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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

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

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

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

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

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

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

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

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

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

Coming soon:  The Habitability Inde

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Enceladus and Water Worlds

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Glittering geysers of water ice erupt from Saturn's enigmatic moon Enceladus as seen during a previous flyby. The plumes are backlit by the sun, which is almost directly behind the moon. The moon's dark side that we see here is illuminated by reflected Saturn-shine. Today, the Cassini spacecraft flew right through the plumes in order to let its instruments 'taste' them. Credit: NASA/JPL/SSI/Ugarkovich
Glittering geysers of water ice erupt from Saturn’s enigmatic moon Enceladus as seen during a previous flyby. The plumes are backlit by the sun, which is almost directly behind the moon. The moon’s dark side that we see here is illuminated by reflected Saturn-shine.  Credit: NASA/JPL/SSI/Ugarkovich

As if the prospect of billions of potentially habitable exoplanets wasn’t enough to get people excited, what about all those watery exo-moons too?

The question arises as the Cassini mission makes its final pass near the now famous geysers at the south pole of the moon Enceladus ,scheduled for Saturday.  The plumes are currently in darkness and so it’s a perfect time to tease out a particularly compelling aspect of the Enceladus story:  how hot is the inside of the mini-moon.  Earlier measurements of the water ice spray took place when the sun was on that southern pole, so this will be the first time Cassini can measure precisely how much of the already detected heat comes from the moon’s interior.

The expectation is that much of the heat does indeed come from inside, warmed substantially by tidal forces and perhaps hydrothermal vents that together serve to keep liquid a subsurface ocean all around the moon.  As a result, the evolving scientific view is that tiny Enceladus, one of 63 moons of Saturn, just may have the ingredients and characteristics that put it into an improbable habitable zone.

“Step by step, we’re learning about an environment that seemed impossible not long ago,” said Cassini Mission Scientist Linda Spilker.  “We know that Enceladus has some rocky core, and that it touches the liquid water.  We also know that some of the compounds identified in the geysers can only be formed when rock is in contact with hot water, and that must be happening at the bottom of the moon’s ocean.  All the pieces are coming together to tell us that the moon has an ocean that might be able to support life.”

NASA's Cassini spacecraft captured this view as it neared icy Enceladus for its closest-ever dive past the moon's active south polar region. The view shows heavily cratered northern latitudes at top, transitioning to fractured, wrinkled terrain in the middle and southern latitudes. The wavy boundary of the moon's active south polar region -- Cassini's destination for this flyby -- is visible at bottom, where it disappears into wintry darkness. This view looks towards the Saturn-facing side of Enceladus. North on Enceladus is up and rotated 23 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 28, 2015. The view was acquired at a distance of approximately 60,000 miles (96,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 45 degrees. Image scale is 1,896 feet (578 meters) per pixel.
The Cassini spacecraft, sponsored by NASA, the European Space Agency and the Italian space Agency,  captured this view on Oct. 28 as it neared Enceladus. The wavy boundary of the moon’s active south polar region — Cassini’s destination for this flyby — is visible at bottom. The image was taken in visible light with the Cassini spacecraft narrow-angle camera from approximately 60,000 miles away. (Cassini Imaging Team, SSI, JPL, ESA, NASA)

That a moon might have habitable conditions is not a new idea:  science fiction great Arthur C. Clarke (as well as many scientists and now members of Congress) have pressed for a mission to Jupiter’s moon Europa because its internal ocean has been identified as similarly promising.

But what is so compelling about Enceladus is that its potential habitability pretty much came out of nowhere.  While Europa is the sixth largest moon in the solar system, Enceladus is but 370 miles in diameter.  It is covered in ice, but in 2004 its four parallel “tiger stripe” fractures were discovered, leading to the conclusion that some kind of volcanic action was taken place beneath them.  Spilker was a scientist with the Voyager mission that passed by Saturn in 1980-81 and said that Enceladus was then in relative darkness and made little impression on the team.  “We definitely missed the tiger stripes,” she said.

