From James Webb Space Telescope: “Simulations Show Webb Telescope Can Reveal Distant Galaxies Hidden in Quasars’ Glare”

From James Webb Space Telescope

October 14, 2020

Media Contact:
Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland
410-338-4366
cpulliam@stsci.edu

Science Contact:
Madeline Marshall
University of Melbourne, Melbourne, Australia
madelinem1@student.unimelb.edu.au

This artist’s illustration portrays two galaxies that existed in the first billion years of the universe. The larger galaxy at left hosts a brilliant quasar at its center, whose glow is powered by hot matter surrounding a supermassive black hole. Scientists calculate that the resolution and infrared sensitivity of NASA’s upcoming James Webb Space Telescope will allow it to detect a dusty host galaxy like this despite the quasar’s searchlight beam. Credit: J. Olmsted (STScI).

These simulated images show how a quasar and its host galaxy would appear to NASA’s upcoming James Webb Space Telescope (top) and Hubble Space Telescope (bottom) at infrared wavelengths of 1.5 and 1.6 microns, respectively. Webb’s larger mirror will provide more than 4 times the resolution, enabling astronomers to separate the galaxy’s light from the overwhelming light of the central quasar. The individual images span about 2 arcseconds on the sky, which represents a distance of 36,000 light-years at a redshift of 7. Credit: M. Marshall University of Melbourne (AU).

Summary
Webb observations will seek dusty galaxies from the first billion years of the universe.

The brightest objects in the distant, young universe are quasars. These cosmic beacons are powered by supermassive black holes consuming material at a ferocious rate. Quasars are so bright that they can outshine their entire host galaxy, making it difficult to study those galaxies and compare them to galaxies without quasars.

A new theoretical study examines how well NASA’s upcoming James Webb Space Telescope, slated for launch in 2021, will be able to separate the light of host galaxies from the bright central quasar. The researchers find that Webb could detect host galaxies that existed just 1 billion years after the big bang.
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Quasars are the brightest objects in the universe and among the most energetic.

Now iconic image of a quasar.
ESO/M.Kornmesser.

They outshine entire galaxies of billions of stars. A supermassive black hole lies at the heart of every quasar, but not every black hole is a quasar. Only the black holes that are feeding most voraciously can power a quasar. Material falling into the supermassive black hole heats up, and causes a quasar to fiercely shine across the universe like a lighthouse beacon.

Although quasars are known to reside at the centers of galaxies, it’s been difficult to tell what those galaxies are like and how they compare to galaxies without quasars. The challenge is that the quasar’s glare makes it difficult or impossible to tease out the light of the surrounding host galaxy. It’s like looking directly into a car headlight and trying to figure out what kind of automobile it is attached to.

A new study [MNRAS] suggests that NASA’s James Webb Space Telescope, set to launch in 2021, will be able to reveal the host galaxies of some distant quasars despite their small sizes and obscuring dust.

“We want to know what kind of galaxies these quasars live in. That can help us answer questions like: How can black holes grow so big so fast? Is there a relationship between the mass of the galaxy and the mass of the black hole, like we see in the nearby universe?” said lead author Madeline Marshall of the University of Melbourne in Australia, who conducted her work within the ARC Centre of Excellence in All Sky Astrophysics in 3D (AU).

Answering these questions is challenging for a number of reasons. In particular, the more distant a galaxy is, the more its light has been stretched to longer wavelengths by the expansion of the universe. As a result, ultraviolet light from the black hole’s accretion disk or the galaxy’s young stars gets shifted to infrared wavelengths.

In a recent study [The Astrophysical Journal], astronomers used the near-infrared capabilities of NASA’s Hubble Space Telescope to study known quasars in hopes of spotting the surrounding glow of their host galaxies, without significant detections. This suggests that dust within the galaxies is obscuring the light of their stars. Webb’s infrared detectors will be able to peer through the dust and uncover the hidden galaxies.

“Hubble simply doesn’t go far enough into the infrared to see the host galaxies. This is where Webb will really excel,” said Rogier Windhorst of Arizona State University in Tempe, a co-author on the Hubble study.

