Technosignatures and the Search for Extraterrestrial Intelligence

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A rendering of a potential Dyson sphere, named after Freeman A. Dyson. As proposed by the physicist and astromomer decades ago, they would collect solar energy on a solar system wide scale for highly advanced civilizations. (SentientDevelopments.com)

The word “SETI” pretty much brings to mind the search for radio signals come from distant planets, the movie “Contact,” Jill Tarter, Frank Drake and perhaps the SETI Institute, where the effort lives and breathes.

But there was a time when SETI — the Search for Extraterrestrial Intelligence — was a significantly broader concept, that brought in other ways to look for intelligent life beyond Earth.

In the late 1950s and early 1960s — a time of great interest in UFOs, flying saucers and the like — scientists not only came up with the idea of searching for distant intelligent life via unnatural radio signals, but also by looking for signs of unexpectedly elevated heat signatures and for optical anomalies in the night sky.

The history of this search has seen many sharp turns, with radio SETI at one time embraced by NASA, subsequently de-funded because of congressional opposition, and then developed into a privately and philanthropically funded project of rigor and breadth at the SETI Institute.  The other modes of SETI went pretty much underground and SETI became synonymous with radio searches for ET life.

But this history may be about to take another sharp turn as some in Congress and NASA have become increasingly interested in what are now called “technosignatures,” potentially detectable signatures and signals of the presence of distant advanced civilizations.  Technosignatures are a subset of the larger and far more mature search for biosignatures — evidence of microbial or other primitive life that might exist on some of the billions of exoplanets we now know exist.

And as a sign of this renewed interest, a technosignatures conference was scheduled by NASA at the request of Congress (and especially retiring Republican Rep. Lamar Smith of Texas.)  The conference took place in Houston late last month, and it was most interesting in terms of the new and increasingly sophisticated ideas being explored by scientists involved with broad-based SETI.

“There has been no SETI conference this big and this good in a very long time,” said Jason Wright, an astrophysicist and professor at Pennsylvania State University and chair of the conference’s science organizing committee.  “We’re trying to rebuild the larger SETI community, and this was a good start.”

 

At this point, the search for technosignatures is often likened to that looking for a needle in a haystack. But what scientists are trying to do is define their haystack, determine its essential characteristics, and learn how to best explore it. (Wiki Commons)

 

During the three day meeting in Houston, scientists and interested private and philanthropic reps. heard talks that ranged from the trials and possibilities of traditional radio SETI to quasi philosophical discussions about what potentially detectable planetary transformations and by-products might be signs of an advanced civilization. (An agenda and videos of the talks are here.)

The subjects ranged from surveying the sky for potential millisecond infrared emissions from distant planets that could be purposeful signals, to how the presence of certain unnatural, pollutant chemicals in an exoplanet atmosphere that could be a sign of civilization.  From the search for thermal signatures coming from megacities or other by-products of technological activity, to the possible presence of “megastructures” built to collect a star’s energy by highly evolved beings.

Michael New is Deputy Associate Administrator for Research within NASA’s Science Mission Directorate. He was initially trained in chemical physics. (NASA)

All but the near infrared SETI are for the distant future — or perhaps are on the science fiction side — but astronomy and the search for distant life do tend to move forward slowly.  Theory and inference most often coming well before observation and detection.

So thinking about the basic questions about what scientists might be looking for, Wright said, is an essential part of the process.

Indeed, it is precisely what Michael New, Deputy Associate Administrator for Research within NASA’s Science Mission Directorate, told the conference. 

He said that he, NASA and Congress wanted the broad sweep of ideas and research out there regarding technosignatures, from the current state of the field to potential near-term findings, and known limitations and possibilities.

“The time is really ripe scientifically for revisiting the ideas of technosignatures and how to search for them,” he said.

He offered the promise of NASA help  (admittedly depending to some extent on what Congress and the administration decide) for research into new surveys, new technologies, data-mining algorithms, theories and modelling to advance the hunt for technosignatures.

 

Crew members aboard the International Space Station took this nighttime photograph of much of the Atlantic coast of the United States. The ability to detect the heat and light from this kind of activity on distant exoplanets does not exist today, but some day it might and could potentially help discover an advanced extraterrestrial civilization. (NASA)

 

Among the several dozen scientists who discussed potential signals to search for were the astronomer Jill Tarter, former director of the Center for SETI Research, Planetary Science Institute astrobiologist David Grinspoon and University of Rochester astrophysicist Adam Frank.  They all looked at the big picture, what artifacts in atmospheres, on surfaces and perhaps in space that advanced civilizations would likely produce by dint of their being “advanced.”

All spoke of the harvesting of energy to perform work as a defining feature of a technological planet, with that “work” describing transportation, construction, manufacturing and more.

Beings that have reached the high level of, in Frank’s words, exo-civilization produce heat, pollutants, changes to their planets and surroundings in the process of doing that work.  And so a detection of highly unusual atmospheric, thermal, surface and orbital conditions could be a signal.

One example mentioned by several speakers is the family of chemical chloroflourohydrocarbons (CFCs,)  which are used as commercial refrigerants, propellants and solvents.

Astronomner Jill Tarter is an iconic figure in the SETI world and led the SETI Institute for 30 years. (AFP)

These CFCs are a hazardous and unnatural pollutant on Earth because they destroy the ozone layer, and they could be doing something similar on an exoplanet.  And as described in the conference, the James Webb Space Telescope — once it’s launch and working — could most likely detect such an atmospheric compound if it’s in high concentration and the project was given sufficient telescope time.