The plumes emerge from the south pole region, not far from the tiger stripe fractures, and appear to come from  near-surface pockets of liquid water. (The oceans of Europa are not nearly as accessible, lying 6 to 20 miles below the icy surface.)  Making Enceladus even more interesting, the 70 geysers spit out organic chemicals known as the building blocks of life along with its water ice.

The chemical composition of the plumes of Enceladus's includes hydrocarbons such as ammonia, methane and formaldehyde in trace amounts similar to the makeup of many comets. (NASA)
The chemical composition of the plumes of Enceladus’s includes hydrocarbons such as ammonia, methane and formaldehyde in trace amounts similar to the makeup of many comets.  The presence of the organic compounds suggests that very interesting chemistry is taking place where the moon’s oceans touch its core. (NASA, ESA)

So Enceladus (and perhaps Europa, too) provide a kind of emerging “proof of concept” that ice-covered water worlds can and do exist elsewhere.  (Jupiter’s giant moons Ganymede and Callisto also have massive ocean, but far below their surfaces and sandwiched between layers of ice.)  In fact, subterranean oceans may be common because, in recent years, scientists have come to understand that water — especially in its vapor and ice stages — is ubiquitous in the solar system, the galaxy and the universe.  Comets, which are generally half water ice and half rock, are one of numerous delivery systems.

And we already have, of course, one good example of what would generally be considered a waterworld without the ice covering — Earth.  But that’s not all, even without leaving our solar system.   We look at planets such as Mars and Venus and now see desiccated landscapes.  Yet it is broadly accepted that Mars once was quite wet on the surface, based on findings from the Curiosity mission and years of satellites imaging, and some have speculated that Venus might once have been wet as well.

So are there many waterworlds or aquaworlds with surface liquid water out there?

“It would be fair to say there is a consensus view that exoplanets and moons with lots of water are all over the place,” said Joel Green, an exoplanet scientist with the Space Telescope Science Institute. “The known presence of so much H2O makes that non-controversial.”

Kepler-62e has been described as being a possible waterworld, with large oceans. UPR Arecibo
Kepler-62e has been described as being a possible waterworld, with large oceans. UPR Arecibo

 

What is indeed controversial is whether any waterworlds, or even potential waterworlds, have been detected. There actually has been wide coverage of waterworld discoveries far, far away,  and even some declared confirmations.  But so far at least, those confirmations have not lasted.  As early as 2004, the Hubble Space Telescope identified the signature of water vapor in the atmosphere of an exoplanet, and similar detections have followed.  But that information and more that scientists have collected and modeled about H2O on exoplanets has never been sufficient to make a confirmation stick.

“It’s a very challenging detection to make, and many don’t think we haven’t gotten there yet,” Green said.  Especially challenging is the detection of liquid water, since it does not show up using optical, infrared, ultraviolet or any other kind of light, and so can’t be identified with a spectroscope. It’s presence can only be inferred based on other conditions.  Water vapor and water ice are, however, detectable via spectroscope.

But waterworld theories abound.  For instance, scientists know that Neptune and Uranus in the outer part of our solar system consist of vast amounts of water ice, and they also know that planets tend to change orbits and sometimes migrate closer to their suns.  Marc Kurchner of NASA’s Goddard Space Flight Center has proposed that if similar planets were to migrate inward in different solar systems, the result could be a very wet planet, with oceans hundreds of miles deep.

There is a general tendency to associate the presence of water with the presence of life, and the ubiquity of water in the galaxies with the likelihood of finding life.   But while life is found everywhere that water exists on Earth, that does not at all mean that the discovery of water elsewhere means life will be present, too.

All it means is that one of many prerequisites for life (as we know it) will have been met. But as Enceladus shows, when water is present, all kinds of interesting things start of happen.

