The paper, titled "Statistics and Habitability of F-type Star-Planet Systems", is published in the September 12 edition of The Astrophysical Journal Supplement Series. Shaan Patel, a UTA doctoral student in physics, is lead author of the paper and is joined by co-authors Manfred Cuntz and Nevin Weinberg, UTA professors of physics.

Stars fall into seven lettered categories according to their surface temperature. They also differ in other factors including mass, luminosity, and radius. F-types are in the middle of the scale, more massive and hotter than our Sun. F-type stars are yellowish-white in color and have surface temperatures in the range of 6200-7200 K.

A habitable zone (HZ) is the distance from a star at which water could exist on orbiting planets' surfaces. In their new study, the physicists presented a detailed statistical analysis of the currently known planet-hosting F-type stars by making use of the NASA Exoplanet Archive.

"F-type stars are usually considered the high-luminosity end of stars with a serious prospect for allowing an environment for planets favorable for life," Cuntz said. "However, those stars are often ignored by the scientific community. Although F-type stars have a shorter lifetime than our Sun, they have a wider HZ. In short, F-type stars are not hopeless in the context of astrobiology."

"F-type star systems are important and intriguing cases when dealing with habitability due to the larger HZs and enhanced UV flux, a measurement of the amount of UV radiation that reaches a planet's surface," Patel said. "HZs are defined as areas in which conditions are right for Earth-type bodies to potentially host exolife."

After disregarding systems for which little or no information about the planets exist, the team identified 206 systems of interest. They also evaluated the stars in each system to see if they were on the main sequence, meaning they fuse hydrogen nuclei to form helium nuclei in their cores. Most stars in the universe are main sequence stars, including the Sun. According to the authors, 60-80 of the stars in those systems were identified as main sequence stars.

"We found in 18 of 206 F-type systems studied that the planet spends at least part of its orbit in the stellar habitable zone," Patel said. "We further broke down those 18 systems into four sub-categories depending on how much time they would spend in the HZ."

In one case, HD 111998, also known as 38 Virginis, the planet is situated in the HZ at all times. The star 38 Virginis is located 108 light-years from Earth and is thus considered to be part of the extended Solar System neighborhood. It is 18 percent more massive than the Sun and has a radius 45 percent greater than the Sun, Cuntz said. It is barely visible with the unaided eye under ideal viewing conditions but can be seen easily with a small telescope, he said.

"The planet in question was discovered in 2016 at La Silla, Chile," Cuntz said. "It is a Jupiter-type planet which is unlikely to permit life itself, but it offers the general prospect of habitable exomoons, an active field of worldwide research, also pursued here at UTA."

"In future studies, our work may serve to investigate the existence of Earth-mass planets and also habitable exomoons hosted by exo-Jupiters in F-type systems," Patel said.

Among possible future projects, the team noted, are studies of planetary orbits, including cases of part-time HZ planets; explorations of the relationships between planetary habitability and stellar evolution, including astrobiological aspects; and assessments of exomoons for distinct systems.

"What makes a study like this possible is the hard work and dedication of the worldwide community of astronomers who have discovered more than 5,000 planets over the last 30 years," Weinberg said. "With so many known planets, we can now carry out statistical analyses of even relatively rare systems, such as planets orbiting F-type stars, and identify those that might reside in the habitable zone."

Cuntz has been studying F-type stars for more than a decade. In 2014, he and former graduate student Satoko Sato, who now pursues research in Japan, teamed with researchers from the University of Guanajuato in Mexico to publish a paper in The International Journal of Astrobiology. The study showed that "at least in the outer portions of F-star habitable zones, UV radiation should not be viewed as an insurmountable hindrance to the existence and evolution of life."

Research Report:Statistics and Habitability of F-type Star-Planet Systems

Related Links
NASA Exoplanet Archive
Lands Beyond Beyond - extra solar planets - news and science
Life Beyond Earth