[Paper Review] The HARPS-N Rocky Planet Search I. HD219134b: A transiting rocky planet in a multi-planet system at 6.5 pc from the Sun

F. Motalebi, S. Udry+37 more|University of Birmingham Research Portal (University of Birmingham)|Jul 30, 2015

Stellar, planetary, and galactic studiesPhysics and Astronomy84 references81 citations

TL;DR

This paper presents the discovery of HD 219134 b, the nearest known transiting rocky exoplanet (6.5 pc away) and the first result from the HARPS-N Rocky Planet Search. Using radial velocity measurements and Spitzer photometry, the study confirms a multi-planet system with a 4.46 M⊕ super-Earth on a 3.09-day orbit, yielding a mean density of 5.89 g cm⁻³ and a core mass fraction of ~22%, indicating a rocky composition consistent with terrestrial planets.

ABSTRACT

We present here the detection of a system of four low-mass planets around the bright (V=5.5) and close-by (6.5 pc) star HD219134. This is the first result of the Rocky Planet Search program with HARPS-N on the TNG in La Palma. The inner planet orbits the star in 3.0937 +/-0.0004 days, on a quasi-circular orbit with a semi-major axis of 0.0382 +/- 0.0003 AU. Spitzer observations allowed us to detect the transit of the planet in front of the star making HD219134b the nearest known transiting planet to date. From the amplitude of the radial-velocity variation (2.33 +/- 0.24 m/s) and observed depth of the transit (359 +/- 38 ppm), the planet mass and radius are estimated to be 4.46 +/- 0.47 M_{\oplus} and 1.606 +/- 0.086 R_{\oplus} leading to a mean density of 5.89 +/- 1.17 g/cc, suggesting a rocky composition. One additional planet with minimum mass of 2.67 +/- 0.59 M_{\oplus} moves on a close-in, quasi-circular orbit with a period of 6.765 +/- 0.005 days. The third planet in the system has a period of 46.78 +/- 0.16 days and a minimum mass of 8.7 +/- 1.1 M{\oplus}, at 0.234 +/- 0.002 AU from the star. Its eccentricity is 0.32 +/- 0.14. The period of this planet is close to the rotational period of the star estimated from variations of activity indicators (42.3 +/- 0.1 days). The planetary origin of the signal is, however, the preferred solution as no indication of variation at the corresponding frequency is observed for activity-sensitive parameters. Finally, a fourth additional longer-period planet of mass of 62 +/- 6 M_{\oplus} orbits the star in 1190 days, on an eccentric orbit (e=0.27 +/- 0.11) at a distance of 2.14 +/- 0.27 AU.

Motivation & Objective

• To detect and characterize low-mass planets around nearby, bright stars using high-precision radial velocity measurements from HARPS-N.
• To search for transits of the innermost planet in the system using Spitzer Space Telescope photometry.
• To determine the physical properties, including mass, radius, and density, of the innermost planet to assess its composition.
• To investigate the dynamical architecture of the multi-planet system around HD 219134, including orbital parameters and potential resonances.
• To establish HD 219134 as a prime target for future atmospheric and interior characterization due to its proximity and brightness.

Proposed method

• High-precision radial velocity measurements were obtained using the HARPS-N spectrograph on the Telescopio Nazionale Galileo in La Palma.
• Spitzer Space Telescope conducted near-infrared photometry to search for transits of the innermost planet, HD 219134 b.
• Radial velocity data were modeled using Keplerian orbital fits to determine orbital elements and minimum masses.
• Transit depth and duration from Spitzer data were used to derive the planet's radius, assuming a circular orbit.
• Planet mass and radius were combined to calculate mean density, which was used to infer composition via comparison to interior structure models.
• Activity indicators were monitored to rule out stellar activity as the source of the radial velocity signal, confirming planetary origin.

Experimental results

Research questions

• RQ1 Is HD 219134 b a transiting planet, and what is its radius and orbital period?
• RQ2 What is the mass and density of HD 219134 b, and does it indicate a rocky or volatile-rich composition?
• RQ3 What is the dynamical architecture of the multi-planet system around HD 219134, including orbital eccentricities and periods?
• RQ4 Could the 46.78-day signal be due to stellar activity, or is it of planetary origin?
• RQ5 How does the system’s proximity and brightness make it a favorable target for future atmospheric and interior characterization?

Key findings

• HD 219134 b is the nearest known transiting exoplanet at 6.5 pc, with a V-magnitude of 5.5, making it the brightest transiting planet system discovered to date.
• The planet has a mass of 4.46 ± 0.47 M⊕ and a radius of 1.606 ± 0.086 R⊕, yielding a mean density of 5.89 ± 1.17 g cm⁻³, consistent with a rocky composition.
• The planet's core mass fraction is estimated at approximately 22%, indicating a composition similar to Earth but with a slightly lower iron content than Earth’s 30%.
• The system hosts three additional planets: a 2.67 M⊕ planet with a 6.765-day period, an 8.7 M⊕ planet with a 46.78-day period and eccentricity 0.32, and a 62 M⊕ planet with a 1190-day period and eccentricity 0.27.
• The 46.78-day signal is not due to stellar activity, as no modulation was detected in activity indicators at that frequency, supporting its planetary origin.
• The system’s proximity and brightness make it an ideal target for future atmospheric studies with JWST, TESS, and other upcoming missions.