A Vision That Could Supercharge NASA

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An artist rendering of an approximately 16-meter telescope in space.  This image was created for an earlier large space telescope feasibility project called ATLAST, but it is similar to what is being discussed inside and outside of NASA as a possible great observatory after the James Webb Space Telescope and the Wide-Field Infrared Survey Telescope.  Advocates say such a large space telescope would revolutionize the search for life on exoplanets, as well as providing the greatest observing ever for general astrophysics. (NASA)

Let your mind wander for a moment and let it land on the most exciting and meaningful NASA mission that you can imagine.  An undertaking, perhaps, that would send astronauts into deep space, that would require enormous technological innovation, and that would have ever-lasting science returns.

Many will no doubt think of Mars and the dream of sending astronauts there to explore.  Others might imagine setting up a colony on that planet, or perhaps in the nearer term establishing a human colony on the moon.  And now that we know there’s a rocky exoplanet orbiting Proxima Centauri — the star closest to our sun — it’s tempting to wish for a major robotic or, someday, human mission headed there to search for life.

All are dream-worthy space projects for sure.  But some visionary scientists (and most especially one well-known former astronaut) have been working for some time on another potential grand endeavor — one that you probably have not heard or thought about, yet might be the most compelling and achievable of them all.

It would return astronauts to deep space and it would have them doing the kind of very difficult but essential work needed for space exploration in the far future. It would use the very costly and very powerful Space Launch System (SLS) rocket and Orion capsule being built now by NASA and Lockheed Martin respectively.  Most important, it would almost certainly revolutionize our understanding of the cosmos near and far.

At a recent meeting of the House Science Committee, chairman Lamar Smith, said of the hearing’s purpose that, “Presidential transitions offer the opportunities to reinvigorate national goals. They bring fresh perspectives and new ideas that energize our efforts.”

That said, here’s the seemingly feasible project that fires my imagination the most.

It has been quietly but with persistence promoted most visibly by John Grunsfeld, the former astronaut who flew to the Hubble Space Telescope three times to fix and upgrade it, who has spent 58 hours on spacewalks outside the Shuttle, and towards the end of his 40 years with the agency ultimately became an associate administrator and head of the agency’s Science Mission Directorate.

A visualization of the assembly in space of a large segmented telescope, with work being done by astronauts and robots.  The honeycomb blocks are parts of the mirror, and the grey cylinders on the right are habitats for astronauts.  (NASA)

His plan:  Build a segmented space telescope mirror that is 16 meters (52 feet) in diameter or larger, package it into one or several payload fairings and launch it into deep space.  Accompanying astronauts would put it together either at its final destination or at a closer point where it could then be propelled to that destination.

This would provide invaluable humans-in-space experience, would put the Orion and SLS to very good use in advance of a projected human mission to Mars, and would deploy the most penetrating telescope observing ever.  By far.

No mirror with a diameter greater than 3.5 meters (11.5 feet)  has ever been deployed in space,  although the the James Webb  Space Telescope mirror will be substantially larger at 6.5 meters (21 feet) when launched in 2018.  The largest ground telescopes are in the 10-meter (33 foot) range.

John Grunsfeld working on the Hubble Space Telescope, some 350 miles above Earth. He said that based on his own experience with spacewalks and space repairs, he thinks that a crew of four astronauts could assembled a 16-meter segmented telescope mirror within four weeks. (NASA)

What Grunsfeld’s space behemoth would provide is an unprecedented power and resolution to see back to the earliest point possible in the history of the universe, and doing that in the ultraviolet and visible wavelengths. But perhaps more significantly and revolutionary, it would supercharge the agency’s ability to search for life beyond Earth.

Like nothing else currently in use or development, it would provide a real chance to answer what is arguably humanity’s most fundamental question:  Are we alone in the universe?

Grunsfeld has been introducing people to the project/vision inside NASA for some time.  He also told me that he has spoken with many members of Congress about it, and that most have been quite supportive.  Now he’s starting to make the case to the public.

“We need our leaders to be bold if we want to stay in the forefront of science and engineering,” he said.  “Assembling a 16-meter telescope in space would not be easy by any means.  But we can do it and — this is the key — it would be transformational. It’s a rational thing to do.”

His confidence in the possibility of launching the segmented mirror parts and having astronauts assemble them in space comes, he says, from experience.  Not only has he flown on the space shuttle five times and has his three very close encounters with the Hubble, but he has also overseen the difficult process of getting the JWST project — with its pioneering segmented, folding mirror — back on track after large budget overruns and delays.  He’s also trained in astrophysics and is enamored of exoplanets.

“If your goal is to search for inhabited planets, you just have to go up to the 16-meter range for the primary telescope mirror,” he said.

“Think about it:  if we sent up something smaller, it will give us important and potentially very intruiging information about what planets might be habitable, that could potentially support life.  But then we’d have to send up a bigger mirror later to actually make any detection.  Why not just go to the 16-meter now?”

