There's no place like home

ANDLiving on one of the seven Earth-sized planets in the TRAPPIST-1 system would be very strange. A huge red star looms ominously in the sky, prone to fiery flashes and seemingly several times larger than the Sun. There are no hours in the day; each planet is tidally attached to the star so that one side is eternally hot and the other is eternally frozen. Along the border separating the day and night sides – the only place with an acceptable climate – an incessant wind blows, and the star hangs on the horizon, in eternal sunset.

A short walk to the dark side results in the appearance of your planetary companions. Every few days, one or more of them float overhead like a floating lantern larger than the Moon. Keen night sky watchers might also notice a bright yellow star, one of the system's close neighbors, and wonder what life might be like next to what humans call the Sun.

But for those of us who live 41 light years away, on a world warmed by this benign yellow star, the search for an answer to the question of whether one of the TRAPPIST-1 planets could make a comfortable home for our imaginary observer has been a disappointment.

When in 2017 it became known about seven planets around TRAPPIST-1, this was a boon not only for science fiction writers, but also for astronomers, who saw this system as the best place to find a habitable planet using the JWST orbital observatory (James Webb), who has been working tirelessly for over two years. It seemed like the perfect combination of tool and purpose. JWST's ability to peer into the infrared atmospheres of exoplanets, where life-giving molecules such as water and carbon dioxide leave their imprints, is unique. And TRAPPIST-1, in addition to being relatively nearby, is a red dwarf slightly larger than Jupiter, so cold and dim that its light does not drown out the radiation of its planets. The rocky worlds orbit in orbits much narrower than Mercury, meaning that despite the faint star, four of the seven planets are in the habitable zone (habitable zone), where liquid water can exist on their surface. Fast orbits (3 weeks or less) also mean that planets regularly intersect the star's orbit from the perspective of Earth and JWST.

These crossings are a boon to observers because if the planet has an atmosphere, some of the starlight will pass through it. Its chemical constituents can selectively absorb light at certain wavelengths, creating differences in the spectrum of starlight. Astronomers have used this technique to find evidence of carbon dioxide, methane and water in the atmospheres of large, hot planets that are uninhabitable. Finding these gases in the atmosphere of the TRAPPIST-1 planets would help prove that they could be habitable.

This hasn't been easy because red dwarfs like TRAPPIST-1 are prone to violent activity, eruptions and flares that can destroy their atmospheres and also interfere with the weak atmospheric signal that JWST is trying to detect. By the end of JWST's third year of operation, TRAPPIST-1 will have logged 175 hours of observations. However, JWST has not yet found any solid evidence of the presence of an atmosphere on the TRAPPIST-1 planets.

Therefore, some astronomers want to expand the range of observations. Working group advising NASA and the Space Telescope Science Institute (S.T.Sc.I.), which operates JWST, is calling for a massive 500-hour study of 15-20 small rocky planets around various red dwarfs to understand once and for all whether such planets could have atmospheres. “If we don't find anything, it will be disappointing, but it would be great to have a definitive answer,” says task force chairman Seth Redfield of Wesleyan University.

Others believe that the search for habitable planets should be expanded to include other types of planets. Should they be rocky and the size of the Earth? Perhaps larger super-Earths, which could be shrouded in global oceans, should be considered. Or maybe an even larger body, mini Neptunecould contain an ocean of water under a thick hydrogen atmosphere that lets in enough light to support life. “These are rather guessworks, and they are very different from the planetary bodies that we know have life,” says Charles Cockell, director of the Center for Astrobiology at the University of Edinburgh. “But any planet with suitable conditions should be explored.”

Regardless of size, few are currently within JWST's range. René Doyon, astronomer at the University of Montreal, counted only six: four temperate worlds of TRAPPIST-1, the search for which has not yet been completed, one potentially aquatic super-Earth called LHS 1140b and one hydrogen-shrouded Neptune-like world called K2-18b. “We only have a handful of facilities,” Doyon says. Michael Gillon of the University of Liege, whose team discovered the TRAPPIST-1 planets, is equally pessimistic. “We may get pleasant surprises, but for now habitable planets remain out of reach,” he says.