To determine what Webb is expected to see, the team used a state-of-the-art computer simulation called BlueTides, developed by a team led by Tiziana Di Matteo at Carnegie Mellon University in Pittsburgh, Pennsylvania.

“BlueTides is designed to study the formation and evolution of galaxies and quasars in the first billion years of the universe’s history. Its large cosmic volume and high spatial resolution enables us to study those rare quasar hosts on a statistical basis,” said Yueying Ni of Carnegie Mellon University, who ran the BlueTides simulation. BlueTides provides good agreement with current observations and allows astronomers to predict what Webb should see.

The team found that the galaxies hosting quasars tended to be smaller than average, spanning only about 1/30 the diameter of the Milky Way despite containing almost as much mass as our galaxy. “The host galaxies are surprisingly tiny compared to the average galaxy at that point in time,” said Marshall.

The galaxies in the simulation also tended to be forming stars rapidly, up to 600 times faster than the current star formation rate in the Milky Way. “We found that these systems grow very fast. They’re like precocious children – they do everything early on,” explained co-author Di Matteo.

The team then used these simulations to determine what Webb’s cameras would see if the observatory studied these distant systems. They found that distinguishing the host galaxy from the quasar would be possible, although still challenging due to the galaxy’s small size on the sky.

“Webb will open up the opportunity to observe these very distant host galaxies for the first time,” said Marshall.

They also considered what Webb’s spectrographs could glean from these systems. Spectral studies, which split incoming light into its component colors or wavelengths, would be able to reveal the chemical composition of the dust in these systems. Learning how much heavy elements they contain could help astronomers understand their star formation histories, since most of the chemical elements are produced in stars.

Webb also could determine whether the host galaxies are isolated or not. The Hubble study found that most of the quasars had detectable companion galaxies, but could not determine whether those galaxies were actually nearby or whether they are chance superpositions. Webb’s spectral capabilities will allow astronomers to measure the redshifts, and hence distances, of those apparent companion galaxies to determine if they are at the same distance as the quasar.

Ultimately, Webb’s observations should provide new insights into these extreme systems. Astronomers still struggle to understand how a black hole could grow to weigh a billion times as much as our Sun in just a billion years. “These big black holes shouldn’t exist so early because there hasn’t been enough time for them to grow so massive,” said co-author Stuart Wyithe of the University of Melbourne.

Future quasar studies will also be fueled by synergies among multiple upcoming observatories. Infrared surveys with the European Space Agency’s Euclid mission, as well as the ground-based Vera C. Rubin Observatory, a National Science Foundation/Department of Energy facility currently under construction on Cerro Pachón in Chile’s Atacama Desert. Both observatories will significantly increase the number of known distant quasars. Those newfound quasars will then be examined by Hubble and Webb to gain new understandings of the universe’s formative years.

The Bluetides simulation (project PI: Tiziana Di Matteo at Carnegie Mellon University) was run at the Blue Waters sustained-petascale computing facility, which is supported by the National Science Foundation.

NCSA U Illinois Urbana-Champaign Blue Waters Cray Linux XE/XK hybrid machine supercomputer, at the National Center for Supercomputing Applications.

See the full article here .

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The James Webb Space Telescope will be a large infrared telescope with a 6.5-meter primary mirror. Launch is planned for later in the decade.

Webb telescope will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.

Webb telescope was formerly known as the “Next Generation Space Telescope” (NGST); it was renamed in Sept. 2002 after a former NASA administrator, James Webb.

Webb is an international collaboration between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center is managing the development effort. The main industrial partner is Northrop Grumman; the Space Telescope Science Institute will operate Webb after launch.

Several innovative technologies have been developed for Webb. These include a folding, segmented primary mirror, adjusted to shape after launch; ultra-lightweight beryllium optics; detectors able to record extremely weak signals, microshutters that enable programmable object selection for the spectrograph; and a cryocooler for cooling the mid-IR detectors to 7K.

There will be four science instruments on Webb: the Near InfraRed Camera (NIRCam), the Near InfraRed Spectrograph (NIRspec), the Mid-InfraRed Instrument (MIRI), and the Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS). Webb’s instruments will be designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range. It will be sensitive to light from 0.6 to 28 micrometers in wavelength.

NASA Webb NIRCam.

NASA Webb NIRspec.