A view of Enceladus’ southern hemisphere in enhanced color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are prominently visible in the center. {NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)
A view of Enceladus’ southern hemisphere in enhanced color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are prominently visible in the center. {NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)
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On Super-Earths, Sub-Neptunes and Some Lessons They Teach

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Part 1 of 2

The discovery of a menagerie of exoplanets sized greater than Earth and smaller than Neptune has changed thinking about planets and solar systems. The radius of Neptune is almost 4 times greater than Earth’s, and the planet’s mass is 17 times greater than our planet. (NASA)
The discovery of a menagerie of exoplanets sized greater than Earth and smaller than Neptune has changed thinking about planets and solar systems. The radius of Neptune is almost 4 times greater than Earth’s, and the planet’s mass is 17 times greater than our planet. (NASA)

When the first exoplanet was identified and confirmed 20 years ago, there was enormous excitement, a sense of historic breakthrough and, with almost parallel intensity, sheer bewilderment. The planet, 51 Pegasi B, was larger than Jupiter yet orbited its parent star in 4 days. In other words, it was much closer to its star than Mercury is to ours and so was extremely hot.

According to theories of the time about planetary formation and solar system organization, a hot Jupiter so close to its sun was impossible. That kind of close-in orbit is where small rocky planets might be found, not Jupiters that belonged much further out and were presumed to always be cold.

That was a soberingly appropriate introduction to the new era of exoplanets, and set the stage for 20 years of surprises and re-evaluations of long held theories and understandings.

While the presence of close-in hot Jupiters certainly remains one of the great puzzles of the exo-planet era, the most consequential exoplanetary revelation has likely been the discovery of many planets larger than Earth and smaller than the next largest planet in our solar system — icy, gaseous Neptune.

These “super-Earths’ and “sub-Neptunes” range greatly in size since Neptune has a radius four times greater than our planet. What’s so surprising about the presence of this class of planets is that they are not just common, they are by far the most frequently detected exoplanets to date.

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Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars, by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)

Perhaps most intriguing of all, however, is their absence in our planetary line-up.

It has long been predicted that the planetary make-up of our solar system would be typical of others, but now we know that is (again) wrong. As Mark Marley, a staff scientist at NASA’s Ames Research Center who studies exoplanets put it, the widespread presence of “super-Earths” elsewhere and their absence in our system “is telling us something quite important.” The work to tease out what that might be has just begun, and will likely keep scientists busy for some time.

“It certainly seems that the universe wants to makes these planets,” Marley told me. “And they’re surprising not only because nobody predicted their vast number but also because they have been intractable – very, very difficult to characterize. It seems like they want to keep their secrets close to the vest.”

How are these planets keeping their secrets – the ingredients of their atmospheres, in particular – from researchers?   Because many seem to be surrounded by thick clouds and layers of sooty smog, like Los Angeles on a very bad day. As a result, the spectroscopy normally used to read exoplanet atmospheres and determine what elements and compounds are present is of little use. The instruments can’t see through the thick film

This helps explain why many astronomers and planetary scientists don’t like the term “super-Earths.” The word implies that they are sized-up Earths, but there’s every reason to believe that very few fit into that category. Nonetheless, the name is so compelling that, for now at least, it seems to have stuck – with that addition of “sub-Neptunes.”

This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft. A new analysis has determined the frequencies of planets of all sizes, from Earths up to gas giants. (C. Pulliam & D. Aguilar, CfA)
This artist’s illustration represents the variety of planets being detected by NASA’s Kepler spacecraft. A new analysis has determined the frequencies of planets of all sizes, from Earths up to gas giants. (C. Pulliam & D. Aguilar, CfA)

Despite the difficulties in characterizing these planets, some progress is being made. Researchers Leslie Rogers of Caltech and Lauren Weiss at Berkeley have separately, for instance, determined which super-Earths and mini-Neptunes are likely to be rocky like Earth and which are likely to be gaseous and icy like Neptune.  The cut-off is by no means precise or across-the-board, but it appears that once a planet has a radius more than 1.5 or 1.6 times the size of Earth, it will most likely have a thick gas envelope of hydrogen, helium and sometimes methane and ammonia around it.