A similar single finding described by Tarter that could be revolutionary is the radioactive isotope tritium, which is a by-product of the nuclear fusion process.  It has a short half-life and so any distant discovery would point to a recent use of nuclear energy (as long as it’s not associated with a recent supernova event, which can also produce tritium.)

But there many other less precise ideas put forward.

Glints on the surface of planets could be the product of technology,  as might be weather on an exoplanet that has been extremely well stabilized, modified planetary orbits and chemical disequilibriums in the atmosphere based on the by-products of life and work.  (These disequilibriums are a well-established feature of biosignature research, but Frank presented the idea of a technosphere which would process energy and create by-products at a greater level than its supporting biosphere.)

Another unlikely but most interesting example of a possible technosignature put forward by Tarter and Grinspoon involved the seven planets of the Trappist-1 solar system, all tidally locked and so lit on only one side.  She said that they could potentially be found to be remarkably similar in their basic structure, alignment and dynamics. As Tarter suggested, this could be a sign of highly advanced solar engineering.

 

Artist rendering of the imagined Trappist-1 solar system that had been terraformed to make the planets similar and habitable.  The system is one of the closest found to our own — about 40 light years.

 

Grinspoon seconded that notion about Trappist-1, but in a somewhat different context.

He has worked a great deal on the question of today’s anthroprocene era — when humans actively change the planet — and he expanded on his thinking about Earth into the galaxies.

Grinspoon said that he had just come back from Japan, where he had visited Hiroshima and its atomic bomb sites, and came away with doubts that we were the “intelligent” civilization we often describe ourselves in SETI terms.  A civilization that may well self destruct — a fate he sees as potentially common throughout the cosmos — might be considered “proto-intelligent,” but not smart enough to keep the civilization going over a long time.

Projecting that into the cosmos, Grinspoon argued that there may well be many such doomed civilizations, and then perhaps a far smaller number of those civilizations that make it through the biological-technological bottleneck that we seem to be facing in the centuries ahead.

These civilizations, which he calls semi-immortal, would develop inherently sustainable methods of continuing, including modifying major climate cycles, developing highly sophisticated radars and other tools for mitigating risks, terraforming nearby planets, and even finding ways to evolve the planet as its place in the habitable zone of its host star becomes threatened by the brightening or dulling of that star.

The trick to trying to find such truly evolved civilizations, he said, would be to look for technosignatures that reflect anomalous stability and not rampant growth. In the larger sense, these civilizations would have integrated themselves into the functioning of the planet, just as oxygen, first primitive and then complex life integrated themselves into the essential systems of Earth.

And returning to the technological civilizations that don’t survive, they could produce physical artifacts that now permeate the galaxy.

 

MeerKAT, originally the Karoo Array Telescope, is a radio telescope consisting of 64 antennas now being tested and verified in the Northern Cape of South Africa. When fully functional it will be the largest and most sensitive radio telescope in the southern hemisphere until the Square Kilometre Array is completed in approximately 2024. (South African Radio Astronomy Observatory)

 

This is exciting – the next phase Square kilometer Array (SKA2) will be able to detect Earth-level radio leakage from nearby stars. (South African Radio Astronomy Observatory)

 

While the conference focused on technosignature theory, models, and distant possibilities, news was also shared about two concrete developments involving research today.

The first involved the radio telescope array in South Africa now called MeerKAT,  a prototype of sorts that will eventually become the gigantic Square Kilometer Array.

Breakthrough Listen, the global initiative to seek signs of intelligent life in the universe, would soon announce the commencement of  a major new program with the MeerKAT telescope, in partnership with the South African Radio Astronomy Observatory (SARAO).

Breakthrough Listen’s MeerKAT survey will examine a million individual stars – 1,000 times the number of targets in any previous search – in the quietest part of the radio spectrum, monitoring for signs of extraterrestrial technology. With the addition of MeerKAT’s observations to its existing surveys, Listen will operate 24 hours a day, seven days a week, in parallel with other surveys.

This clearly has the possibility of greatly expanded the amount of SETI listening being done.  The SETI Institute, with its radio astronomy array in northern California and various partners, have been listening for almost 60 years, without detecting a signal from our galaxy.

That might seem like a disappointing intimation that nothing or nobody else is out there, but not if you listen to Tarter explain how much listening has actually been done.  Almost ten years ago, she calculated that if the Milky Way galaxy and everything in it was an ocean, then SETI would have listened to a cup full of water from that ocean.  Jason Wright and his students did an updated calculation recently, and now the radio listening amounts to a small swimming pool within that enormous ocean.

 

The NIROSETI team with their new infrared detector inside the dome at Lick Observatory. Left to right: Remington Stone, Dan Wertheimer, Jérome Maire, Shelley Wright, Patrick Dorval and Richard Treffers. (Laurie Hatch)

The other news came from Shelley Wright of the University of California, San Diego, who has been working on an optical SETI instrument for the Lick Observatory.

The Near-Infrared Optical SETI (NIROSETI) instrument she and her colleagues have developed is the first instrument of its kind designed to search for signals from extraterrestrials at near-Infrared wavelengths. The near-infrared regime is an excellenr spectral region to search for signals from extraterrestrials, since it offers a unique window for interstellar communication.

