The NASA-UC Eta-Earth Program: I. A Super-Earth Orbiting HD 7924

We report the discovery of the first low-mass planet to emerge from the NASA-UC Eta-Earth Program, a super-Earth orbiting the K0 dwarf HD 7924. Keplerian modeling of precise Doppler radial velocities reveals a planet with minimum mass M_P sin i = 9.26 M_Earth in a P = 5.398 d orbit. Based on Keck-HIRES measurements from 2001 to 2008, the planet is robustly detected with an estimated false alarm probability of less than 0.001. Photometric observations using the Automated Photometric Telescopes at Fairborn Observatory show that HD 7924 is photometrically constant over the radial velocity period to 0.19 mmag, supporting the existence of the planetary companion. No transits were detected down to a photometric limit of ~0.5 mmag, eliminating transiting planets with a variety of compositions. HD 7924b is one of only eight planets known with M_P sin i < 10 M_Earth and as such is a member of an emerging family of low-mass planets that together constrain theories of planet formation.

💡 Research Summary

The paper reports the first low‑mass planet discovered by the NASA‑UC Eta‑Earth Program: a super‑Earth orbiting the nearby K0 dwarf HD 7924. Using the Keck‑I telescope’s HIRES spectrograph, the authors obtained 198 high‑precision radial‑velocity (RV) measurements spanning 2001–2008. A Lomb‑Scargle periodogram of the RV data shows a dominant peak at 5.398 days. Extensive bootstrap simulations (10⁴ trials) yield a false‑alarm probability (FAP) of less than 0.001, confirming the robustness of the signal.

A single‑planet Keplerian model fits the data with an orbital period P = 5.398 days, eccentricity e ≈ 0.17 ± 0.09, and velocity semi‑amplitude K = 3.87 ± 0.31 m s⁻¹. Assuming a stellar mass of 0.83 M⊙, the minimum mass of the companion (M sin i) is 9.26 M⊕, placing it firmly in the super‑Earth regime. The semi‑major axis is a ≈ 0.057 AU, corresponding to an insolation roughly 150 times that of Earth, implying a very hot surface environment.

To rule out stellar activity as the source of the RV variation, the team conducted an intensive photometric campaign with the Automated Photometric Telescopes (APTs) at Fairborn Observatory. Over 1,200 observations were obtained in 2009–2010, achieving a typical precision of 0.15 mmag per measurement. The photometry shows no variability at the 5.398‑day period down to 0.19 mmag, indicating that starspots, plages, or pulsations cannot account for the RV signal.

The authors also searched for transits. Given the orbital geometry, a central transit would produce a depth of order 0.5 mmag for a rocky planet of Earth‑like radius. The APT data, however, reveal no transit events at this level, effectively excluding transiting configurations with radii larger than ~1.5 R⊕ for a range of plausible compositions. Consequently, the planet’s inclination remains unknown, and the true mass could be modestly higher than the minimum value.

HD 7924b joins a small but growing sample of planets with M sin i < 10 M⊕ (now eight known). Its host star’s modest metallicity (