This month will mark a new chapter in the search for extraterrestrial life when the most powerful space telescope ever built begins to spy on planets orbiting other stars. Astronomers hope the James Webb Space Telescope will reveal whether some of these planets have an atmosphere that could support life.

Finding an atmosphere in another solar system would be cold enough. But there’s a chance — however small — that one of these atmospheres has something called a biosignature: a reference to life.

“I think we can find planets that look interesting — you know, good possibilities for life,” said Megan Mansfield, an astronomer at the University of Arizona. “But we don’t always recognize life right away.”

To this day, Earth remains the only planet in the universe known to harbor life. Scientists have been sending probes to Mars for almost 60 years and have yet to find Mars. But it’s conceivable that life lurks beneath the surface of the red planet, or is waiting to be discovered on the moons of Jupiter or Saturn. Some scientists have expressed hope for this VenusDespite its atmosphere scorched by clouds of sulfur dioxide, it may be home to the children of Venus.

Even if Earth turns out to be the only life-bearing planet in our solar system, many other solar systems in the universe have so-called exoplanets.

In 1995, Swiss astronomers discovered the first exoplanet orbiting a Sun-like star. The exoplanet known as 51 Pegasi b is a giant gas giant larger than Jupiter and 1,800 degrees Fahrenheit, making it unlikely home for life.

In subsequent years, scientists discovered More than 5000 other exoplanets. Some are very similar to Earth—about the same size, made of rock rather than gas, and orbiting in the “Goldilocks Zone” around their star, not close enough to cook food but not close enough to freeze.

Unfortunately, the relatively small size of these exoplanets has made it difficult to study them so far. The James Webb Space Telescope, launched last Christmas, will serve as a magnifying glass allowing astronomers to get a closer look at these worlds.

The telescope has been around since it was launched from Kourou, French Guiana I travel Millions of miles from Earth, it enters an orbit around the Sun. There, his 21-foot mirror is protected by a shield from the heat or light of the sun or earth. In this deep darkness, the telescope can detect faint and distant rays of light, including those that could reveal new details about distant planets.

The space telescope is “the first major space observatory to carry the study of exoplanet atmospheres into its design,” said Dr. Mansfield said.

NASA engineers began photographing various objects using the Webb Telescope in mid-June, and will release their first images to the public on July 12.

Eric Smith, the program’s chief scientist, said that this first batch will contain exoplanets. Because the telescope only observes exoplanets for a relatively short time, Dr. Smith considered these first images a “quick and dirty” look at the telescope’s power.

These rapid surveys will be followed by a series of longer observations starting in July, providing a clearer picture of exoplanets.

Several teams of astronomers plan to test it Seven planets orbiting the star Trappist-1. Previous observations have indicated that three planets are in the habitable zone.

“It’s an ideal place to look for signs of life outside the solar system,” said University of Montreal graduate student Olivia Lim, who has been observing planets aboard TRAPIST-1 since July 4.

Because Trappist-1 is a small, cool star, its habitable zone is closer than our own solar system. As a result, its habitable planets orbit at close range, taking only a few days to orbit the star. Each time the planets pass in front of TRAPIST-1, scientists can address a basic but important question: Do any of them have atmospheres?

“If there was no air, it wouldn’t be habitable, even if it were in a habitable zone,” said Cornell University astronomer Nicole Lewis.

If Trappist-1 does not detect the atmosphere around the planets Dr. Lewis and other astronomers would not be surprised. Although atmospheres formed when the planets formed, the star probably blew them away long ago with ultraviolet and X-rays.

“They can strip a planet of its entire atmosphere before it has a chance to produce life,” Dr Mansfield said. “That’s the first question we’re trying to answer here: Could these planets have enough atmosphere to support life?”

A planet passing by TRAPIST-1 will cast a small shadow, but the shadow will be too small for a space telescope to pick up. Instead, the telescope will detect a slight dimming of the star’s light.

