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 that the James Webb Space Telescope will reveal whether some of these planets contain atmospheres that could support life.
Identifying an atmosphere in another solar system would be remarkable enough. But there’s even a chance—however slim—that one of these atmospheres might offer what’s known as a biosignature: a signal of life itself.
“I think we’re going to 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 identify life right away.”
For now, 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 yet to find any Martians. But it is possible that life is hiding beneath the surface of the Red Planet or waiting to be discovered on a moon of Jupiter or Saturn. Some scientists hoped 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 life-bearing planet in our solar system, many other solar systems in the universe have so-called exoplanets.
In 1995, Swiss astronomers spotted the first exoplanet orbiting a Sun-like star. Known as 51 Pegasi b, the exoplanet proved to be an unpromising home for life—a fluffy gas giant larger than Jupiter and a toasty 1,800 degrees Fahrenheit.
In the years since, scientists have discovered more than 5,000 other exoplanets. Some are far more Earth-like – about the same size, made of rock rather than gas, and orbiting in a “Goldilocks zone” around their star, not close enough to be cooked, but not too far away , to be frozen.
Unfortunately, the relatively small size of these exoplanets has made them extremely difficult to study so far. The James Webb Space Telescope, launched last Christmas, will change that, acting as a magnifying glass to allow astronomers to take a closer look at these worlds.
Since its launch from Kourou, French Guiana, the telescope has traveled millions of 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 ones that could reveal new details about distant planets.
The space telescope “is the first major space observatory to factor the study of exoplanet atmospheres into its design,” Dr Mansfield said.
NASA engineers began taking pictures of a range 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 this first batch of images, said Eric Smith, the program’s lead scientist. Because the telescope will spend relatively little time observing exoplanets, Dr. Smith considers these first images a “quick and dirty” look at the telescope’s power.
These 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 plan to look at the seven planets that orbit a star called Trappist-1. Earlier observations indicated that three of the planets occupy the habitable zone.
“This is an ideal place to look for signs of life outside the solar system,” said Olivia Lim, a graduate student at the University of Montreal who will observe the Trappist-1 planets from around July 4.
Because Trappist-1 is a small, cool star, its habitable zone is closer to it than it is to 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 tackle a basic but crucial question: Do any of them have atmospheres?
“If there’s no air, it’s not habitable, even if it’s in the habitable zone,” said Nicole Lewis, an astronomer at Cornell University.
Dr. Lewis and other astronomers would not be surprised to find a lack of atmosphere around Trappist-1’s planets. Even if the planets developed atmospheres when they formed, the star may have blasted them long ago with ultraviolet and X-ray radiation.
“It’s possible that they could simply remove the entire atmosphere of a planet before it even had a chance to start forming life,” Dr Mansfield said. “That’s the first-order question we’re trying to answer here: can these planets have atmospheres long enough for life to develop.”
A planet passing in front of Trappist-1 will cast a small shadow, but the shadow will be too small for the space telescope to pick up. Instead, the telescope will detect a slight dimming of the light traveling from the star.
“It’s like looking at a solar eclipse with your eyes closed,” said Jacob Lustig-Yager, an astronomer who is a postdoctoral fellow at the Johns Hopkins Laboratory for Applied Physics. “You may feel that the light is 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 only look at starlight at these wavelengths, the planet will eclipse Trappist-1 even more.
The telescope will send these Trappist-1 observations back to Earth. “And then you get an email like, ‘Hi, your data is available,'” Dr Mansfield said.
But the light coming from Trappist-1 will be so faint that it will take time to understand it. “Your eye is used to dealing with millions of photons per second,” Dr. Smith said. “But these telescopes, they just collect a few photons per second.”
Before Dr. Mansfield or her fellow astronomers can analyze exoplanets passing in front of Trappist-1, they will first have to distinguish it from small fluctuations produced by the telescope’s own machinery.
“A lot of the work that I’m actually doing is making sure that we’re carefully correcting for anything weird that the telescope is doing so that we can see these tiny signals,” Dr Mansfield said.
It is possible that at the end of these efforts, Dr. Mansfield and her colleagues will discover an atmosphere around the planet Trappist-1. But this result alone will not reveal the nature of the atmosphere. It could be rich in nitrogen and oxygen like Earth, or closer 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 Rathke, an astronomer at the Technical University of Denmark. “We have ideas and simulations and all that stuff, but we really have no idea. We should go and have a look.
The James Webb Space Telescope, sometimes called JWST, may prove powerful enough to determine the specific ingredients of exoplanet atmospheres because each type of molecule absorbs a different range of wavelengths of light.
But these discoveries will depend on the timing of the exoplanets. A bright, reflective blanket of clouds can block any starlight from entering an exoplanet’s atmosphere, thwarting any attempt to find alien air.
“It’s really hard to tell the difference between an atmosphere with clouds or no atmosphere,” Dr Rathke 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 this would probably be a good starting point for looking for life,” Dr Mansfield said.
But a watery atmosphere doesn’t necessarily mean an exoplanet harbors life. To confirm that a planet is alive, scientists will need to detect a biosignature, a molecule or combination of molecules that was created 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 a product of plants and algae. But oxygen can also be produced without the help of life when the water molecules in the air are split apart. Methane can also be released by living microbes, but also by volcanoes.
There may be a certain balance of gases that can provide a distinct biosignature that cannot be maintained without the help of life.
“We need extremely favorable scenarios to detect these biosignatures,” said Dr Rathke. “I’m not saying it’s not possible. I just think it’s overkill. We must be extremely lucky.”
Joshua Krissansen-Totten, 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 that repeatedly passes in front of Trappist-1.
“If someone comes along in the next five years and says, ‘Yes, we’ve found life with the JWST,’ I’ll be very skeptical of that claim,” Dr Krissansen-Totten said
It is possible that the James Webb Space Telescope simply cannot detect biosignatures. That task may have to wait for the next generation of space telescopes, more than a decade from now. They will study exoplanets 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 watching them pass in front of a star.
“Most of all, we will make the very important foundation for future telescopes,” predicts Dr. Rathke. “I would be very surprised if JWST provided biosignature detections, but I hope that . . .
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