The study of exoplanets in the habitable zones of red dwarfs raises questions and hopes, but requires prioritization
Space Telescope «James Webb» (JWST) has great power and potential for scientific discovery. One of his main goals — study of the atmospheres of exoplanets in order to determine their chemical composition — a task that only such a powerful tool can accomplish. However, even JWST takes time to effectively exploit opportunities, especially when it comes to one of the key tasks — studying rocky exoplanets orbiting red dwarfs.
Red dwarfs are the most common type of star in the Milky Way. Observations show that there are many rocky planets in the habitable zones of these stars. But questions about how suitable these planets are for life and about the possibility of the presence of an atmosphere in the habitable zones of red dwarfs still remain unanswered. Astronomers are eager to study the atmospheres of such planets and look for biosignatures and other information about the composition of their atmosphere.
JWST will need hundreds of hours of observations to detect and help study the atmospheres of exoplanets, according to a new study. The sole author of this study is René Doyon from the Department of Physics at the University of Montreal in Canada. Doyon notes that while studying the atmospheres of exoplanets is one of the main goals of JWST, work has so far only been done on a few planets: Trappist-1d, e, f, g, LHS1140b and mini-Neptune K2-18b.
The study's results show that JWST is indeed capable of studying the atmospheres of exoplanets, but also indicate that stellar activity is a barrier to successful results and high-quality, conclusive data. The telescope analyzes the atmospheres of exoplanets by watching the planet pass by its star. It analyzes starlight passing through the exoplanet's atmosphere, looking for signatures of different molecules.
One of the most important questions in the study of exoplanets is related to rocky planets located in the habitable zones of red dwarfs. Do they have an atmosphere? Fortunately, low-mass red dwarfs and their planets are the best objects for spectrometric study.
However, every opportunity comes with its own challenges, and when it comes to low-mass red dwarfs, the obstacles become especially significant. These stars are known for their intense flares, which can make nearby planets unsuitable for life. The flares emit powerful X-ray and ultraviolet radiation that can destroy a planet's atmosphere. Over billions of years, such flares can thin the atmosphere so much that there is virtually no chance of habitation.
Red dwarf flares pose another problem: this stellar activity can complicate spectroscopic studies of exoplanet atmospheres.
JWST — the best tool for studying exoplanet atmospheres. However, its service life is limited. It is designed to last up to 10 years, and has already completed its mission for approximately 18 months. When it comes to studying exoplanets, which is just one of his goals, how to best use his time?
In the author's opinion, it is very important to use JWST time not only for studying the atmosphere, but also for preparing for future flagship missions and reconnaissance of future targets for ground-based observatories.
Doyon says priority should be given to what he calls Golden-J — the most promising exoplanets. This group of planets is cool enough to avoid the greenhouse effect. And it is also necessary to have accurate measurements of radius and mass, which leads to an accurate understanding of their density.
«These criteria limit the selection to only a few rocky planets: Trappist-1d, e, f, g and LHS1140b. And as an exception, mini-Neptune K2-18b, despite the uncertainty of its mass», — writes Doyon.
JWST, like Hubble, has already explored these exoplanets. But the results contain uncertainty. Doyon describes JWST's early looks at Golden-J exoplanets as exploratory and believes that to resolve much of this uncertainty, these planets should be studied more thoroughly in the remaining time.
The exoplanet LHS-1140 b received much attention in Doyon's works. «LHS1140b is perhaps the best temperate planet, the state of its liquid surface can be indirectly judged based on the detection of CO2 in its atmosphere», — writes the author. But JWST can only observe 4 transits of the planet and 4 eclipses in one year. JWST may require 12 «looks» within three years to collect sufficiently convincing evidence of the presence of liquid water on the surface.
Doyon calls this attempt to take a closer look at LHS-1140 b and other exoplanets in the Golden-J selection as a “deep habitability survey.”
Perhaps the most important question that JWST should and can answer — this is the title of my article: Do rocky temperate planets around M-dwarfs have atmospheres?, — writes Doyon.
This will take time. Doyon estimated how long it would take to observe JWST: “A comprehensive survey would require a minimum of 700 hours, including approximately 225 hours dedicated to eclipse photometry.” However, these hours may not be enough: «Introducing a higher detection threshold for the reconnaissance program – as published for the Trappist-1b and Trappist-c observations using the MIRI instrument – will significantly increase the total observation time, potentially ranging from 1300 to 2000 hours».
This may take even longer, especially if promising results emerge that require additional observations. This may seem to take a long time for a small number of exoplanets. However, JWST was created to find answers, and if it takes that much time, then it will be put to good use.