This month will mark a new chapter in the search for extraterrestrial life, when the most powerful space telescope still built will begin spying on planets orbiting other stars. Astronomers expect the James Webb Space Telescope to reveal whether some of these planets are home to atmospheres that could support life.
Identifying an atmosphere in another solar system would be remarkable enough. But there is even the possibility, albeit a tiny one, that one of these atmospheres offers what is known as biosignature: a sign of life itself.
“I think we can 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 immediately.”
To date, 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 hidden beneath 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 clouds of sulfur dioxide, could be home to Venusians.
Even though Earth turns out to be the only planet that harbors life in our own solar system, many other solar systems in the universe contain 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: an inflated gas giant larger than Jupiter and a 1,800-degree Fahrenheit toast.
In later years, scientists have found more than 5,000 more exoplanets. Some of them are much more similar to Earth: about the same size, made of rock instead of gas and orbiting in a “zone of golden streaks” around their star, not as close as to to cook but not so far as to freeze.
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 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 the Earth. In this deep darkness, the telescope can detect light and distant lights, including those that could reveal new details about distant planets.
The space telescope “is the first large space observatory that takes into account the study of the atmospheres of exoplanets in their design,” Dr. Mansfield said.
NASA engineers began taking photographs of a series of objects with the Webb telescope in mid-June and will release their first images to the public on July 12th.
The exoplanets will be in this first batch of images, said Eric Smith, the show’s lead scientist. Because the telescope will spend relatively little time observing exoplanets, Dr. Smith considered these early images a “quick, dirty” look at the telescope’s power.
These quick glances will be followed by a series of much longer observations, starting in July, offering a much clearer picture of the exoplanets.
Several teams of astronomers are planning to look at 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 look for traces of life outside the solar system,” said Olivia Lim, a graduate student at the University of Montreal who will observe the planets Trappist-1 starting July 4th.
Because Trappist-1 is a small, cool star, its habitable zone is closer to it than 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. Each time the planets pass in front of Trappist-1, scientists will be able to address a basic but crucial question: are there any of them with 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 any atmosphere around the planets of Trappist-1. 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 is possible that they could only remove the entire atmosphere of a planet before it had a chance to start forming life,” Dr. Mansfield said. “This is the first-class question we are trying to answer here: whether these planets could have an atmosphere long enough to be able to develop 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 decrease 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 Laboratory of Applied Physics. “You may have the feeling that the light has dimmed.”
A planet with an atmosphere would darken the star behind it in a different way than a naked planet would. Part of the star’s light will pass directly through the atmosphere, but the gases will absorb light at certain wavelengths. If astronomers look only at the light of the stars at these wavelengths, the planet will further darken the Trappist-1.
The telescope will send these Trappist-1 observations back to Earth. “And then you get an email that says,‘ Hello, your data is available, ’” Dr. Mansfield.
But the light from Trappist-1 will be so dim that it will take time to make sense. “Your eyes are used to processing millions of photons per second,” Dr. Smith said. “But these telescopes are only picking up a few photons per second.”
Before Dr. Mansfield or his fellow astronomers can analyze the exoplanets that pass in front of Trappist-1, they will first have to distinguish it from the small fluctuations produced by the telescope’s own machinery.
“Much of the work I do is make sure we’re carefully correcting anything weird the telescope does, so we can see those tiny signals,” Dr. Mansfield.
It is possible that, at the end of these efforts, Dra. Mansfield and his colleagues discover an atmosphere around a planet Trappist-1. But this result alone will not reveal the nature of the atmosphere. It can be rich in nitrogen and oxygen, as on Earth, or more similar to the toxic stew of carbon dioxide and sulfuric acid from Venus. Or it could be a mixture scientists had 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 that stuff, but we really have no idea. We have to go look.”
The James Webb Space Telescope, sometimes called the JWST, can be powerful enough to determine the specific ingredients of exoplanet atmospheres because each type of molecule absorbs a different range of light wavelengths.
But these discoveries will depend on the climate on the exoplanets. A bright, reflective cloud blanket could prevent any starlight from entering an exoplanet’s atmosphere, ruining any attempt to find alien air.
“It’s very difficult to distinguish between an atmosphere with clouds or without an atmosphere,” Dr. Rathcke said.
If time cooperates, astronomers are especially anxious to see if exoplanets have water in their atmospheres. At least on Earth, water is an essential requirement for biology. “We think it would probably be a good starting point to look for life,” Dr. Mansfield said.
But an aqueous atmosphere does not necessarily mean that an exoplanet is home to life. To make sure a planet is alive, scientists will have to detect a biosignature, a molecule, or a combination of several molecules made in a distinctive way by living things.
Scientists are still debating what a reliable biosignature would be. The Earth’s atmosphere is unique to our solar system because 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 water molecules in the air divide. Methane, in the same way, can be released by living microbes but also by volcanoes.
It is possible that there is a particular gas balance that can provide a clear biosignature, which cannot be maintained without the help of life.
“We need extremely favorable scenarios to find these biosignatures,” Dr. Rathcke said. “I’m not saying it’s not possible. I just think it’s crazy. We have to be very 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 repeatedly passing in front of Trappist-1.
“If someone shows up in the next five years and says,‘ Yes, we’ve found life with JWST, ’I’ll be very skeptical of that statement,” Dr. Krissansen-Totton.
The James Webb Space Telescope may simply not be able to find biosignatures. This task may have to wait for the next generation of space telescopes, in addition to a decade. These will study exoplanets in the same way that people look at Mars or Venus in the night sky: observing the light of stars reflected on the black background of space, instead of observing them as they pass in front of them. a star.
“Above all, we will make the very important foundations for future telescopes,” Dr. Rathcke predicted. “I would be very surprised if JWST offers biosignature detections, but I hope …