Time is also limited. JWST is expected to operate for up to 20 years, twice as long as predicted at launch, but finding and studying habitable atmospheres has proven more difficult than originally expected and could push it beyond that. “Small exoplanets are very difficult to observe. It will be extremely difficult,” says Redfield. “We are on the edge of the abyss, but how close is difficult to say.” (Here, to be honest, I didn’t quite understand what was meant. But I dare to suggest that this is how the scientist laments the fact that we do not have enough time for observations, just in such a metaphorical form – approx. lane).

ASTRONOMERS ALWAYS believed that most likely home for life near another star is a rocky planet, just like Earth. “We can't search everywhere,” says Cockell, “and the best way to narrow our search is to follow the constraints of life as we know it.” Atmospheric oxygen is not a necessary condition, since life on Earth existed without it. But at a minimum, most astrobiologists agree that you need water, a source of energy, and a place for prebiotic molecules to concentrate and react. This may be in rock pools along sunny shorelines, in hot springs, or around hydrothermal ocean vents. And to keep water liquid, you need a temperate climate, which is supported by greenhouse gases such as carbon dioxide.

But Earth's true twins—planets similar in size in Earth-like orbits around stars similar to the Sun—are not easy to study. It is impossible to photograph such a planet using modern telescopes, which can only directly image the largest and hottest planets in wide orbits that take them away from the blinding light of the star. Even getting data on the transit of Earth's twin will be difficult: it will pass through its star only once every 365 days, and when it does, the drop in brightness of a star as large and bright as the Sun will be too small to measure.

So instead, attention was paid to the small red dwarf starsor M-dwarfs, which are 10 to 60% the size of the Sun and less than 7% brighter. Exoplanet studies show that M dwarfs are replete with small, rocky planets in narrow orbits, some of which transit every few hours. And there are a lot of them in the galactic environs. Of the 60 stars closest to Earth, 50 are M-dwarfs. “That's why M dwarfs are so attractive to us from an observational point of view,” Redfield says.

But the results of transit observations of planets around TRAPPIST-1, an M-dwarf, did not reveal clear signs of the presence of atmospheres. Results from a technique called eclipse photometry – measuring the brightness just before and just after a planet disappears behind its star – were also disappointing. By subtracting the brightness of the star individually from the brightness of the star and planet together, researchers can determine how bright the planet's dayside glows, which is an indicator of temperature. A planet that appears cooler than expected when illuminated by a star likely has an atmosphere that transfers heat to its night side.

A study of the innermost planet, TRAPPIST-1b, receiving four times more radiation than Earth, conducted in March 2023, did not reveal no sign of having an atmosphere. In July 2023, researchers turned to its neighbor TRAPPIST-1c, whose similarity to Venus in size and the radiation received from its star raised hopes of a dense atmosphere. They found that it was very hot there too, there was practically no airwhich would transfer heat to its night side. However, the team could not rule out the presence of a thin layer of gas, and later modeling attempts allowed us to assumethat oxygen or water vapor may also fit these data.

Finding atmospheres around TRAPPIST-1b and TRAPPIST-1c has always been unlikely given their proximity to the star. But there is a more fundamental concern: TRAPPIST-1 planets are susceptible to losing their atmosphere to their star, says Laura Kreidberg of the Max Planck Institute for Astronomy. M-dwarfs are particularly aggressive when young, causing bursts of ultraviolet and X-ray radiation on nearby planets that can strip them of their atmosphere. Even if a planet's atmosphere survives the star's turbulent youth, M dwarfs continue to flare throughout their long lives. These slow-burning stars are expected to last trillions of years, hundreds of times longer than the Sun, and TRAPPIST-1 is already nearly twice the age of the Sun.

