Common, relatively cool and potentially life-supporting, red dwarf stars are of particular interest to astronomers hunting down distant worlds. The latest study of Kepler data has now identified 15 new exoplanets around red dwarfs, including a system of three "Super-Earths" – one of which appears to be orbiting within the host star's habitable zone.

According to NASA's latest count, there are 3,706 confirmed exoplanets – worlds orbiting a star other than the Sun – with many more candidates awaiting closer inspection. The 15 newest confirmations come courtesy of researchers at Tokyo Institute of Technology. The team studied data gathered by K2, the second phase of the planet-hunting Kepler mission, before following up with observations by the Subaru Telescope in Hawaii and the Nordic Optical Telescope in Spain.

Of the 15 new exoplanets, three in particular caught the team's eye, orbiting a relatively bright red dwarf known as K2-155, about 200 light-years away. These were spotted using the transition method, where planets give their presence away when they pass in front of the star, causing dips in its observed brightness.

K2-155's three planets are what astronomers call Super-Earths, meaning they're about 1.5 to two times bigger than our home world. Planets of this size are thought to be the most likely to house life-friendly conditions, especially when they fall within their host star's habitable zone, the region around a star where conditions are just right for liquid water to exist on the surface.

The system's outermost planet, dubbed K2-155d, seems to orbit within that Goldilocks zone. According to the researchers' calculations, the planet has a radius about 1.6 times that of Earth, and has a similar atmosphere and composition. Their global climate simulations suggest it may even host liquid water.

That said, it's still very early days, and these findings are largely based on simulations. Before any concrete conclusions can be made about its habitability, more precise measurements will need to be taken of the planet and the star.

Analysis of the 15 new exoplanets provides evidence that exoplanets orbiting red dwarfs have similar characteristics to those that orbit other types of stars. There seems to be a "gap" in the number of exoplanets with a radius between about 1.5 and 2 times that of Earth. This trend has been well observed in Sun-like star systems, but this marks the first time it's been noticed in red dwarfs too. The team also found a connection between the size of the planets and the amount of metal in the host star – another trend only previously seen in solar-type stars.

"Large planets are only discovered around metal-rich stars," says Teruyuki Hirano, lead researcher of the team. "And what we found was consistent with our predictions. The few planets with a radius about three times that of Earth were found orbiting the most metal-rich red dwarfs."

The research was published in two articles in The Astronomical Journal.

Source: Tokyo Institute of Technology

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