The European Space Agency (ESA) has released a transmission spectrum it made by comparing starlight filtered through a planet’s atmosphere as it moves across the star, to the unfiltered starlight detected when the planet is beside the star. Each of the 141 data points (white circles) on this graph represents the amount of a specific wavelength of light that is blocked by the planet and absorbed by its atmosphere.
ESA say that in this observation, the wavelengths detected by NIRISS range from 0.6 µm (red) to 2.8 µm (in the near infrared). The amount of starlight blocked ranges from about 13,600 parts per million (1.36 %) to 14,700 parts per million (1.47 %).
The grey lines extending above and below each data point are error bars that show the uncertainty of each measurement, or the reasonable range of actual possible values. For a single observation, the error on these measurements is remarkably small. The blue line is a best-fit model that takes into account the data, the known properties of WASP-96 b and its star (e.g., size, mass, temperature), and assumed characteristics of the atmosphere. Researchers can vary the parameters in the model—changing unknown characteristics like cloud height in the atmosphere and abundances of various gases—to get a better fit and further understand what the atmosphere is really like. The difference between the best-fit model shown here and the data simply reflects the additional work to be done in analysing and interpreting the data and the planet.
Although full analysis of the spectrum will take additional time, it is possible to draw a number of preliminary conclusions. The labelled peaks in the spectrum indicate the presence of water vapour. The height of the water peaks, which is less than expected based on previous observations, is evidence for the presence of clouds that suppress the water vapour features. The gradual downward slope of the left side of the spectrum (shorter wavelengths) is indicative of possible haze. The height of the peaks along with other characteristics of the spectrum is used to calculate an atmospheric temperature of about 725 °C.
This is the most detailed infrared exoplanet transmission spectrum ever collected, the first transmission spectrum that includes wavelengths longer than 1.6 µm at such high resolution and accuracy, and the first to cover the entire wavelength range from 0.6 µm (visible red light) to 2.8 µm (near infrared) in a single shot. The speed with which researchers have been able to make confident interpretations of the spectrum is further testament to the quality of the data.
The observation was made using NIRISS’s Single-Object Slitless Spectroscopy (SOSS) mode, which involves capturing the spectrum of a single bright object, like the star WASP-96, in a field of view.
WASP-96 b is a hot gas giant exoplanet that orbits a Sun-like star roughly 1150 light years away, in the constellation Phoenix. The planet orbits extremely close to its star (less than 1/20th the distance between Earth and the Sun) and completes one orbit in less than 3½ Earth-days. The planet’s discovery, based on ground-based observations, was announced in 2014. The star, WASP-96, is somewhat older than the Sun, but is about the same size, mass, temperature and colour.
The background illustration of WASP-96 b and its star is based on current understanding of the planet from both NIRISS spectroscopy and previous ground- and space-based observations. Webb has not captured a direct image of the planet or its atmosphere.
NIRISS was contributed by the Canadian Space Agency. The instrument was designed and built by Honeywell in collaboration with the Université de Montréal and the National Research Council Canada.
