Tagged: NASA/WFIRST Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 1:31 pm on September 28, 2019 Permalink | Reply
    Tags: "Two Eyes to Hunt Stray Planet Masses", , , , , , , NASA/WFIRST   

    From AAS NOVA: “Two Eyes to Hunt Stray Planet Masses” 

    AASNOVA

    From AAS NOVA

    1
    Artist’s impression of a gas-giant exoplanet that has been ejected from its star system and now has no host. [NASA/Caltech]

    How can we measure the masses of free-floating planets wandering around our galaxy? A new study identifies one approach that combines the power of two upcoming missions.

    Finding Invisible Planets

    Most exoplanets we’ve found so far have relied on measurements of their host stars, either via dips in the host star’s light as the planet passes in front (transit detections), or via wiggling of lines in the host star’s spectra caused by the planet’s gravitational tug (radial velocity detections).

    Planet transit. NASA/Ames

    Radial velocity Image via SuperWasp http:// http://www.superwasp.org/exoplanets.htm


    Radial Velocity Method-Las Cumbres Observatory

    But free-floating planets have no hosts and are therefore effectively invisible, since they don’t give off much light of their own. To find these rogues, we rely on another method: gravitational microlensing.

    Gravitational microlensing, S. Liebes, Physical Review B, 133 (1964): 835

    In microlensing, the mass of a passing foreground planet — either free-floating or bound to a host star — can act as a lens, briefly gravitationally focusing the light of a background star behind it.

    4
    A diagram of how planets are detected via gravitational microlensing. In this case, the planet is in orbit around a foreground lens star, but this same diagram can also apply to a free-floating planet acting alone as the lens. [NASA]

    As a result, the background star temporarily brightens (on timescales of perhaps seconds to years) in our observations. Though we never directly see the foreground planet, we can infer its presence from the spike in the background star’s brightness.

    Masses from Parallax

    By itself, a microlensing observation usually can’t tell us about the mass of a free-floating planet; this is because the timescale of a brightening event depends on both the mass of the lens and on the relative proper motion between the background source and the foreground lensing planet.

    But if we could simultaneously observe a microlensing event from two different locations, separated by a large enough distance? Then the parallax would allow us to break that degeneracy: the differences in peak brightness and its timing at the two locations would allow us to calculate both the speed of lens relative to the source and the planet mass.

    Vantage Points in Space

    Where do we find two sensitive eyes located far enough apart to make this work? In space, of course!

    NASA’s Wide Field Infrared Survey Telescope (WFIRST) is set for launch in the mid-2020s, and one of its primary mission objectives is to perform wide-field imaging that may allow for the detection of hundreds of free-floating planets — and many additional bound planets — via microlensing.

    NASA/WFIRST

    As for the second eye, scientists Etienne Bachelet (Las Cumbres Observatory) and Matthew Penny (The Ohio State University) propose that ESA’s upcoming Euclid mission is exactly what we need.

    ESA/Euclid spacecraft depiction

    Euclid, launching in 2022, will have similar wide-field imaging capabilities to WFIRST, and it will be able to make complementary microlensing parallax measurements as long as the two satellites are 100,000 km or more apart.

    Making Use of Gaps

    Though Euclid’s primary science goal is to study dark energy and dark matter, Bachelet and Penny demonstrate that a modest investment of Euclid observing time — approximately 60 days during its primary mission, and another 60 days during its extended mission — during scheduling gaps would be enough to obtain the masses for 20 free-floating planets and many more bound planets.

    So what are we waiting for? Let’s go learn more about the rogue planets sneaking through our galaxy!

    Citation

    “WFIRST and EUCLID: Enabling the Microlensing Parallax Measurement from Space,” Etienne Bachelet and Matthew Penny 2019 ApJL 880 L32.
    https://iopscience.iop.org/article/10.3847/2041-8213/ab2da5

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 2:20 pm on November 29, 2018 Permalink | Reply
    Tags: , , , , , NASA/WFIRST   

    From AAS NOVA: “Simulating WFIRST’s Search for Supernovae” 

    AASNOVA

    From AAS NOVA

    NASA/WFIRST

    New exoplanets, distant galaxies, unexpected transients — the successful discoveries of major astronomical missions get splashed across news headlines. What generally isn’t seen, however, is the often decades-long development process that led to these successful missions — a process that includes not only technology and engineering feats, but also the meticulous planning necessary to optimize the use of an observatory with a limited lifetime.

    Want a closer look? A recently published study provides an insider’s view of these complex planning stages for a proposed upcoming mission, the Wide Field Infrared Survey Telescope (WFIRST).

    A New Eye in the Sky

    In 2010, the astronomy community selected WFIRST as the highest-ranked large space-based mission for the next decade. Like most major missions, WFIRST has been through its share of ups, downs, and funding scares in the planning process — but as of this writing, it’s on the books for a planned launch in the mid 2020s.

