[Paper Review] The HARPS search for southern extra-solar planets V. A 14 Earth-masses planet orbiting HD 4308

S. Udry, M. Mayor+8 more|ArXiv.org|Oct 12, 2005

Stellar, planetary, and galactic studiesPhysics and Astronomy38 references86 citations

TL;DR

This paper reports the discovery of a 14 Earth-mass planet orbiting the metal- deficient star HD 4308 using high-precision radial-velocity measurements from the HARPS spectrograph. With a 15.56-day orbital period and a radial-velocity semi-amplitude of 4.1 m s⁻¹, the planet represents one of the lowest-mass exoplanets detected at the time, demonstrating HARPS' capability to detect sub-Earth-mass planets and challenging assumptions about planet formation in low-metallicity environments.

ABSTRACT

We present here the discovery and characterisation of a very light planet around HD4308. The planet orbits its star in 15.56 days. The circular radial-velocity variation presents a tiny semi-amplitude of 4.1 m/s that corresponds to a planetary minimum mass m2sin(i)=14.1 Earth masses. The planet was unveiled by high-precision radial-velocity measurements obtained with the HARPS spectrograph on the ESO 3.6-m telescope. The radial-velocity residuals around the Keplerian solution are 1.3 m/s, demonstrating the very high quality of the HARPS measurements. Activity and bisector indicators exclude any significant perturbations of stellar intrinsic origin, which supports the planetary interpretation. Contrary to most planet-host stars, HD4308 has a marked sub-solar metallicity ([Fe/H]=-0.31), raising the possibility that very light planet occurrence might show a different coupling with the parent star's metallicity than do giant gaseous extra-solar planets. Together with Neptune-mass planets close to their parent stars, the new planet occupies a position in the mass-separation parameter space that is constraining for planet-formation and evolution theories. The question of whether they can be considered as residuals of evaporated gaseous giant planets, ice giants, or super-earth planets is discussed in the context of the latest core-accretion models.

Motivation & Objective

• To detect low-mass planets around solar-type stars using high-precision radial-velocity measurements.
• To characterize the physical and orbital properties of a newly discovered planetary companion to HD 4308.
• To investigate the relationship between planetary mass, orbital distance, and host star metallicity, particularly for low-mass planets.
• To assess whether such planets could be remnants of evaporated gas giants or formed via core accretion in low-metallicity environments.
• To demonstrate the performance of HARPS in detecting sub-Earth-mass planets by minimizing stellar noise from oscillations and activity.

Proposed method

• High-precision radial-velocity measurements were obtained using the HARPS spectrograph on the ESO 3.6-m telescope at La Silla Observatory.
• Averaging of 205 individual spectra improved signal-to-noise ratio and reduced stellar oscillation noise.
• Orbital parameters were derived by fitting a Keplerian model to the radial-velocity data, yielding a period of 15.56 days and a semi-amplitude of 4.1 m s⁻¹.
• Stellar activity and bisector span variations were monitored to rule out false positives from stellar surface inhomogeneities.
• Stellar parameters, including metallicity ([Fe/H] = -0.31), were derived from LTE analysis of high-resolution HARPS spectra.
• The radial-velocity residuals around the Keplerian fit were as low as 1.3 m s⁻¹, confirming measurement precision.

Experimental results

Research questions

• RQ1 Can HARPS detect planets with masses as low as 14 Earth masses around solar-type stars?
• RQ2 Does the occurrence of low-mass planets correlate with host star metallicity, especially in metal-poor systems like HD 4308?
• RQ3 Is the HD 4308 b planet a remnant of a more massive, evaporated gas giant, or a direct product of core accretion?
• RQ4 How do the orbital and physical properties of HD 4308 b compare to other Neptune-mass planets in the mass-separation plane?
• RQ5 Can the low radial-velocity residuals (1.3 m s⁻¹) confirm the planetary origin of the signal and rule out stellar activity?

Key findings

• A planet with a minimum mass of 14.1 Earth masses was detected orbiting HD 4308 with a period of 15.56 days.
• The radial-velocity semi-amplitude of 4.1 m s⁻¹ corresponds to a planetary signal with a precision of 1.3 m s⁻¹ in residuals, confirming HARPS' high stability.
• The host star HD 4308 has a sub-solar metallicity of [Fe/H] = -0.31, which is unusual for a planet-host star and challenges the metallicity-planet formation correlation seen in giant planets.
• Stellar activity and bisector span measurements showed no significant variations, supporting the planetary origin of the signal.
• The planet lies in the Neptune-mass regime at a short orbital distance (0.115 AU), placing it in a region of parameter space that constrains core-accretion and evaporation models.
• The discovery suggests that low-mass planets may form independently of high-metallicity conditions, indicating a different formation pathway than gas giants.