This month will mark a new chapter in the search for extraterrestrial life, when the most powerful space telescope ever built begins to spy on planets that orbit other stars. Astronomers hope the James Webb Space Telescope will reveal whether any of these planets harbor atmospheres that could support life.
Identifying an atmosphere in another solar system would be quite remarkable. But there’s even a chance, albeit a tiny one, that one of these atmospheres offers what’s called a biosignature: a signal from 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.”
So far, 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 any Martians. But it’s conceivable 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 have hoped that even Venus, despite its scorching atmosphere of sulfur dioxide clouds, might harbor Venusians.
Even though Earth happens to be the only life-supporting planet in our own solar system, many other solar systems in the universe are home to exoplanets.
In 1995, French astronomers spotted the first exoplanet orbiting a sun-like 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 temperature of 1,800 degrees Fahrenheit.
In the years that followed, scientists discovered 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 orbiting in a “Goldilocks zone” around their star, not so close to be cooked but not 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 allow astronomers to take a closer look at these worlds.
Since launching from Kourou, French Guiana, the telescope has traveled a million miles from Earth, entering its own orbit around the sun. There, a shield shields its 21-foot mirror from any heat or light from the sun or earth. In this deep darkness, the telescope can detect faint distant glimmers of light, including those 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 set of objects with the Webb Telescope in mid-June and will release its first images to the public on July 12.
Exoplanets will feature in this first batch of images, said program lead scientist Eric Smith. Since the telescope will spend relatively little time observing exoplanets, Dr Smith considered these early images a “quick and dirty” look at the power of the telescope.
These quick glances will be followed by a series of much longer observations, starting in July, providing a much clearer picture of exoplanets.
A number of teams of astronomers plan to examine the seven planets that orbit a star called Trappist-1. Previous observations have suggested that three of the planets occupy the habitable zone.
“It’s a great place to look for traces of life outside the solar system,” said Olivia Lim, a graduate student at the University of Montreal who will be observing 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 only a few days to orbit the star. Each time the planets pass Trappist-1, scientists will be able to grapple with a fundamental but crucial question: does any of them have an atmosphere?
“If there’s no air, it’s not habitable, even if it’s in the habitable zone,” said Cornell University astronomer Nikole Lewis.
Dr. Lewis and other astronomers wouldn’t be surprised to find no atmosphere around the Trappist-1 planets. Even if the planets had developed atmospheres when they formed, the star might have destroyed them long ago with ultraviolet rays and X-rays.
“It’s possible that they could just wipe out all of a planet’s atmosphere before it’s even had a chance to start forming life,” Dr Mansfield said. “That’s the first-order question we’re trying to answer here: Could these planets 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 dimming of light coming from the star.
“It’s like watching 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 might feel like the light has faded.”
A planet with an atmosphere would obscure the star behind it differently than a bare planet. Some light from the star will pass directly through the atmosphere, but the gases will absorb light at certain wavelengths. If astronomers only look at starlight at these wavelengths, the planet will darken Trappist-1 even further.
The telescope will send these observations of Trappist-1 back to Earth. “And then you get an email like ‘Hello, your data is 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 processing millions of photons per second,” Dr. Smith said. “But these telescopes, they only collect a few photons per second.”
Before Dr. Mansfield or his fellow astronomers can analyze exoplanets passing in front of Trappist-1, they will first need to distinguish it from the tiny fluctuations produced by the telescope’s own machinery.
“A lot of the work I do is making sure that we carefully correct anything that the telescope does that is weird, so that we can see these tiny signals,” Dr Mansfield said.
It is possible that at the end of these efforts, Dr. Mansfield and his colleagues will discover an atmosphere around a Trappist-1 planet. But this result alone will not reveal the nature of the atmosphere. It could be rich in nitrogen and oxygen, like on Earth, or closer to the poisonous 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 these atmospheres are made of,” said Alexander Rathcke, an astronomer at the Technical University of Denmark. “We have ideas, simulations and all that, but we really have no idea. We have to go and see.”
The James Webb Space Telescope, sometimes referred to as the 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 weather on the exoplanets. A bright, reflective cover of clouds could block any starlight from entering an exoplanet’s atmosphere, ruining 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 that an exoplanet harbors life. To be sure that a planet is alive, scientists will need to detect a biosignature, a molecule or a combination of several molecules that is distinctly 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, much of it the product of plants and algae. But oxygen can also be produced without the help of life, 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 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 far-fetched. We must be extremely lucky.
Joshua Krissansen-Totton, a planetary scientist at the University of California, Santa Cruz, said striking such a balance could require the Webb telescope to observe a planet repeatedly passing in front of Trappist-1.
“If someone comes forward in the next five years and says, ‘Yes, we’ve found life with JWST,’ I’d be very skeptical of that claim,” Dr. Krissansen-Totton said.
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, 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.
“We’re mainly going to do the very important groundwork for future telescopes,” predicted Dr Rathcke. “I would be very surprised if JWST provides biosignature detections, but I hope to be corrected. I mean, that’s basically why I’m doing this work.