HD 97658 is a star 70 light years away in the constellation of Leo. The star is slightly too dim to be seen with the naked eye, though almost any additional equipment will allow it to be seen. The star itself is a metal-poor early K-dwarf that is somewhat older than the sun.
Planetary system
On November 1, 2010, a super-Earth was announced orbiting the star along with Gliese 785 b as part of the NASA-UC Eta-Earth program. The planet orbits in just under 9.5 days and was originally thought to have a minimum mass of 8.2 ± 1.2 M⊕. Spurred by the possibility of transits, additional data was acquired for less than a year which found a lower mass for the star and hence reduced the minimum mass of the planet to 6.4 ± 0.7 M⊕, and improved certainty on the time of possible transit. Transits of the planet were apparently detected and announced on September 12, 2011; this would make HD 97658 the second-to-brightest star with a transiting planet after 55 Cancri and indicating a low-density planet like Gliese 1214 b. However, the occurrence of transits was quietly retracted on April 11, 2012, and three days later it was announced that observations by the MOST space telescope could not confirm transits. Transits of radii larger than 1.87 R⊕ were ruled out. Further transit measurements were taken in April 2012 and were confirmed with transit readings made in the following year, March and April 2013. It was determined that HD 97658 b had a diameter 2.34 times that of Earth. Using a radial velocity mass of 7.86 M⊕ and the radius measured from the transits taken in 2012 and 2013 and in early 2014, the density of the planet was calculated as 3.44 g cm−3. It is likely therefore that the super-Earth exoplanet HD 97658 b has a large rocky core covered with a thick layer of volatiles, either a deep ocean of water or a thick atmosphere possibly made up of a mixture of helium and hydrogen. The gravity on this exoplanet's surface is about 1.6 times greater than that of Earth's. The planetary transmission spectrum of HD 97658 b taken in 2020 have revealed presence of clouds up to millibar pressure. Although no conclusion can be made on atmosphere composition, best model fitting is obtained with hydrogen-helium envelope with carbon monoxide and methane admixture. No helium was detected at HD 97658 b in 2020 though.