The NIROSETI instrument utilizes two near-infrared photodiodes to be able to detect artificial, very fast (nanosecond) pulses of infrared radiation.

The NIROSETI instrument, which is mounted on the Nickel telescope at Lick Observatory, splits the incoming near-infrared light onto two channels, and then checks for coincident events, which indicate signals that are identified by both detectors simultaneously.

Jason Wright is an assistant professor of astronomy and astrophysics at Penn State. His reading list is here.

Wright of Penn State was especially impressed by the project, which he said can look at much of the sky at once and was put together with on very limited budget.

Wright, who teaches a course on SETI at Penn State and is a co-author of a recent paper trying to formalize SETI terminology, said his own take-away from the conference is that it may well represent an important and positive moment in the history of technosignatures.

“Without NASA support, the whole field has lacked the normal structure by which astronomy advances,” he said.  “No teaching of the subject, no standard terms, no textbook to formalize findings and understandings.

“The Seti Institiute carried us through the dark times, and they did that outside of normal, formal structures. The Institute remains essential, but hopefully that reflex identification will start to change.”

 

Participants in the technosignatures conference in Houston last month, the largest SETI gathering in years.  And this one was sponsored by NASA and put together by the NExSS for Exoplanet Systems Science (NExSS,)  an interdisciplinary agency initiative. (Delia Enriquez)
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Water Worlds, Aquaplanets and Habitability

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This artist rendering may show a water world — without any land — or an aquaplanet with lots of more shallow water around a rocky planet. (NASA)

 

The more exoplanet scientists learn about the billions and billions of celestial bodies out there, the more the question of unusual planets — those with characteristics quite different from those in our solar system — has come into play.

Hot Jupiters, super-Earths, planets orbiting much smaller red dwarf stars — they are all grist for the exoplanet mill, for scientists trying to understand the planetary world that has exploded with possibilities and puzzles over the past two decades.

Another important category of planets unlike those we know are the loosely called “water worlds” (with very deep oceans) and their “aquaplanet” cousins (with a covering of water and continents) but orbiting stars very much unlike our sun.

Two recent papers address the central question of habitability in terms of these kind of planets — one with oceans and ice hundreds of miles deep, and one particular and compelling planet (Proxima Centauri b, the exoplanet closest to us) hypothesized to have water on its surface as it orbits a red dwarf star.

The question the papers address is whether these watery worlds might be habitable.  The conclusions are based on modelling rather than observations, and they are both compelling and surprising.

In both cases — a planet with liquid H20 and ice many miles down, and another that probably faces its red dwarf sun all or most of the time — the answers from modelers is that yes, the planets could be habitable.   That is very different from saying they are or even might be inhabited.  Rather,  the conclusions are based on computer models that take into account myriad conditions and come out with simulations about what kind of planets they might be.

This finding of potential watery-world habitability is no small matter because predictions of how planets form point to an abundance of water and ice in the planetesimals that grow into planets.

As described by Eric Ford, co-author of one of the papers and a professor of astrophysics at Pennsylvania State University, “Many scientists anticipate that planets with oceans much deeper than Earths could be a common outcome of planet formation. Indeed, one of the puzzling properties of Earth is that it has oceans that are just skin deep” compared to the radius of the planet.

“While some planets very close to their star might loose all their water, it would take a delicate balancing act to remove many ocean’s worth of water and to leave a planet with oceans as shallow as those on Earth.”

An interesting place to start.

 

Artist’s conception of a planet covered with a global ocean. A new study finds that these wate rworlds could maintain stable climates and perhaps sustain life under certain conditions. (ESO/M. Kornmesser)

 

It should first be said that many scientists are dubious that extreme water worlds can support life or can support detectable life.  My colleague Elizabeth Tasker wrote a column — Can You Overwater a Planet? — focused on this view last year.

The first of the two new exoplanets/ocean papers involves planets with very deep oceans. Written by Edwin Kite of the University of Chicago and Ford of Penn State, the paper in the Astrophysical Journal concludes that even a planet with such super deep oceans could — under certain conditions — provide habitable conditions.

This finding is at odds with previous simulations, and Kite says that is part of its significance. The scientific community has largely assumed that planets covered in a deep ocean would not support the cycling of minerals and gases that keeps the climate stable on Earth, and thus wouldn’t be friendly to life.

But the Kite and Ford study found that ocean planets (with 10 to 1000 times as much water as Earth) could remain habitable much longer than previously assumed. The authors performed more than a thousand simulations to reach that conclusion.

Eric Ford is a professor astrophysics at Penn State and a specialist in planet formation. (Penn State)

“This really pushes back against the idea you need an Earth clone—that is, a planet with some land and a shallow ocean,” said Edwin Kite, assistant professor of geophysical sciences at the University of Chicago and lead author of the study.

Edwin Kite is an assistant professor of planetary sciences at the University of Chicago. (Univ. of Chicago)

Because life needs an extended period to evolve — and because the light and heat on planets can change as their stars age — scientists usually look for planets that have both some water and some way to keep their climates stable over time. The method for achieving this steady state that we know is, of course, how it works on Earth. Over eons, our planet has cooled itself by drawing down atmospheric greenhouse gases into minerals and warms itself up by releasing them via volcanoes.