NASA Webb MIRI.

CSA Webb Fine Guidance Sensor-Near InfraRed Imager and Slitless Spectrograph FGS/NIRISS.

Webb has four main science themes: The End of the Dark Ages: First Light and Reionization, The Assembly of Galaxies, The Birth of Stars and Protoplanetary Systems, and Planetary Systems and the Origins of Life.

Launch is scheduled for later in the decade on an Ariane 5 rocket. The launch will be from Arianespace’s ELA-3 launch complex at European Spaceport located near Kourou, French Guiana. Webb will be located at the second Lagrange point, about a million miles from the Earth.

From Goddard: “How Hardy Is Webb? A Q&A About the Toughness of NASA’s Webb Telescope”

NASA Goddard Space Flight Center

Dec. 5, 2017
Eric Villard
eric.s.villard@nasa.gov
NASA’s Goddard Space Flight Center

[Finally an article on Webb worth exploring.]

Paul Geithner is the deputy project manager – technical for James Webb Space Telescope at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Credits: NASA/Chris Gunn

NASA/ESA/CSA Webb Telescope annotated

Just how resilient does a space telescope have to be to survive both Earth’s environment and the frigid, airless environment of space? Paul Geithner, the deputy project manager – technical for James Webb Space Telescope at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, answered some questions about the design challenges of building the telescope and the gauntlet of tests it has endured in the years leading up to launch. James Webb Space Telescope, or Webb, is NASA’s upcoming infrared space observatory, which will launch in 2019.

Q: What kind of conditions do Webb and its instruments need to withstand?

Paul: The entire observatory must survive the mechanically stressing conditions from the violent vibration of launch. In addition to this, the “cold” half of the observatory—the telescope and its instruments—must survive the thermal shrinkage that occurs when they are cooling from room temperature to the cryogenic temperatures at which they operate in the cold of space.

The engineering challenge is to operate Webb at extremely cold temperatures, since Webb is built at room temperature. Materials typically shrink at various temperature rates as they get cold. We have to build the Webb telescope in a way so that it shrinks to precisely the right shape and dimensions when it’s extremely cold. Webb has to survive the stresses of shrinking and expanding during cold temperature tests and warming it back up again—things that will happen when it goes into space.

Webb has to survive years in space, exposed to the radiation from the Sun and the galaxy.

NASA’s James Webb Space Telescope will orbit the Sun, 1 million miles away from the Earth at what is called the second Lagrange point, or L2. What is special about this orbit is that it lets the telescope stay in line with the Earth as it moves around the Sun. Credit: NASA

LaGrange Points map. NASA

Q: Why is vibration testing so important, and how does it show Webb is ready for the rigors of launch?

Paul: Vibration testing is done for two reasons. One reason is to validate that Webb can handle the rigorous shaking it will experience while riding a rocket into space, and the other reason is to verify the workmanship of Webb’s construction and prove the design was engineered and assembled correctly.

We use two complementary methods of vibration. For lower frequencies of vibration, meaning from about 5 hertz (cycles per second) to 100 hertz, we put the hardware on a surface—basically a big metal plate—that rides on bearings so it can move back and forth, and this surface is connected to essentially a big electromagnet that generates the shaking motion.

For higher frequencies, above 100 hertz, it is very difficult or impossible to achieve the necessary vibration with a big vibration table system, so instead we put the hardware in an acoustics chamber. This is a thick-walled room with large speakers that produce literally deafening levels of sound.

Taken together, the vibration table and acoustics chamber produce the vibration environments we need to properly test the Webb. Typically for a one-of-a-kind article like Webb, the levels of vibration we subject it to in testing on the ground is about twice what it will endure during the mission. This testing gives us confidence that Webb has been put together correctly, will survive the actual flight, and will work as designed once in space.

NASA engineers and technicians perform vibration testing on the James Webb Space Telescope at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in February 2017.
Credits: NASA/Chris Gunn

Q: Why is cryogenic testing so important, and how does it show Webb is ready for the stresses of space?

Paul: Super cold or “cryogenic” tests are part of demonstrating and verifying that Webb’s instruments and components operate like they should and will operate properly once at Earth’s second Lagrange point (L2). The L2 point is about 1 million miles from Earth.

