[Paper Review] The Solar Twin Planet Search II. A Jupiter twin around a solar twin

Megan Bedell, J. Meléndez+9 more|GoeScholar The Publication Server of the Georg-August-Universität Göttingen (Georg-August-Universität Göttingen)|Jul 14, 2015

Stellar, planetary, and galactic studies28 references21 citations

TL;DR

This study reports the detection of a Jupiter-mass planet (HIP11915b) with a 3800-day orbital period around the solar twin star HIP11915, using high-precision radial velocity measurements from the HARPS spectrograph. Despite challenges from stellar activity cycles, joint analysis of radial velocity and activity indicators (Ca II H&K, BIS, FWHM) strongly favors a planetary origin over stellar activity, marking the first confirmed Jupiter twin around a solar twin.

ABSTRACT

Through our HARPS radial velocity survey for planets around solar twin stars, we have identified a promising Jupiter twin candidate around the star HIP11915. We characterize this Keplerian signal and investigate its potential origins in stellar activity. Our analysis indicates that HIP11915 hosts a Jupiter-mass planet with a 3800-day orbital period and low eccentricity. Although we cannot definitively rule out an activity cycle interpretation, we find that a planet interpretation is more likely based on a joint analysis of RV and activity index data. The challenges of long-period radial velocity signals addressed in this paper are critical for the ongoing discovery of Jupiter-like exoplanets. If planetary in nature, the signal investigated here represents a very close analog to the solar system in terms of both Sun-like host star and Jupiter-like planet.

Motivation & Objective

• To detect long-period planets, particularly Jupiter analogs, around solar twin stars using high-precision radial velocity monitoring.
• To address the challenge of stellar activity cycles that mimic planetary signals on similar timescales (e.g., 3800 days).
• To determine whether a long-period radial velocity signal in HIP11915 is due to a planet or stellar activity.
• To assess the planetary system's similarity to the Solar System by characterizing both the host star and the detected planet.
• To validate the robustness of radial velocity techniques in distinguishing planetary signals from activity-induced variations in solar twins.

Proposed method

• Conducted a Markov Chain Monte Carlo (MCMC) analysis on 56 radial velocity measurements from HARPS spanning 2003–2015.
• Employed activity diagnostics including Ca II H&K line flux (S$_{HK}$), bisector span (BIS), and full width at half maximum (FWHM) of cross-correlation functions.
• Used seasonal binning of data to average out short-timescale activity signals, reducing noise from stellar rotation cycles.
• Performed correlation analysis between RV residuals and activity indicators before and after subtracting the Keplerian signal to test for spurious correlations.
• Applied statistical tests to assess whether activity signals could explain the 3800-day radial velocity variation.
• Evaluated phase coherence between the 3800-day signal and activity cycle trends to test for physical linkage.

Experimental results

Research questions

• RQ1 Is the 3800-day radial velocity signal in HIP11915 caused by a planetary companion or by stellar activity?
• RQ2 Can activity indicators such as S$_{HK}$, BIS, and FWHM account for the observed radial velocity variation?
• RQ3 Does the phase relationship between the radial velocity signal and activity cycle trends support a stellar origin?
• RQ4 How robust is the planetary signal after removing potential activity-related noise using seasonal binning?
• RQ5 To what extent does the host star HIP11915 and its planet resemble the Sun and Jupiter in physical and chemical properties?

Key findings

• A Keplerian signal with a period of 3800 days (approximately 10.4 years) and a semi-amplitude of 69.5 m s⁻¹ was detected, consistent with a Jupiter-mass planet.
• The planet has a low orbital eccentricity (e ≈ 0.04), indicating a near-circular orbit.
• After removing the Keplerian signal, the correlation between RV residuals and activity indicators (S$_{HK}$, BIS, FWHM) decreased significantly—from 7.7 m s⁻¹ to 3.0 m s⁻¹—indicating the signal is not driven by stellar activity.
• Activity indicators showed a rise in mid-2013, coinciding with the signal maximum, but this did not correlate with the 3800-day signal in a way that supports an activity origin.
• Seasonal binning of data preserved the 3800-day signal but eliminated statistically significant correlations with activity tracers, further supporting a planetary interpretation.
• The host star HIP11915 is a solar twin with T$_{\textrm{eff}}$ = 5760 ± 4 K, log g = 4.46 ± 0.01, [Fe/H] = -0.059 ± 0.004, and age ≈ 4.0 ± 0.6 Gyr, closely matching the Sun's properties.