But this model doesn’t work on a water world, with deep water covering the rock and suppressing volcanoes.

Kite and Ford wanted to know if there was another way to achieve a balance. They set up a simulation with thousands of randomly generated planets, and tracked the evolution of their climates over billions of years.

“The surprise was that many of them stay stable for more than a billion years, just by luck of the draw,” Kite said. “Our best guess is that it’s on the order of 10 percent of them.”

These planets sit in the right location around their stars. They happened to have the right amount of carbon present, and they don’t have too many minerals and elements from the crust dissolved in the oceans that would pull carbon out of the atmosphere. They have enough water from the start, and they cycle carbon between the atmosphere and ocean only, which in the right concentrations is sufficient to keep things stable.

None of this means that such a planet exists — our ability to detect oceans worlds is in its infancy.  The issue is rather that Kite and Ford conclude that a deep ocean planet could potentially be habitable if other conditions were met.

 

Artist rendering of Proxima Centauri b orbiting its red dwarf host star. (ESO/L.Calçada/Nick Risinger)

 

Anthony Del Genio and his team of modelers at NASA’s Goddard Institute for Space Studies in New York used their state-of-the-art climate simulations to look at another aspect of the exoplanet water story, and they chose Proxima Centauri b as their subject.  The roughly Earth-sized planet was discovered in 2016 and is the closest exoplanet to Earth.

Scientists determined early on that it is a rocky (as opposed to gaseous) planet and that it orbits its host star every 11 days.  If that star was as powerful as our sun, there would be no talk of possible habitability on close-in Proxima b.  But the star is a red dwarf and puts out only a fraction of the radiation coming from a host star like our sun.

Still, the case for habitability on Proxima b was initially considered to be weak, in part because the planet is tidally locked by its closeness to the host star.  In other words, it would most likely not spin to create days and nights as it orbits, but rather would have a sun-facing side and a space-facing side — making the temperature differences great.

Our ability to characterize a small planet like Proxima b remains very limited, and so it is unknown whether the planet has water or whether it has an atmosphere.  So those two essential components of the habitability question are missing.

But Del Genio’s team decided to model the dynamics of Proxima b with a presumed ocean, though not one that is many miles deep.  In Earth science parlance, what Del Genio referred to as an “aquaplanet.” And using their sophisticated models, they would simulate “dynamic” oceans with currents like our own, rather than the stationary oceans modeled earlier on exoplanets.

And rather to their surprise, they reported in the journal Astrobiology that their model of ocean behavior showed that the planet would not have only small areas of potential habitability — the earlier proposed habitable “eyeball” scenario — but rather much of the planet could be habitable.  That could include some of the normally space-facing side.

 

One type of possible water world is an “eyeball” planet, where the star-facing side is able to maintain a liquid-water ocean, while the rest of the surface is ice. (Image via eburacum45/DeviantArt)

 

“Our group said let’s hook up an atmosphere to a dynamic ocean rather than a static one,” Del Genio said.  “That way you get ocean currents like those on our coasts, and they move water of different temperatures around.

“If you have a real and dynamic ocean in your model, then we found that the eyeball goes away.  Usually the currents go west to east and they carry warmer water even to the night side.”

Anthony Del Genio, leader of NASA’s GISS team that  uses cutting edge Earth climate models to better understand conditions on exoplanets.

So using this more sophisticated model, not insignificant areas of Proxima b, or any other planet like it orbiting a red dwarf star, could be habitable, they concluded.  But again, that is assuming some pretty big “ifs” — the presence of an ocean and an atmosphere.

And then the team added variables such as a thick nitrogen and carbon atmosphere or a thin one, fresh water or salty water, a planet that is firmly locked and never rotating, or one that rotates a modest amount — giving the dark side some light.  Del Genio said that with all these added factors, a substantial portion of the surface of Proxima b, or a planet like it, would have liquid water and potentially habitable conditions.

This focus on watery worlds — including those that would be extreme compared with Earth today — makes sense in the context of the history of Earth.

While there is no direct evidence of this, many scientists think that the very early Earth was covered for a period of time with water with little or no land.

And then after land appeared, it still took some three billion years for any life form — bacteria, early planets — to colonize the land, and another half billion years for animals to come ashore.  Yet the oceans were long habitable and inhabited, as early a 3.8 billion years ago.

So until astronomy and exoplanet science develop the needed instruments and scientists acquire the observed knowledge of conditions on water worlds, progress will come largely from modelling that tells us what might be possible.

 

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A National Strategy for Finding and Understanding Exoplanets (and Possibly Extraterrestrial Life)

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The National Academies of Science, Engineering and Medicine took an in-depth look at what NASA, the astronomy community and the nation need to grow the burgeoning science of exoplanets — planets outside our solar system that orbit a star. (NAS)

 

An extensive, congressionally-directed study of what NASA needs to effectively learn how exoplanets form and whether some may support life was released today, and it calls for major investments in next-generation space and ground telescopes.  It also calls for the adoption of an increasingly multidisciplinary approach for addressing the innumerable questions that remain unanswered.

While the recommendations were many, the top line calls were for a sophisticated new space-based telescope for the 2030s that could directly image exoplanets, for approval and funding of the long-delayed and debated WFIRST space telescope, and for the National Science Foundation and to help fund two of the very large ground-based telescopes now under development.

