The Occurrence Rate of Earth Analog Planets Orbiting Sunlike Stars (1103.1443v3)

Abstract: Kepler is a space telescope that searches Sun-like stars for planets. Its major goal is to determine {\eta}_Earth, the fraction of Sunlike stars that have planets like Earth. When a planet 'transits' or moves in front of a star, Kepler can measure the concomitant dimming of the starlight. From analysis of the first four months of those measurements for over 150,000 stars, Kepler's science team has determined sizes, surface temperatures, orbit sizes and periods for over a thousand new planet candidates. In this paper, we characterize the period probability distribution function of the super-Earth and Neptune planet candidates with periods up to 132 days, and find three distinct period regimes. For candidates with periods below 3 days the density increases sharply with increasing period; for periods between 3 and 30 days the density rises more gradually with increasing period, and for periods longer than 30 days, the density drops gradually with increasing period. We estimate that 1% to 3% of stars like the Sun are expected to have Earth analog planets, based on the Kepler data release of Feb 2011. This estimate of is based on extrapolation from a fiducial subsample of the Kepler planet candidates that we chose to be nominally 'complete' (i.e., no missed detections) to the realm of the Earth-like planets, by means of simple power law models. The accuracy of the extrapolation will improve as more data from the Kepler mission is folded in. Accurate knowledge of {\eta}_Earth is essential for the planning of future missions that will image and take spectra of Earthlike planets. Our result that Earths are relatively scarce means that a substantial effort will be needed to identify suitable target stars prior to these future missions.

• The paper presents a probabilistic model that estimates a 1%-3% occurrence rate of Earth-like planets around Sunlike stars using Kepler data.
• It employs power-law extrapolation and a Monte Carlo approach to adjust for detection biases and sample incompleteness.
• The study’s findings guide future exoplanet missions by refining habitable zone criteria and contrasting results with radial velocity surveys.

The Occurrence Rate of Earth Analog Planets Orbiting Sunlike Stars

In their research, Catanzarite and Shao present an analysis centered on determining the occurrence rate of Earth analog planets orbiting Sun-like stars using data collected by the Kepler space telescope. This observational paper is of pertinent interest to researchers seeking to understand the prevalence of Earth-like conditions in the vast expanse of the galaxy, potentially guiding future exoplanet detection missions.

The researchers worked with data from over 150,000 stars, gathered during the first four months of the Kepler Mission. Through their analysis, they established a probabilistic model of super-Earth and Neptune-sized planet candidates, elucidating three distinct period regimes—characterized by sharp increases and gradual declines in planet candidate densities across different orbital period ranges. They provided estimates for the proportion of Sun-like stars that host Earth-like planets, pegging this fraction at around 1% to 3% based on Kepler data available at that time.

Their methodology incorporated the habitable zone (HZ) criteria where liquid water could potentially exist on a planet's surface. The boundaries of this zone were evaluated using solar models, reflecting recent understandings of stellar atmospheric conditions and the potential reflective properties of planetary atmospheres. The core of their analysis revolved around extrapolating the characteristics from the "fiducial" region of detected planets to the target region housing Earth analogs. This extrapolation was performed using power-law models for planet mass and semi-major axis, with consideration given to potential incompleteness in detections.

For the robustness of the results, Catanzarite and Shao assumed completeness of their sample within specified bounds and applied geometric corrections to account for the transit probability relative to the observers' line of sight. Notably, the partnership employed a Monte Carlo approach to estimate biases arising from false positive detections, which they estimated at a mean probability of 23%.

In juxtaposition to other studies, notably those employing radial velocity (RV) methods, the research delineated significant differences in the occurrence rates of planet types across different orbital periods and mass ranges. The analysis highlighted that the Kepler data, focusing mostly on smaller planets, presents an occurrence pattern varying from the larger gas giants typically found in RV surveys.

The implications of Catanzarite and Shao’s findings are profound, particularly in the context of future astronomical endeavors. Knowing the fraction of Sun-like stars with Earth analogs is vital for planning missions to image and analyze these planets spectroscopically. The result also stresses the necessity for comprehensive target identification to enhance the scientific yield of such efforts. It opens space for future developments in astronomy and astrophysics while indicating the challenges inherent in direct imaging missions that seek to corroborate the habitability associated with Earth-like exoplanets.

Their rigorous analysis, based on a solid foundation of data collection and statistical modeling, advances understanding in the field and provides a framework for subsequent studies that plan to explore these territories more deeply. As Kepler and subsequent telescopes continue to provide richer datasets, the refinement of these models will enhance predictive accuracies, forming the basis of what will become standard practices in exoplanet exploratory missions.