Getting Real About the Oxygen Biosignature

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Oxygen, which makes up about 21 percent of the Earth atmosphere, has been embraced as the best biosignature for life on faraway exoplanets. New research shows that detecting distant life via the oxygen biosignature is not so straight-forward, though it probably remains the best show we have. (NASA)

I remember the first time I heard about the atmospheres of distant exoplanets and how could and would let us know whether life was present below.

The key was oxygen or its light-modified form, ozone.  Because both oxygen and ozone molecules bond so quickly with other molecules — think rust or iron oxide on Mars, silicon dioxide in the Earth’s crust — it was said that oxygen could only be present in large and detectable quantities if there was a steady and massive source of free oxygen on the planet.

On Earth, this of course is the work of photosynthesizers such as planets, algae and cyanobacteria, which produce oxygen as a byproduct.  No other abiotic, or non-biological, ways were known at the time to produce substantial amounts of atmospheric oxygen, so it seemed that an oxygen signal from afar would be a pretty sure sign of life.

But with the fast growth of the field of exoplanet atmospheres and the very real possibility of having technology available in the years ahead that could measure the components of those atmospheres, scientists have been busy modelling exoplanet formations, chemistry and their atmospheres.

One important goal has been to search for non-biological ways to produce large enough amounts of atmospheric oxygen that might fool us into thinking that life has been found below.

And in recent years, scientists have succeeded in poking holes in the atmospheric oxygen-means-life scenario.

Oxygen bonds quickly with many other molecules. That means has to be resupplied regularly to be present as O2 in an atmosphere . On Earth, O is mostly a product of biology, but elsewhere it might be result of non-biological processes. Here is an image of oxygen bubbles in water.

Especially researchers at the University of Washington’s Virtual Planetary Laboratory (VPL) have come up with numerous ways that exoplanets atmospheres can be filled (and constantly refilled) with oxygen that was never part of plant or algal or bacteria photo-chemistry.

In other words, they found potential false positives for atmospheric oxygen as a biosignature, to the dismay of many exoplanet scientists.

In part because she and her own team were involved in some of these oxygen false-positive papers, VPL director Victoria Meadows set out to review, analyze and come to some conclusions about what had become the oxygen-biosignature problem.

The lengthy paper (originally planned for 6 pages but ultimately 34 pages because research from so many disciplines was coming in) was published last month in the journal Astrobiology.  It seeks to both warn researchers about the possibilities of biosignature false-positives based on oxygen detection, and then it assures them that there are ways around the obstacles.

“There was this view in the community that oxygen could only be formed by photosynthesis, and that no other process could make O2,”  Meadows told me.  “It was a little simplistic.  We now see the rich complexity of what we are looking at, and are thinking about the evolutionary paths of these planets.

 

Artist’s impression of the exoplanet GJ 1132 b, which orbits the red dwarf star GJ 1132.  Earlier this year, astronomers managed to detect the atmosphere of this Earth-sized planet and have determined that water and methane are likely prevalent in the atmosphere.  (Max Planck Institute for Astronomy)

“What I see is a maturing of the field.  We have models that show plausible ways for oxygen to be produced without biology, but that doesn’t mean that oxygen is no longer an important biosignature.

“It is very important.  But it has to be seen and understood in the larger context of what else is happening on the planet and its host star.”

Before moving forward, perhaps we should look back a bit at the history of oxygen on Earth.

For substantial parts of our planet’s history there was only minimal oxygen in the atmosphere, and life survived in an anaerobic environment.  When exactly oxygen went from a small percentage of the atmosphere to 21 percent of the atmosphere is contested, but there is broader agreement about the source of the O2 in the atmosphere.  The source was photosynthesis, most importantly coming from cyanobacteria in the oceans.

As far back as four billion years ago, photosynthesis occurred on Earth based on the capturing of the energy of near infrared light by sulfur-rich organisms, but it did not involve the release of oxygen as a byproduct.

A chart showing the percentage rise in oxygen in Earth’s atmosphere over the past 3.8 billion years. The great oxidation event occurred some 2.3 billion years ago, but it took more than a billion additional years for the build-up to have much effect on the composition of the planet’s atmosphere.

Then came the the rise of cyanobacteria in the ocean and their production of oxygen.  With their significantly expanded ability to use photosynthesis, this bacterium was able to generate up to 16 times more energy than its counterparts, which allowed it to out-compete and explode in reproduction.

