[Paper Review] The HARPS survey for southern extra-solar planets II. A 14 Earth-masses exoplanet around mu Arae

N. C. Santos, F. Bouchy+14 more|CERN Bulletin|Aug 25, 2004

Stellar, planetary, and galactic studiesPhysics and Astronomy25 references145 citations

TL;DR

This paper reports the discovery of a 14 Earth-mass exoplanet orbiting the G5V star mu Arae using radial velocity measurements from the HARPS spectrograph, achieving a precision of 0.9 m s⁻¹. The planet, with a 9.5-day orbital period, represents the lowest-mass planet detected at the time, challenging formation models and suggesting it is a super-Earth rather than a failed ice giant due to its low mass and formation environment.

ABSTRACT

In this letter we present the discovery of a very light planetary companion to the star mu Ara (HD160691). The planet orbits its host once every 9.5days, and induces a sinusoidal radial velocity signal with a semi-amplitude of 4.1 m/s, the smallest Doppler amplitude detected so far. These values imply a mass of m2 sini = 14 earth-masses. This detection represents the discovery of a planet with a mass slightly smaller than that of Uranus, the smallest ``ice giant" in our Solar System. Whether this planet can be considered an ice giant or a super-earth planet is discussed in the context of the core-accretion and migration models.

Motivation & Objective

• To detect low-mass exoplanets in the southern sky using high-precision radial velocity measurements.
• To investigate the formation and nature of a newly discovered planetary companion around mu Arae with a minimum mass of 14 M⊕.
• To determine whether the planet is a super-Earth or a failed ice giant based on orbital and stellar characteristics.
• To assess the detectability of Earth-mass planets with current instrumentation like HARPS.
• To contribute to statistical understanding of planetary systems by identifying low-mass planets in multi-planet systems.

Proposed method

• Radial velocity measurements were obtained using the HARPS spectrograph on the 3.6-m ESO telescope at La Silla Observatory.
• The data were analyzed using a Keplerian orbital fit combined with a linear trend to account for long-period companions.
• Stellar parameters including effective temperature, surface gravity, metallicity, and mass were derived from high-S/N HARPS spectra.
• The radial velocity signal was modeled to extract the planet's orbital period (9.5 days) and semi-amplitude (4.1 m s⁻¹).
• Stellar activity and rotational modulation were ruled out as false positives using low log R′HK and low projected rotational velocity (2.4 km s⁻¹).
• Formation models including core accretion and disk migration were used to interpret the planet's origin and composition.

Experimental results

Research questions

• RQ1 Can HARPS detect planets with masses below 20 Earth masses using radial velocity techniques?
• RQ2 Is the 14 M⊕ planet around mu Arae a super-Earth or a failed ice giant, based on its orbital and stellar environment?
• RQ3 What constraints do the planet's low mass and short orbital period place on core accretion and migration models?
• RQ4 How does the presence of a massive planet at 660 days affect the formation scenario of the inner 14 M⊕ planet?
• RQ5 What is the likelihood that stellar activity or rotational modulation mimics the observed radial velocity signal?

Key findings

• A planetary companion with a minimum mass of 14 M⊕ was detected via a radial velocity semi-amplitude of 4.1 m s⁻¹, the smallest Doppler signal measured at the time.
• The planet orbits mu Arae every 9.5 days, placing it in a short-period orbit close to its host star at 0.09 AU.
• Stellar activity and rotational modulation were ruled out as causes of the signal, given the star's low chromospheric activity (log R′HK = -5.034) and low projected rotational velocity (2.4 km s⁻¹).
• The planet's low mass and formation environment suggest it never reached the critical core mass required for runaway gas accretion, indicating it is a super-Earth rather than a failed ice giant.
• The system hosts a more massive planet with a 660-day period, suggesting the inner planet formed later, possibly inside the ice line at ≤3 AU.
• The detection demonstrates HARPS's capability to reach sub-1 m s⁻¹ precision, enabling the discovery of low-mass planets near Earth-mass thresholds.