The strongest driver on the size of the LUVOIR telescope is the desire to have a large sample of exoEarth candidates to study. This figure shows the real stars in the sky for which a planet in the habitable zone can be observed. The color coding shows the probability of observing an exoEarth candidate if it’s present around that star (green is a high probability, red is a low one). This is a visualization of the work of Chris Stark at Space Telescope Science Institute, who created an advanced code to calculate yields of exoplanet observations with different facilities.  (C. Stark and J. Tumlinson, STScI)

While all this may sound to many like science fiction, NASA actually has a team in place studying the science and technology involved with a very large space telescope, and has funded studies of in-space assembly as well.

The current team is one of four studying different projects for a grand observatory for the 2030s.  Their mission is called LUVOIR (the Large UV/Optical/IR Surveyor), and both it and a second mission under study (Hab-Ex) have exoplanets as a primary focus. It was Grunsfeld and Paul Hertz, director of NASA’s astrophysics division, who selected the four concepts for more in-depth study based in large part on astronomy and astrophysics community thinking and aspirations, especially as laid out in the 2013 Thirty-Year Astrophysics Visionary Roadmap.

The LUVOIR team started out with the intention of studying the engineering and technological requirements — and science returns — of a space telescope between 8 and 16 meters in diameter, while Hab-Ex would look at the 4 to 7 meter option for a telescope designed to find exoplanets.  Grunsfeld addressed the LUVOIR study team and encouraged them to be ambitious in their thinking — a message delivered by quite a few others as well.  What’s more, a number of study team members were inclined towards the 16-meter version from the onset.

Aki Roberge of the Goddard Space Flight Center is the team scientist for the LUVOIR Science and Technology Definition Team.

The LUVOIR team has not addressed the issue of assembly in space — their goals are to understand the science made possible with telescopes of different sizes, to design an observatory that can be repaired and upgraded, and to determine if the technology to pull it all together is within reach for the next decade or two.

A key issue is how large a folded up mirror the launch vehicle rocket nose cone (the fairing) can hold.  While the current version of the SLS would certainly not accommodate a 16-meter segmented mirror, team study scientist Aki Roberge — an astrophysicist at the Goddard Space Flight Center — said that the team just recently got the good news that a next generation SLS fairing looks like it could well hold a folded mirror of up to 15 meters. Quite a few “ifs” involved, but still promising.

“We’re still in the midst of our work, but it’s clear that a LUVOIR with a large aperture (mirror) gives us a major science return,” she said.  “Going up to nine meters would be a major leap forward, and going to 16 would be a dramatic advance on that.”

“But we have to assess what we gain in terms of going large and what we might lose in terms of added technical difficulty, cost and time.”  As is, the 9 or 16-meter project — if selected — would not be ready to launch until the mid 2030s.  All the great space observatories and missions have had decades-long gestation periods.

The results from the LUVOIR and other formal NASA study teams will be reviewed by the agency and then assessed by a sizeable group of experts convened by the National Academy of Sciences for the 2020 Astrophysics Decadal Survey.  They set the next decade’s topic and mission priorities for the astronomy and astrophysics communities (as well as others) — assessments that are sent back to NASA and generally followed.

One of Grunsfeld’s goals, he told me, is to make the assembled-in-space 16-meter telescope a top Decadal Survey priority.  While supportive of the LUVOIR efforts, he believes that including astronauts in the equation, deploying a somewhat larger mirror even if the difference in size is not great, and making a mirror that he says will be easier to fix and upgrade than a folded up version, gives the assembled-in-space option the advantage.

These images, which are theoretical simulations using the iconic Hubble Deep Field image, are adjusted to reflect the light collected by telescopes of different sizes. They show the increased resolution and quality of images taken by a 16-meter telescope, a 9-meter, and the Hubble Space Telescope, which is 2.4 meters in diameter.  They illustrate pretty clearly why astronomers and exoplanet hunters want ever larger telescope mirrors to collect those photons from galaxies, stars and planets.

Simulated views of galaxies in deep space, as seen with a proposed 16-meter telescope. This and the two images below are of the same part of the sky. The exposure time for each image was assumed to be the same, to make them comparable. Scientists get higher resolution images with the larger telescopes.  (G. Snyder, STScI /M. Postman, STScI.)

 

Deep space galaxies as seen with nine meter telescope.

 

Once again the same view, taken with Hubble’s 2.4-meter telescope for the same period of time as the images above.  The iconic Hubble Deep Field images are much clearer than this one, and that’s because the telescope was collecting light for a much longer period of time.

Whether or not the LUVOIR project is selected to be a future NASA flagship observatory, and whether or not it will be an assembled-in-space version of it, many at the agency clearly see human activity and habitation in space (as well as on planets or moons) as a necessary and inevitable next step.

Harley Thronson is the senior scientist for Advanced Concepts in Astrophysics at Goddard, and he has worked on several projects related to how and where astronauts might live and work in space.

Harley Thronson, the senior scientist for Advanced Concepts in Astrophysics at the Goddard Space Flight Center, standing outside the JWST clean room. (NASA)

He said this research goes back decades, having gained the attention of then-NASA Administrator Dan Goldin around 2000.  It has recently experienced another spurt of interest as the agency has been assessing opportunities for human operations beyond the immediate vicinity of the Earth.

