Do the two exoplanet system families reflect star formation environment or observational bias?

A selection-centered reading warns that apparent families may partly reflect how planets are found and classified. Transit surveys favor favorable orbital orientations and detectable planet-to-star size contrasts, while Doppler surveys favor suitable mass, period, and stellar-noise conditions. Results involving singles, multis, and planet ordering need completeness and detection-bias corrections before being assigned to formation environments.

• Transit detections require near edge-on orbital orientations, making system geometry a direct filter on the sample.
• Because most known planets come from transit and Doppler methods, their combined biases are strongly embedded in exoplanet classifications.
• Differences between apparent singles and multis are not automatically evidence for distinct underlying populations; some contrasts weaken after removing hot Jupiters and comparing host types.
• Planet-radius ordering studies that account for observational biases still find trends, while metallicity dependence indicates that selection and initial conditions must be analyzed together.

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