Three Super-Earths Orbiting HD 7924

HD 7924, a nearby K-dwarf star located ~55 light-years away, hosts a planetary system comprised of three super-Earths. High precision radial velocity measurements made using the High Resolution Echelle Spectrometer (HIRES) at Keck Observatory in Hawaii and the Automated Planet Finder Telescope (APF) at Lick Observatory in California robustly detected the three super-Earths around HD 7924. They are identified as HD 7924 b, HD 7924 c and HD 7924 d, with the lower case alphabetical suffixes denoting their planetary nature.

Super-Earths are a class of planets intermediate in size and mass between Earth and Neptune. Such planets do not exist in the Solar System even though observations have shown that they are ubiquitous around stars. The three super-Earths around HD 7924 were found using the radial velocity method. It involves measuring the tiny “wobbling” of a star due to gravitational tugging from an unseen planet that is orbiting the star. In the case of HD 7924, the way it “wobbles” indicates the presence of three super-Earths in orbit around it.

HD 7924 b has an orbital period of 5.40 days, is at least 8.7 times the mass of Earth, receives 114 times more insolation than Earth gets from the Sun and has an estimated equilibrium temperature of roughly 830 K.

HD 7924 c has an orbital period of 15.30 days, is at least 7.9 times the mass of Earth, receives 28 times more insolation than Earth gets from the Sun and has an estimated equilibrium temperature of roughly 580 K.

HD 7924 d has an orbital period of 24.45 days, is at least 6.4 times the mass of Earth, receives 15 times more insolation than Earth gets from the Sun and has an estimated equilibrium temperature of roughly 500 K.

Of the three super-Earths around HD 7924, HD 7924 c and HD 7924 d are new discoveries, while HD 7924 b was previously discovered in 2009. The orbits of all three super-Earths fit within a region of space less than half the size of Mercury’s orbit around the Sun. Although HD 7924 has only about one-third the Sun’s luminosity, the three super-Earths orbit way too close and receive too much stellar irradiation from HD 7924 to be habitable. 

Deconvoluted radial velocity curves of HD 7924 b (top), HD 7924 c (middle) and HD 7924 d (bottom). Open black squares indicate pre-upgrade Keck/HIRES data, open black circles are post-upgrade Keck/HIRES data, and filled green diamonds are APF data. Fulton et al. (2015).

Reference:
Fulton et al. (2015), “Three Super-Earths Orbiting HD 7924”, arXiv:1504.06629 [astro-ph.EP]