    WFIRST will use a telescope the size of Hubble’s (i.e., a 2.37-m mirror) that was donated in 2012 by the National Reconnaissance Office. It will host two main instruments: a coronagraph that will be used for exoplanet and planetary disk studies, and a wide-field instrument that will be used to probe dark-energy models. The wide-field instrument will have two components: a wide-field channel imager, and an integral field channel spectrometer.

    Vying for Time

    Looking at just the dark-energy science objective, we can already see timing challenges emerge. WFIRST seeks to constrain the nature of dark energy by discovering and measuring the distance to Type Ia supernovae, thereby measuring the evolution of dark energy over time.

    But though WFIRST’s proposed mission duration is five years, only a total of 6 months of observing time can be devoted to the supernova survey. Should this time be primarily spent on wide-field imaging to detect as many supernovae as possible? Or should we employ a targeted strategy, using the spectrometer to better determine redshifts of the supernovae discovered? What areas should the survey cover, at what depth? How frequently should we look at the same patches of sky?

    These are just some of the many questions survey designers must wrestle with in order to optimize the WFIRST mission and give the project the best chance of answering our questions. To aid decision-making, a team of scientists led by Rebekah Hounsell (University of California, Santa Cruz and University of Illinois at Urbana-Champaign) has now conducted a series of simulations to explore different supernova survey strategies for WFIRST.

    An Optimized Reference

    Hounsell and collaborators realistically simulated supernova light curves and spectra as viewed by WFIRST’s instruments. They then explored 11 survey strategies with different time allocations between the imager and the spectrometer, taking into account various uncertainties. Their results suggest an imaging-focused strategy would be the most successful at increasing our understanding of the dark-energy equation of state.

    Though we won’t know exactly which strategy is the most optimal until we’ve determined some of the specific systematic uncertainties of the mission, Hounsell and collaborators’ study has laid the groundwork for future planning of the mission. What’s more, their results confirm that WFIRST will have the potential to significantly advance our understanding of dark energy — so keep an eye on this project in the future!

    Citation

    “Simulations of the WFIRST Supernova Survey and Forecasts of Cosmological Constraints,” R. Hounsell et al 2018 ApJ 867 23.
    http://iopscience.iop.org/article/10.3847/1538-4357/aac08b/meta

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Societyis to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 1:34 pm on December 22, 2017 Permalink | Reply
    Tags: , , , , NASA/WFIRST   

    From Goddard: “NASA’s Next Major Telescope to See the Big Picture of the Universe” 

    NASA Goddard Banner
    NASA Goddard Space Flight Center

    Dec. 22, 2017
    Claire Saravia
    NASA’s Goddard Space Flight Center

    NASA is beginning to design its next big astrophysics mission, a space telescope that will provide the largest picture of the universe ever seen with the same depth and clarity as the Hubble Space Telescope.

    NASA/WFIRST

    Scheduled to launch in the mid-2020s, the Wide Field Infrared Survey Telescope (WFIRST) will function as Hubble’s wide-eyed cousin. While just as sensitive as Hubble’s cameras, WFIRST’s 300-megapixel Wide Field Instrument will image a sky area 100 times larger. This means a single WFIRST image will hold the equivalent detail of 100 pictures from Hubble.


    Watch to learn more about the NASA’s Wide Field Infrared Survey Telescope (WFIRST).

    “A picture from Hubble is a nice poster on the wall, while a WFIRST image will cover the entire wall of your house,” said David Spergel, co-chair of the WFIRST science working group and the Charles A. Young professor of astronomy at Princeton University in New Jersey.

    The mission’s wide field of view will allow it to generate never-before-seen big pictures of the universe, which will help astronomers explore some of the greatest mysteries of the cosmos, including why the expansion of the universe seems to be accelerating. One possible explanation for this speed-up is dark energy, an unexplained pressure that currently makes up 68 percent of the total content of the cosmos and may have been changing over the history of the universe. Another possibility is that this apparent cosmic acceleration points to the breakdown of Einstein’s general theory of relativity across large swaths of the universe. WFIRST will have the power to test both of these ideas.

    To learn more about dark energy, WFIRST will use its powerful 2.4-meter mirror and Wide Field Instrument to do two things: map how matter is structured and distributed throughout the cosmos and measure how the universe has expanded over time. In the process, the mission will study galaxies across cosmic time, from the present back to when the universe was only half a billion years old, or about 4 percent of its current age.

    “To understand how the universe evolved from a hot, uniform gas into stars, planets, and people, we need to study the beginnings of that process by looking at the early days of the universe,” said WFIRST Project Scientist Jeffrey Kruk at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We’ve learned much from other wide-area surveys, but WFIRST’s will be the most sensitive and give us our farthest look back in time.”

    WFIRST will do this through multiple observational strategies, including surveys of exploding stars called supernovae and galaxy clusters, and mapping out the distribution of galaxies in three dimensions. Measuring the brightness and distances of supernovae provided the first evidence for the presence of dark energy. WFIRST will extend these studies to greater distances to measure how dark energy’s influence increased over time.

