Lesson 3 of 6 · Direct imaging
Direct imaging
10 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish separated planetary light from a resolved surface, and evaluate angular separation and contrast together.
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Separate the light, not necessarily the surface
detects light associated with a planet at a position separated from its host star. The light can include thermal radiation from the planet and reflected starlight, depending on wavelength and system. It does not follow that the telescope resolves continents, clouds, or a planetary disk. A distant unresolved source is spread into a finite image by the telescope’s optical response, just as a star becomes more than one detector pixel.
The central obstacle is both geometric and photometric: a faint source lies close to a much brighter one. A larger separation helps only if enough planetary light can be distinguished from stellar glare and noise. Young, warm giant planets at wide projected separations have consequently been favorable targets for infrared imaging. Their visibility does not make them representative of all planets, nor does their brightness directly specify a unique mass without an evolutionary model.
Understand what a coronagraph removes
A suppresses host-star light using optical components within the telescope. It does not physically extinguish the star, and it does not remove every trace of its light. Diffraction, imperfect wavefront control, and detector effects can leave structures that resemble faint companions. Reference observations and carefully tested processing help separate those structures from a real source; processing also affects the fraction of planetary light retained.
The James Webb Space Telescope observed the previously discovered planet HIP 65426 b in several infrared bands in 2022. Its published display assigns visible colors to infrared measurements. White star symbols mark the host location after suppression and subtraction. Bar-like features in the near-infrared panels arise from the optics. Those bars are not rings or moons. Different blob shapes across filters describe the observing system as well as the source, not changing planetary coastlines.
The Digitized Sky Survey supplies the large background star field. Insets show the same planet in Webb’s 3.00, 4.44, 11.4, and 15.5 micrometer bands, mapped to visible colors. White symbols locate the suppressed host star. Near-infrared bars are optical artifacts; the differently shaped blobs do not resolve the planet’s surface.
Look closely
Are the four colored points four different planets, and do their different outlines measure different planetary radii?
See what to notice
They are observations of one planet through four infrared filters. Wavelength-dependent optics and processing shape each unresolved image. The white host markers are annotations, and the bars in the first two panels are instrumental structure.
Check 1 of 3
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Work through a decision
Choose between two fictional targets
At small angles, in astronomical units divided by distance in parsecs gives angular separation in arcseconds. Imagine target A has a projected separation of 10 au at 20 pc: 10/20 = 0.5 . Target B has 10 au at 50 pc: 10/50 = 0.2 arcsecond. A fictional instrument’s useful inner separation limit is 0.3 arcsecond for this band and contrast. A clears that limit; B does not.
That geometric test is necessary, not sufficient. If A’s planet-to-star flux ratio is 1/100,000 but the instrument reaches only 1/10,000 at 0.5 arcsecond under the actual exposure conditions, A is still too faint for that threshold. Nor is the observed projected separation automatically the orbital semimajor axis. The current orbital phase, , and eccentricity matter. Do not replace a three-dimensional orbit with one distance measured across an image.
Check 2 of 3
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Test a faint companion over time
A background star can appear close to a target by chance. Repeated observations test whether the candidate shares the host’s motion and whether its changing relative position is consistent with an orbit. Spectra and additional bands can supply further constraints. A single bright residual beside a masked star is a starting hypothesis, and the evidence for association must accompany the attractive picture.
For a follow-up proposal, state the band, expected separation, , exposure conditions, and competing explanations. Distinguish an observation of a known planet from its original discovery; Webb’s 2022 image was not humanity’s first image. This method provides valuable planetary light for characterization, while the sample’s strong visibility biases must remain part of any claim about how frequently different kinds of planets occur.
Check 3 of 3
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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: Webb’s first direct exoplanet observationSeptember 2022 image, exact band colors, and optical-artifact explanation.
- Read: HIP 65426 b: the imaging analysisOriginal research using multiple infrared bands and high-contrast processing.
- Read: The original 2017 discoverySPHERE discovery paper; projected separation is distinct from a measured full orbit.