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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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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Exoplanet Science Flying High

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An artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Credit: NASA/JPL-Caltech

 

Early this spring, the organizers of an exoplanet science gathering at Cambridge University put out the word that they would host a major meeting this summer.  Within a week, the 300 allotted slots had been filled by scientists aspiring and veteran, and within a short time the waiting list was up to 150 more.

Not the kind of reaction you might expect for a hardcore, topic-specific meeting, but exoplanet science is now in a phase of enormous growth and excitement.  With so many discoveries already made and waiting to be made, so many new (and long-standing) questions to be worked on, so much data coming in to be analyzed and turned into findings,  the field has something of a golden shine.

What’s more, it has more than a little of the feel of the Wild West.

Planet hunters Didier Queloz and Michel Mayor at the European Southern Observatory’s La Silla site. (L. Weinstein/Ciel et Espace Photos)

Didier Queloz, a professor now at Cambridge but in the mid 1990s half of the team that identified the first exoplanet, is the organizer of the conference.

“It sometimes seems like there’s not much exploration to be done on Earth, and the opposite is the case with exoplanets,” he told me outside the Cambridge gathering.

“I think a lot of young scientists are attracted to the excitement of exoplanets, to a field where there’s so much that isn’t known or understood.”

Michel Mayor of the Observatory of Geneva — and the senior half of the team that detected the first exoplanet orbiting a star like our sun, 51 Pegasi b– had opened the gathering with a history of the search for extra-solar planets.

That search had some conceptual success prior to the actual 1995 announcement of an exoplanet discovery, but several claims of having actually found an exoplanet had been made and shown to be wanting.  Except for the relative handful of scientists personally involved, the field was something of a sideshow.

“At the time we made our first discovery, I basically knew everyone in the field.  We were on our own.”

Now there are thousands of people, many of them young people, studying exoplanets.  And the young people, they have to be smarter, more clever, because the questions are harder.”

And enormous progress is being made.

The pace of discovery is charted here by Princeton University physicist and astronomer Joshua Winn. First is a graphic of all the 3,735 exoplanet discoveries made since 1995, and then the 1943 planets found just from 2016 to today.

The total number and distribution of known exoplanets, identified by the mass of the planet and their distance from their host star. A legend to the four major techniques for finding exoplanets is in the lower right The circled planets in green are those in our solar system. All the data comes from the NASA Exoplanet archive. (Joshua Winn, Princeton University)

 

Based on published papers, Winn found that the discovery of 1,943 new planets had been announced in papers between 2016 and today. Winn said the number is not formal as some debate remains whether a small number are planets or not.

Many of the planets discovered via the transit method come from the Kepler and K2 missions.  Kepler revolutionized the field with its four years of intensively observing a region of the sky for planet transits in front of their star.

The K2 mission began after the second of Kepler’s four stabilizing wheels failed. But adjustments were made and the second incarnation of Kepler has continued to find planets, though in a different way.

While a majority of exoplanets have been detected via the transit method, the first exoplanet was discovered by Mayor and Queloz via the radial velocity method — which involves ground-based measurements of the “wobble” of a star caused by the gravitational pull of a planet.

Many astronomers continue to use the technique because it provides more information about the minimum mass and orbital eccentricity of planet.  In addition, two high-precision, next-generation spectrometers for radial velocity measuring are now coming on line and are expected to significantly improve the detection of smaller planets using that method.

One is the ESPRESSO instrument (the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic) recently installed by the European Southern Observatory on the Very Large Telescope in Chile. The other newcomer is EXPRES, developed by scientists at Yale University, with support for the National Science Foundation.  The instrument, designed go look for Earth-sized planets, has been installed on the Lowell Observatory Discovery Channel Telescope in Arizona.

 

The Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) will search for exoplanets with unprecedented precision by looking at the minuscule changes in the properties of light coming from their host stars. This picture shows the front-end structure where the light beams coming from the four Very Large Telescopes are brought together and fed into fibers. They then deliver the photons to a spectrograph in another room, which makes the radial velocity measurements. (Giorgio Calderone, INAF Trieste)

The conference, which will go through the week, focuses both generally and in great detail on many of the core questions of the field:  how exoplanets are formed, what kind of stars are likely to produce what kinds of planets, the makeup and dynamics of exoplanet atmospheres, planet migration, the architecture of planetary systems.

And, of course, where new exoplanets might be found.  (Mostly around red dwarf stars, several scientists argued, and many in the relatively near neighborhood.)

