Exoplanet Detection
Secondary Eclipse
Catching a planet’s own glow as it hides
A secondary eclipse is the moment an exoplanet slides behind its host star and its own light — reflected starlight plus its thermal glow — is briefly hidden, so the total system brightness drops by a tiny step called the eclipse depth. That step equals the planet-to-star flux ratio at the observed wavelength, which means measuring it in the infrared reveals the planet's dayside temperature and albedo without ever resolving the planet itself. First detected in 2005 by Spitzer for HD 209458b and TrES-1b, the secondary eclipse turned exoplanets from points of inference into worlds with measured climates.
- Also calledOccultation
- Eclipse depthF_planet / F_star (~10⁻⁶–10⁻³)
- Hot Jupiter dayside~1000–3000 K
- Best wavelengthsInfrared (~3–24 μm)
- First detected2005 (Spitzer; HD 209458b, TrES-1b)
- RevealsTemperature · albedo · emission spectrum
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What happens during a secondary eclipse
An exoplanet on a transiting orbit crosses our line of sight to its star twice each revolution. When it passes in front, it blocks starlight: that is the transit, or primary eclipse, and it tells us the planet's size. Half an orbit later the planet passes behind the star. We can no longer see the planet — but we were already seeing it, faintly, mixed into the combined light of the system. As the star occults the planet, that faint extra light disappears and the total brightness drops by a small step. When the planet emerges on the far side, the brightness recovers. That dip is the secondary eclipse, also called an occultation.
The crucial difference from a transit is what gets blocked. During a transit, the planet's opaque disk subtracts a fraction of the bright stellar surface, so the depth is purely geometric: (Rp/Rs)². During a secondary eclipse, the star subtracts the planet's own emitted and reflected light, so the depth equals the ratio of the planet's flux to the star's flux. We never resolve the planet as a separate dot; we simply notice that the system got slightly dimmer for a few hours.
The eclipse depth: a planet's flux laid bare
Write the planet's flux as the sum of two contributions: thermal emission (the planet glows because it is hot) and reflected starlight (it acts like a mirror, set by its albedo). The measured depth is
depth = Fp / Fs = (Rp/Rs)² × [ Bλ(Tday) / Bλ(Tstar) ] + Ag (Rp/a)²
The first term is thermal emission, the ratio of the planet's blackbody brightness at temperature Tday to the star's, scaled by their projected areas. The second term is reflected light, set by the geometric albedo Ag and the orbital distance a. Which term dominates depends on wavelength. In the optical, reflection usually wins and the depth measures albedo. In the infrared, thermal emission dominates and the depth measures the dayside temperature.
For a typical hot Jupiter (Rp ≈ RJupiter) orbiting a Sun-like star (Rs ≈ RSun ≈ 10 RJupiter), the area ratio (Rp/Rs)² is about 1%. In the mid-infrared, where a 1500 K planet and a 5800 K star differ less in brightness, the bracketed factor is of order a few percent, so eclipse depths land near 0.1–0.3%. In the optical the same planet's depth from reflected light may be only tens of parts per million — which is why secondary eclipses were first found in the infrared.
Why the infrared is the planet's home turf
Every warm body radiates a blackbody spectrum that peaks at a wavelength set by its temperature. A 5800 K star peaks near 0.5 μm (visible light); a 1500 K planet peaks near 1.9 μm (near-infrared). Toward longer wavelengths the star's spectrum falls steeply down the Rayleigh–Jeans tail while the planet's is still climbing, so the planet-to-star contrast keeps improving. The table below shows roughly how a single hot Jupiter's eclipse depth grows with wavelength.
| Wavelength band | Dominant signal | Typical eclipse depth |
|---|---|---|
| 0.5 μm (optical) | Reflected light (albedo) | ~10–100 ppm |
| 1.4 μm (HST/WFC3) | Thermal emission (H₂O band) | ~300–1000 ppm |
| 3.6 μm (Spitzer/JWST) | Thermal emission | ~1500–2500 ppm |
| 4.5 μm (Spitzer/JWST) | Thermal emission | ~2000–3500 ppm |
| 8–24 μm (Spitzer/JWST MIRI) | Thermal emission | ~3000–6000 ppm |
This is why the historic firsts came from space infrared telescopes. In 2005 the Spitzer Space Telescope independently detected the secondary eclipses of HD 209458b and TrES-1b at 8–24 μm — the first direct measurements of light from planets beyond the Solar System. The dayside temperatures, near 1000 K, confirmed these worlds were genuinely roasted by their stars rather than merely inferred to be close in.
From one dip to a planet's climate
A single eclipse depth at one wavelength gives a brightness temperature. Measure depths across many wavelengths and you assemble an emission spectrum — the planet's thermal fingerprint. Dips and bumps in that spectrum reveal molecules (water, carbon dioxide, carbon monoxide, methane) seen in emission if the upper atmosphere is hotter than the layers below (a thermal inversion), or in absorption if it cools with altitude. This is fundamentally different from the transmission spectrum gathered during transit, which probes only the thin terminator ring; the secondary eclipse probes the full illuminated dayside.
