How the James Webb Space Telescope will examine stars for signs of Earth 2.0

It's one thing to find them, another to prove their habitability.
Brad Bergan
Webb in space (left), and the novel 'mask' designed to help NIRISS detect distinct objects (right).1, 2

In space-based astronomy, it all comes down to what's under the hood.

And for Webb, which is nearing completion of the final stages of commissioning for its 17 science instrument modes, one device — specifically designed to capture both images and spectra from varying kinds of cosmic objects — has the potential to go beyond detecting where alien worlds may lie.

Called the Near-Infrared Imager and Slitless Spectrograph (NIRISS), the Canadian-built scientific instrument could reveal signals of Earth 2.0 — just by looking at a viable candidate's host star.

Canada's contribution to the James Webb Space Telescope

Crucially, the NIRISS instrument will capably examine universal phenomena in near-infrared light, "at wavelengths up to 5.0 microns," said Nathalie Ourellette of the Université de Montréal, in a blog post on NASA's website. "The NIRISS team has developed four instrument modes to collect different kinds of data that are well-suited for different targets and scientific objectives."

The NIRISS' SOSS mode will enable the James Webb Space Telescope to gather highly precise spectra "from one bright object at a time," added Ourellette in the post. "This mode is optimized to carry out time-series observations, which are ideal for studying a phenomenon that changes over the length of a typically hours-long observation, such as an exoplanet transiting in front of its host star."

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"Using a technique called transit spectroscopy, the NIRISS instrument can collect a spectrum of an exoplanet’s atmosphere, which contains different markers that allow astronomers to determine its composition, temperature, potential habitability signatures, and other important characteristics," explained Ourellette in the blog post.

Then there's the WFSS mode, which will enable the NIRISS to help Webb gather information from thousands of objects, "such as galaxies, at the same time over the detector's entire field of view" — which is 4.84 square arc-minutes, according to Ourellette. "The spectra of thousands of galaxies will enable measurement of their distances, ages, and other physical parameters to trace how galaxies evolve over the lifetime of the universe."

And there's more. NIRISS is capable of gathering information from many spectra at once in WFSS mode, which means discrete spectra might overlap (if the sources of light are too close from our point of view). "There are thus two orthogonal grisms, GR150C and GR150R, that can produce spectra horizontally and vertically, respectively, which helps to disentangle blended spectra from different galaxies," said Ourellette.

European Southern Observatory Webb
In SOSS mode, Webb's NIRISS instrument can examine the atmospheres of alien worlds as they transit their host star. Source: European Southern Observatory / NASA

Webb's NIRISS could reveal new alien worlds with habitable conditions

Additionally, the NIRISS' AMI mode will enable Webb to examine cosmic objects that are in close proximity (from our point of view, of course). This will be done with a special technique known as interferometry. "A mask inside the instrument allows light from only certain parts of the primary mirror to pass through," explained Ourellette. "Astronomers can increase the resolution of the telescope by a factor of nearly 2.5 by looking at the patterns created as the carefully chosen beams of light interfere with each other."

By using this method, two objects that are in close apparent proximity can still appear as two distinct points of light — instead of a nonsensical blur. However, since the mask blocks a lot of light, the observed cosmic objects need to be fairly bright, for detection to happen. But, given sufficient brightness, the AMI mode of the NIRISS instrument will enable the James Webb Space Telescope to observe brown dwarfs, protoplanetary disks, and even exoplanets themselves. It's the first time we've ever used such a method to set our eyes on alien worlds, and it's only a small portion of the riches of scientific knowledge in store once Webb begins its science missions in earnest this summer.

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