“It’s inevitable that the astronomy community will want and need larger space observatories, and so we have to work out how to design and build them, how and where they might be assembled in space, and how they can be serviced,” Thronson said.  The JWST will not be reachable for upgrades and servicing, and Congress responded to that drawback by telling NASA will make sure future major observatories can be serviced if at all possible.

Thronson said that he supports and is inspired by the idea of a 16-meter space telescope, and he agrees with Grunsfeld that assembly in space is the wave of the future.  But he said “I’m not quite as optimistic as John that we’re ready to attack that now, though it would be terrific if we were.”

Part of Thronson’s work involves understanding operation sites where space telescopes would be most stable, and that generally involves the libration points, where countervailing gravity pulls are almost neutralized.  LUVOIR, like JWST, is proposed for the so-called Sun-Earth L-2 point, about one million miles outward from Earth where the Earth and sun create a gravitational equilibrium of sorts.

Thronson said there has been some discussion about the possibility of assembling a telescope at a closer Earth-moon libration point and then propelling it towards its destination.  That assembly point could, over time, become a kind of depot for servicing space telescopes and as well as other tasks.

As a sign of the level of interest in these kind of space-based activities, NASA last year awarded $65 million to six companies involved in creating space habitats for astronauts on long-duration missions in deep space.

One of the locations in relatively nearby space where a space telescope would have a stable gravitational environment. (NASA

At the time, the director of NASA’s Advanced Exploration Systems, Jason Crusan,  said that “the next human exploration capabilities needed beyond the Space Launch System rocket and Orion capsule are deep space, long duration habitation and in-space propulsion. We are now adding focus and specifics on the deep space habitats where humans will live and work independently for months or years at a time, without cargo supply deliveries from Earth.”

Not surprisingly, building and maintaining telescopes and habitats in space will be costly (though less so than any serious effort to send humans to Mars).  As a result, how much support NASA gets from the White House, Congress and the public — as well as the astronomy and astrophysics communities — will determine whether and when this kind of space architecture becomes a reality.

John Grunsfeld, who has walked the walk like nobody else, plans to be stepping up his own effort to explain how and why this is a vision worth embracing.

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Storming the One-Meter-Per-Second Barrier

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Kitt Peak National Observatory mountain top at Dusk looking north. Visible in the picture are the NOAO 4-meter Mayall, the Steward Observatory 90-inch, the University of Arizona Lunar and Planetary Laboratory Spacewatch Telescopes, LOTIS, 0.4-meter Visitor Center Telescope, Case Western Reserve University Observatory and the SARA Observatory. Credit: P. Marenfeld (NOAO/AURA/NSF)
The Kitt Peak National Observatory, on the Tohono O’odham reservation outside Tucson, will be home to a next-generation spectrometer and related system which will allow astronomers to detect much smaller exoplanets through the radial velocity method.  P. Marenfeld (NOAO/AURA/NSF)

When the first exoplanet was identified via the radial velocity method, the Swiss team was able to detect a wobble in the star 51 Pegasi at a rate of 50 meters per second.   The wobble is the star’s movement back and forth caused by the gravitational pull of the planet, and in that first case it was dramatic — the effects of a giant Jupiter-sized planet orbiting extremely close to the star.

Many of the early exoplanet discoveries were of similarly large planets close to their host stars, but it wasn’t because there are so many of them in the cosmos.  Rather, it was a function of the capabilities of the spectrographs and other instruments used to view the star.  They were pioneering breakthroughs, but they didn’t have the precision needed to measure wobbles other than the large, dramatic ones caused by a close-in, huge planet.

That was the mid 1990s, and radial velocity astronomers have worked tirelessly since to “beat down” that 50 meters per second number.  And twenty years later, RV astronomers using far more precise instruments and more refined techniques have succeeded substantially:  1 meter per second of wobble is now achieved for the quietest stars.  That has vastly improved their ability to find smaller exoplanets further from their stars and is a major achievement.  But it has nonetheless been a major frustration for astronomers because to detect terrestrial exoplanets in the Earth-sized range, they have to get much more precise  — in the range of tens of centimeters per second.

A number of efforts to build systems that can get that low are underway, most notably the ESPRESSO spectrograph scheduled to begin work on the High Accuracy Radial Vlocity Planet Searcher (HARPS) in Chile next year. Then earlier this month an ambitious NASA-National Science Foundation project was awarded to Penn State University to join the race.  The next-generation spectrograph is scheduled to be finished in 2019 and installed at the Kitt Peak National Observatory in Arizona, and its stated goal is to reach the 20 to 30 centimeters per second range.

Suvrath Mahadevan, an assistant professor at Penn State, is principal investigator for the project.  It is called NEID, which means ‘to see’ in the language of the Tohono O’odham, on whose land the Kitt Peak observatory is located.

“For many reasons, the (radial velocity) community has been desperate for an instrument that would allow for detections of smaller planets, and ones in habitable zones,” he said.  “We’re confident that the instrument we’re building will — in time — provide that capability.”