Weiss, a Ken & Gloria Levy Graduate Student Fellow, described some other super-Earth/sub-Neptune characteristics that she and others have found. These exoplanets, for instance, very often have nearby companions in the same class. Many of these larger ones (above 1.5 Earth radii) also tend to be fluffy; quite big but not particularly dense. Weiss likens the least dense super-Earths to macarons – a light, French meringue-based confection (that is definitely not a macaroon.)

They may well have cores of iron and some inner rockiness, but they are so light that they have to consist in large part of hydrogen, helium, water and other gases. It is common to find super-Earths and even sub-Neptunes that have much larger diameters than Earth, but have less mass than Earth.

While some of the super-Earths and sub-Neptunes were, and still are being detected using ground-based radial velocity and other techniques, most were found by Kepler.  That means the field is very young because that early data came out only a few years ago.  But it represents such an important and compelling paradigm shift in astronomy and planetary science that a large and growing contingent of researchers has quickly assembled to search for and study these properly high-profile planets – their orbits, their planetary neighbors, their masses, and now to some extent the make-up of their atmospheres and cores. Some of the work involves observation, some theory and some modeling.

As Mark Marley pointed out, these planets are not giving up their secrets easily. And inevitably, given the great interest and limited data, conclusions and findings will be published that appear strong at the time, but are quickly eclipsed by new information.

Take, for instance, the announced interpretation in 2009, 2012 and 2013 of a sub-Neptune size “water world.” While the papers that introduced the possibility of a very wet exoplanet Gliese 1214b contained caveats, the news stories that went around the world reported that the first water world had apparently been discovered. Exciting news, for sure.

The planet Gliese 1214b was initially described as a possible "water world," and the idea caught the public imagination. But subsequent examination, and the characterizing of other super-Earths and sub-Neptunes, has led to a different conclusion: that the planet is most likely covered by a hydrogen/helium envelope and a thick film of sooty dust. (Artist rendering by L. Calçada, European Space Observatory.)
The planet Gliese 1214b was initially described as a possible “water world,” and the idea caught the public imagination. But subsequent examination, and the characterizing of other super-Earths and sub-Neptunes, has led to a different conclusion: that the planet is most likely covered by a hydrogen/helium envelope and a thick film of sooty dust. (Artist rendering by L. Calçada, European Space Observatory.)

But several years later, it is clear that the water world story was premature. The presence of water had never been confirmed for Gleise 1214b, but rather had been inferred by other limited measurements involving mass, radius, and the absence of spectral data, which were together interpreted to mean the possible, or even probable, presence of a steamy, wet atmosphere.

It still may be the case that the planet has abundant water. But follow-up investigation using the Hubble Space Telescope showed conclusively that the planet was covered in clouds of unknown make-up and origin, and that the presence of massive amounts of water could not be properly inferred from the data at hand.

Zachory Berta-Thompson of MIT was one of the key participants in the Gliese 1214b papers, and he agrees that the evidence today does not point to a water world.  “There was a very deep investigation of the GJ 1214b atmosphere with the Hubble, and if water was there it would have been detected,” he said.  (The lead author of that paper was Laura Kreidberg of the University of Chicago.)

“We used the data we had when the planet was discovered, and made calculations and inferences that made sense at the time,” Berta-Thompson said.  “But the field moves quickly and with the discovery of many other sub-Neptunes, we would draw other conclusions.”  Gliese 1214b, he said, is most likely a puffy planet (with an envelope of hydrogen and helium) rather than a water world.

This is not, it should be noted, a knock on the initial paper. If anything, it’s a knock on journalists (of which I have long been one) who highlighted the water world story. But primarily, the Gliese 1214b research is one of numerous examples of the exciting new science of super-Earths and sub-Neptunes playing out at very high speed, with inevitable potholes on a bumpy and terribly hard-to-navigate road.

 

Many Worlds will continue this discussion of super-Earths and sub-Neptunes on Friday, with an emphasis on thinking about whether they might be conducive or anathema to life.

