Public Universe · Student worksheet
Which planets do our surveys find?
Does a difference between discovery samples tell us how common planets really are?
Data and starting point
Use the exoplanet directory’s discovery-method filter. Before collecting values, define a rule such as all listed planets whose names begin with a chosen prefix, or a recorded list of identifiers. Apply the same rule to Transit and Radial Velocity. A manually chosen subset is exploratory, not a representative survey.
Explore the exoplanet catalogue ·
Investigation
- Record the selection rule, date, method and object identifiers. From each detail page collect period in days and radius in Earth radii when available. Record the total selected count and usable count for each variable and method.
- Keep unknown values blank. Store upper/lower limits separately and exclude them from an ordinary measured-value median. State this exclusion and count it; excluding missing values can itself bias a result.
- For positive measured values, plot period against radius on logarithmic axes, marking discovery method. If there are too few usable radii, compare period distributions instead and explain the change.
- Report a median and sample size for each method. Discuss transit alignment, observing duration and measurement sensitivity as explanations for differences. Do not label the groups controlled or random samples.
- Extension: compare mass only after checking provenance. A radial-velocity minimum mass M sin(i) is not generally a true mass. Explain what survey completeness, target-star counts and detection efficiencies would be needed to estimate occurrence rates.
Submit
Submit a frozen selection table, a missingness/limit count table, one labelled comparison plot and a 250-word argument stating what the sample can and cannot establish.
Audit the comparison
Selection rule and selected / usable counts for each method:
One missingness pattern and one selection effect:
A conclusion justified by this sample, and a claim it cannot support:
Public Universe · Teaching notes
Which planets do our surveys find?
Learning outcomes
- Define a repeatable selection before looking at outcomes.
- Report missingness and limits separately from measured values.
- Distinguish a discovery sample from an occurrence-rate estimate.
Discussion and interpretation
This activity has no fixed count or expected median: catalogues evolve and the selection rule matters. A reproducible small sample is preferable to a large sample with an undocumented stopping rule.
Transit geometry and repeated-event detection affect what can be found; radial velocity measures stellar motion along the line of sight. Encourage explanations grounded in the method, not a claim that catalogue fractions equal population fractions.
For API work, discovery_method is a top-level field. Archive values live under source_data: pl_orbper (days), pl_rade (Earth radii), and pl_bmasse (Earth masses or minimum mass). Check the corresponding lim flags, err1/err2 fields and mass_provenance; absent keys remain missing.
Assessment criteria
- A selection another student can reproduce, with counts before and after exclusions.
- Correct units, treatment of limits and readable plots.
- A clear distinction between catalogue demographics and occurrence rates.
Primary sources and further reading
- NASA: how we find and characterize exoplanetshttps://science.nasa.gov/exoplanets/how-we-find-and-characterize/
- NASA Exoplanet Archive: parameter definitions and flagshttps://exoplanetarchive.ipac.caltech.edu/docs/API_PS_columns.html