Las Cumbres Observatory Global Telescope Network.
A illustration of how the radial velocity method of planet hunting works.  The wobble of the stars is far away miniscule in galactic terms, making extreme precision essential in measuring the movement. (Las Cumbres Observatory Global Telescope Network)

Project scientist Jason Wright, associate professor of astronomy and astrophysics at Penn State, put it this way:  “NEID will be more stable than any existing spectrograph, allowing astronomers around the world to make the precise measurements of the motions of nearby, Sun-like stars.”  He said his Penn State team will use the instrument “to discover and measure the orbits of rocky planets at the right distances from their stars to host liquid water on their surfaces.”

NASA and the NSF wanted the new spectrograph built on an aggressive timetable to meet major coming opportunities and needs, Mahadevan said.

The speedy three-year finish date is a function of the role that radial velocity detection plays in exoplanet research.  While many planets have been, and will be, first detected through the technique, it is also essential in the confirming of candidate planets identified by NASA space telescopes such as Kepler, the soon-to-be launched TESS (the Transiting Exoplanet Survey Satellite) and others into the future.  There is a huge backlog of planets to be confirmed, and many more expected in the relatively near future.

What’s more, as Mahadevan explained, an instrument like NEID could significantly help NASA’s planning for a possible 2030s Flagship space telescope mission focused on exoplanets.  Two of the four NASA contenders under study are in that category — LUVOIR (Large Ultraviolet Visible Infrared) Surveyor and Hab-Ex — and their capabilities, technologies, timetables and cost are all now under consideration.

If NEID can identify some clearly Earth-sized planets in habitable zones, he said, then the planning for LUVOIR or Hab-Ex could be more focused (and the proposal potentially less costly.)  This is because the observatory could be designed to look at a limited number of exoplanets and their host stars, rather than scanning the skies for a clearly Earth-like planet.

“Right now we have no definite Earth-sized planets in a habitable zone, so a LUVOIR or Hab_ex design would have to include a blind search.  But if we know of maybe 15 planets we’re pretty sure are in their habitable zones, the targets get more limited and the project becomes a lot cheaper.”

Suvrath Mahadevan, assistant professor of Astronomy and Astrophysics at Penn State, and principal investigator for a new-generation high precision spectrometer. (Penn State)
Suvrath Mahadevan, assistant professor of Astronomy and Astrophysics at Penn State, and principal investigator for a new-generation high precision spectrometer. (Penn State)

These possibilities, however, are for the future.  Now, Mahadevan said, the Penn State team has to build a re-considered spectrograph, a significant advance on what has come before.  With its track record of approaching their work through interdisciplinary collaboration, the Penn State team will be joined by collaborators from NASA Goddard Space Flight Center, University of Colorado, National Institute of Standards and Technology, Macquarie University in Australia, Australian Astronomical Observatory, and Physical Research Laboratory in India.  Much of the work will be done over the next three years at Penn State, but some at the partner institutions as well.

Key to their assembly approach is that the instrument will be put together in vacuum-sealed environment and will have no vibrating or moving parts.  This design stability will prevent, or minimize, instrument-based misreadings of the very distant starlight being analyzed.

A major issue confronting radial velocity astronomers is that light from stars can fluctuate for many reasons other than a nearby planet — from sunspots, storms, and other magnetic phenomena.  The NEID instrument will try to minimize these stellar disruptors by providing the broadest wavelength coverage so far in an exoplanet spectrograph, Mahadevan said, collecting light from well into the blue range of the spectrum to almost the end of the red.

“We’re not really building a spectorograph but a radial velocity system, he said.  That includes upgrades to the telescope port, the data pipeline and more.

This is how Lori Allen, Associate Director for Kitt Peak, described that new “system”: “The extreme precision (of NEID) results from numerous design factors including the extreme stability of the spectrometer environment, image stabilization at the telescope, innovative fiber optic design, as well as state-of-the-art calibration and data reduction techniques”.

 

The new generation spectrograph will be installed on the 3.5 meter WYN telescope at Kitt Peak. Operated by National Optical Astronomy Observatory, the $10 million project is a collaboration of NASA and the National Science Foundation.
The new generation spectrograph will be installed on the 3.5 meter WYN telescope at Kitt Peak. The site is managed by the National Optical Astronomy Observatory, and $10 million spectrograph project is a collaboration of NASA and the National Science Foundation.

Sixteen teams ultimately competed to build the spectrograph, and the final two contenders were Penn State and MIT.  Mahadevan said that, in addition to its spectrograph design, he believed several factors helped the Penn State proposal prevail.

His team has worked for several years on another advanced spectrograph for the Hobby-Eberly Telescope in Texas, one that required complex vacuum-sealed and very cold temperature construction.  Although the challenges slowed the design, the team ultimately succeeded in demonstrating the environmental stability in the lab.  So Penn State had a track record. What’s more, the school and its Center for Exoplanets and Habitable Worlds have a history of working in an interdisciplinary manner, and have been part of several NASA Astrobiology Institute projects. (The instrument has a blog of its own: NEID.)

