Lesson 5 of 6 · Confirmation and false positives
Confirmation and false positives
10 minute lesson · 3 checks along the way
Your goal for this lessonDesign a discriminating follow-up observation and quantify how unresolved light can dilute an eclipse.
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A candidate is a testable explanation
Detecting a statistically interesting pattern is not the same as establishing its physical cause. A -like signal might come from a planet, an eclipsing pair of stars blended with the target, variability on a star, or an instrumental problem. The question is not whether one can draw a plausible planet orbit. It is whether competing explanations can reproduce the measurements and survive further tests.
Different tests address different alternatives. A shift in the source’s measured light centroid during a dip can point toward a neighboring variable object. Detailed imaging can reveal contaminating stars. Spectra may disclose a stellar companion or an activity-related signal. Odd and even eclipse depths can expose alternating events in a binary system. No single checklist item proves every candidate; its value depends on which alternative it actually constrains.
Extra light changes the denominator
A can include light from several unresolved stars. Suppose a target supplies flux Ftarget and a constant neighbor supplies Fneighbor. A loss ΔF from the target is recorded as fractional depth ΔF/(Ftarget + Fneighbor). Relative to the target alone, the depth would be ΔF/Ftarget. Adding constant light therefore makes the fractional event shallower without making the occulting object physically smaller.
This creates two different problems. A real planet’s radius may be underestimated if is ignored. Alternatively, a deep stellar eclipse in a faint blended system may imitate a shallow planetary transit of the brighter target. Identifying which source changes is consequently as important as measuring the total dip precisely. More photons do not fix a wrong assignment of the event to a particular star.
Check 1 of 3
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Work through a decision
Compare two explanations of one dip
Build a fictional aperture from a bright constant star contributing 9,900 flux units and a faint eclipsing system contributing 100 units outside eclipse. If the faint system loses half its light, the total falls from 10,000 to 9,950 units. The measured depth is 50/10,000 = 0.005, or 0.5%. A deep 50% event in the faint system has become a shallow dip in their combined light.
A planet transiting the bright star could also produce a small combined dip, so depth alone does not decide between these explanations. Propose resolved observations during the predicted event. If the faint system dims while the bright star remains steady, that observation specifically supports the blend explanation. Repeating the same unresolved measurement with smaller random errors may confirm the dip’s existence while still failing to locate its origin.
Blend denominators
Original instructional model. Each bar is normalized to its own out-of-eclipse total, so the bars have different flux scales. The same 50-unit loss creates a 50% eclipse in the faint system and a 0.5% dip in the combined aperture.
Open full-size diagram in a new tab: Blend denominatorsCheck 2 of 3
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Join independent evidence without claiming certainty
The Transiting Exoplanet Survey Satellite identifies candidates that follow-up observatories can examine. A consistent radial-velocity orbit can establish a companion’s dynamical effect, while compares a planet model with constrained false-positive populations. These are related but distinct routes to confidence. A validated transit need not have an independently measured mass; do not invent one merely because the object enters a catalog.
Published conclusions remain revisable as improved data or calibration arrive. Report the object classification, the evidence behind it, and the uncertainties on individual properties separately. Detecting a planet also does not establish habitability or life. A convincing detection result can be both exciting and bounded: a companion of constrained size and orbit is present, while its composition and environment await observations designed to answer those different questions.
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Put it together
Your lesson progress
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Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: Hidden stars and underestimated planet sizesNASA account of contaminating stellar light and host identification.
- Read: Kepler-9 and statistical validationOriginal BLENDER study: testing blends without automatically measuring a planet’s mass.
- Read: Vetting K2 candidatesResearch record on secondary eclipses, depth, and photocenter tests.