Notably, many of the exoplanet questions being studied have clear implications for better understanding our own solar system.  In fact, it is often said that we won’t really understand the workings and history of our solar system, planets, moons, asteroids and more until we know a lot more about the billions and billion of other planetary systems in our galaxy.

Also notable for this conference is the lack of emphasis on biosignatures, habitability and the search for life beyond Earth.  The conference is billed as being about “exoplanet science,” and Queloz explained the absence of habitability and life-detection talks was based on the scientific progress made, or not made, in the past two years.

When it comes to planet detection, however, theory and practice are coming together in searches for exoplanets around smaller and cooler stars, and even around young stars where planets are just forming.  Such a planet discovery was announced this week coming from the European Space Agency’s Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument.

 

The first clear image of a planet caught while being formed,around the dwarf star PDS 70. The planet is visible as a bright point to the right of center. The star at the center is blacked out by a coronagraph mask that blocks its blinding light. The SPHERE instrument is on the European Southern Observatory’s Very Large Telescope (A. Müller et al./ESO)

 

The Cambridge exoplanet conference is the second in a series begun two years ago by Queloz and Kevin Heng, an exoplanet atmosphere theoretician at the University of Bern and director of the Center for Space and Habitability.

The two had been struck by how European exoplanet conferences seemed to be dominated by senior scientists, with little time or space for the many younger men and women coming up in the field.  The presentations also seemed more long and formal than needed.

So using funds from their own institutions to seed the conferences, Heng set up the first in Davos, Switzerland and Didier the second in Cambridge.  The idea has caught on, and similar gathering are now scheduled at two year intervals in Heidelberg, Las Vegas, Amsterdam, Porto and hopefully later in Asia, too.

There is no dearth of other exoplanet gatherings around the world, and attendees report that they are also very well attended.

But given sheer amount of work now being done in the field that was so lonely only twenty years ago,  they surely appear to be warranted.

And newsworthy, though no always reportable.

Three of the papers discussed in the Cambridge conference, for instance, are under reporting embargo from the journal Nature. And information from George Ricker, principal investigator for NASA’s Transiting Exoplanet Survey Satellite (TESS), about the early days of the mission are also under embargo.  Suffice it to say, however, that Ricker reported that things are going well for the exoplanet-hunting telescope.

 

This test image from one of the four cameras aboard the Transiting Exoplanet Survey Satellite (TESS) captures a swath of the southern sky along the plane of our galaxy. TESS is designed to study exoplanets around the brightest stars, and is expected to cover more than 400 times the amount of sky shown in this image. (NASA/MIT/TESS)

While the initial discovery of an exoplanet was difficult for sure, what the much, much larger field is grappling with now is clearly even more challenging.  With that in mind, I asked Queloz what he hoped to see from exoplanets in the years ahead.

“We have reached the point where we know stars usually have planets.  But what we are still looking for is an Earth twin — a planet clearly like ours.  That we have not found.  Before I retire, what I hope for is the discovery of that Earth twin.”

 

 

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Planets Still Forming Detected in a Protoplanetary Disk

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An artist rendering of infant star HD 163296 with three protoplanets forming in its disk  The planets were discovered using a new mode of detection — identifying unusual patterns in the flow of gas within a protoplanetary disk. (NRAO/AUI/NSF; S. Dagnello)

Just as the number of planets discovered outside our solar system is large and growing — more than 3,700 confirmed at last count — so too is the number of ingenious ways to find exoplanets ever on the rise.

The first exoplanets were found by measuring the “wobble” in their host stars caused by the gravitational pull of the planets, then came the transit technique that measured dips in the light from stars as planets passed in front of them, followed by the direct imaging of moving objects deemed to be planets, and numerous more.

A new technique can now be added to the toolkit, one that is useful only in specific galactic circumstances but is nonetheless ingenious and intriguing.

By detecting unusual patterns in the flow of gas within the protoplanetary disk of a young star, two teams of astronomers have confirmed the distinct, telltale hallmarks of newly formed planets orbiting the infant star.

In other words, the astronomers found planets in the process of being formed, circling a star very early in its life cycle.

These results came thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), and are presented in a pair of papers appearing in the Astrophysical Journal Letters.

Richard Teague, an astronomer at the University of Michigan and principal author on one of the papers, said that his team looked at “the localized, small-scale motion of gas in a star’s protoplanetary disk. This entirely new approach could uncover some of the youngest planets in our galaxy, all thanks to the high-resolution images coming from ALMA.”