Despite the setbacks, researchers are aiming to move beyond TRAPPIST-1b and TRAPPIST-1c to cooler planets in the system that are farther away from the star and are more likely to retain atmospheres. But this comes at the cost of telescope runtime, as cooler planets emit less infrared radiation, making eclipse photometry more difficult. Additionally, the cooler atmosphere will shrink closer to the planet, weakening any transit signal. For example, it is estimated that JWST will need more than 100 hours of observations to detect carbon dioxide around one of the cooler planets, TRAPPIST-1. “A lot of astronomers are shocked by how many hours of observation you need on a planet,” says Kreidberg.

TIME ON JWST precious. Every year, STScI distributes more than 10,000 hours among hundreds of observing teams, and only about 30% of that time is spent studying exoplanets. In 2023, STScI Acting Director Nancy Levenson, knowing how much time it takes to thoroughly study cool, rocky exoplanets, offered 500 hours of special “director time” to exoplanet researchers in Cycle Three, the third year of observations that begins July 1. “This is something that would not happen within our normal time management processes,” she says.

STScI created a working group, led by Redfield, to recommend how to use this time. Over the past year, the group held three general meetings and received 42 program proposal documents. April 2 group recommended dedicate 500 hours to studying 15-20 rocky planets around M-dwarfs to definitively answer the question of whether such worlds can have atmospheres. “We'll try to study a wide enough sample that we'll get an answer one way or another,” Redfield says.

The proposal does not call for characterizing atmospheres by painstakingly collecting transit spectra. Instead, astronomers must use eclipse photometry to simply determine whether an atmosphere exists. Some see the lack of spectroscopy and the move away from TRAPPIST-1 as a missed opportunity. “I would like to have a more balanced program,” Doyon says.

Of additional concern to TRAPPIST-1 proponents is that only one program dedicated to this system has been approved for the third cycle. This study, covering 129 hours over cycles 3 and 4, will test a new strategy to eliminate the influence of stellar activity, which can vary from day to day. On 15 occasions, JWST will observe a star as TRAPPIST-1b and TRAPPIST-1e transit in quick succession. During transit “b”, observers will receive a spectrum without an atmosphere. When “e” transits, either shortly before or after it, the star should be in a very similar state, so that if there are differences between the two spectra, they must be due only to absorption of “e” by the atmosphere.

While the tactic would be extremely valuable if successful, some had hoped for a more systematic study of the TRAPPIST-1 planets, says Julien de Wit, a planetary scientist at the Massachusetts Institute of Technology. “We hope that research into the system will not be suspended until this program is completed.”

While M-Dwarfs may be the most common star, rocky worlds like Earth are not the most common type of planet. Of the more than 5,600 exoplanets, confirmed by far, the vast majority lie somewhere between rocky Earths and gas giants, a range not found in the solar system. Exactly what the planet, halfway between rocky and gaseous, looks like and whether it could be habitable remains unclear, although its density may provide clues.

If a planet's density is high, it likely has an Earth-like composition: an iron core surrounded by rock and a thin atmosphere. If the density is lower, then a variety of possibilities open up: It could have a rocky core surrounded by a deep and dense hydrogen atmosphere, or a somewhat thinner atmosphere and a global ocean of water or a layer of ice. To get to the bottom of this, researchers need to study the planet's atmosphere and see if their results fit climate models based on the planet's composition.

One such planet that JWST is studying is LHS 1140b, a world within the habitable zone of its M dwarf star, 49 light-years from Earth. In January, Doyon's team reported about precise measurements of its mass (5.6 times the mass of Earth) and size (1.73 times the size of Earth), making it less dense than if it were simply an enlarged Earth.

In March, other researchers described first LHS 1140b transit spectrum received from JWST in July 2023. The data, published in a preprint on the arXiv website, is preliminary, but it shows hints of what could be a nitrogen-dominated atmosphere like Earth's and no indication at all of hydrogen, leaving water or ice as the preferred explanation for the low density. “That's why we're very excited about this planet,” says team member Renyu Hu of the California Institute of Technology.

