[Paper Review] Close-in planets around giant stars. Lack of hot-Jupiters and prevalence of multi-planetary systems

J. Lillo-Box, D. Barrado+1 more|arXiv (Cornell University)|Mar 31, 2016

Stellar, planetary, and galactic studies25 references8 citations

TL;DR

This study analyzes 13 close-in planets around giant and subgiant stars, finding that 70.0±6.6% are part of multiplanetary systems—significantly higher than the 42.4±0.1% for main-sequence hosts. The results suggest that hot Jupiters in orbits <0.06 au are likely engulfed during stellar evolution, leaving only multiplanetary systems in the 0.06–0.5 au range, which supports tidal engulfment models and highlights a key observational trend in planetary system evolution post-main sequence.

ABSTRACT

Extrasolar planets abound in almost any possible configuration. However, until five years ago, there was a lack of planets orbiting closer than 0.5 au to giant or subgiant stars. Since then, recent detections have started to populated this regime by confirming 13 planetary systems. We discuss the properties of these systems in terms of their formation and evolution off the main sequence. Interestingly, we find that $70.0\pm6.6$ % of the planets in this regime are inner components of multiplanetary systems. This value is 4.2$σ$ higher than for main-sequence hosts, which we find to be $42.4\pm0.1$ %. The properties of the known planets seem to indicate that the closest-in planets (a &lt; 0.06 au) to main-sequence stars are massive (i.e., hot Jupiters) and isolated and that they are subsequently engulfed by their host as it evolves to the red giant branch, leaving only the predominant population of multiplanetary systems in orbits 0.06 &lt; a &lt; 0.5 au. We discuss the implications of this emerging observational trend in the context of formation and evolution of hot Jupiters.

Motivation & Objective

• To investigate the multiplicity and orbital distribution of close-in planets around evolved giant and subgiant stars.
• To determine whether the lack of hot Jupiters at very short periods (<0.06 au) around evolved stars is due to planetary engulfment.
• To compare the multiplicity frequency of close-in planets around evolved stars with that around main-sequence stars to infer evolutionary processes.
• To provide observational constraints for theoretical models of planet-star interactions during post-main-sequence evolution.

Proposed method

• The authors compiled a sample of 13 confirmed or validated exoplanets orbiting stars with log g < 3.8 and semi-major axis a < 0.5 au, identifying them as subgiant or red giant branch hosts.
• They classified systems as single-planet or multiplanetary based on the presence of additional companions, focusing on the innermost planet in multiplanetary systems.
• The multiplicity fraction was calculated as the ratio of multiplanetary systems to total systems in the close-in regime (0.06 < a < 0.5 au), with statistical uncertainties propagated via bootstrapping.
• The results were compared to the multiplicity fraction of planets around main-sequence stars (42.4±0.1%), using a 4.2σ significance test to assess the difference.
• The study used radial velocity and transit data from missions like Kepler and ground-based surveys (e.g., TAPAS, EXPRESS) to identify and validate the sample.
• Theoretical models of tidal engulfment and Kozai-Lidov migration were referenced to interpret the observed trends.

Experimental results

Research questions

• RQ1 Why are hot Jupiters (a < 0.06 au) absent around giant and subgiant stars despite their prevalence around main-sequence stars?
• RQ2 What is the multiplicity frequency of close-in planets (a < 0.5 au) around evolved stars compared to main-sequence hosts?
• RQ3 Does the observed absence of isolated hot Jupiters around evolved stars support the hypothesis of tidal engulfment during stellar evolution?
• RQ4 Can gravitational interactions or mean-motion resonances explain the survival of multiplanetary systems in the 0.06–0.5 au range around evolved stars?
• RQ5 What role do planetary mass and orbital distance play in determining survivability during the subgiant and red giant phases?

Key findings

• 70.0±6.6% of close-in planets (0.06 < a < 0.5 au) around giant and subgiant stars are part of multiplanetary systems, significantly higher than the 42.4±0.1% observed for main-sequence stars.
• The difference in multiplicity frequency is statistically significant at 4.2σ, indicating a non-random evolutionary process shaping planetary systems post-main sequence.
• No planets were found in orbits closer than 0.06 au in the evolved star sample, suggesting that hot Jupiters in such tight orbits are likely engulfed during stellar evolution.
• The surviving population of close-in planets around evolved stars is predominantly found in multiplanetary systems, implying that isolated hot Jupiters are preferentially destroyed.
• Only Kepler-56 shows a near 2:1 mean-motion resonance, while Kepler-108 and Kepler-432 are near 4:1 and 8:1 resonances, indicating resonant configurations are rare but not absent.
• The results support theoretical models in which massive planets are more readily engulfed during the subgiant and red giant phases, especially when orbiting within 2–3 stellar radii.