The Kitt Peak observatory, which saw first light in 1994, has been the sight of many discoveries, but in recent years has faced cutbacks in NSF funding.  There was some discussion of reducing its use, and the NASA-NSF decision t0 upgrade the spectrograph was in part an effort to make it highly relevant again.  And given the scientific need to confirm so many planets — a need that will grow substantially after TESS launches in 2017 or 2018 and begins sending back information on thousands of additional transiting exoplanets — enhancing the capabilities of the Kitt Peak 3.5 meter telescope made sense.

Kitt Peak is unusual in being open to all comers with a great proposal, whether they’re from the U.S. or abroad.  The Penn State team and partners will get a certain number of dedicated night to observe, but many others will be allocated through competitive reviews.  And so when NEID is completed, astronomers from around will have a shot at using this state-of-the-art planet finder.

 

 

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The Habitable Zone Gets Poked, Tweaked and Stretched to the Limits

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To find another planet like Earth, astronomers are focusing on the "Goldilocks" or habitable zone around stars--where it's not too hot and not too cold for liquid water to exist on the surface. (NASA)
To find another planet like Earth, astronomers are focusing on the “Goldilocks” or habitable zone around stars–where it’s not too hot and not too cold for liquid water to exist on the surface. (NASA)

For more than 20 years now — even before the first detection of an extra-solar planet — scientists have posited, defined and then debated the existence and nature of a habitable zone.  It’s without a doubt a central scientific concept, and  the idea has caught on with the public (and the media) too.  The discovery of “habitable zone planets” has become something of a staple of astronomy and astrophysics.

But beneath the surface of this success is a seemingly growing discomfort about how the term is used. Not only do scientists and the general public have dissimilar understandings of what a habitable zone entails, but scientists have increasingly divergent views among themselves as well.

And all this is coming to the fore at a time when a working definition of the habitable zone is absolutely essential to planning for what scientists and enthusiasts hope will be a long-awaited major space telescope focused first and foremost on exoplanets.  If selected by NASA as a flagship mission for the 2030s, how such a telescope is designed and built will be guided by where scientists determine they have the best chance of finding signs of extraterrestrial life — a task that has ironically grown increasingly difficult as more is learned about those distant solar systems and planets.

Most broadly, the habitable zone is the area around a star where orbiting planets could have conditions conducive to life.  Traditionally, that has mean most importantly orbiting far enough from a star that it doesn’t become a desiccated wasteland and close enough that it is not forever frozen.  In this broad definition, the sometimes presence of liquid water on the surface of a planet is the paramount issue in terms of possible extraterrestrial life.

 The estimated habitable zones of A stars, G stars and M stars are compared in this diagram. More refinement is needed to better understand the size of these zones. Image credit: NASA/JPL-Caltech/MSSS.

The estimated habitable zones of A stars, G stars and M stars are compared in this diagram. More refinement is needed to better understand the size of these zones. Image credit: NASA/JPL-Caltech/MSSS.

It was James Kasting of Penn State University, Daniel Whitmire, then of Louisiana State University, and Ray Reynolds of NASA’s Ames Research Center who defined the modern outlines of a habitable zone, though others had weighed in earlier.  But Kasting and the others wrote with greater detail and proposed a model that took into account not only distance from the host star, but also the presence of planetary systems that could maintain relatively stable climates by cycling essential compounds.

Their concept became something of a consensus model, and remains an often-used working definition.

But with the detection now of thousands of exoplanets, as well as a better understanding of potential habitability in our solar system and the workings of atmospheric gases around planets, some scientists argue the model is getting outdated.  Not wrong, per se, but perhaps not broad enough to account for the flood of planetary and exoplanetary research and discovery since the early 1990s.

Consider, first our own habitable zone:  Two bodies often discussed as potentially habitable are the moons Europa and Enceladus. Both are far from the solar system’s traditional habitable zone, and are heated by gravitational forces from Jupiter and Saturn.

And then there’s the Mars conundrum.  The planet, now viewed as unable to support life on the surface, is currently within the range of our sun’s habitable zone.  Yet when Mars was likely quite wet and warmer and “habitable” some 3.5 billion years ago — as determined by the Curiosity rover team — it was outside the traditional habitable zone because the sun was less luminous and so Mars would ostensibly be frozen.

Remnants of an ancient alluvial fan have been found at Gale Crater, Mars, indicating that water flowed there for long periods of time billions of years ago.
Remnants of an ancient alluvial fan have been found at Gale Crater, Mars, indicating that water flowed there for long periods of time billions of years ago. Traditional habitable zone models cannot account for this wet and warm period on ancient Mars.  (NASA/JPL-Caltech)

Just as the source of heat keeping water on the moons liquid is not the sun, scientists have also proposed that even giant and distant planets with thick atmospheres of molecular hydrogen, a powerful greenhouse gas, could maintain liquid water on their surfaces.  Some have suggested that a hydrogen-rich atmosphere could keep a planet ten times further from the sun than Earth warm enough for possible life.

