[Paper Review] The HARPS search for southern extra-solar planets. XXIV. Companions to HD 85390, HD 90156 and HD 103197: A Neptune analogue and two intermediate mass planets

C. Mordasini, M. Mayor+12 more|arXiv (Cornell University)|Oct 5, 2010

Stellar, planetary, and galactic studiesPhysics and Astronomy50 references18 citations

TL;DR

This paper reports the discovery of three new extrasolar planets using radial velocity measurements from the HARPS spectrograph: a Neptune analogue (17.98 M⊕, 49.8-day orbit) around HD 90156, and two intermediate-mass planets (31.2 M⊕ and 42.0 M⊕) around HD 103197 and HD 85390, respectively. The findings suggest these planets formed during the final phase of gas disk dissipation, with low gas accretion rates leading to their intermediate compositions between Neptune-like and Jupiter-like planets.

ABSTRACT

We report the detection of three new extrasolar planets orbiting the solar type stars HD 85390, HD 90156 and HD 103197 with the HARPS spectrograph mounted on the ESO 3.6-m telescope at La Silla observatory. HD 85390 has a planetary companion with a projected intermediate mass (42.0 Earth masses) on a 788-day orbit (a=1.52 AU) with an eccentricity of 0.41, for which there is no analogue in the solar system. A drift in the data indicates the presence of another companion on a long period orbit, which is however not covered by our measurements. HD 90156 is orbited by a warm Neptune analogue with a minimum mass of 17.98 Earth masses (1.05 Neptune masses), a period of 49.8 days (a=0.25 AU) and an eccentricity of 0.31. HD 103197 has an intermediate mass planet on a circular orbit (P=47.8 d, Msini=31.2 Earth masses). We discuss the formation of planets of intermediate mass (about 30-100 Earth masses) which should be rare inside a few AU according to core accretion formation models.

Motivation & Objective

• To identify and characterize low-mass planetary companions around solar-type stars using high-precision radial velocity measurements.
• To investigate the formation mechanisms of intermediate-mass planets (30–100 M⊕), which are rare in core accretion models but observed in radial velocity surveys.
• To constrain the gas accretion rate during runaway phase by analyzing the composition and orbital properties of newly detected planets.
• To assess the role of disk evolution and gap formation in limiting the final mass of gas-accreting planets.
• To improve theoretical planet formation models by comparing observed planetary frequency and mass distribution with synthetic populations.

Proposed method

• Radial velocity measurements were obtained using the HARPS spectrograph on the ESO 3.6 m telescope at La Silla Observatory.
• Orbits were determined by fitting Keplerian models to the radial velocity data, accounting for eccentricities and long-term trends.
• Stellar parameters including effective temperature, metallicity, and mass were derived from high-resolution HARPS spectra and literature data.
• Planetary masses were estimated using the projected mass (M sin i), with real masses inferred assuming random orbital inclination.
• Theoretical planet formation models were applied to interpret the observed masses and orbital parameters, particularly focusing on gas accretion rates during the runaway phase.
• Synthetic planetary populations were generated using Monte Carlo simulations to compare observed frequencies with theoretical predictions under different accretion scenarios.

Experimental results

Research questions

• RQ1 What are the orbital and physical properties of the newly detected planets around HD 85390, HD 90156, and HD 103197?
• RQ2 How do the masses and compositions of intermediate-mass planets (30–100 M⊕) constrain the rate of gas accretion during the runaway phase?
• RQ3 Why are intermediate-mass planets rare in core accretion models, and what conditions allow their formation?
• RQ4 To what extent do disk evolution and gap formation limit the final mass of gas-accreting planets?
• RQ5 Can the observed planetary population in the 30–100 M⊕ range be used to improve theoretical models of planet formation?

Key findings

• A Neptune analogue with a minimum mass of 17.98 M⊕ (1.05 MNeptune) and an orbital period of 49.8 days was detected around HD 90156, with a semi-major axis of 0.25 AU and eccentricity of 0.31.
• An intermediate-mass planet with a projected mass of 42.0 M⊕ and an orbital period of 788 days (semi-major axis 1.52 AU) was discovered around HD 85390, with an eccentricity of 0.41.
• A second intermediate-mass planet with a projected mass of 31.2 M⊕ and an orbital period of 47.8 days was detected around HD 103197, on a circular orbit at 0.25 AU.
• The two intermediate-mass planets are likely formed during the final phase of gas disk dissipation, when gas accretion rates had already declined to 10⁻⁴–10⁻³ M⊕/yr.
• Synthetic models indicate that these planets have compositions between Neptune-like and Jupiter-like, with typical gas mass fractions of 40% for the more massive planet (HD 85390 b) and 30% for the less massive one (HD 103197 b).
• The observed frequency of intermediate-mass planets can be used to constrain the efficiency of gas accretion and the role of gap formation in planet formation models.