This month will be 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 some of those planets contain atmospheres that can sustain life.
Identifying an atmosphere in another solar system would be remarkable enough. But there is even a chance – albeit small – that one of these atmospheres known as a biosignature will present: a sign of life itself.
“I think we’ll be able to find planets that we think are interesting – you know, great potential for life,” said Megan Mansfield, an astronomer at the University of Arizona. “But we will not necessarily be able to identify life immediately.”
So far, the 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 Martians. But it is conceivable that life lurks beneath the surface of the Red Planet or waits to be discovered on a moon of Jupiter or Saturn. Some scientists have hoped that even Venus, despite its scorching atmosphere of sulfur dioxide clouds, could be the home of Venusians.
Even though the earth appears to be the only planet that houses life in our own solar system, many other solar systems in the universe contain so-called exoplanets.
In 1995, French astronomers saw the first exoplanet orbiting a solar star. Known as 51 Pegasi b, the exoplanet has proven to be an unpromising home for life – a puffy gas giant larger than Jupiter, and a glorious 1,800 degrees Fahrenheit.
In the years since, scientists have found more than 5,000 other exoplanets. Some of them are much more similar to Earth – about the same size, made of rock rather than gas and revolve in a “Goldilocks zone” around their star, not so close that it cooks, but not so far that it not be frozen.
Unfortunately, the relatively small size of these exoplanets has made them extremely difficult to study, until now. Launched last Christmas, the James Webb Space Telescope will change it and serve 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 a million miles from Earth and entered 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 dim, distant luminosities, including those that can reveal new details about distant planets.
The space telescope “is the first large space observatory to consider the study of exoplanet atmospheres in its design,” said dr. Mansfield said.
NASA engineers began taking photos of a variety 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 group of pictures, said Eric Smith, the program’s lead scientist. Because the telescope will spend relatively little time observing the exoplanets, dr. Smith viewed those first images as a “quick and dirty” look at the telescope’s power.
Those quick looks will be followed by a series of much longer observations, beginning in July, which provide a much clearer picture of the exoplanets.
A number of teams of astronomers plan to look at the seven planets orbiting a star called Trappist-1. Earlier observations have suggested that three of the planets occupy the habitable zone.
“This is 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 Trappist-1 planets from July 4th.
Because Trappist-1 is a small, cool star, its habitable zone is closer to it than in our own solar system. Consequently, its potentially habitable planets orbit at short distances, which only take a few days to orbit the star. Each 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 an atmosphere?
“If it does not have air, it is not habitable, even if it is in the habitable zone,” said Nikole Lewis, an astronomer at Cornell University.
Dr. Lewis and other astronomers will not be surprised to find no atmosphere around Trappist-1’s planets. Even if the planets evolved when they formed, the star might have blown them away long ago with ultraviolet and X-rays.
“It is possible that they could just strip away all the atmosphere on a planet before it even had a chance to start forming life,” said Dr. Mansfield said. “This is the first-order question we are trying to answer here: whether these planets can have an atmosphere long enough for them to develop life.”
A planet moving 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 eclipse in the light moving from the star.
“It’s like looking at a solar eclipse with your eyes closed,” says 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 will dim the star behind it differently than a bare planet. Some of the star’s light will move straight through the atmosphere, but the gases will absorb light at certain wavelengths. If astronomers were just looking at starlight at those wavelengths, the planet Trappist-1 would fade 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 is available,'” he said. Mansfield said.
But the light coming from Trappist-1 will be so dim that it will take time to make sense of it. “Your eye is used to handling millions of photons per second,” said Dr. Smith said. “But these telescopes, they only collect a few photons per second.”
Before Dr. Mansfield or her fellow astronomers will be able to analyze exoplanets that pass before Trappist-1, they will first have to distinguish it from small fluctuations produced by the telescope’s own machinery.
“A lot of the work I actually do is make sure we carefully correct for anything weird that the telescope does so we can see those tiny signals,” said dr. Mansfield said.
It is possible that Dr. Mansfield and his colleagues at the end of those efforts will discover an atmosphere around a Trappist-1 planet. But that result alone will not reveal the nature of the atmosphere. It may be rich in nitrogen and oxygen, as on Earth, or more similar to the toxic stew of carbon dioxide and sulfuric acid on Venus. Or it could be a mixture that scientists have never seen before.
“We have no idea what this atmosphere is 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 see. ”
The James Webb Space Telescope, sometimes called the JWST, can be powerful enough to determine the specific constituents 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 the exoplanets. A bright, reflective blanket of clouds can prevent any starlight from entering an exoplanet’s atmosphere, destroying any attempt to find alien air.
“It is really difficult to distinguish between an atmosphere with clouds or no atmosphere,” said Dr. Rathcke said.
If the weather works together, astronomers are especially eager to see if the exoplanets have water in their atmosphere. At least on earth, water is an essential requirement for biology. “We think it will probably be a good starting point to look for life,” said Dr. Mansfield said.
But a watery atmosphere will not necessarily mean that an exoplanet will house life. To be sure that a planet is alive, scientists will have to find a biosignature, a molecule or a combination of several molecules that are typically made by living things.
Scientists are still debating what a reliable biosignature would be. The 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 the help of life, when water molecules are distributed in the air. Methane can also be released by living microbes, but also by volcanoes.
It is possible that there is a specific 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 bio-signatures,” said Dr. Rathcke said. “I am not saying that this is not possible. I just think it’s far-fetched. We must be extremely happy. ”
Joshua Krissansen-Totton, a planetary scientist at the University of California, Santa Cruz, said finding such a balance might require the Webb Telescope to observe a planet repeatedly in front of Trappist-1.
“If someone comes forward in the next five years and says, ‘Yes, we found life with JWST,’ I would be very skeptical about that allegation,” he said. Krissansen-Totton said.
It is possible that the James Webb Space Telescope will simply not be able to find bio-signatures. That task may have to wait for the next generation of space telescopes, more than a decade away. It will study exoplanets in the same way that humans look at Mars or Venus in the night sky: by observing starlight that reflects them against the black background of space, rather than observing them as they pass in front of a star.
“We will mostly do the very important groundwork for future telescopes,” said dr. Rathcke predicts. “I would be very surprised if JWST provides biosignature detection, but I hope to be corrected. I mean, that’s basically what I’m doing this job for. ”