    WFIRST will measure precise distances to galaxy clusters to map how they grew over time. The mission will also pinpoint the distances to millions of galaxies by measuring how their light becomes redder at greater distances, a phenomenon called redshift. The farther off a galaxy is, the redder its light appears when we see it. Mapping out the 3-D positions of galaxies will allow astronomers to measure how the distribution of galaxies has changed over time, providing another measure of how dark energy has affected the cosmos.

    The Wide Field Instrument will also allow WFIRST to measure the matter in hundreds of millions of distant galaxies through a phenomenon dictated by Einstein’s relativity theory. Massive objects like galaxies curve space-time in a way that bends light passing near them, creating a distorted, magnified view of far-off galaxies behind them. Using this magnifying glass effect, called weak gravitational lensing, WFIRST will paint a broad picture of how matter is structured throughout the universe, allowing scientists to put the governing physics of its assembly to the ultimate test.

    Gravitational Lensing NASA/ESA

    Gravitational microlensing, S. Liebes, Physical Review B, 133 (1964): 835

    WFIRST can use this same light-bending phenomenon to study planets beyond our solar system, known as exoplanets. In a process called microlensing, a foreground star in our galaxy acts as the lens. When its motion randomly aligns with a distant background star, the lens magnifies, brightens and distorts the background star. As the lensing star drifts along in its orbit around the galaxy and the the alignment shifts, so does the apparent brightness of the star. The precise pattern of these changes can reveal planets orbiting the lensing star because the planets themselves serve as miniature gravitational lenses. Such alignments must be precise and last only hours.

    WFIRST’s microlensing survey will monitor 100 million stars for hundreds of days and is expected to find about 2,500 planets, with significant numbers of rocky planets in and beyond the region where liquid water may exist. This planet-detection method is sensitive enough to find planets smaller than Mars, and will reveal planets orbiting their host stars at distances ranging from closer than Venus to beyond Pluto.

    These results will make WFIRST an ideal companion to missions like NASA’s Kepler and the upcoming Transiting Exoplanet Survey Satellite (TESS), which are best suited to find larger planets orbiting closer to their host stars.

    NASA/Kepler Telescope

    NASA/TESS

    Together, discoveries from these three missions will help complete the census of planets beyond our solar system, helping us learn how planets form and migrate into systems like our own. The combined data from these missions provide insight into planets in the critical area known as the habitable zone, the orbiting distance from a host star that would permit a planet’s surface to harbor liquid water — and potentially life.

    WFIRST will also feature a coronagraph technology demonstration instrument designed to directly image exoplanets by blocking out a star’s light, allowing the much fainter planets to be observed. As NASA’s first advanced coronagraph in space, it will be 1,000 times more capable than any previously flown. This is a key step toward future direct imaging missions that will study truly Earth-like planets discovered nearby. The instrument will be able to image gas giant planets orbiting mature Sun-like stars, allowing scientists to study them in ways that haven’t been possible before. Scientists are hoping to use the coronagraph to determine important properties about these planets, such as their atmospheric composition.

    WFIRST will serve as an important tool for the science community through its General Observer and archival data analysis programs. All WFIRST data will be publicly available immediately after processing and delivery to the archive. Also, by submitting proposals through the competitive program, scientists around the world will be able to use the observatory to study the cosmos in their own way, from the nearest exoplanets out to clusters of distant galaxies.

    The mission will complement other missions expected to operate in the next decade, notably the James Webb Space Telescope, scheduled to launch in 2019.

    NASA/ESA/CSA Webb Telescope annotated

    Webb provides a detailed look at rare and interesting objects, while WFIRST will take a wide look at the universe. WFIRST will also complement new ground-based observatories such as the Large Synoptic Survey Telescope (LSST) currently in development.

    LSST


    LSST Camera, built at SLAC



    LSST telescope, currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    By combining data from WFIRST and LSST, scientists will be able to view the universe in nine different wavelengths, data that will provide the most detailed wide-angle view of the universe to date.

    By pioneering an array of innovative technologies, WFIRST will serve as a multipurpose mission, furnishing a big picture of the universe and helping us answer some of the most profound questions in astrophysics, such as how the universe evolved into what we see today, its ultimate fate and whether we are alone.

    “By building this telescope we’re enabling a wealth of science and the capability to address those kinds of questions,” Spergel said. “It’s deeply interesting not only to scientists, but anyone who looks up at the sky and wonders.”

    WFIRST is managed at Goddard, with participation by NASA’s Jet Propulsion Laboratory and Caltech/IPAC, also in Pasadena, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from research institutions across the United States.

    For more information about NASA’s WFIRST mission, visit: http://www.nasa.gov/wfirst

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    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

     
  • richardmitnick 6:27 am on November 2, 2017 Permalink | Reply
    Tags: , , , , , , Earth-sized alien worlds are out there. Now astronomers are figuring out how to detect life on them, Exobiology, , , NASA Deep Space Climate Observatory, NASA HabEx, , NASA/WFIRST, ,   

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

    ScienceMag
    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.

    1
    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.

    2
    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.

    3
    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.

    4
    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.”

    4
    NASA HabEx: The Planet Hunter

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: