This month will mark a new chapter in the search for extraterrestrial life, when the most powerful space telescope ever built will begin spying on planets orbiting other stars. Astronomers hope the James Webb Space Telescope will reveal whether any of those planets harbor atmospheres that could support life.
Identifying an atmosphere in another solar system would be remarkable enough. But there is even a chance, albeit a small one, that one of these atmospheres offers what is known as a biosignature: a sign of life itself.
“I think we’ll be able to find planets that we think are interesting, you know, good chances for life,” said Megan Mansfield, an astronomer at the University of Arizona. “But we won’t necessarily be able to identify life right away.”
Until now, Earth remains the only planet in the universe where life is known to exist. Scientists have been sending probes to Mars for nearly 60 years and have yet to find Martians. But it is conceivable that life is hiding under the surface of the Red Planet or is waiting to be discovered on a moon of Jupiter or Saturn. Some scientists have hoped that even Venusdespite its scorching atmosphere of sulfur dioxide clouds, it could be home to Venusians.
Even if Earth turns out to be the only life-bearing planet in our own solar system, many other solar systems in the universe are home to so-called exoplanets.
In 1995, Swiss astronomers detected the first exoplanet orbiting a Sun-like star. Known as 51 Pegasi b, the exoplanet turned out to be an unpromising home for life: a bloated gas giant larger than Jupiter and a temperature of 1,800 degrees Fahrenheit.
In the years since then, scientists have discovered over 5,000 other exoplanets. Some of them are much more similar to Earth: about the same size, made of rock instead of gas, and orbit in a “Goldilocks zone” around their star, not close enough to cook but not far away. to freeze.
Unfortunately, the relatively small size of these exoplanets has made them extremely difficult to study, until now. The James Webb Space Telescope, launched last Christmas, will change that, acting as a magnifying glass that will allow astronomers to take a closer look at these worlds.
Since its launch from Kourou, French Guiana, the telescope has traveled a million miles from Earth, entering its own orbit around the sun. There, a shield protects its 21-foot mirror from any heat or light from the sun or Earth. In this deep darkness, the telescope can detect faint, distant flashes of light, including those that could reveal new details about distant planets.
The space telescope “is the first large space observatory to take into account the study of exoplanet atmospheres in its design,” said Dr. Mansfield.
NASA engineers began taking pictures of a number of objects with the Webb telescope in mid-June and will release their first images to the public on July 12.
Exoplanets will be in that first batch of images, said Eric Smith, lead scientist for the program. Because the telescope will spend relatively little time observing exoplanets, Dr. Smith considered those first images a “quick and dirty” look at the power of the telescope.
Those quick looks will be followed by a much longer series of observations, starting in July, that will offer a much clearer picture of exoplanets.
Several teams of astronomers are planning to observe the seven planets orbiting a star called Trappist-1. Previous observations have suggested that three of the planets occupy the habitable zone.
“It’s an ideal place to search for traces of life outside the solar system,” said Olivia Lim, a graduate student at the University of Montreal who will observe the Trappist-1 planets beginning July 4.
Because Trappist-1 is a small, cool star, its habitable zone is closer to it than in our own solar system. As a result, its potentially habitable planets orbit at close range, taking only a few days to orbit the star. Every time the planets pass in front of Trappist-1, scientists will be able to address a basic but crucial question: Do any of them have atmospheres?
“If it doesn’t have air, it’s not habitable, even if it’s in the habitable zone,” said Nikole Lewis, an astronomer at Cornell University.
Dr. Lewis and other astronomers would not be surprised if they did not find atmospheres around the Trappist-1 planets. Even if the planets had developed atmospheres when they formed, the star could have destroyed them long ago with ultraviolet and X-ray radiation.
“It’s possible that they could simply remove all of a planet’s atmosphere before it has a chance to start forming life,” Dr. Mansfield said. “That’s the first-order question we’re trying to answer here: whether these planets could have an atmosphere long enough for them to support life.”
A planet passing in front of Trappist-1 will create a small shadow, but the shadow will be too small for the space telescope to capture. Instead, the telescope will detect a slight dimming in the light traveling from the star.