“It’s like closing your eyes and looking at a solar eclipse,” said astronomer Jacob Lustig-Jaeger, who did a postdoctoral fellowship at the Johns Hopkins Applied Physics Laboratory. “You can feel something that the light is dim.”

A planet with an atmosphere obscuring a star is different from a bare planet. Some of the star’s light will pass directly through the atmosphere, but gases will absorb light at certain wavelengths. If astronomers looked only for starlight at these wavelengths, Trappist-1 would be fainter.

The telescope will transmit these TRAPIST-1 observations back to Earth. Then you get an email saying, “Hey, your data is available,” Dr. Mansfield.

But the light from TRAPIST-1 will be so faint that it will take time to perceive. “Your eye is used to handling millions of photons per second,” Dr. Smith said. “But these telescopes only collect a few photons per second.”

Dr. Before Mansfield or her fellow astronomers can analyze the exoplanets that TRAPIST-1 passes by, they first need to separate them from tiny fluctuations caused by the telescope’s special mechanism.

“Most of the work I do is we carefully correct all the weird things the telescope does so we can see these very small signals,” Dr. Mansfield.

After these efforts Dr. Mansfield and her colleagues were able to detect the atmosphere around Trappist-1. But this result alone does not reveal the nature of the atmosphere. It can be as rich in nitrogen and oxygen as on Earth, or more like a toxic soup of carbon dioxide and sulfuric acid on Venus. Or it could be a combination that scientists haven’t seen before.

“We don’t know what these atmospheres are made of,” said Alexander Rathke, an astronomer at the Technical University of Denmark. “We have ideas and simulations and all that, but we really don’t have a clue. Let’s go and see.

The James Webb Space Telescope, sometimes called JWST, may prove powerful enough to detect specific components of exoplanet atmospheres because each type of particle absorbs light in a different wavelength range.

But these findings may depend on the climate of the exoplanets. The bright, reflective cloud cover would block any starlight from entering the exoplanet’s atmosphere, defeating any attempt to detect space air.

“It’s really hard to tell the difference between an atmosphere with clouds and one without,” Dr. Ratke.

Astronomers are particularly interested in knowing if the outer planets have water in their atmospheres if the weather is favorable. Water is a prerequisite for biology, at least on Earth. “We think this might be a good starting point for the search for life,” Dr. Mansfield said.

But a watery atmosphere doesn’t necessarily mean an exoplanet has life. To confirm that the planet is alive, scientists need to find a biomarker, a molecule or group of molecules produced by living organisms.

Scientists are still debating what constitutes a reliable biosignature. Earth’s atmosphere is unique in our solar system in that it is rich in oxygen, a product primarily of plants and algae. But oxygen can also be produced without the help of life when water molecules in the air break down. Methane can be released not only by living microbes, but also by volcanoes.

It is probable that a certain balance of gases exists capable of giving a definite vital impression which life cannot maintain without the aid of life.

“We need very favorable conditions to find these important fingerprints,” said Dr. Ratke. “I’m not saying it’s impossible. I thought it was far off. We should be very lucky.”

To find such a balance, the Webb telescope would need to observe a planet that frequently transits TRAPIST-1, said Joshua Christensen-Totten, a planetary scientist at the University of California, Santa Cruz.

“If someone comes out in the next five years and says, ‘Yes, we found life on the JWST,’ I would be very skeptical of that claim,” Dr. Christensen-Totton.

The James Webb Space Telescope is likely unable to detect biometrics. That mission may have to wait more than a decade for the next generation of space telescopes. These exoplanets will be studied the same way people look at Mars or Venus in the night sky: by watching the starlight reflected off them against the black background of space, rather than passing by the star.

“For the most part, we will be doing very important groundwork for future telescopes,” Dr. Ratke. “I would be very surprised if JWST implements biometric fingerprinting, but I hope it gets resolved. Basically I think I do this job.