stellar surface in which plages surround the spots.

In concluding remarks, the authors of the paper underlined that re-investigation of archival data, like in the case of GJ 536, may uncover alien worlds that have previously escaped detection.

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