This month will mark a new chapter in the search for extraterrestrial life, when the most powerful space telescope yet built will start spying on planets that orbit other stars. Astronomers hope that the James Webb Space Telescope will reveal whether some of those planets harbor atmospheres that might support life.
Identifying an atmosphere in another solar system would be remarkable enough. But there is even a chance — albeit tiny — that one of these atmospheres will offer what is known as a biosignature: a signal of life itself.
“I think we will be able to find planets that we think are interesting — you know, good possibilities for life,” said Megan Mansfield, an astronomer at the University of Arizona. “But we won’t necessarily be able to just identify life immediately.”
So far, Earth remains the only planet in the universe where life is known to exist. Scientists have been sending probes to Mars for almost 60 years and have not yet found Martians. But it is conceivable that life is hiding under the surface of the Red Planet or waiting to be discovered on a moon of Jupiter or Saturn. Some scientists have held out hope that even Venus, despite its scorching atmosphere of sulfur dioxide clouds, might be home to Venusians.
Even if Earth turns out to be the only planet harboring life in our own solar system, many other solar systems in the universe hold so-called exoplanets.
In 1995, French astronomers spotted the first exoplanet orbiting a sunlike star. Known as 51 Pegasi b, the exoplanet turned out to be an unpromising home for life — a puffy gas giant bigger than Jupiter, and a toasty 1,800 degrees Fahrenheit.
In the years since, scientists have found more than 5,000 other exoplanets. Some of them are far more similar to Earth — roughly the same size, made of rock rather than gas and orbiting in a “Goldilocks zone” around their star, not so close as to get cooked but not so far as to be frozen.
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 to let astronomers look more closely 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 profound darkness, the telescope can detect faint, distant glimmers of light, including those that could reveal new details about faraway planets.
The space telescope “is the first big space observatory to take the study of exoplanet atmospheres into account in its design,” Dr. Mansfield said.
NASA engineers began taking pictures of an array of objects with the Webb telescope in mid-June and will release its first images to the public on July 12.
Exoplanets will be in that first batch of pictures, said Eric Smith, the program’s lead scientist. Because the telescope will spend relatively little time observing the exoplanets, Dr. Smith considered those first images a “quick and dirty” look at the telescope’s power.
Those quick looks will be followed by a series of much longer observations, starting in July, offering a much clearer picture of the exoplanets.
A number of teams of astronomers are planning to look at the seven planets that orbit a star called Trappist-1. Earlier observations have suggested that three of the planets occupy the habitable zone.
“It’s an ideal place to look for traces of life outside of the solar system,” said Olivia Lim, a graduate student at the University of Montreal who will be observing the Trappist-1 planets starting around 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 just a few days to circle the star. Every time the planets pass in front of Trappist-1, scientists will be able tackle a basic but crucial question: Do any of them have an atmosphere?
“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 to find no atmospheres surrounding Trappist-1’s planets. Even if the planets had developed atmospheres when they formed, the star might have blasted them away long ago with ultraviolet and X-ray radiation.
“It’s possible that they could just strip away all of the atmosphere on a planet before it even had a chance to like start forming life,” Dr. Mansfield said. “That’s the first-order question that we’re trying to answer here: whether these planets could have an atmosphere long enough that they’d be able to develop life.”
A planet passing in front of Trappist-1 will create a tiny 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 shut,” said Jacob Lustig-Yaeger, an astronomer doing a postdoctoral fellowship at the Johns Hopkins Applied Physics Laboratory. “You might have some sense that the light has dimmed.”
A planet with an atmosphere would dim the star behind it differently than a bare planet would. Some of the star’s light will pass straight through the atmosphere, but the gases will absorb light at certain wavelengths. If astronomers look only at starlight at those wavelengths, the planet will dim Trappist-1 even more.
The telescope will send these observations of Trappist-1 back to Earth. “And then you get an email that’s like, ‘Hello, your data are available,’” Dr. Mansfield said.
But the light coming from Trappist-1 will be so faint that it will take time to make sense of it. “Your eye is used to dealing with millions of photons per second,” Dr. Smith said. “But these telescopes, they’re just collecting a few photons a second.”
Before Dr. Mansfield or her fellow astronomers will be able to analyze exoplanets passing in front of Trappist-1, they will have to first distinguish it from tiny fluctuations produced by the telescope’s own machinery.
“A lot of the work that I actually do is making sure that we’re carefully correcting for anything weird that the telescope is doing, so that we can see those teeny-tiny signals,” Dr. Mansfield said.
It is possible that at the end of those efforts, Dr. Mansfield and her colleagues will discover an atmosphere around a Trappist-1 planet. But that result alone will not reveal the nature of the atmosphere. It might be rich in nitrogen and oxygen, like on Earth, or more akin to 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 this stuff, but we really have no idea. We have to go and look.”
The James Webb Space Telescope, sometimes called the J.W.S.T., may prove powerful enough to determine the specific ingredients of exoplanet atmospheres because each kind of molecule absorbs a different range of wavelengths of light.
But those discoveries will depend on the weather on the exoplanets. A bright, reflective blanket of clouds could prevent any starlight from entering an exoplanet’s atmosphere, ruining any attempt to find alien air.
“It is really hard to distinguish between an atmosphere with clouds or no atmosphere,” Dr. Rathcke said.
If the weather cooperates, astronomers are especially eager to see if the 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 will not necessarily mean that an exoplanet harbors life. To be sure 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 in that it contains a lot of oxygen, largely the product of plants and algae. But oxygen can also be produced without life’s help, when water molecules in the air are 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 biosignature, one that cannot be maintained without the help of life.
“We need extremely favorable scenarios to find these biosignatures,” said Dr. Rathcke. “I’m not saying that it’s not possible. I just think it’s far-fetched. We need to be extremely lucky.”
Joshua Krissansen-Totton, a planetary scientist at the University of California, Santa Cruz, said that finding such a balance may require the Webb telescope to observe a planet repeatedly passing in front of Trappist-1.
“If anyone comes forward in the next five years and says, ‘Yes, we’ve found life with J.W.S.T.,’ I’ll be very skeptical of that claim,” Dr. Krissansen-Totton said
It is possible that the James Webb Space Telescope simply will not be capable of finding biosignatures. That task may have to wait for the next generation of space telescopes, more than a decade away. These will study exoplanets the same way that 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.
“Mostly, we’ll be doing the very important groundwork for future telescopes,” Dr. Rathcke predicted. “I would be very surprised if J.W.S.T. delivers biosignature detections, but I hope to stand corrected. I mean, this is basically what I’m doing this work for.”
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