It was Raymond Pierrehumbert  at University of Chicago and Eric Gaidos of the University of Hawaii who first proposed this possibility in 2011, but others have taken it further.  Perhaps most forcefully has been Sara Seager at MIT, who has argued that the exoplanet community’s definition of a habitable zone needs to be broadened to keep up with new thinking and discoveries.  This is what she wrote in an influential 2013 Science paper:

“Planet habitability is planet specific, even with the main imposed criterion that surface liquid water must be present. This is because the huge range of planet diversity in terms of masses, orbits, and star types should extend to planet atmospheres and interiors, based on the stochastic nature of planet formation and subsequent evolution. The diversity of planetary systems extends far beyond planets in our solar system. The habitable zone could exist from about 0.5 AU out to 10 AU (astronomical units, the distance from the sun to the Earth) for a solar-type star, or even beyond, depending on the planet’s interior and atmosphere characteristics. As such, there is no universal habitable zone applicable to all exoplanets.”

Seager even makes room for the many rogue planet floating unconnected to a solar system as possible candidates, with the same kind of warming deep hydrogen covering that Pierrehumbert proposed. Clearly, her goal is to add exoplanets that are far less like Earth to the possible habitable mix.

 

In this artist's concept shows "The Behemoth," an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. The hydrogen is evaporating from the planet due to extreme radiation from the star, but on many exoplanets it remains a thick covering. (NASA, ESA, and G. Bacon, STScI)
In this artist’s concept shows “The Behemoth,” an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. The hydrogen is evaporating from the planet due to extreme radiation from the star, but on many exoplanets it remains a thick covering. (NASA, ESA, and G. Bacon, STScI)

Meanwhile, scientists have been adding numerous conditions beyond liquid surface water to enable a planet to turn from a dead to a potentially habitable one.  Kasting and Whitmore did include some of these conditions in their initial 1993 paper, but the list is growing.  A long-term stable climate is considered key, for instance, and that in turn calls for the presence of features akin to plate tectonics, volcanoes, magnetic fields and cycling into the planet interior of carbon, silicates and more.  Needless to say, these are not planetary features scientists will be able to identify for a long time to come.

So the disconnect grows between how exoplanet hunters and researchers use the term “habitable zone” and how the public understands its meaning.  Scientists describe a myriad of conditions and add that they are “necessary but not sufficient.”  Meanwhile, many exoplanet enthusiasts in the public are understandably awaiting a seemingly imminent discovery of extraterrestrial life on one of the many habitable zone planets announced.  (In fairness, no Earth-sized planet orbiting a sun-like star has been identified so far.)

Kasting, for one, does not see all this questioning of the necessary qualities of a habitable zone as a problem.

“Push back is what scientists do; we’re brought up to question authority.  My initial work is over 20 years old and a lot has been learned since then.  Not all things that are written down are correct.”

James Kasting of Penn State University, a pioneer in defining a habitable zone.
James Kasting of Penn State University, a pioneer in defining a habitable zone.

But in this case, he says, a lot of the conventional habitable zone concept is pretty defensible.

What’s more, it’s practical and useful.  While not discounting the possibility of life on exo-moons, on giant planets surrounded by warming molecular hydrogen or other possibilities, he says that the technical challenges to making a telescope that could capture the light necessary to analyze these moons or far-from-their-star planets would be so faint as to be undetectable given today’s (or even tomorrow’s) technology.  With those two exoplanet-focused telescopes (LUVOIR and Hab-Ex) now under formal study for a possible mission in the 2030s, Kasting thinks it’s essential to think inside, rather than outside, the box.

“I think that when the teams sit down and think about the science and technology of those projects, our habitable zone is the only one that make sense.  If you design a telescope to capture possible evidence of life as far out as 10 AU, you give up capability to study with the greatest precision planets close in the traditional habitable zone.  That doesn’t mean the telescope can’t look for habitable worlds outside the traditional habitable zone, but but don’t design the telescope with that as a high priority.  Better to focus on what we know does exist.”

Coming soon:  The Habitability Inde

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Movement in The Search For ExoLife

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A notional version of an observatory for the 2030s that could provide revolutionary direct imaging of exoplanets. GSFC/JPL/STScI
A notional version of an observatory for the 2030s that could provide revolutionary direct imaging of exoplanets. GSFC/JPL/STScI

Assuming for a moment that life exists on some exoplanets, how might researchers detect it?

This is hardly a new question.  More than ten years ago, competing teams of exo-scientists and engineers came up with proposals for a NASA flagship space observatory capable of identifying possible biosignatures on distant planets. No consensus was reached, however, and no mission was developed.

But early this year, NASA Astrophysics Division Director Paul Hertz announced the formation of four formal Science and Technology Definition Teams to analyze proposals for a grand space observatory for the 2030s.  Two of them in particular would make possible the kind of super-high resolution viewing needed to understand the essential characteristics of exoplanets.  As now conceived, that would include a capability to detect molecules in distant atmospheres that are associated with living things.

These two exo-friendly missions are the Large Ultraviolet/Optical/Infrared (LUVOIR) Surveyor and the Habitable Exoplanet (HabEx) Imaging Mission.   Both would be on the scale of, and in the tradition of, scientifically and technically ground-breaking space observatories such as the Hubble and the James Webb Space Telescope, scheduled to launch in 2018.  These flagship missions provide once in a decade opportunities to move space science dramatically forward, and not-surprisingly at a generally steep cost.