We put the Webb telescope’s hardware into a big vacuum chamber, close the door, pump all the air out, and then run liquid nitrogen and extremely cold helium gas through plumbing that crisscrosses the surface of thin “shells” that are nested Russian-doll style inside the vacuum chamber.

The shells are also called shrouds, and they are very cold. The outer one approaches 77 Kelvin (about minus 321 degrees Fahrenheit/minus 196 degrees Celsius—the temperature of the liquid nitrogen). The inner shell runs between 10 and 20 Kelvin (between minus 442 degrees Fahrenheit/minus 263 degrees Celsius and minus 424 degrees Fahrenheit/minus 253 degrees Celsius—the temperature of the cold helium gas). Anything nestled inside the shrouds will radiate its latent heat to them and get really cold, too.

The effect is similar to what happens on a clear night, when the heat from the previous day radiates freely into the night sky. By morning, the temperature can be quite cold. Think about the desert, where skies are typically dry and clear. Even though it’s scorching hot during the day, it gets frigid at night because the heat radiates away from the surface.

Engineers pose by NASA’s James Webb Space Telescope shortly after it emerged from Chamber A at NASA’s Johnson Space Center in Houston on Dec. 1, 2017. Credits: NASA/Chris Gunn

Q: Why does Webb need a “sunshield,” and what kind of protection does it provide?

Paul: The instruments are shaded from the Sun by a tennis court-sized, five-layer, deployable sunshield. The sunshield consists of deployable booms and gossamer polyimide membranes, essentially sheets of special plastic (DuPont Kapton®), each only about one one-thousandth of an inch thick and coated with reflective aluminum and protective silicon. Basically, it looks like a five-layered, giant silver kite in space.

We need a sunshield to keep the telescope and instruments cold because Webb is an infrared telescope, which means it sees infrared light. Infrared light is light that is of slightly longer, or redder, wavelengths than visible light. We cannot see it with our eyes, but we can feel it as radiant heat.

For an infrared telescope to be as sensitive as possible, its optics and scientific instruments need to be very cold, so their own heat does not blind them to the faint infrared signals they are trying to observe from astronomical objects. In space and shaded from the Sun by the sunshield, the telescope and scientific instruments will face the extreme cold of deep space and will get very cold themselves.

The sunshield of NASA’s James Webb Space Telescope will prevent the background heat from the Sun from interfering with the telescope’s infrared sensors. It was designed by Northrop
Grumman Aerospace Systems in Redondo Beach, California. Credits: Northrop Grumman

Q: What materials were used to build Webb, and how do these materials increase Webb’s resiliency?

Paul: We used beryllium for many of Webb’s mirrors and some of the structures because it is simultaneously lightweight, stiff, strong, and dimensionally stable (stops shrinking and expanding) at the telescope’s operating temperature. Beryllium changes dimensions a lot with temperature, but it virtually stops shrinking once it goes below a temperature of 100 Kelvin (about minus 280 degrees Fahrenheit or minus 173 degrees Celsius).

We used many other materials on the Webb, including aluminum for some things, stainless steel for fasteners, titanium for structures and fasteners, invar (an alloy) for structural nodes, and many other metals. We also have non-metals like graphite-epoxy composites for most structures and silicon carbide ceramic for one of the scientific instruments (the near infrared spectrograph — NIRSpec).

NASA/ESA/CSA NIRSpec

Because the weight of a payload is so critical to cost-effectively launch it into space, each one of Webb telescope’s primary mirror segments was “light-weighted.” The process involved cutting away most of the backside of each mirror segment while leaving structural ribs for support, thus reducing the weight while maintaining the strength and integrity of the mirror. Credit: NASA

Q: Webb’s orbit at Earth’s second Lagrange point (L2) is beyond the protective sheath of Earth’s magnetic field, meaning the telescope is more susceptible to the Sun’s radiation and to solar flares. How is Webb insulated from these threats?

