Astronomy & Celestial Mechanics
the sky, computed
Concept

Exozodiacal dust — where it appears

Warm dust in the inner planetary system of another star, the analogue of the Solar System's zodiacal cloud. Its glow is the background against which an Earth-like planet must be seen, and its level is quoted in units of the Sun's own dust.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

What shares a resolution element with an Earth twin. Light collected in one resolution element around an Earth-like planet in the habitable zone of a Sun-like star, against the system's distance, both on logarithmic scales, for a 6 metre telescope at visible wavelengths, in units of the planet's own light at 10 parsecs. The planet and the leaked starlight both dim as the square of distance. The exozodiacal dust, at 3 times the Solar System's level, and the Solar System's own zodiacal light do not dim at all: each is a surface brightness, and a resolution element at a greater distance simply contains more dust. The exozodi equals the planet at 9.1 parsecs and is 11 times it at 30.

Dust that does not dim with distance

An Earth-like planet around a nearby star is seen against the glow of the dust in its own system, and the two behave differently with distance. The planet is a point, and it fades as the square of how far away it is. The dust is a surface, and a surface does not fade at all — a more distant system packs more dust into each resolution element in exactly the proportion its light is diluted. So the dust around a star, not the telescope's contrast, sets how far away an Earth can be found, and how many stars a search can reach.

exoplanets · Direct imaging Where a ten-micron zodi comes from. How much each logarithmic interval of radius contributes to the ten-micron glow of a Sun-like star's exozodiacal dust, for blackbody grains in the Solar System's radial profile, seen face-on from 10 pc: with no instrument (dashed) and through a 14.4 m nulling interferometer whose fringes suppress the light near the star (solid, with the averaged transmission dotted). Without the null the glow is concentrated far inside the habitable zone at 1 AU (marked), because the grains' emission at ten microns rises steeply with their temperature: half of it comes from inside 0.34 AU. The null removes the innermost part and pushes the sensitivity outward, but half the nulled signal still comes from inside 0.69 AU, peaking at 0.74. A zodi level measured this way is a measurement of the dust between a few tenths of an AU and the habitable zone, extrapolated to the habitable zone with the Sun's profile.

A zodi measured in heat and needed in light

The dust around nearby stars that will limit the search for Earth-like planets in reflected light has been measured — but in its own glow at ten microns, by an interferometer whose fringes hide the dust nearest the star. Converting that measurement into the visible brightness at the habitable zone needs the dust's radial profile and its grains' albedo and temperature, none of which was measured. The same ten-micron zodi can mean a quarter or ten times the visible dust a coronagraph will see.

exoplanets · Direct imaging

Named alongside it

The objects these essays reach for when they reach for this one.

CoronagraphDirect imagingHabitable zoneSurface brightnessSurvey yieldZodiacal lightAlbedoBlackbodyIntegration timeNulling interferometry

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