Astronomy & Celestial Mechanics
the sky, computed
Concept

Transmission spectrum — where it appears

The apparent size of a transiting planet as a function of wavelength, built from the depth of its transit in many colours. Starlight filtered through the atmosphere's rim makes the planet look larger where gases absorb. The signal is small, and it is also affected by the star's own surface.

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

Three planets that are the same spectrum. A model transmission spectrum, in scale heights of apparent radius, drawn three times: once as it is, once with the reference radius raised by 0.45 scale heights and the abundance reduced to compensate, and once with a cloud deck truncating the features. The three differ by 0.21 scale heights root-mean-square against features of 2.1, which is well inside the error bars of any real observation. The reason is structural rather than observational: a transmission spectrum measures a difference in apparent radius with wavelength and never an absolute radius, so the level is a free parameter, and shifting the level trades against the abundance almost exactly. Adding a cloud deck adds a third parameter that flattens features and trades against both. Three unknowns and one curve is why the quoted abundance uncertainties from transmission spectroscopy are so much larger than the photometric precision suggests.

A spectrum flattened by cloud, or by nothing

A transmission spectrum measures how a planet's apparent radius changes with wavelength, and never the radius itself. That missing level is a free parameter, it trades almost exactly against the abundance of whatever is absorbing, and a cloud deck adds a third unknown to a curve that constrains two.

exoplanets · Exoplanet atmospheres Water that is on the star and not on the planet. Excess transit depth, in parts per million, against wavelength, for a planet of depth 7000 ppm around a star of 2600 K. The upper curve is a bare rock with no atmosphere on a star whose disc is 10% covered by 2300 K spots that hold more water than the rest of the photosphere: it shows a broad slope and a band near 1.4 μm, 36 ppm from its shoulder. The lower curve is the signal of an Earth-like air column with water on an Earth-sized planet, about 93 ppm at the band. The band in the spot spectrum is an imposed illustration rather than a model atmosphere, but its sign is measured: it adds where a planet's own water would add.

Water that is on the star and not the planet

A transit depth is the planet's shadow divided by the brightness of the whole star, and the shadow falls on only one strip of it. If the strip is cleaner than the rest of the disc, the depth comes out too large, by an amount that changes with colour. The star's own spots hold water, so the error has water's fingerprint. On the smallest red dwarfs, whose rocky planets are the best targets there are, that false signal is larger than the real atmosphere being looked for.

exoplanets · Exoplanet atmospheres

Named alongside it

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

Abundance degeneracyBayesian inferenceCloud deckFaculaGrey opacityMean molecular weightRayleigh scatteringRed dwarfReference radiusRetrievalScale heightStarspot

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