 

 

 

 

 

 

 

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The Exoplanet Era

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Many, and perhaps most stars have solar systems with numerous planets, as in this artist rendering of Kepler 11. (NASA)

Throughout the history of science, moments periodically arrive when new fields of knowledge and discovery just explode.

Cosmology was a kind of dream world until Edwin Hubble established that the universe was expanding, and doing so at an ever-faster rate. A far more vibrant and scientific discipline was born. On a more practical level, it was only three decades ago that rudimentary personal computers were still a novelty, and now computer-controlled, self-driving cars are just on the horizon. And not that long ago, genomics and the mapping of the human genome also went into hyperspeed, and turned the mysterious into the well known.

Most frequently, these bursts of scientific energy and progress are the result of technological innovation, coupled with the far-seeing (and often lonely and initially unsupported) labor and insights of men and women who are simply ahead of the curve.

We are at another of those scientific moments right now, and the subject is exoplanets – the billions (or is it billions of billions?) of planets orbiting stars other than our sun.

The 20th anniversary of the breakthrough discovery of the first exoplanet orbiting a sun, 51 Pegasi B, is being celebrated this month with appropriate fanfare. But while exoplanet discovery remains active and planet hunters increasingly skilled and inventive, it is no longer the edgiest frontier.

Now, astronomers, astrophysicists, astrobiologists, planetary scientists, climatologists, heliophysicists and many more are streaming into a field made so enticing, so seemingly fertile by that discovery of the apparent ubiquitiousness of exoplanets.

The new goal: Identifying the most compelling mysteries of some of those distant planets, and gradually but inexorably finding ever-more inventive ways to solve them. This is a thrilling task on its own, but the potential prize makes it into quite an historic quest. Because that prize is the identification of extraterrestrial life.

The presence of life beyond Earth is something that humans have dreamed about forever – with a seemingly intuitive sense that there just had to be other planets out there, and that it made equal sense that some of them supported life. Hollywood was on to this long ago, but now we have the beginning technology and fast-growing knowledge to transform that intuitive sense of life out there into a working science.

The thin gauzy rim of the planet in foreground is an illustration of its atmosphere. (NASA’s Goddard Space Flight Center)
The thin gauzy rim of the planet in foreground is an illustration of its atmosphere. (NASA’s Goddard Space Flight Center)

Already the masses and orbits of several thousand exoplanets have been measured. Some planets have been identified as rocky like Earth (as opposed to gaseous like Jupiter.) Some have been found in what the field calls “habitable zones” – regions around distant suns where liquid water could plausibly run on a surface –as it does on Earth and once did on Mars. And some exoplanets have even been determined to have specific compounds – carbon dioxide, water, methane, even oxygen – in their atmospheres.

This and more is what I will be exploring, describing, hopefully bringing to life through an on-going examination of this emerging field of science and the inventive scientists working to understand planets and solar systems many light-years away. Theirs is a daunting task for sure, and progress may be halting. But many scientists are convinced that the goal is entirely within reach – that based on discoveries already made, the essential dynamics and characteristics of very different kinds of planets and solar systems are knowable. Thus the name of this offering: “Many Worlds.”

Artist rendering of early stages of planet formation in the swirl and debris of the disk of a new star. (NASA/JPL-Caltech)
Artist rendering of early stages of planet formation in the swirl and debris of the disk of a new star. (NASA/JPL-Caltech)

I was first introduced to, and captivated by, this cosmic search in a class for space journalists taught by scientists including Sara Seager, a dynamic young professor of physics and planetary science at M.I.T., a subsequently-selected MacArthur “genius,” and a pioneer in the field not of discovering exoplanets, but of characterizing them and their atmospheres. And based on her theorizing and the observations of many others, she was convinced that this characterizing would lead to the discovery of very distant extraterrestrtial life, or at least to the discovery of planetary signatures that make the presence of life highly probable. Just this week, she predicted the discovery could take place within a decade.

