Lesson 4 of 6 · Microlensing and astrometry
Microlensing and astrometry
10 minute lesson · 3 checks along the way
Your goal for this lessonTell gravitational brightening from sky-position reflex motion, and identify the extra information needed to infer a planet’s properties.
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A foreground system changes background light
uses the gravity of a foreground object to alter the apparent brightness of a more distant source star. As their alignment changes, the source can brighten and fade even though its intrinsic luminosity stays steady. A planet associated with the foreground lens can introduce a deviation from a simple single-lens . The observed brightening usually belongs to the background source, not to the planet turning on a lamp.
A planetary deviation need not be a neat second positive spike. Lens geometry can produce more complicated excesses and deficits relative to a single-lens prediction. Several configurations may fit limited data. Timing, sampling, and independent constraints help distinguish them. The alignment normally passes rather than repeating each planetary year, so waiting one orbit is not a dependable way to replay the same microlensing event. Later observations can nevertheless add valuable information about the lens.
Measure the star’s changing position
instead follows precise positions on the sky. A star’s path combines its overall , the annual apparent shift from the observer’s changing location, and possibly orbital reflex motion around a shared center of mass. Calibration and a model are needed to separate these components. A small bend in a path is not automatically a planet; a stellar companion or imperfect modeling can also produce it.
Gaia’s repeated positional measurements have supplied astrometric companion candidates. Its Gaia-4b result, reported with radial-velocity follow-up in February 2025, illustrates why a second observable helps. Astrometry provides orbital orientation information that alone can lack. Conversely, spectra test a predicted line-of-sight component. Both methods measure the host’s response to a companion, unlike photometry’s temporary blocking of the host’s light.
Gaia’s 2018 DR2 map combines detections from July 2014 to May 2016. Brightness encodes detected star density, with dust and scanning effects visible. It is a population map, not a plotted astrometric orbit of an individual exoplanet host.
Look closely
What information needed to identify a stellar reflex orbit is absent from this map?
See what to notice
The map compresses many detections into a sky-density display. A companion analysis needs repeated positions with timestamps, uncertainties, calibration, and separation of proper motion and parallax. Dense-looking parts of the map do not directly locate individual planets.
Check 1 of 3
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Work through a decision
Compute an ideal angular wobble
Take a fictional two-body circular orbit with star mass 1,000 units and planet mass one unit. Let their separation be 10.01 au. The star’s distance from the center of mass is aStar = [Mp/(Mstar + Mp)] × a = (1/1,001) × 10.01 = 0.01 au. This exact mass-ratio form avoids treating the star as an immovable origin. The planet travels on the much larger companion orbit.
Place the system 10 pc away and view it face-on. Its ideal angular reflex radius is 0.01/10 = 0.001 , or one . At 20 pc the same physical wobble subtends 0.5 milliarcsecond. These numbers are not an instrument performance promise. Real observations sample finite times with uncertainty; a partially observed long orbit can resemble a trend, and a luminous companion can make the measured photocenter differ from the host star’s center.
Check 2 of 3
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Choose the information a model lacks
A microlensing light curve can constrain a planet-to-host mass ratio and separation scaled to the lens geometry, while absolute masses and distances often need further information. Do not infer an exact planet mass merely from the height of an isolated brightness peak. Parallax in the lensing event, finite-source effects, or later resolved observations can help, depending on the event and available data.
Match the follow-up to the question. Dense monitoring during a brief lensing deviation preserves information that cannot simply be recollected next year. A longer astrometric baseline helps distinguish curved orbital motion from steady drift. Neither approach is a substitute for every other method. Their different selection effects widen the range of planetary systems accessible to observation and create opportunities to test one physical model against several measured quantities.
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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: Gaia: positional wobbles and follow-upObserved complementarity of astrometry and radial velocity.
- Read: Planet parameters in microlensing eventsOriginal analysis of mass-ratio/separation constraints and degeneracies.
- Read: Microlensing masses and geometryResearch explanation of the mass–distance–timescale ambiguity and additional constraints.