A simulated spiral galaxy as viewed by Hubble, and the proposed High Definition Space Telescope (HDST) at a lookback time of approximately 10 billion years (z = 2) The renderings show a one-hour observation for each space observatory. Hubble detects the bulge and disk, but only the high image quality of HDST resolves the galaxy’s star-forming regions and its dwarf satellite. The zoom shows the inner disk region, where only HDST can resolve the star-forming regions and separate them from the redder, more distributed old stellar population. Image credit: D. Ceverino, C. Moody, G. Snyder, and Z. Levay (STScI)500 light years away, as imaged by Hubble and potential of the kind of telescope the exoplanet community is working towards.
A simulated spiral galaxy as viewed by Hubble, and as viewed by the kind of high definition space telescope now under study.   Hubble detects the bulge and disk, but only the high definition image resolves the galaxy’s star-forming regions and its dwarf satellite. The zoom shows the inner disk region, where only high definition can resolve the star-forming regions and separate them from the redder, more distributed old stellar population. (D. Ceverino, C. Moody, G. Snyder, and Z. Levay (STScI)

Because the stakes are so high, planning and development takes place over decades — twenty years is the typical time elapsed between the conception of a grand flagship mission and its launch.  So while what is happening now with the science and technology definition teams  is only a beginning — albeit one with quite a heritage already — it’s an essential, significant and broadly-supported start.  Over the next three years, the teams will undertake deep dives into the possibilities and pitfalls of LUVOIR and HabEx, as well as the two other proposals.  There’s a decent chance that a version of one of the four will become a reality.

Aki Roberge, an astrophysicist at the Goddard Space Flight Center and staff scientist of the LUVOIR study, said that the explicit charge to the teams is to cooperate rather than compete.  Any of the four observatories under consideration, she said, would enable transformative science. But from an exoplanet perspective, the possibilities she described are pretty remarkable.

“What we’re aiming for is the capability to really search for the true Earth analogues out there, the Earth-sized planets in the habitable zones of sun-like stars.  We need to understand their atmospheres, their climates, their compositions.  And ultimately, the goal is to search for life.”

The co-chair of the HabEx team, Bertrand Menneson of the Jet Propulsion Lab, said the goals are the same:  A major jump forward in our ability to understand exoplanets and a serious effort to find life.

actual image of venus crossing in front of the sun. Exoplanets will not be imaged like this in our lifetimes, but this is the goal.
Actual image of Venus crossing in front of the sun in 2012 taken by NASA’s Solar Dynamics Observatory. Exoplanets will not be imaged like this in our lifetimes, but this is the ultimate goal.

The field of exoplanet detection and research has exploded over the past two decades, with an essential boost from increasingly capable observatories on Earth and in space.  With at least three more major exoplanet-friendly space telescopes scheduled (or planned) for the next decade — as well as first light at several enormous ground-based mirrors — the brisk pace of discoveries is sure to continue.

So why are so many scientists in the field convinced that a grand, Flagship-class NASA space observatory is essential, and that it needs to be developed and built ground-up with exoplanet research in mind?  Can’t the instruments in use today, and planned for the next decade, provide the kind of observing power needed to continue making breakthroughs?

Well, no, they can’t and won’t.  That has been the conclusion of numerous studies over the years, and most recently an in-depth effort by the Association of Universities for Research in Astronomy (AURA,)   http://www.hdstvision.org/report which last summer called for development of a 12-meter (about 44 feet across) High Definition Space Telescope with the super high resolution needed to study exoplanets.  Generally speaking, a larger light-collecting mirror allows astronomers and astrophysicists to see further and better.

 

A direct, to-scale, comparison between the primary mirrors of the Hubble Space Telescope, James Webb Space Telescope, and the proposed High Definition Space Telescope (HDST). In this concept, the HDST primary is composed of 36 1.7 meter segments. Smaller segments could also be used. An 11 meter class aperture could be made from 54 1.3 meters segments. Image credit: C. Godfrey (STScI)
A direct, to-scale, comparison between the primary mirrors of the Hubble Space Telescope, James Webb Space Telescope, and the High Definition Space Telescope (HDST) proposed by the AURA group. In this concept, the HDST primary is composed of 36 1.7 meter segments.  The LUVOIR mirror under consideration is in the eight to twelve meters range. C. Godfrey (STScI)

The group, headed by Julianne Dalcanton of the University of Washington and Sara Seager of MIT, began with this overview of the state of play when it comes to exoplanets, instruments, and what is possible now and might be in the future:

While we now have a small sample of potentially habitable planets around other stars, our current telescopes lack the power to confirm that these alien worlds are truly able to nurture life. This small crop of worlds may have temperate, hospitable surface conditions, like Earth. But they could instead be so aridly cold that all water is frozen, like on Mars, or so hot that all potential life would be suffocated under a massive blanket of clouds, like on Venus. Our current instruments cannot tell the difference for the few rocky planets known today, nor in general, for the larger samples to be collected in the future. Without better tools, we simply cannot see their atmospheres and surfaces, so our knowledge is limited to only the most basic information about the planet’s mass and/ or size, and an estimate of the energy reaching the top of the planet’s atmosphere. But if we could directly observe exoplanet atmospheres, we could search for habitability indicators (such as water vapor from oceans) or for signs of an atmosphere that has been altered by the presence of life (by searching for oxygen, methane, and/or ozone).