The eclipse also constrains the energy budget. If a planet absorbed all incident starlight and instantly re-radiated it from the dayside alone, it would reach the maximum dayside temperature. If winds spread heat efficiently to the night side, the dayside is cooler. Comparing the measured dayside temperature to these limits reveals the heat-redistribution efficiency — and whether the planet has reflective clouds raising its albedo.
| Property | Transit (primary) | Secondary eclipse (occultation) |
|---|---|---|
| Geometry | Planet in front of star | Planet behind star |
| What is blocked | Starlight | Planet's own light |
| Depth equals | (Rp/Rs)² — area ratio | Fp/Fs — flux ratio |
| Best wavelength | Optical / near-IR | Infrared |
| Reveals | Radius, transmission spectrum, atoms at terminator | Dayside temperature, albedo, emission spectrum |
| Region probed | Terminator (limb ring) | Illuminated dayside |
Eclipse timing, phase curves, and hot-spot offsets
The eclipse is not just a depth — its timing is information. For a perfectly circular orbit, the secondary eclipse falls exactly halfway between transits. If it arrives early or late, the orbit is eccentric, and the offset measures e·cos(ω) — a clean geometric handle on the orbit's shape. Eclipse arrival is also delayed by the light-travel time across the orbit (tens of seconds for close-in planets), a small but real relativistic-era bookkeeping correction.
Stretching the measurement across the whole orbit produces a phase curve: the system brightens as the hot dayside rotates into view and dims as the cooler night side faces us, bottoming out at the secondary eclipse when the full dayside is hidden. If the brightness peaks before the eclipse, the planet's hottest spot is offset eastward from the substellar point — the signature of a super-rotating equatorial jet dragging heat downwind. HD 189733b showed exactly such an offset in Spitzer 8 μm data, implying winds of several thousand kilometers per hour.
Measuring depths of a few parts in ten thousand
Detecting a 1000 ppm dip — let alone a 30 ppm optical one — demands extraordinary photometric stability. The combined star-plus-planet signal must be tracked through ingress, the flat eclipse bottom, and egress while instrumental drifts, detector ramps, and stellar variability are modeled out. Spitzer's warm-mission 3.6 and 4.5 μm channels became the workhorses for a decade; the Hubble WFC3 grism opened the 1.1–1.7 μm water band; and JWST's NIRSpec, NIRCam, and MIRI now deliver eclipse spectra with precisions of tens of parts per million across 0.6–28 μm, resolving CO₂, H₂O, and even silicate clouds on individual worlds. The same technique works on rocky planets too: JWST eclipse photometry of the TRAPPIST-1 planets at 15 μm has begun ruling out thick atmospheres by measuring how hot their bare daysides run.
Frequently asked questions
What is a secondary eclipse?
A secondary eclipse, also called an occultation, is the moment an exoplanet passes behind its host star and the planet's own light is hidden from view. Because the planet contributes a small amount of flux — its reflected starlight plus its thermal glow — the total system brightness drops slightly while the planet is occulted, then recovers. That dip, the eclipse depth, equals the planet-to-star flux ratio at the observed wavelength.
How is a secondary eclipse different from a transit?
A transit (the primary eclipse) is the planet crossing in front of the star, blocking starlight; its depth equals the area ratio (Rp/Rs)² and reveals the planet's radius. A secondary eclipse is the planet passing behind the star, hiding the planet's own emission; its depth equals the planet-to-star flux ratio and reveals dayside brightness and temperature. Transits are deepest in visible light; secondary eclipses are deepest in the infrared, where a hot planet glows.
What does the eclipse depth tell us?
The eclipse depth is the planet's flux divided by the star's flux at the observed wavelength. In the thermal infrared this directly gives the planet's dayside brightness temperature; for a hot Jupiter that is often 1000–3000 K. In the visible, the depth is dominated by reflected light and yields the geometric albedo. Measuring depth across many wavelengths builds an emission spectrum that reveals molecules, thermal inversions, and clouds.
Why are secondary eclipses observed in the infrared?
A planet at 1500 K radiates a blackbody curve peaking near 2 microns, while a 5000 K star peaks near 0.6 microns. Toward longer infrared wavelengths the planet-to-star contrast improves dramatically — a depth that is a few parts per million in the optical can grow to thousands of parts per million at 4.5 microns. Spitzer pioneered these measurements; JWST now resolves them with exquisite precision.
What was the first secondary eclipse detected?
In 2005 the Spitzer Space Telescope independently measured the secondary eclipses of HD 209458b and TrES-1b in the mid-infrared — the first direct detection of light from planets outside the Solar System. The depths, a fraction of a percent at 8–24 microns, implied dayside temperatures near 1000 K and launched the field of exoplanet thermal characterization.
Can secondary eclipses measure heat redistribution?
Yes. The eclipse measures the dayside temperature at one instant. By tracking brightness through the whole orbit — a phase curve — and timing the peak relative to the eclipse, astronomers measure how efficiently winds carry heat to the night side. A hot-spot offset from the substellar point, seen on HD 189733b and others, reveals super-rotating equatorial jets thousands of kilometers per hour.