The study of exoplanets has seen remarkable discoveries in the past two decades.  But the in-depth study from the private, non-profit National Academies of Sciences, Engineering and Medicine concludes that there is much more that we don’t understand than that we do, that our understandings are “substantially incomplete.”

So the two overarching goals for future exoplanet science are described as these:

 

  • To understand the formation and evolution of planetary systems as products of star formation and characterize the diversity of their architectures, composition, and environments.
  • To learn enough about exoplanets to identify potentially habitable environments and search for scientific evidence of life on worlds orbiting other stars.

 

Given the challenge, significance and complexity of these science goals, it’s no wonder that young researchers are flocking to the many fields included in exoplanet science.  And reflecting that, it is perhaps no surprise that the NAS survey of key scientific questions, goals, techniques, instruments and opportunities runs over 200 pages. (A webcast of a 1:00 pm NAS talk on the report can be accessed here.)

 


Artist’s concept showing a young sun-like star surrounded by a planet-forming disk of gas and dust.
(NASA/JPL-Caltech/T. Pyle)

These ambitious goals and recommendations will now be forwarded to the arm of the National Academies putting together 2020 Astronomy and Astrophysics Decadal Survey — a community-informed blueprint of priorities that NASA usually follows.

This priority-setting is probably most crucial for the two exoplanet direct imaging missions now being studied as possible Great Observatories for the 2030s — the paradigm-changing space telescopes NASA has launched almost every decade since the 1970s.

HabEx (the Habitable Exoplanet Observatory) and LUVOIR (the Large UV/Optical/IR Surveyor) are two direct-imaging exoplanet projects in conception phase that would indeed significantly change the exoplanet field.

Both would greatly enhance scientists’ ability to detect and characterize exoplanets. But the more ambitious LUVOIR in particular, would not only find many exoplanets in all stages of formation, but could readily read chemical components of the atmospheres and thereby get clear data on whether the planet was habitable or even if it supported life.  The LUVOIR would provide either an 8 meter or a record-breaking 15-meter space telescope, while HabEx would send up a 4 meter mirror.

HabEx and LUVOIR are competing with two other astrophysics projects for that Great Observatory designation, and so NAS support now and prioritizing later is essential if they are to become a reality.

 

An artist notional rendering of an approximately 15-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. (NASA)

These two potential Great Observatories will be costly and would take many years to design and build.  As the study acknowledges and explains, “While the committee recognized that developing a direct imaging capability will require large financial investments and a long time scale to see results, the effort will foster the development of the scientific community and technological capacity to understand myriad worlds.”

So a lot is at stake.  But with budget and space priorities in flux, the fate of even the projects given the highest priority in the Decadal Survey remains unclear.

That’s apparent in the fact that one of the top recommendations of today’s study is the funding of the number one priority put forward in the 2010 Astronomy and Astrophysics Decadal Survey — the Wide Field Infrared Survey Telescope (WFIRST.)

The project — which would boost the search for exoplanets further from their stars than earlier survey mission using microlensing– was cancelled in the administration’s proposed 2019 federal budget.  Congress has continued funding some development of this once top priority, but its future nonetheless remains in doubt.

WFIRST could have the capability of directly imaging exoplanets if it were built with technology to block out the blinding light of the star around which exoplanets would be orbiting — doing so either with internal coronagraph or a companion starshade.  This would be novel technology for a space-based telescope, and the NAS survey recommends it as well.

 

An artist’s rendering of a possible “starshade” that could be launched to work with WFIRST or another space telescope and allow the telescope to take direct pictures of other Earth-like planets. (NASA/JPL-Caltech)

The list of projects the study recommends is long, with these important additions:

That “ground-based astronomy – enabled by two U.S.-led telescopes – will also play a pivotal role in studying planet formation and potentially terrestrial worlds, the report says. The future Giant Magellan telescope (GMT) and proposed Thirty Meter Telescope (TMT) would allow profound advances in imaging and spectroscopy – absorption and emission of light – of entire planetary systems. They also could detect molecular oxygen in temperate terrestrial planets in transit around close and small stars, the report says.”

The committee concluded that the technology road map to enable the full potential of GMT and TMT in the study of exoplanets is in need of investments, and should leverage the existing network of U.S. centers and laboratories. To that end, the report recommends that the National Science Foundation invest in both telescopes and their exoplanet instrumentation to provide all-sky access to the U.S. community.

And for another variety of ground-based observing the study called for the funding of a project to substantially increase the precision of instruments that find and measure exoplanets using the detected “wobble” of the host star.  But stars are active with or without a nearby exoplanet, and so it has been difficult to achieve the precision that astronomers using this “radial velocity” technique need to find and characterize smaller exoplanets.

Several smaller efforts to increase this precision are under way in the U.S., and the European Southern Observatory has a much larger project in development.

Additionally, the report recommends that the administrators of the James Webb Space Telescope give significant amounts of observing time to exoplanet study, especially early in its time aloft (now scheduled to begin in 2021.)  The atmospheric data that JWST can potentially collect could and would be used in conjunction with results coming from other telescopes, and to further study of exoplanet targets that are already promising based on existing theories and findings.

 

Construction has begun on the Giant Magellan Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. This artist rendering shows what the 24.5 meter (80 foot) segmented mirror and observatory will look like when completed, estimated to be in 2024. (Mason Media Inc.)

 

While the NAS report gives a lot of attention to instruments and ways to use them, it also focuses as never before on astrobiology — the search for life beyond Earth.