It took hundreds of millions of years more, but that steady increase in the cyanobacteria population led to what is called the “Great Oxidation Event” of some 2.3 billion years ago, when oxygen levels began to really climb in Earth’s atmosphere.  They did level off and remained well below current levels for another billion years, but then shot up in the past billion years.

As Meadows (and others) point out, this means that life existed on Earth for at least two billion years years without producing a detectable oxygen biosignature.  It’s perhaps the ultimate false negative.

But as biosignatures go, oxygen offers a lot.  Because it bonds so readily with other elements and compounds, it remains unbonded or “free” O2 only if it is being constantly produced.  On Earth, the mode of production is overwhelmingly photosynthesis and biology.  What’s more, phototrophs — organism that manufacture their own food from inorganic substances using light for energy — often produce reflections and seasonally dependent biosignatures that can serve as secondary confirmations of biology as the source for abundant Oin an atmosphere.

So in a general way, it makes perfect sense to think that O in the atmosphere of an exoplanet would signify the presence of photosynthesis and life.

Victoria “Vikki” Meadows is the director of the Virtual Planetary Institute at the University of Washington, which has been an important engine for NASA’s Astrobiology Institute (NAI) since 2001.  Among its many lines of research, her group focuses on the Earth as a template for understanding exoplanets, and so Meadows is holding up a rock here as a whimsical nod to that approach.  (University of Washington.)

The problem arises because other worlds out there orbiting stars very different than our own can have quite different chemical and physical dynamics and evolutionary histories, with results at odds with our world.

For instance, when it comes to the non-biological production of substantial amounts of oxygen that could collect in the atmosphere, the dynamics involved could include the following:

Perhaps the trickiest false positive involves the possible non-biological release of O2 via the photolysis of water — the breaking apart of H2O molecules by light.  On Earth, the water vapor in the atmosphere condenses into liquids after reaching a certain height and related temperature, and ultimately falls back down to the surface.  How and why that happens is related to the presence of large amounts of nitrogen in our atmosphere.

But what if an exoplanet atmosphere doesn’t have a lot of an element like nitrogen that allows the water to condense?  Then the water would rise into the stratosphere, where it would be subject to intense UV light,. The molecule would be split, and an H atom would fly off into space — leaving behind large amounts of oxygen that had nothing to do with life.  This conclusion was reached by Robin Wordsworth and Raymond Pierrehumbert of the University of Chicago and was published by the The Astrophysical Journal.

Another recently proposed mechanism to generate high levels of abiotic oxygen, first described by Rodrigo Luger and Rory Barnes of Meadow’s VPL team, focuses on the effects of the super-luminous phase of young stars on any rocky planets that might be orbiting them.

Small-mass M dwarfs in particular can burn much brighter when they are young, exposing potential planets around those stars to very high levels of radiation for as long as one billion years.

Modeling suggests that during this super-luminous phase a terrestrial planet that forms within what will become the main sequence habitable zone around an M dwarf star may lose up to several Earth ocean equivalents of water due to evaporation and hydrodynamic escape, and this can lead to generation of large amounts of abiotic O via the same H2O photolysis process.

Red dwarf, or M stars, are the most common in the cosmos.  They start off with a long period of extremely high luminosity and radiation before evolving into low-energy cool (and red) stars.  While a mature red dwarf star might have habitable zone planets that appear today to have characteristics conducive to life,  exoplanet modelers have determined that many of those red dwarf stars may well have lost their oceans during their early  long exposure to intense radiation. This is an artist rendering of three exoplanets around a red dwarf star. (ESO/M. Kornmesser)

Non-biological oxygen can also build up on an exoplanet, according to a number of researchers, if the host star sends out a higher proportion of far ultraviolet light than near ultraviolet.  The dynamics of photo-chemistry are such, they argue, that the excess far ultraviolet radiation would split CO2 to an extent that O2 would build up in the atmosphere.

There are other potential scenarios that would produce an oxygen false positive, and almost all of them involve radiation from the host star driving chemistry in the planet’s atmosphere, with the planetary environment then allowing O2 to build up.  While some of these false positive mechanisms can produce enough oxygen to make a big impact on their planets, some may not produce enough to even be seen by telescopes currently being planned.

As Meadows tells it, it was Shawn Domagal-Goldman of NASA Goddard and VPL who first brought the issue of oxygen false-positives to her attention. It was back in 2010 after he found an anomaly in his photo-chemical code results regarding atmospheric oxygen and exoplanets, and followed it. Since that initial finding, several other VPL researchers discovered new ways to produce O2 without life, and often while undertaking research focused on a different scientific goal.

