[Paper Review] Observational Techniques for Detecting Planets in Binary Systems

Matthew W. Muterspaugh, M. Konacki+2 more|arXiv (Cornell University)|May 21, 2007

Stellar, planetary, and galactic studies39 references4 citations

TL;DR

This paper evaluates observational techniques for detecting planets in binary star systems, focusing on S-type (orbiting one star) and P-type (circumbinary) configurations. It compares radial velocity, astrometry, and eclipse timing methods, demonstrating that eclipse timing with space-based telescopes offers superior sensitivity for circumbinary planets, especially for longer-period systems, while radial velocity is limited by stellar rotation in short-period binaries.

ABSTRACT

Searches for planets in close binary systems explore the degree to which stellar multiplicity inhibits or promotes planet formation. There is a degeneracy between planet formation models when only systems with single stars are studied--several mechanisms appear to be able to produce such a final result. This degeneracy is lifted by searching for planets in binary systems; the resulting detections (or evidence of non-existence) of planets in binaries isolates which models may contribute to how planets form in nature. In this chapter, we consider observational efforts to detect planetary companions to binary stars in two types of hierarchical planet-binary configurations: first ``S-type'' planets which orbit just one of the stars, with the binary period being much longer than the planet's; second, ``P-type'' or circumbinary planets, where the planet simultaneously orbits both stars, and the planetary orbital period is much longer than that of the binary. The S-type planet finding techniques are different for binaries that can or cannot be spatially resolved. For wider systems, techniques reviewed include dualstar interferometric differential astrometry and precision radial velocities. Alternatively, unresolved binaries can be studied using modified dualstar "PHASES-style" differential astrometry or a modification of the radial velocity technique for composite spectra. Should a fortunately aligned--but still long period--binary be found, eclipse timing can also reveal the presence of S-type planets. Methods for detecting P-type planets include the composite-spectra variant of the radial velocity technique and eclipse timing.

Motivation & Objective

• To investigate how studying planets in binary systems helps resolve degeneracies in planet formation models.
• To evaluate the effectiveness of astrometry, radial velocity, and eclipse timing techniques for detecting S-type and P-type planets in binaries.
• To quantify the sensitivity limits of each method under realistic observational constraints, including stellar rotation and telescope aperture.
• To identify optimal observational strategies for detecting circumbinary planets, especially in systems with short orbital periods.
• To assess the impact of atmospheric turbulence and instrumental precision on astrometric measurements in interferometric systems.

Proposed method

• Uses interferometric narrow-angle astrometry with dual beam combiners to measure optical path differences (delay) with sub-micron precision, enabling micro-arcsecond astrometry.
• Applies radial velocity techniques with a 20 m s⁻¹ precision limit, accounting for rotational line broadening in tidally locked stars with orbital periods ≤5 days.
• Employs eclipse timing variations (ETVs) to detect planetary perturbations in eclipsing binaries, modeling timing residuals from gravitational tugs.
• Derives sensitivity curves using equations for timing precision (eq. 28), incorporating number of observations, magnitude, telescope diameter, and system parameters.
• Compares detection limits across methods by combining radial velocity and timing observables, assuming solar-mass stars and varying telescope apertures.
• Incorporates atmospheric turbulence effects on fringe visibility and path delay, requiring short exposures and precise baseline knowledge (±100 μm) for high-precision astrometry.

Experimental results

Research questions

• RQ1 How do observational techniques like astrometry, radial velocity, and eclipse timing compare in detecting S-type and P-type planets in binary systems?
• RQ2 What are the fundamental sensitivity limits of radial velocity measurements in short-period binaries due to stellar rotation and line broadening?
• RQ3 How does eclipse timing precision scale with telescope aperture, magnitude, and number of observations for detecting circumbinary planets?
• RQ4 What role does atmospheric turbulence play in limiting astrometric accuracy in optical interferometers?
• RQ5 At what orbital periods do radial velocity and eclipse timing techniques cross in sensitivity for detecting planetary companions in binary systems?

Key findings

• Eclipse timing with a 2.5 m space telescope (e.g., HST or SOFIA) achieves sensitivity down to 0.19 M_J for circumbinary planets, outperforming radial velocity in long-period systems.
• Radial velocity measurements are limited to 20 m s⁻¹ precision, but this is reduced to worse than 20 m s⁻¹ for stars with orbital periods ≤5 days due to tidal locking and line broadening.
• For a 10-day binary, eclipse timing with a 1 m ground-based telescope can detect companions down to ~0.19 M_J, while a 10 m ground-based telescope improves sensitivity by a factor of ~3.
• The sensitivity of eclipse timing to outer planets in single-star systems with hot Jupiters drops by a factor of 8 compared to circumbinary systems, limiting detection to massive companions.
• For systems with V=10 magnitude, 3 m s⁻¹ radial velocity observations are more sensitive than half-meter telescope transit timing for companions with periods up to 60 years.
• The transition point where eclipse timing surpasses radial velocity sensitivity occurs at ~15-year orbital periods for HST/SOFIA-class telescopes, but not for ground-based 0.5 m telescopes.