Much work has been done on how to determine whether life exists on a distant planet through modeling and theorizing about biosignatures.  The report encourages scientists to expand that work and embraces it as a central aspect of exoplanet science.

The study also argues that interdisciplinary science — bringing together researchers from many disciplines — is the necessary way forward.  It highlights the role of the Nexus for Exoplanet System Science, a NASA initiative which since 2015 has brought together a broad though limited number  of science teams from institutions across the country to learn about each other’s work and collaborate whenever possible.

The initiative itself has not required much funding, instead bringing in teams that had been supported with other grants.   However, that may be changing. One of the study co-chairs, David Charbonneau of Harvard University, said after the release of the study that the “promise of NExSS is tremendous…We really want that idea to grow and have a huge impact.”

The NAS study itself recommends that “building on the NExSS model, NASA should support a cross-divisional exoplanet research coordination network that includes additional membership opportunities via dedicated proposal calls for interdisciplinary research.”

The initiative, I’m proud to say, sponsors this interdisciplinary column in addition to all that interdisciplinary science.

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A New Frontier for Exoplanet Hunting

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The spectrum from the newly-assembled EXtreme PREcision Spectrometer (EXPRES)  shines on Yale astronomy professor Debra Fischer, who is principal investigator of the project. The stated goal of EXPRES is to find many Earth-size planets via the radial velocity method — something that has never been done. (Ryan Blackman/Yale)

The first exoplanets were all found using the radial velocity method of measuring the “wobble” of a star — movement caused by the gravitational pull of an orbiting planet.

Radial velocity has been great for detecting large exoplanets relatively close to our solar system, for assessing their mass and for finding out how long it takes for the planet to orbit its host star.

But so far the technique has not been able to identify and confirm many Earth-sized planets, a primary goal of much planet hunting.  The wobble caused by the presence of a planet that size has been too faint to be detected by current radial velocity instruments and techniques.

However, a new generation of instruments is coming on line with the goal of bringing the radial velocity technique into the small planet search.  To do that, the new instruments, together with their telescopes. must be able to detect a sun wobble of 10 to 20 centimeters per second.  That’s quite an improvement on the current detection limit of about one meter per second.

At least three of these ultra high precision spectrographs (or sometimes called spectrometers) are now being developed or deployed.  The European Southern Observatory’s ESPRESSO instrument has begun work in Chile; Pennsylvania State University’s NEID spectrograph (with NASA funding) is in development for installation at the Kitt Peak National Observatory in Arizona; and the just-deployed EXPRES spectrograph put together by a team led by Yale University astronomers (with National Science Foundation funding) is in place at the Lowell Observatory outside of Flagstaff, Arizona.

The principal investigator of EXPRES, Debra Fischer, attended the recent University of Cambridge Exoplanets2 conference with some of her team, and there I had the opportunity to talk with them. We discussed the decade-long history of the instrument, how and why Fischer thinks it can break that 1-meter-per-second barrier, and what it took to get it into attached and working.

 

This animation shows how astronomers use very precise spectrographs to find exoplanets. As the planet orbits its gravitational pull causes the parent star to move back and forth. This tiny radial motion shifts the observed spectrum of the star by a correspondingly small amount because of the Doppler shift. With super-sensitive spectrographs the shifts can be measured and used to infer details of a planet’s mass and orbit. ESO/L. Calçada)

One of the earliest and most difficult obstacles to the development of EXPRES, Fischer told me, was that many in the astronomy community did not believe it could work.

Their view is that precision below that one meter per second of host star movement cannot be measured accurately.  Stars have flares, sunspots and a generally constant churning, and many argue that the turbulent nature of stars creates too much “noise” for a precise measurement below that one-meter-per-second level.

Yet European scientists were moving ahead with their ESPRESSO ultra high precision instrument aiming for that 10-centimeter-per-second mark, and they had a proven record of accomplishing what they set out to do with spectrographs.

In addition to the definite competiti0n going on, Fisher also felt that radial velocity astronomers needed to make that leap to measuring small planets “to stay in the game” over the long haul.

She arrived at Yale in 2009 and led an effort to build a spectrograph so stable and precise that it could find an Earth-like planet.  To make clear that goal, the instrument is at the center of a project called “100 Earths.”

Building on experience gained from developing two earlier spectrographs, Fischer and colleagues began the difficult and complicated process of getting backers for EXPRES, of finding a telescope observatory that would house it (The Discovery Channel Telescope at Lowell) and in the end adapting the instrument to the telescope.

And now comes the actual hard part:  finding those Earth-like planets.

As Fischer described it:  “We know from {the Kepler Telescope mission} that most stars have small rocky planets orbiting them.  But Kepler looked at stars very far away, and we’ll be looking at stars much, much closer to us.”

Nonetheless, those small planets will still be extremely difficult to detect due to all that activity on the host suns.

 

EXPRES in its vacuum-sealed chamber at the Lowell Observatory. will help detect Earth-sized planets in neighboring solar systems. (Ryan Blackman/Yale)

 

 

The 4.3 meter Discovery Channel Telescope in the Lowell Observatory in Arizona.  The photons collected by the telescope are delivered via optical fiber to the EXPRES instrument. (Boston University)

Spectrographs such as EXPRES are instruments astronomers use to study light emitted by planets, stars, and galaxies.

