Rounding up the wanderers: optimizing coronagraphic searches for extrasolar planets

Abstract

I derive analytic scalings for coronagraphic imaging searches for extrasolar planets. I compute the efficiency of detecting planets about any given star, and from this compute dimensionless distribution functions for the detected planets as a function of planet–star distance and distance to the host stars. I find the following for blind planet surveys: (1) the optimum wavelength is between 4000 and 5000 Å for Earth‐like planets and 4200 and 5800 Å for Jovian planets; (2) between 21 and 32 per cent of the number of planets per decade of radius can be detected with an optimized survey; (3) target stars should be ranked from greatest to least by their luminosity divided by distance to the sixth or eighth power, depending on the dominant source of noise for the survey; (4) surveys targeting all main‐sequence stars will detect ∼3 times as many planets as surveys only targeting G‐type stars and (5) stellar populations with different metallicities should have exposure times that vary with the cube of the metallicity. I apply these results to the current suite of proposed coronagraphic satellite telescopes, of which TPF‐C is the most powerful, but a much smaller telescope, TOPS, may have a significant chance of detecting Earth‐sized planets due to its small inner working angle and high throughput. The most significant uncertainty in these results is the noise contribution of exo‐zodiacal light. These results can be applied to designing coronagraphs, comparing proposed telescope designs, optimizing the observing strategies, determining the properties of detected planet populations and selecting target stars.

Imaging Young Giant Planets From Ground and Space

“…There have been a number of investigations of coronagraphic imaging of planets, particularly in the context of designs for the Terrestrial Planet Finder-Coronagraph (TPF-C), including Agol (2007), Beckwith (2009), andBrown (2009). These authors have investigated the challenging task of finding planets, both gas or ice giants and terrestrial planets, through their reflected light.…”

Characterizing Rocky and Gaseous Exoplanets with 2 m Class Space-based Coronagraphs

“…Agol (2007) presented analytic models of the detection efficiency of coronagraph searches and investigated dependencies on key parameters, including survey duration, telescope diameter, and telescope inner working angle (IWA). Brown & Soummer (2010) discussed methods for optimizing survey "completeness" (Brown 2004(Brown , 2005, while other studies (Savransky et al 2010;Turnbull et al 2012) have emphasized techniques for evaluating the effectiveness of proposed missions.…”

The Center of Light: Spectroastrometric Detection of Exomoons

“…If (exo-) zodiacal light is a significant source of noise, then shorter wavelengths may be favored. We estimate (Agol 2007) the ratio of the flux of the exozodi flux within the planet's PSF, F EZ , to flux of the host star, F * , to be: ( )…”

“…A deeper analysis of this calculation can be found in Agol (2007); note that R l l = D is the spectral resolving power. We assume that the PSF of the planet is well approximated by an Airy disk, which we in turn approximate as a Gaussian with angular profile I exp 1 2…”

“…Such a large telescope would be capable of rapid detection of Earth-sized exoplanets (in of order a few weeks) with a directimaging survey of nearby Sun-like stars (Agol 2007). The planets found with this initial coronagraphic imaging survey could then be prioritized based upon, for example, their presence within their host-star habitable zone and potential for the existence of a large, detectable exomoon, to motivate the additional observing time for detailed spectral and spectroastrometric characterization.…”