Roadmap details how to improve exoplanet exploration using the JWST

A coalition of scientists created a step-by-step guide to unlock the potential of the James Webb Space Telescope for identifying habitable worlds in the universe

by Paige Colley (EAPS News, MIT), adapted by Nathalie Ouellette

The launch of the James Webb Space Telescope (JWST) in 2021 kicked off an exciting new era for exoplanet research, especially for scientists looking at terrestrial planets orbiting stars other than our Sun. But three years into the telescope’s mission, some scientists have run into challenges that have slowed down progress.

In a paper published in Nature Astronomy, the TRAPPSIT-1 JWST Community Initiative lays out a step-by-step roadmap to overcome the challenges they faced while studying the TRAPPIST-1 system by improving the efficiency of data gathering to benefit the astronomy community at large.

“A whole community of experts came together to tackle these complex cross-disciplinary challenges to design the first multi-year observational strategy to give JWST a fighting chance at identifying habitable worlds over its lifetime,” says Julien de Wit, a professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) and one of the lead authors of the paper.

Two-for-one deal

Located 41 light years from Earth, the TRAPPIST-1 system and its seven planets presents a unique opportunity to study a large system with multiple planets of different compositions, similar to our own Solar System.

“It’s a dream target: you have not one but maybe three planets in the habitable zone, so you have a way to actually compare in the same system,” says René Doyon, Director of the Trottier Institute for Research on Exoplanets (iREx) and Professor at the Université de Montréal, who co-led the study with de Wit. “There are only a handful of well-characterised temperate rocky planets for which we can hope to detect their atmosphere, and most of them are within the TRAPPIST-1 system.”

Astronomers like de Wit and Doyon study exoplanet atmospheres through a technique called transmission spectroscopy, where they look at the way starlight passes through a planet’s potential atmosphere to see what elements are present. Transmission spectra are collected when the planet passes in front of its host star.

The planets within the TRAPPIST system have short orbital periods. As a result, their transits frequently overlap. Transit observation times are usually allotted in five-hour windows, and when scheduled properly, close to half of these can catch at least two transits. This “two-for-one” saves both time and money while doubling data collection.

Stellar contamination

A reflection of a star and some exoplanets in the mirror shape of the James Webb Space Telescope. (Credit: D. Berardo/MIT)

Stars are not uniform; their surfaces can vary in temperature, creating spots that can be hotter or cooler. Molecules like water vapor can condense in cool spots and interfere with transmission spectra. Stellar information like this can be difficult to disentangle from the planetary signal and give false indications of a planet’s atmospheric composition, creating what’s known as “stellar contamination”. While it has often been ignored, the improved capabilities of the JWST have revealed the challenges stellar contamination introduces when studying planetary atmospheres.

These challenges related to stellar contamination have already been highlighted in multiple JWST result papers, including those of iREx’s Olivia Lim on the TRAPPIST-1 b exoplanet and Marylou Fournier-Tondreau on the hot Saturn HAT-P-18 b.

“Considering the system’s host star makes a big difference! In our paper on HAT-P-18 b, it was actually the first time that we clearly disentangle the signature of hazes versus starspots, thanks to the Canadian NIRISS instrument on JWST,” explained Marylou Fournier-Tondreau.

“In addition to the contamination from stellar spots and faculae, we saw a stellar flare, an unpredictable event during which the star looks brighter for several minutes to hours,” mentions Olivia Lim, recounting her own experience with the TRAPPIST-1 system. “This flare affected our measurement of the amount of light blocked by the planet. Such signatures of stellar activity are difficult to model but we need to account for them to ensure that we interpret the data correctly.”

Beyond TRAPPIST-1

The James Webb Space Telescope points to the TRAPPIST-1 planets in a row out in space. A red dotted line meanders from the telescope to the plants. Along the way it circles around a signpost reading “exoplanet observation”. (Credit: Paige Colley/NASA/JPL-Caltech/ ESA/CSA/STScI)

The current roadmap was born from the efforts of the TRAPPIST JWST Community Initiative to bring together separate programs focused on individual planets, which prevented them from leveraging the optimal transit observation windows.

“We understood early on that this effort would take a village to avoid the efficiency traps of small observation programs,” says de Wit. “Our hope now is that a large-scale community effort guided by the roadmap can be initiated to yield deliverables at a timely pace”. De Wit hope that it could result in identifying habitable, or inhabitable, worlds around TRAPPIST-1 within a decade.

Both de Wit and Doyon believe that the TRAPPIST-1 system is the best place for conducting fundamental research on exoplanet atmospheres that will extend to studies in other systems. Doyon thinks that “the TRAPPIST-1 system will be useful not only for TRAPPIST-1 itself, but also to learn how to do very precise correction of stellar activity which will be beneficial to many other transmission spectroscopy programs also affected by stellar activity.”

“We have within reach fundamental and transforming answers with a clear roadmap to them,” says de Wit. “We just need to follow it diligently.”

For more information

• Community paper on Nature Astronomy