Paul: Earth’s magnetic field acts like a deflector shield for protons and electrons spewing all the time from the Sun. Protection for satellites within Earth’s magnetic field includes putting some metal—like aluminum panels—between electronics and the space environment, implementing good electrical grounding, and making electronic components resistant to radiation. Because Webb is outside Earth’s magnetic field, it will be bombarded by charged particles streaming from the Sun, and so it needs extra protection. These charged particles are hard on electronics, and they can accumulate on surfaces to build up static charge that can cause damaging discharges.

Webb will also be vulnerable to the occasional massive “belch” from the Sun that happens with solar flares and coronal mass ejections, which are phenomena in which the Sun releases slugs of maybe a few years’ worth of protons and electrons in just hours. To enable Webb to weather such stormy solar weather as well as ordinary “nice days,” almost all of its electronics are shielded inside metal boxes and behind multiple layers of metal or metal-coated film.

The electronics on the cold side of Webb’s sunshield get some benefit of being behind the shield’s five layers, which are coated in aluminum. The electronics inside the spacecraft bus, which faces the Sun, are hardened, shielded, and grounded. Webb used tried and true design practices and satellite building codes to ensure it will survive and function in the harshness of the L2 environment.

Image of M5.7 solar flare on April 2, 2017, as seen by NASA’s Solar Dynamics Observatory in a blend of 131 and 171 angstroms.
Credits: NASA/SDO

NASA/SDO

Q: Webb was not designed to be serviced, but could it eventually be repaired or refueled during a robotic service mission?

Paul: Conceivably, some robotic servicing of Webb could be possible. A robot could grapple Webb at the same place where it was attached to the Ariane launch vehicle, which is the launcher interface ring on the Sun-facing spacecraft bus, and then add fuel to its propulsion tank. Given that Webb is an exquisitely sensitive infrared observatory, and much of it is at cryogenic temperatures, opportunities and benefits of servicing are limited.

The James Webb Space Telescope is the world’s premier infrared space observatory of the next decade. A barrier-breaking mission for engineers and astronomers, Webb will solve mysteries of our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, the European Space Agency (ESA), and the Canadian Space Agency (CSA).

For more information about the Webb telescope visit: http://www.webb.nasa.gov or http://www.nasa.gov/webb

See the full article here.

Please help promote STEM in your local schools.

Stem Education Coalition

NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.

NASA/Goddard Campus

From Science: “Earth-sized alien worlds are out there. Now, astronomers are figuring out how to detect life on them”

Science Magazine

Nov. 1, 2017
Daniel Clery

Stephen Kane spends a lot of time staring at bad pictures of a planet. The images are just a few pixels across and nearly featureless. Yet Kane, an astronomer at the University of California, Riverside, has tracked subtle changes in the pixels over time. They are enough for him and his colleagues to conclude that the planet has oceans, continents, and clouds. That it has seasons. And that it rotates once every 24 hours.

He knows his findings are correct because the planet in question is Earth.

An image from the Deep Space Climate Observatory satellite (left), degraded to a handful of pixels (right), is a stand-in for how an Earth-like planet around another star might look through a future space telescope.
(LEFT TO RIGHT) NASA EPIC TEAM; STEPHEN KANE

Kane took images from the Deep Space Climate Observatory satellite, which has a camera pointing constantly at Earth from a vantage partway to the sun, and intentionally degraded them from 4 million pixels to just a handful.

NASA Deep Space Climate Observatory

The images are a glimpse into a future when telescopes will be able to just make out rocky, Earth-sized planets around other stars. Kane says he and his colleagues are trying to figure out “what we can expect to see when we can finally directly image an exoplanet.” Their exercise shows that even a precious few pixels can help scientists make the ultimate diagnosis: Does a planet harbor life?

Finding conclusive evidence of life, or biosignatures, on a planet light-years away might seem impossible, given that space agencies have spent billions of dollars sending robot probes to much closer bodies that might be habitable, such as Mars and the moons of Saturn, without detecting even a whiff of life. But astronomers hope that a true Earth twin, bursting with flora and fauna, would reveal its secrets to even a distant observer.

Detecting them won’t be easy, considering the meager harvest of photons astronomers are likely to get from such a tiny, distant world, its signal almost swamped by its much brighter nearby star. The new generation of space telescopes heading toward the launch pad, including NASA’s mammoth James Webb Space Telescope (JWST), have only an outside chance of probing an Earth twin in sufficient detail.