In a preprint based on two transit observations of LHS 1140b made in December 2023, Doyon and his colleagues confirmed the absence of hydrogen on the planet. According to their climate modeling, two plausible scenarios remain: the planet could be a rocky, airless body covered in water ice, like Jupiter's moon Europa; or, if its atmosphere is thicker and warmed by carbon dioxide, that ice could be permeated by a warm ocean at the center of the star's side, half the size of the Atlantic Ocean, he says.

Of the six potentially habitable worlds, LHS 1140b is the most likely to have an atmosphere, Doyon said. “We need more observations from JWST!” But this will not be an easy task. LHS 1140b's location in the sky means it is often out of sight of JWST, which can only observe it transiting its star four times a year. Doyon says at least a dozen transits will be needed to confirm the presence of an atmosphere and determine its carbon dioxide content, an indicator of water surfaces.

More speculative is the idea that a planet even closer in size to Neptune could support life under a thick hydrogen-rich atmosphere. One candidate is planet K2-18b, which has 8.6 times the mass of Earth and is located 124 light-years away. A team led by Nikku Madhusudhan from the University of Cambridge studied the planet's spectrum taken by the Hubble Space Telescope in 2019 and saw evidence of the hydrogen expected for large gaseous planets. Team modeledwhat kind of climate could exist around mini-Neptune under such an atmosphere. They found a small number of options in which an ocean of liquid water could exist, and the temperature would be maintained at the desired level due to the greenhouse effect from a moderately dense atmosphere.

Last year, the team took another look at K2-18b using JWST. In the solar system, planets with hydrogen-rich atmospheres, such as Saturn, also contain small amounts of ammonia and methane. Sunlight destroys these compounds in the upper atmosphere, but they are restored below. On K2-18b, a thinner atmosphere and water ocean could interfere with this recycling, especially of ammonia, according to modeling by Madhusudhan's team. JWST transit spectrum coincided with this forecastshowing the presence of methane and carbon dioxide, but not ammonia, which, according to Madhusudhan, indicates the existence of an ocean of water. He calls K2-18b “hycean” (the name is a portmanteau of the Latin words hydrogenium (hydrogen) and ocean (ocean) – approx. translator). “So far, no other hypotheses match the data,” he says. The team is currently studying JWST data from several other candidate hybrid worlds.

However, not everyone is ready to accept the idea of ​​Hykeans being habitable without stronger evidence. “Most modelers are quite pessimistic about the presence of liquid water on the surface of sub-Neptune,” says Gillon. The level of greenhouse effect must be finely tuned to achieve moderate conditions because the thick hydrogen atmospheres around such planets can quickly cause an explosive greenhouse effect, heating the surface above the boiling point. In addition, they can be so thick that they become opaque to light. “Could there be life there? Because you need ultraviolet photons to pass through it and create complex chemistry.”

Overall, Kreidberg doubts that worlds with deep global oceans can support life, since any molecular building blocks would be too dilute. “You'll never meet” other complex molecules, she says. However, Cockell is not so quick to dismiss such unusual places for life to originate. “They are different, but have physical and chemical conditions similar to the places we know,” he says. “This is an exciting hypothesis to test.”

Despite the slow progress, those searching for habitable worlds can't help but be excited about the surprises that JWST is steadily and steadily discovering on big, hot worlds. He discovered snow-like quartz flakes filling the sky WASP-17 b. He identified supersonic winds that transfer heat to the cloud-filled night side WASP-43b. And he found grainy clouds of silt and sand high in the atmosphere VHS 1256 b. “It's already a miracle that we find out,” says Gillon.

The hunt for cooler, wetter, and more habitable planets is sure to take unexpected turns as well. A few months ago, NASA's Transiting Exoplanet Survey Satellite (TESS), designed to detect exoplanets by observing star transits, has found a planet that could be another promising target for JWST: temperate world size c Earth around the red dwarf Gliese 12, which is even closer to Earth than TRAPPIST-1. “These are our first steps in studying rocky planets,” says Kreidberg. “We're in an amazing position right now.” Gillon agrees. “This is a very exciting time. Life is, of course, a goal, but we will have to be patient.”

Original version of this article was published in Science, June 20, 2024, Volume 384, Issue 6702.