ALMA image of the protoplanetary disk surrounding the young star HD 163296 as seen in dust. ( ALMA: ESO/NAOJ/NRAO; A. Isella; B. Saxton NRAO/AUI/NSF.

To make their respective discoveries, each team analyzed the data from various ALMA observations of the young star HD 163296, which is about 4 million years old and located about 330 light-years from Earth in the direction of the constellation Sagittarius.

Rather than focusing on the dust within the disk, which was clearly imaged in an earlier ALMA observation, the astronomers instead studied the distribution and motion of carbon monoxide (CO) gas throughout the disk.

As explained in a release from the National Radio Astronomy Observatory, which manages the American operations of the multi-national ALMA, molecules of carbon monoxide naturally emit a very distinctive millimeter-wavelength light that ALMA can observe. Subtle changes in the wavelength of this light due to the Doppler effect provide a glimpse into the motion of the gas in the disk.

If there were no planets, gas would move around a star in a very simple, predictable pattern known as Keplerian rotation.

“It would take a relatively massive object, like a planet, to create localized disturbances in this otherwise orderly motion,” said Christophe Pinte of Monash University in Australia and lead author on the other of the two papers. 

And that’s what both teams found.

ALMA is a radio astronomy array located in Chile and set 16,000 feet above sea level. It’s a partnership between the European Southern Observatory (ESO), the National Science Foundation (NSF) of the United States and the National Institutes of Natural Sciences (NINS) of Japan in collaboration with the Republic of Chile. ALMA, which began operations in 2013, is used to observe light from space in comparatively long radio wavelengths. ((ESO/José Francisco Salgado )

Detecting planets within a protoplanetary disk — or finding theorized planets within those disks — is a big deal. 

That’s because information about the characteristics of very young planets orbiting young stars can potentially add substantially to one of the long-debated questions of planetary science:  How exactly did those billions upon billions of planets out there form?

The leading theory of planet formation, the “core accretion model,” has planets forming slowly — with dust, small objects and then planetesimals smashing into a rocky core and leaving matter behind.  In this model, the planet building takes place in a region close to the protoplanet’s stars.

Another theory looks to gravitational instabilities in the disk, arguing that giant planets can form quickly and far from their host stars.

The distribution of current solar system planets and beyond can give some clues based on the size, type and distribution of those planets.  But planets migrate and evolve, and they have never been studied before they had a chance to do much of either.

The techniques currently used for finding exoplanets in fully formed planetary systems — such as measuring the wobble of a star or how a transiting planet dims starlight — don’t lend themselves to detecting protoplanets.

With this new method for looking into those early protoplanetary disks, the hunt for infant planets becomes possible.  And the results in terms of understanding planet formation look to be very promising.

“Though thousands of exoplanets have been discovered in the last few decades, detecting protoplanets is at the frontier of science,” said Pinte.

 

These earlier images from ALMA reveal details in the planet-forming disk around a nearby sun-like star, TW Hydrae, including an intriguing gap at the same distance from the star as the Earth is from the sun. This structure may mean that an infant version of our home planet is beginning to form there, although these dust gaps are considered to be suggestive rather than conclusive. ( S. Andrews; Harvard-Smithsonian CfA, ALMA (ESO/NAOJ/NRAO)}ALMA

This is not the first time that ALMA images of protoplanetary disks have been used to identify what seem to be protoplanets.

In 2016, a team led by Andrea Isella of Rice University reported the possible detection of two planets, each the size of Saturn, orbiting the same star that is the subject of this week’s report, HD 163296.

These possible planets, which are not yet fully formed, revealed themselves by the dual imprint they left in both the dust and the gas portions of the star’s protoplanetary disk.

But at the time that paper was published, in Physical Review Letters, Isella said the team was focused primarily on the dust in the disks and the gaps they created, and as a result they could not be certain that the features they found were created by a protoplanet.

Teague’s team also studied the dust gaps in the disk of HD 163296, and concluded they provided only  circumstantial evidence of the presence of protoplanets.  What’s more, that kind of detection could not be used to accurately estimate the masses of the planets.

“Since other mechanisms can also produce ringed gaps in a protoplanetary disk,” he said, “it is impossible to say conclusively that planets are there by merely looking at the overall structure of the disk.”

But studying the behavior of the gas allowed for a much greater degree of confidence.