They are connected to either a ground-based or orbital telescope and they stretch out or split a beam of light into a spectrum of frequencies.  That spectrum is then analyzed to determine an object’s speed, direction, chemical composition, or mass.  With planets, the work involves determining (via the Doppler shift seen in the spectrum) whether and how much a sun is moving to and away from Earth due to the pull of a planet.

As Fisher and EXPRES postdoctoral fellow John Brewer explained it, the signal (noise) coming from the turbulence of the star is detectably different from the signal made by the wobble of a star due to the presence of an orbiting planet.

While these differences — imprinted in the spectrum captured by the spectrograph — have been known for some time, current spectrographs haven’t had sufficient resolving power to actually detect the difference.

If all works as planned for the EXPRES, Espresso and NEID spectrographs, they will have that necessary resolving power and so can, in effect, filter out the noise from the sun and identify what can only come from a planet-caused wobble.  If they succeed, they provide a major new pathway to  for astronomers to search for Earth-sized worlds.

“This is my dream machine, the one I always wanted to build,” Fischer said. “I had a belief that if we went to higher resolution, we could disentangle (the stellar noise from the planet-caused wobble.)

“I could still be wrong, but I definitely think that trying was the right choice to make.”

This image shows spectral data from the first light last December of the ESPRESSO instrument on ESO’s Very Large Telescope in Chile. The light from a star has been dispersed into its component colors. This view has been colorised to indicate how the wavelengths change across the image, but these are not exactly the colors that would be seen visually. (ESO/ESPRESSO)

While Fischer and others have very high hopes for EXPRES, it is not the sort of  big ticket project that is common in astronomy.  Instead, it was developed and built primarily with a $6 million grant from National Science Foundation.

It was completed on schedule by the Yale team, though the actual delivering of EXPRES to Arizona and connecting it to the telescope turned out to be a combination of hair-raising and edifying.

Twice, she said, she drove from New Haven to Flagstaff with parts of the instrument; each trip in a Penske rental truck and with her son Ben helping out.

And then when the instrumentation was in process late last year, Fischer and her team learned that funds for the scientists and engineers working on that process had come to an end.

Francesco Pepe of the University of Geneva. He is the principal scientist for the ESPRESSO instrument and gave essential aid to the EXPRES team when they needed it most.

She was desperate, and sent a long-shot email to Francesco Pepe of University of Geneva, the lead scientist and wizard builder of several European spectrographs, including ESPRESSO. In theory, he and his instrument — which went into operation late last year at the ESO Very Large Telescope in Chile — will be competing with EXPRES for discoveries and acknowledgement.

Nonetheless, Pepe heard Fischer out and understood the predicament she was in.  ESPRESSO had been installed and so he was able to contact an associate who freed up two instrumentation specialists who flew to Flagstaff to finish the work.  It was, Fischer said, an act of collegial generosity and scientific largesse that she will never forget.

Fischer is at the Lowell observatory now, using the Arizona monsoon as a time to clean up many details before the team returns to full-time observing.  She write about her days in an EXPRES blog.  Earlier, in March after the instrumentation had been completed and observing had commenced, she wrote this:

“Years of work went into EXPRES and as I look at this instrument, I am surprised that I ever had the audacity to start this project. The moment of truth starts now. It will take us a few more months of collecting and analyzing data to know if we made the right design decisions and I feel both humbled and hopeful. I’m proud of the fact that our design decisions were driven by evidence gleaned from many years of experience. But did I forget anything?”

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The Architecture of Solar Systems

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The architecture of planetary systems is an increasingly important factor to exoplanet scientists.  This illustration shows the Kepler-11 system where the planets are all roughly the same size and their orbits spaced at roughly the same distances from each other.  The the planets are, in the view of scientists involved with the study, “peas in a pod.” (NASA)

Before the discovery of the first exoplanet that orbits a star like ours, 51 Pegasi b, the assumption of solar system scientists was that others planetary systems that might exist were likely to be like ours.  Small rocky planets in the inner solar system, big gas giants like Jupiter, Saturn and Neptune beyond and, back then, Pluto bringing up the rear

But 51 Peg b broke every solar system rule imaginable.  It was a giant and hot Jupiter-size planet, and it was so close to its star that it orbited in a little over four days.  Our Jupiter takes twelve years to complete an orbit.

This was the “everything we knew about solar systems is wrong” period, and twenty years later thinking about the nature and logic of solar system architecture remains very much in flux.

But progress is being made, even if the results are sometimes quite confounding. The umbrella idea is no longer that solar, or planetary, systems are pretty much like ours, but rather that the galaxy is filled with a wild diversity of both planets and planetary systems.

Detecting and trying to understand planetary systems is today an important focus 0f  exoplanet study, especially now that the Kepler Space Telescope mission has made clear that multi-planet systems are common.

As of early July, 632 multi planet systems have been detected and 2,841 stars are known to have at least one exoplanets.  Many of those stars with a singular planet may well have others yet to be found.

An intriguing newcomer to the diversity story came recently from University of Montreal astronomer Lauren Weiss, who with colleagues expanded on and studied some collected Kepler data.

What she found has been deemed the “peas in a pod” addition to the solar system menagerie.

Weiss was working with the California-Kepler Survey, which included a team of scientists pouring over, elaborating on and looking for patterns in, among other things, solar system architectures.