A central goal for both LUVOIR and HabEx is to provide that “seeing” through much more sophisticated direct imaging — that is, capturing the actual reflected light from exoplanets rather than relying on indirect techniques and measurements.  The many indirect methods of finding and studying exoplanets have played and will continue to play an essential role.  But there is now a community consensus that next generation direct imaging from space is the gold standard.

 

Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars, by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)
There are more than 4,000 Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars.  This graphic shows their distribution by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)

That a major space observatory for the 2030s just might be exoplanet-focused reflects a definite maturing of the field.  From a science perspective, the discoveries of the Kepler mission in particular made clear that exoplanets are everywhere, and not infrequently orbiting in habitable zones.  The work of the Curiosity rover on Mars, and especially the conclusion that the planet once was wet and “habitable,” added to the general interest and excitement about possible life beyond Earth.

And then there are the lessons learned from the earlier bruising battles among exoplanet scientists, who had developed a reputation for serious in-fighting.  THEIA, the Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy, was put forward as a flagship direct imaging mission in 2010, when the Astronomy and Astrophysics Decadal Survey that sets priorities for the field was being put together by the National Academy of Sciences.  But THEIA was not adopted.

A cartoon from Chas Beichman’s ExoPAG presentation illustrates the infighting within the exoplanet science community during the 2010 decadal survey, with cosmologists, represented by “dark energy” to the side, ready to reap the benefits of that debate.
A cartoon from a exoplanet science presentation illustrates the infighting within the exoplanet science community during the 2010 decadal survey, with cosmologists, represented by “dark energy” to the side, ready to reap the benefits of that debate. ( Chas Beichman)

With the 2020 Decadal Survey on the horizon, exoplanet scientists have tried to limit conflicts and to work with the larger astronomy community.  The formal NASA/community study group, the Exoplanet Exploration Program Analysis Group (ExoPAG), brought two related groups together and ultimately recommended the intensified study for LUVOIR, HabEx and the two other proposals —  which focus on black holes, ancient galaxy formation, and other aspects of the early cosmos.  https://exep.jpl.nasa.gov/files/exep/ExoPAG_Large_Missions.pdf

When completed, the studies will go to the National Academy of Sciences for further review, discussion, and ultimately a recommendation to NASA regarding which project should go forward.

The leader of the ExoPAG  group was astronomer Scott Gaudi of Ohio State University, who specializes in characterizing exoplanets but played no favorites in the ExoPAG report and recommendations.

“What we want is to set up a fair process of intense review so the most compelling science can be chosen to go forward.  At this point, we don’t know if the necessary technologies will be available in time, and we don’t know what the costs will be.  There’s only so much money that comes from NASA for our (astrophysics) community, and maybe a top choice will cost more than the community is willing to spend.  So there are so many factors to consider.”

(The LUVOIR mission is generally considered to be somewhat more ambitious than HabEx, and would require a larger telescope mirror — greater than 8 meters across –and more funding.  Flagship missions are expensive, as NASA learned once again with the James Webb telescope, which will have cost $8.8 billion by the time of its scheduled launch.)

I asked Gaudi if the seemingly substantial public interest in exoplanets could play any role in subsequent decision-making, and he replied that it possibly would.  “In the past five or ten years, exoplanets have become a prominent topic for sure.  And the public is clearly very, very interested in that topic.”  But that public interest, he said, won’t mean much if the science and technical feasibility isn’t there.

Scott Gaudi, chairman of ExoPAG in 2015.
Scott Gaudi, chairman of ExoPAG in 2015.

We won’t know for some years if the stars will align in a way that will lead to a major observatory with direct imaging and exoplanets at its center.  But for those active in the field, the opportunity to take part in a major effort to formally determine its scientific merit and feasibility is irresistible.

Shawn Domagal-Goldman, a research space scientist at Goddard, was selected to be a deputy on the LUVOIR science and technology team, which he sees as a much-anticipated “proof of concept” effort for the exoplanet research of the future.

Between 12 and 18 scientists and engineers will be selected by NASA headquarters for each team, and Domagal-Goldman said it’s essential that they make up a broad and inter-disciplinary group, including people from industry.  Scientists from abroad not associated with an American institution can’t be formal members, but they can observe and may become more involved if their national space agencies decide to join in the effort. He encourages researchers — from newly minted PhDs to career scientists — to nominate themselves to join.

“Nobody gets paid for this, it’s a labor of love,” he said.  “But what would be more satisfying than having some of your intellectual contribution go into the formulation of missions like these.

“Direct imaging of exoplanets is clearly a direction where the community is headed. These are the missions of the future in one form or another, and if you’re a PhD or postdoc who’s qualified, this could be your career.”

Of course, it just might make the greatest discovery of modern science — finding life beyond Earth.

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