NASA/ESA/CSA Webb Telescope annotated

But they will be able to sample light from a range of other planets, and astronomers are already dreaming of a space telescope that might produce an image of an Earth-like planet as good as Kane’s pixelated views of Earth. To prepare for the coming flood of exoplanet data, and help telescope designers know what to look for, researchers are now compiling lists of possible biosignatures, from spectral hints of gases that might emanate from living things to pigments that could reside in alien plants or microbes.

There is unlikely to be a single smoking gun. Instead, context and multiple lines of evidence will be key to a detection of alien life. Finding a specific gas—oxygen, say—in an alien atmosphere isn’t enough without figuring out how the gas could have gotten there. Knowing that the planet’s average temperature supports liquid water is a start, but the length of the planet’s day and seasons and its temperature extremes count, too. Even an understanding of the planet’s star is imperative, to know whether it provides steady, nourishing light or unpredictable blasts of harmful radiation.

“Each [observation] will provide crucial evidence to piece together to say if there is life,” says Mary Voytek, head of NASA’s astrobiology program in Washington, D.C.

In the heady early days following the discovery of the first exoplanet around a normal star in 1995, space agencies drew up plans for extremely ambitious—and expensive—missions to study Earth twins that could harbor life. Some concepts for NASA’s Terrestrial Planet Finder and the European Space Agency’s Darwin mission envisaged multiple giant telescopes flying in precise formation and combining their light to increase resolution. But neither mission got off the drawing board. “It was too soon,” Voytek says. “We didn’t have the data to plan it or build it.”

Instead, efforts focused on exploring the diversity of exoplanets, using both ground-based telescopes and missions such as NASA’s Kepler spacecraft.

NASA/Kepler Telescope

Altogether they have identified more than 3500 confirmed exoplanets, including about 30 roughly Earth-sized worlds capable of retaining liquid water. But such surveys give researchers only the most basic physical information about the planets: their orbits, size, and mass. In order to find out what the planets are like, researchers need spectra: light that has passed through the planet’s atmosphere or been reflected from its surface, broken into its component wavelengths.

Most telescopes don’t have the resolution to separate a tiny, dim planet from its star, which is at least a billion times brighter. But even if astronomers can’t see a planet directly, they can still get a spectrum if the planet transits, or passes in front of the star, in the course of its orbit. As the planet transits, starlight shines through its atmosphere; gases there absorb particular wavelengths and leave characteristic dips in the star’s spectrum.

Astronomers can also study a transiting planet by observing the star’s light as the planet’s orbit carries it behind the star.

Planet transit. NASA/Ames

Before the planet is eclipsed, the spectrum will include both starlight and light reflected from the planet; afterward, the planet’s contribution will disappear. Subtracting the two spectra should reveal traces of the planet.

Teasing a recognizable signal from the data is far from easy. Because only a tiny fraction of the star’s light probes the atmosphere, the spectral signal is minuscule, and hard to distinguish from irregularities in the starlight itself and from absorption by Earth’s own atmosphere. Most scientists would be “surprised at how horrible the data is,” says exoplanet researcher Sara Seager of the Massachusetts Institute of Technology in Cambridge.

In spite of those hurdles, the Hubble and Spitzer space telescopes, plus a few others, have used these methods to detect atmospheric gases, including sodium, water, carbon monoxide and dioxide, and methane, from a handful of the easiest targets.

NASA/ESA Hubble Telescope

NASA/Spitzer Infrared Telescope

Most are “hot Jupiters”—big planets in close-in orbits, their atmospheres puffed up by the heat of their star.

In an artist’s concept, a petaled starshade flying at a distance of tens of thousands of kilometers from a space telescope blocks a star’s light, opening a clear view of its planets. NASA/JPL.

The approach will pay much greater dividends after the launch of the JWST in 2019. Its 6.5-meter mirror will collect far more light from candidate stars than existing telescopes can, allowing it to tease out fainter exoplanet signatures, and its spectrographs will produce much better data.

https://jwst.nasa.gov/mirrors.html

And it will be sensitive to the infrared wavelengths where the absorption lines of molecules such as water, methane, and carbon monoxide and dioxide are most prominent.