Weiss is part of the California-Kepler Survey team, which used the Keck Observatory to obtain high-resolution spectra of 1305 stars hosting 2025 transiting planets originally discovered by Kepler.

From these spectra, they measured precise sizes of the stars and their planets, looking for patterns in, among other things, solar system architectures.  They focused on 909 planets belonging to 355 multi-planet systems. By improving the measurements of the radii of the stars, Weiss said, they were able to recalculate the radii of all the planets.

So Weiss studied hundreds systems and did find a number of surprising, unexpected patterns.

In many systems, the planets were all roughly the same size as the planet in orbit next to them. (No tiny-Mars-to-gigantic-Jupiter transitions.)  This kind of planetary architecture was not found everywhere but it was quite common — more common than random planet sizing would predict.

“The effect showed up with smaller planets and larger ones,” Weiss told me during last week’s University of Cambridge Exoplanets2 conference. “The planets in each system seemed to know about the sizes of the neighbors,” and for thus far unknown reasons maintained those similar sizes.

What’s more, Weiss and her colleagues found that the orbits of these “planets in a pod” were generally an equal distance apart in “multi” of three planets or more. In other words, the distance between the orbits of planet A and planet B was often the same distance as between the orbits of planet B and planet C.

Lauren Weiss at the W.M Keck Observatory.

So not only were many of the planets almost the same size, but they were in orbits spaced at distances from each other that were once again much more similar than a random distribution would predict. In the Astronomical Journal article where she and her colleagues described the phenomena, they also found a “wall” defining how close together the planets orbited.

The architecture of these systems, Weiss said, reflected the shapes and sizes of the protoplanetary in which they were formed.  And it would appear that the planets had not been disrupted by larger planets that can dramatically change the structure of a solar system — as happened with Jupiter in our own.

But while those factors explain some of what was found, Weiss said other astrophysical dynamics needed to be at play as well to produce this common architecture.  The stability of the system, for instance, would be compromised if the orbits were closer than that “wall,” as the gravitational pull of the planets would send them into orbits that would ultimately result in collisions.

The improved spectra of the Kepler planets were obtained from 2011 to 2015, and the targets are mostly located between 1,000 and 4,000 light-years away from Earth.

The architectures of California-Kepler study multi-planet systems with four planets or more.  Each row corresponds to the planets around one and the circles represent the radii of planets in the system.  Note how many have lines of planets that are roughly the same size. (Lauren Weiss, The Astronomical Journal.)

Planetary system architecture was a significant topic at the Cambridge Exoplanets2 conference.  While the detection of individual exoplanets remains important in the field, it is often treated as a precursor to the ultimate detection of systems with more planets. 

The TRAPPIST-1 system, discovered in 2015 by a Belgian team, is probably the most studied and significant of those discovered so far.

The ultra-cool dwarf star hosts seven Earth-sized, temperate exoplanets in or near the “habitable zone.” As described by one of those responsible for the discovery, Brice-Olivier Demory of the Center for Space and Habitability University of Bern, the system “represents a unique setting to study the formation and evolution of terrestrial planets that formed in the same protoplanetary disk.”

The Trappist-1 architecture features not only the seven rocky planets, but also a resonance system whereby the planets orbits at paces directly related to the planets nearby them.  In other words, one planet may make two orbits in exactly the time that it takes for the next planet to make three orbits.

All the Trappist-1 planets are in resonance to another system planet, though they are not all in resonance to each other.

The animation above from the NASA Ames Research Center shows the orbits of the Trappist-1 system.  The planets pass so close to one another that gravitational interactions are significant, and to remain stable the orbital periods are nearly resonant. In the time the innermost planet completes eight orbits, the second, third, and fourth planets complete five, three, and two respectively.

The system is very flat and compact. All seven of TRAPPIST-1’s planets orbit much closer to their star than Mercury orbits the sun. Except for TRAPPIST-1b, they orbit farther than the Galilean moons — three of which are also in resonance around Jupiter.

The distance between the orbits of TRAPPIST-1b and TRAPPIST-1c is only 1.6 times the distance between the Earth and the Moon.  A year on the closest planet passes in only 1.5 Earth days, while the seventh planet’s year passes in only 18.8 days.

Given the packed nature of the system, the planets have to be in particular orbits that keep them from colliding.  But they also have to be in orbits that ensure that all or most of the planets aren’t on the same side of the star, creating a severe imbalance that would result in chaos.

“The Trappist-1 system has entered into a zone of stability,” Demory told me, also at the Exoplanets2 conference.  “We think of it as a Darwinian effect — the system survives because of that stability created through the resonance.  Without the stability, it would die. ”

He said the Trappist-1 planets were most likely formed away from their star and migrated inward.  The system had rather a long time to form, between seven and eight billion years.

The nature of some of the systems now being discovered brings to mind that early reaction to the detection of 51 Pegasi b, the world’s first known exoplanet.

The prevailing consensus that extra-solar systems would likely be similar to ours was turned on its head by the giant planet’s closeness to its host star.  For a time many astronomers thought that hot Jupiter planets would be found to be common.

But 20 years later they know that hot Jupiters — and the planetary architecture they create — are rather unusual, like the architecture of our own solar system.

With each new discovery of a planetary system, the understanding grows that while solar systems are governed by astrophysical forces, they nonetheless come in all sizes and shapes. Diversity is what binds them together.

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