 

Composite image of the protoplanetary disk surrounding the young star HD 163296. The inner red area shows the dust of the protoplanetary disk. The broader blue disk is the carbon monoxide gas in the system. ALMA observed dips in the concentration and behavior of carbon monoxide in outer portions of the disk, strongly suggesting the presence of planets being formed. ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)

The team led by Teague identified two distinctive planet-like patterns in the disk, one at approximately 80 astronomical units (AU) from the star and the other at 140 AU. (An astronomical unit is the average distance from the Earth to the sun.)  The other team, led by Pinte, identified the third at about 260 AU from the star. The astronomers calculate that all three planets are similar in mass to Jupiter.

The two teams used variations on the same technique, which looked at anomalies in the flow of the gas – as seen in the shifting wavelengths of the CO emission — that would indicate it was interacting with a massive object.

Teague and his team measured variations in the gas’s velocity. This revealed the impact of several planets on the gas motion nearer to the star.

Pinte and his team more directly measured the gas’s actual velocity, which is better precise method when studying the outer portion of the disk and can more accurately pinpoint the location of a potential planet.

“Although dust plays an important role in planet formation and provides invaluable information, gas accounts for 99 percent of a protoplanetary disks’ mass,” said coauthor Jaehan Bae of the Carnegie Institute for Science.

So while those images of patterns within the concentric rings of a protoplanetary disk are compelling and seem to be telling an important story, it’s actually the gas that is the key.

This is all an important coup for ALMA, which saw its first light in 2013.  The observatory was not designed with protoplanet detection and characterization as a primary goal, but it is now front and center.

Coauthor Til Birnstiel of the University Observatory of Munich said the precision provided by ALMA is “mind boggling.” In a system where gas rotates at about 5 kilometers per second, he said,  ALMA detected velocity changes as small as a few meters per second.

“Oftentimes in science, ideas turn out not to work or assumptions turn out to be wrong,” he said. “This is one of the cases where the results are much more exciting than what I had imagined.

 

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Can You Overwater a Planet?

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Water worlds, especially if they have no land on them, are unlikely to be home to life, or at least lifewe can detect.  Some of the basic atmospheric and mineral cycles that make a planet habitable will be absent. Cool animation of such a world. (NASA)

By guest columnist Elizabeth Tasker

 

Wherever we find water on Earth, we find life. It is a connection that extends to the most inhospitable locations, such as the acidic pools of Yellowstone, the black smokers on the ocean floor or the cracks in frozen glaciers. This intimate relationship led to the NASA maxim, “Follow the Water”, when searching for life on other planets.

Yet it turns out you can have too much of a good thing. In the November NExSS Habitable Worlds workshop in Wyoming, researchers discussed what would happen if you over-watered a planet. The conclusions were grim.

Despite oceans covering over 70% of our planet’s surface, the Earth is relatively water-poor, with water only making up approximately 0.1% of the Earth’s mass. This deficit is due to our location in the Solar System, which was too warm to incorporate frozen ices into the forming Earth. Instead, it is widely — though not exclusively — theorized that the Earth formed dry and water was later delivered by impacts from icy meteorites. It is a theory that two asteroid missions, NASA’s OSIRIS-REx and JAXA’s Hayabusa2, will test when they reach their destinations next year.

But not all planets orbit where they were formed. Around other stars, planets frequently show evidence of having migrated to their present orbit from a birth location elsewhere in the planetary system.

One example are the seven planets orbiting the star, TRAPPIST-1. Discovered in February this year, these Earth-sized worlds orbit in resonance, meaning that their orbital times are nearly exact integer ratios. Such a pattern is thought to occur in systems of planets that formed further away from the star and migrated inwards.

 

Trappist-1 and some of its seven orbiting planets.  They would have been sterilized by high levels of radiation in the early eons of that solar system — unless they were formed far out and then migrated in.  That scenario would also allow for the planets to contain substantial amounts of water. (NASA)

The TRAPPIST-1 worlds currently orbit in a temperate region where the levels of radiation from the star are similar to that received by our terrestrial worlds. Three of the planets orbit in the star’s habitable zone, where a planet like the Earth is most likely to exist.

However, if these planets were born further from the star, they may have formed with a high fraction of their mass in ices. As the planets migrated inwards to more clement orbits, this ice would have melted to produce a deep ocean. The result would be water worlds.

With more water than the Earth, such planets are unlikely to have any exposed land. This does not initially sound like a problem; life thrives in the Earth’s seas, from photosynthesizing algae to the largest mammals on the planet. The problem occurs with the planet itself.