Once astronomers have such spectra, one of the main gases that they hope to find is oxygen. Not only does it have strong and distinctive absorption lines, but many believe its presence is the strongest sign that life exists on a planet.

Oxygen-producing photosynthesis made Earth what it is today. First cyanobacteria in the oceans and then other microbes and plants have pumped out oxygen for billions of years, so that it now makes up 21% of the atmosphere—an abundance that would be easily detectable from afar. Photosynthesis is evolution’s “killer app,” says Victoria Meadows, head of the NASA-sponsored Virtual Planet Laboratory (VPL) at the University of Washington in Seattle. It uses a prolific source of energy, sunlight, to transform two molecules thought to be common on most terrestrial planets—water and carbon dioxide—into sugary fuel for multicellular life. Meadows reckons it is a safe bet that something similar has evolved elsewhere. “Oxygen is still the first thing to go after,” she says.

Fifteen years ago, when exoplanets were new and researchers started thinking about how to scan them for life, “Champagne would have flowed” if oxygen had been detected, Meadows recalls. But since then, researchers have realized that things are not that simple: Lifeless planets can have atmospheres full of oxygen, and life can proliferate without ever producing the gas. That was the case on Earth, where, for 2 billion years, microbes practiced a form of photosynthesis that did not produce oxygen or many other gases. “We’ve had to make ourselves more aware of how we could be fooled,” Meadows says.

To learn what a genuine biosignature might look like, and what might be a false alarm, Meadows and her colleagues at the VPL explore computer models of exoplanet atmospheres, based on data from exoplanets as well as observations of more familiar planets, including Earth. They also do physical experiments in vacuum chambers. They recreate the gaseous cocktails that may surround exoplanets, illuminate them with simulated starlight of various kinds, and see what can be measured.

Over the past few years, VPL researchers have used such models to identify nonbiological processes that could make oxygen and produce a “false positive” signal. For example, a planet with abundant surface water might form around a star that, in its early years, surges in brightness, perhaps heating the young planet enough to boil off its oceans. Intense ultraviolet light from the star would bombard the resulting water vapor, perhaps splitting it into hydrogen and oxygen. The lighter hydrogen could escape into space, leaving an atmosphere rich in oxygen around a planet devoid of life. “Know thy star, know thy planet,” recites Siddharth Hegde of Cornell University’s Carl Sagan Institute.

Discovering methane in the same place as oxygen, however, would strengthen the case for life. Although geological processes can produce methane, without any need for life, most methane on Earth comes from microbes that live in landfill sites and in the guts of ruminants. Methane and oxygen together make a redox pair: two molecules that will readily react by exchanging electrons. If they both existed in the same atmosphere, they would quickly combine to produce carbon dioxide and water. But if they persist at levels high enough to be detectable, something must be replenishing them. “It’s largely accepted that if you have redox molecules in large abundance they must be produced by life,” Hegde says.

Some argue that by focusing on oxygen and methane—typical of life on Earth—researchers are ignoring other possibilities. If there is one thing astronomers have learned about exoplanets so far, it is that familiar planets are a poor guide to exoplanets’ huge diversity of size and nature. And studies of extremophiles, microbes that thrive in inhospitable environments on Earth, suggest life can spring up in unlikely places. Exobiology may be entirely unlike its counterpart on Earth, and so its gaseous byproducts might be radically different, too.

But what gases to look for? Seager and her colleagues compiled a list of 14,000 compounds that might exist as a gas at “habitable” temperatures, between the freezing and boiling points of water; to keep the list manageable they restricted it to small molecules, with no more than six nonhydrogen atoms. About 2500 are made of the biogenic atoms carbon, nitrogen, oxygen, phosphorus, sulfur, and hydrogen, and about 600 are actually produced by life on Earth. Detecting high levels of any of these gases, if they can’t be explained by nonbiological processes, could be a sign of alien biology, Seager and her colleagues argue.