It was in 2010 that she began her book “Exoplanet Atmospheres” with the statement: “A new era in planetary science is upon us.” I would take it further: A new era has arrived in the human drive to understand the universe and our place in it. Exoplanets and their solar systems are a magnet to young scientists, says Paul Hertz, the head of NASA’s Astrophysics Division. Almost a third of the papers presented at astronomy conferences these days involve exoplanets, he said, and “it’s hard to find scientists in our field under thirty not working on exoplanets.” Go to a major geology conference, or a planetary science meeting, and much the same will be true.

And why not? I think of this moment as akin to the time in the 17th century when early microscopes revealed a universe of life never before seen. So many new questions to ask, so many discoveries to make, so much exciting and ultimately world-changing science ahead.

But the challenge of characterizing exoplanets and some day identifying signs of life does not lend itself to the kind of solitary or small group work that characterized microbiology (think the breakthrough NASA Kepler mission and the large team needed to make it reality and to analyze its results.) Not only does it require costly observatories and telescopes and spectrometers, but it also needs the expertise that scientists from different fields can bring to the task – rather like the effort to map the human genome.

That is the organizing logic of astrobiology – the more general hunt for life elsewhere in our solar system and far beyond, alongside the search for clues into how life may have started on our planet. NASA is eager to encourage that same spirit in the more specific but nonetheless equally sprawling exploration of exoplanets, their atmospheres, their physical makeup, their climates, their suns, their neighborhoods.

The Earth alongside “Super-Earth-” sized exoplanets identified with the Kepler Space Telescope. (NASA Ames / JPL-Caltech)
The Earth alongside “Super-Earth-” sized exoplanets identified with the Kepler Space Telescope. (NASA Ames / JPL-Caltech)

The result was the creation this summer of the the Nexus for Exoplanet System Science (NExSS), a group that will be led by 17 teams of scientists from around the country already working on some aspect of the rich exoplanet opportunity. The group was selected from teams that had applied for grants from NASA’s Astrobiology Institute, an arm of its larger NASA Astrobiology Program, as well as other NASA programs in the Planetary Sciences, Astrophysics and Astronomy divisions.

Their mandate is to spark new approaches in the effort to understand exoplanets by identifying areas without consensus in the broader community, and then fostering collaborations here and abroad to address those issues. “Many Worlds” grew out of the NExSS initiative, and will chronicle and explain the efforts of some team members as they explore how exo-plants and exo-creatures might be detected; what can be learned from afar about the surfaces and cores of exoplanets and how both play into the possibility of faraway life; the presence and dynamics of exo-weather, what we can learn about exoplanets from our own planet and solar system, and so much more.

A few of the teams are small, but many are quite large, established and mature – perhaps most especially the Virtual Planetary Laboratory at the University of Washington, and run by Victoria Meadows. Since 2001, the virtual lab has collaborated with researchers representing many disciplines, and from as many as 20 institutions, to understand what factors might best predict whether an exoplanet harbors life, using Earth as a model.

But just as I will be venturing beyond NExSS in my writing about this new era of exploration, so too will NExSS be open to the involvement of other scientists in the field. The original group has been tasked with identifying an agenda of sorts for NASA exoplanet missions and efforts ahead. But its aim is to be inclusive and its conclusions and recommendations will only be as useful and important as the exoplanet community writ large determines them to be.

The Carina Nebula, one of many regions where stars come together and planets later form made out of the surrounding dust, gas and later rock. (NASA, ESA, and the Hubble SM4 ERO Team)
The Carina Nebula, one of many regions where stars come together and planets later form made out of the surrounding dust, gas and later rock. (NASA, ESA, and the Hubble SM4 ERO Team)

This is a moment pregnant with promise. Systematically investigating exoplanets and their environs is an engine for discovery and a pathway into that largest question of whether or not we are alone in the universe.

Will scientists some day find worlds where donkeys talk and pigs can fly (as at least one “everything is possible” philosopher has posited)? Unlikely.

But just as microscopes and the scientists using them led to the science of microbiology and most of modern medicine, so too are our orbiting observatories, Earth-based telescopes and the scientists who analyze their results are regularly opening up a world of myriad and often surprising marvels.

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