The clement environment on the Earth’s surface is dependent on our atmosphere. If this envelope of gas was stripped away, the Earth’s average global temperature would be about -18°C (-0.4°F): too cold for liquid water. Instead, this envelope of gases results in a global average of 15°C (59°F).

Exactly how much heat is trapped by our atmosphere depends on the quantity of greenhouse gases such as carbon dioxide. On geological timescales, the carbon dioxide levels can be adjusted by a geological process known as the “carbon-silicate cycle”.

In this cycle, carbon dioxide in the air dissolves in rainwater where it splashes down on the Earth’s silicate rocks. The resulting reaction is termed “weathering”. Weathering forms carbonates and releases minerals from the rocks that wash into the oceans. Eventually, the carbon is released back into the air as carbon dioxide through volcanoes.

Continents are not only key for habitability because they sources of minerals and needed elements but also because they allow for plate tectonics — the movements and subsequent crackings of the planet’s crust that allow gases to escape.  Those gases are needed to produce an atmosphere.  (National Oceanic and Atmospheric Administration)

The rate of weathering is sensitive to temperature, slowing when he planet is cool and increasing when the temperature rises. This allows the Earth to maintain an agreeable climate for life during small variations in our orbit due to the tug of our neighboring planets or when the sun was young and cooler. The minerals released by weathering are used by all life on Earth, in particular phosphorous which forms part of our DNA.

However, this process requires land. And that is a commodity a water world lacks. Speaking at the Habitable Worlds workshop, Theresa Fisher, a graduate student at Arizona State University, warned against the effects of submerging your continents.

Fisher considered the consequences of adding roughly five oceans of water to an Earth-sized planet, covering all land in a global sea. Feasible, because weathering could still occur with rock on the ocean floor, though at a much reduced efficiency. The planet might then be able to regulate carbon dioxide levels, but the large reduction in freed minerals with underwater weathering would be devastating for life.

Despite being a key element for all life on Earth, phosphorus is not abundant on our planet. The low levels are why phosphorous is the main ingredient in fertilizer. Reduce the efficiency with which phosphorous is freed from rocks and life will plummet.

Such a situation is a big problem for finding a habitable world, warns Steven Desch, a professor at Arizona State University. Unless life is capable of strongly influencing the composition of the atmosphere, its presence will remain impossible to detect from Earth.

“You need to have land not to have life, but to be able to detect life,” Desch concludes.

However, considerations of detectability become irrelevant if even more water is added to the planet. Should an Earth-sized planet have fifty oceans of water (roughly 1% of the planet’s mass), the added weight will cause high pressure ices to form on the ocean floor. A layer of thick ice would seal the planet rock away from the ocean and atmosphere, shutting down the carbon-silicate cycle. The planet would be unable to regulate its surface temperature and trapped minerals would be inaccessible for life.

Add still more water and Cayman Unterborn, a postdoctoral fellow at Arizona State, warns that the pressure will seal the planet’s lid. The Earth’s surface is divided into plates that are in continual motion. The plates melt as they slide under one another and fresh crust is formed where the plates pull apart. When the ocean weight reaches 2% of the planet’s mass, melting is suppressed and the planet’s crust grinds to a halt.

A stagnant lid would prevent any gases trapped in the rocks during the planet’s formation from escaping. Such “degassing” is the main source of atmosphere for a rocky planet. Without such a process, the Earth-sized deep water world could only cling to an envelop of water vapor and any gas that may have escaped before the crust sealed shut.

Unterborn’s calculations suggest that this fate awaits the TRAPPIST-1 planets, with the outer worlds plausibly having hundreds of oceans worth of water pressing down on the planet.

So can we prove if TRAPPIST-1 and similarly migrated worlds are drowning in a watery grave? Aki Roberge, an astrophysicist at NASA Goddard Space Flight Center, notes that exoplanets are currently seen only as “dark shadows” briefly reducing their star’s light.

However, the next generation of telescopes such as NASA’s James Webb Space Telescope, will aim to change this with observations of planetary atmospheres. Intertwined with the planet’s geological and biological processes, this cloak of gases may reveal if the world is living or dead.

 

Elizabeth Tasker is a planetary scientist and communicator at the Japanese space agency JAXA and the Earth-Life Science Institute (ELSI) in Tokyo.  She is also author of a new book about planet formation titled “The Planet Factory.”

 

 

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