“It’s like looking at a solar eclipse with your eyes closed,” said Jacob Lustig-Yaeger, an astronomer doing a postdoctoral fellowship at the Johns Hopkins Applied Physics Laboratory. “You may have a feeling that the light has dimmed.”
A planet with an atmosphere would darken the star behind it differently than a bare planet would. Some of the starlight will pass directly through the atmosphere, but the gases will absorb light at certain wavelengths. If astronomers only look at starlight at those wavelengths, the planet will dim Trappist-1 even more.
The telescope will send these Trappist-1 observations back to Earth. “And then you get an email that says, ‘Hello, your data is available.'” Mansfield said.
But the light from Trappist-1 will be so faint that it will take time to understand. “Her eye of hers is used to dealing with millions of photons per second,” Dr. Smith said. “But these telescopes, they’re only collecting a few photons per second.”
Before Dr. Mansfield or her fellow astronomers can analyze the exoplanets passing in front of Trappist-1, they will first have to distinguish it from the tiny fluctuations produced by the telescope’s own machinery.
“A lot of the work that I do is making sure that we carefully correct for whatever weird stuff the telescope is doing, so that we can see those tiny signals,” Dr. Mansfield said.
It is possible that at the end of those efforts, Dr. Mansfield and his colleagues will discover an atmosphere around a Trappist planet-1. But that result alone will not reveal the nature of the atmosphere. It could be rich in nitrogen and oxygen, like on Earth, or more like the toxic stew of carbon dioxide and sulfuric acid on Venus. Or it could be a mix that scientists have never seen before.
“We have no idea what these atmospheres are made of,” said Alexander Rathcke, an astronomer at the Technical University of Denmark. “We have ideas, simulations and all these things, but we really have no idea. We have to go and look.”
The James Webb Space Telescope, sometimes called JWST, may be powerful enough to determine the specific ingredients of exoplanet atmospheres because each type of molecule absorbs a different range of wavelengths of light.
But those discoveries will depend on the weather on exoplanets. A bright, reflective blanket of clouds could prevent starlight from entering an exoplanet’s atmosphere, derailing any attempts to find extraterrestrial air.
“It’s really hard to distinguish between an atmosphere with clouds or without an atmosphere,” Dr. Rathcke said.
If the weather cooperates, astronomers are especially eager to see if exoplanets have water in their atmospheres. At least on Earth, water is an essential requirement for biology. “We think that would probably be a good starting point to look for life,” Dr. Mansfield said.
But a watery atmosphere doesn’t necessarily mean an exoplanet harbors life. To be sure that a planet is alive, scientists will have to detect a biosignature, a molecule or a combination of several molecules that is distinctively made by living things.
Scientists are still debating what a reliable biosignature would be. Earth’s atmosphere is unique in our solar system because it contains a lot of oxygen, much of it produced by plants and algae. But oxygen can also be produced without the help of life, when water molecules in the air split. Methane, likewise, can be released by living microbes but also by volcanoes.
It is possible that there is a particular balance of gases that can provide a clear biological signature, which cannot be maintained without the help of life.
“We need extremely favorable scenarios to find these biosignatures,” said Dr. Rathcke. “I’m not saying it’s not possible. I think it’s exaggerated. We need to be extremely lucky.”
Joshua Krissansen-Totton, a planetary scientist at the University of California, Santa Cruz, said finding that balance may require the Webb telescope to observe a planet that repeatedly passes in front of Trappist-1.
“If someone comes forward in the next five years and says, ‘Yes, we have found life with JWST,’ I will be very skeptical of that claim,” Dr. krissansen-totton said.
The James Webb Space Telescope may simply not be able to find biosignatures. That task may have to wait for the next generation of space telescopes, more than a decade from now. These will study exoplanets the same way people look at Mars or Venus in the night sky: by observing starlight reflecting off them against the black background of space, rather than observing them as they pass in front of a star.
“Primarily, we will do the very important groundwork for future telescopes,” Dr. Rathcke predicted. “I would be very surprised if JWST offers biosignature detections, but I hope they correct me. I mean, this is basically what I’m doing this job for.”
