Finding Atmospheres on Red Dwarf Planets Will Take Hundreds of Hours of Webb Time

The JWST is enormously powerful. One of the reasons it was launched is to examine exoplanet atmospheres to determine their chemistry, something only a powerful telescope can do. But even the JWST needs time to wield that power effectively, especially when it comes to one of exoplanet science’s most important targets: rocky worlds orbiting red dwarfs.

Red dwarfs are the most common type of star in the Milky Way. Observations show that red dwarfs host many rocky planets in their habitable zones. There are unanswered questions about red dwarf habitable zones and whether the rocky planets in these zones are truly habitable because of well-documented red dwarf flaring. Astronomers want to examine these planets’ atmospheres and look for biosignatures and other atmospheric information.

New research suggests that it could take the capable JWST hundreds of hours of observing time to detect these atmospheres to a greater degree of certainty. The new research is “Do Temperate Rocky Planets Around M Dwarfs Have an Atmosphere?” The sole author is Rene Doyon from the Physics Department at the University of Montreal, Canada. The paper hasn’t been peer-reviewed yet.

Doyon points out that even though one of the JWST’s main goals is to probe exoplanet atmospheres, it’s only done that for a small handful of planets: Trappist-1d, e, f, g, LHS1140b, and the mini-Neptune K2-18b.

These results have shown that the JWST has the power to probe exoplanet atmospheres. But the effort has also shown how stellar activity poses an obstacle to even more success. The JWST examines exoplanet atmospheres by watching as the planet transits its star. The telescope dissects the light from the star as it passes through the exoplanet’s atmosphere, looking for the light signatures of different molecules.

One of the biggest questions in exoplanet science concerns rocky planets in red dwarf habitable zones. Do they have atmospheres? Without atmospheres and liquid water, they may as well be way outside of the habitable zones. Fortunately, M-dwarfs have lower masses and radii, making them and their planets better targets for spectrometry. “This is known as the ‘M-dwarf opportunity,” Doyon writes in his research.

But each opportunity has an obstacle attached to it, and when it comes to M-dwarfs, the obstacle is flaring. M-dwarfs are known for their powerful flaring, and in some cases, the flares are powerful enough to render nearby planets uninhabitable. The flares emit powerful X-ray and UV radiation that can erode their atmospheres. Over billions of years, they can be so degraded that they have no chance of being habitable.

Red dwarf flaring introduces another problem. All that stellar activity can make it harder for the JWST to study exoplanet atmospheres spectroscopically.

The JWST is our best tool for examining exoplanet atmospheres. But it won’t last forever. It should last up to ten years and is already about 18 months into its mission. When it comes to studying exoplanets, which is only one of its jobs, how can its time be best spent?

According to the author, it’s essential that we use the JWST’s exoplanet time not only to study atmospheres but also to prepare for future flagship missions and ground-based observatories that can pick up where the JWST leaves off.

“We contend that given JWST’s limited lifetime, a comprehensive program of both eclipse and transmission spectroscopy of a key sample of temperate planets is an essential way forward,” Doyon writes.

Doyon says that the JWST should prioritize what he calls the Golden-J sample of the best temperate exoplanets. This group of planets is cool enough to avoid the runaway greenhouse effect. They also need to have accurate radius and mass measurements, which leads to an accurate understanding of their density.

“These criteria limit the selection to only a handful of rocky planets: Trappist-1d, e, f, g, and LHS1140b,” Doyon writes. “We make an exception to include the temperate mini-Neptune K2-18b as a potential habitable world, despite its mass uncertainty of 18%.” This is the Golden-J sample.

The JWST has already examined these exoplanets, and so have other telescopes, including the Hubble. But the results contain some uncertainty. Doyon describes the JWST’s first looks at the Golden-J sample exoplanets as reconnaissance and thinks that in order to remove more of that uncertainty, the JWST should examine these planets more thoroughly with some of its remaining time.

The exoplanet LHS-1140 b has a prominent spot in Doyon’s work. “LHS1140b is arguably the best temperate planet from which liquid surface condition may be inferred indirectly through the detection of CO2 in its atmosphere,” he explains. But the JWST can only observe 4 of the planet’s transits and 4 of its eclipses in one year. The JWST could require 12 visits over three years to gather strong enough evidence in favour of liquid surface water.

Doyon calls this effort to observe LHS-1140 b and the other exoplanets in the Golden-J sample more thoroughly ‘deep habitability reconnaissance.’

“Arguably, the single most important question that JWST should and can answer under the scientific theme, Planetary Systems and the Origin of Life, is the title of this paper: Do temperate rocky planets around M dwarfs have an atmosphere?” Doyon writes.

It’ll take time to do that, and Doyon has calculated how much JWST observing time is required. “This comprehensive reconnaissance effort would necessitate a minimum of 700 hours, including approximately 225 hours dedicated to eclipse photometry,” he explains.

“However, this 715-hour effort may not be enough in all circumstances. Imposing a higher detection threshold would require even more time. “Imposing a higher detection threshold (4-5?) for this reconnaissance program – as was published for the MIRI observations of Trappist-1b and c – would
significantly increase the total observing time, potentially ranging from 1300 to 2000 hours,” Doyon explains.

It could take even more time than that, especially if there are any particularly tantalizing results that require more follow-up observations.

This may sound like a good chunk of time to spend on a small handful of exoplanets. But the JWST was built to find answers, and if it takes this much time, it’s time well spent.

“Initiating such an extensive habitability program at the earliest opportunity is paramount,” Doyon writes.