A. CUADRA/SCIENCE

Light shining through the atmospheres of transiting exoplanets is likely to be the mainstay of biosignature searches for years to come. But the technique tends to sample the thin upper reaches of a planet’s atmosphere; far less starlight may penetrate the thick gases that hug the surface, where most biological activity is likely to occur. The transit technique also works best for hot Jupiters, which by nature are less likely to host life than small rocky planets with thinner atmospheres. The JWST may be able to tease out atmospheric spectra from small planets if they orbit small, dim stars like red dwarfs, which won’t swamp the planet’s spectrum. But these red dwarfs have a habit of spewing out flares that would make it hard for life to establish itself on a nearby planet.

To look for signs of life on a terrestrial planet around a sunlike star, astronomers will probably have to capture its light directly, to form a spectrum or even an actual image. That requires blocking the overwhelming glare of the star. Ground-based telescopes equipped with “coronagraphs,” which precisely mask a star so nearby objects can be seen, can now capture only the biggest exoplanets in the widest orbits. To see terrestrial planets will require a similarly equipped telescope in space, above the distorting effect of the atmosphere. NASA’s Wide Field Infrared Survey Telescope (WFIRST), expected to launch in the mid-2020s, is meant to fill that need.

NASA/WFIRST

Even better, WFIRST could be used in concert with a “starshade”—a separate spacecraft stationed 50,000 kilometers from the telescope that unfurls a circular mask tens of meters across to block out starlight. A starshade is more effective than a coronagraph at limiting the amount of light going into the telescope. It not only blocks the star directly, but also suppresses diffraction with an elaborate petaled edge. That reduces the stray scattered light that can make it hard to spot faint planets. A starshade is a much more expensive prospect than a coronagraph, however, and aligning telescope and starshade over huge distances will be a challenge.

Direct imaging will provide much better spectra than transit observations because light will pass through the full depth of the planet’s atmosphere twice, rather than skimming through its outer edges. But it also opens up the possibility of detecting life directly, instead of through its waste gases in the atmosphere. If organisms, whether they are plants, algae, or other microbes, cover a large proportion of a planet’s surface, their pigments may leave a spectral imprint in the reflected light. Earthlight contains an obvious imprint of this sort. Known as the “red edge,” it is the dramatic change in the reflectance of green plants at a wavelength of about 720 nanometers. Below that wavelength, plants absorb as much light as possible for photosynthesis, reflecting only a few percent. At longer wavelengths, the reflectance jumps to almost 50%, and the brightness of the spectrum rises abruptly, like a cliff. “An alien observer could easily tell if there is life on Earth,” Hegde says.

There’s no reason to assume that alien life will take the form of green plants. So Hegde and his colleagues are compiling a database of reflectance spectra for different types of microbes. Among the hundreds the team has logged are many extremophiles, which fill marginal niches on Earth but may be a dominant life form on an exoplanet. Many of the microbes on the list have not had their reflectance spectra measured, so the Cornell team is filling in those gaps. Detecting pigments on an exoplanet surface would be extremely challenging. But a tell-tale color in the faint light of a distant world could join other clues—spectral absorption lines from atmospheric gases, for example—to form “a jigsaw puzzle which overall gives us a picture of the planet,” Hegde says.

None of the telescopes available now or in the next decade is designed specifically to directly image exoplanets, so biosignature searches must compete with other branches of astronomy for scarce observing time. What researchers really hanker after is a large space telescope purpose-built to image Earth-like alien worlds—a new incarnation of the idea behind NASA’s ill-fated Terrestrial Planet Finder.

The Habitable Exoplanet Imaging Mission, or HabEx, a mission concept now being studied by NASA, could be the answer. Its telescope would have a mirror up to 6.5 meters across—as big as the JWST’s—but would be armed with instruments sensitive to a broader wavelength range, from the ultraviolet to the near-infrared, to capture the widest range of spectral biosignatures. The telescope would be designed to reduce scattered light and have a coronagraph and starshade to allow direct imaging of Earth-sized exoplanets.

Such a mission would reveal Earth-like planets at a level of detail researchers can now only dream about—probing atmospheres, revealing any surface pigments, and even delivering the sort of blocky surface images that Kane has been simulating. But will that be enough to conclude we are not alone in the universe? “There’s a lot of uncertainty about what would be required to put the last nail in the coffin,” Kane says. “But if HabEx is built according to its current design, it should provide a pretty convincing case.”

NASA HabEx: The Planet Hunter

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