Six years later, when she was writing up a VPL annual report, it jumped out that the group (and others) had found quite a few potential oxygen false positives — a significant development in the field of biosignature detection and interpretation.  That’s when she decided that an analysis and summary of the findings would be useful and important for the exoplanet community.  “Never let it be said that administrative tasks can’t lead to inspiration!” she wrote to me.

While Meadows does not downplay the new challenges to defining oxygen and ozone as credible biosignatures, she does say that these new understandings can be worked around.

Some of that involves targeting planets and stars for observation that don’t have the characteristics known to produce abiotic oxygen.  Some involves finding signatures of this abiotic oxygen that can be identified and then used to discard potential false positives.  And perhaps most telling, the detection of methane alongside free oxygen in an exoplanet atmosphere would be considered a powerful signature of life.

The Virtual Planetary Laboratory investigates the potential habitability of extrasolar planets. The research will help in predicting the habitability of discovered bodies like the Earth-size planets orbiting TRAPPIST-1 and the planet orbiting our closest neighbor, Proxima Centauri. (NASA)

The official goal of Meadows’ VPL is to wrestle with this question: “How would we determine if an extrasolar planet were able to support life or had life on it already?”

This has led her to a highly interdisciplinary approach, bringing together fifty researchers from twenty institutions.  In addition to its leading role in the NASA Astrobiology Institute, the VPL is also part of a broad NASA initiative to bring together scientists from different locales and disciplines to work on issues and problems of exoplanet research — the Nexus for Exoplanet System Science, or NExSS.

Given this background and these approaches, it is hardly surprising that Meadows would be among the first to see the oxygen-false positive issue in both scientific and collective terms.

“I wanted the community to have some place to go to when thinking about Ofalse positives,” she said. “We’re learning now about the complexity and richness of exoplanets, and this is essential for preparing to do the best job possible {in terms of looking for signs of life on exoplanets} when we get better and better observations to work with.”

“This story needed to be told now. Forewarned is forearmed.”

 

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Proxima b Is Surely Not “Earth-like.” But It’s A Research Magnet And Just May Be Habitable.

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Simulated comparison of a sunset on Earth and Proxima b. The red-dwarf star Proxima Centauri appears almost three times bigger than the Sun in a redder and darker sky. Red-dwarf stars appear bigger in the sky than sun-like stars, even though they are smaller. This is because they are cooler and the planets have to be closer to them to maintain temperate conditions. The original photo of the beach was taken at Playa Puerto Nuevo in Vega Baja, Puerto Rico. Credit: PHL @ UPR Arecibo.
A simulated comparison of a sunset on Earth and Proxima b. The images sets out to show that the red-dwarf star Proxima Centauri appears almost three times bigger than our sun in a redder and darker sky. There is value in illustrating how conditions in different solar systems would change physical conditions on the planets, but there is a real danger that the message conveyed becomes the similarities between planets such as Earth and Proxima b.  At this point, there is no evidence that Proxima b is “Earth-like” at all. The original photo of the beach was taken at Playa Puerto Nuevo in Vega Baja, Puerto Rico. (PHL @ UPR Arecibo))

It is often discussed within the community of exoplanet scientists that a danger lies in the description of intriguing exoplanets as “Earth-like.”

Nothing discovered so far warrants the designation, which is pretty nebulous anyway.  Size and the planet’s distance from a host star are usually what earn it the title “Earth-like,” with its inescapable expectation of inherent habitability. But residing in a habitable zone is just the beginning; factors ranging from the make-up of the planet’s host star to the presence and content of an atmosphere and whether it has a magnetic field can be equally important.

The recent announcement of the detection of a planet orbiting Proxima Centauri, the closest star to our own, set off another round of excitement about an “Earth-like” planet.  It was generally not scientists who used that phrase — or if they did, it was in the context of certain “Earth-like” conditions.  But the term nonetheless became a kind of shorthand for signalling a major discovery that just might some day even yield a finding of extraterrestrial life.

Consider, however, what is actually known about Proxima b:

  • The planet, which has a minimum mass of 1.3 Earths and a maximum of many Earths, orbits a red dwarf star.  These are the most common class of star in the galaxy, and they put out considerably less luminosity than a star like our sun — about one-tenth of one percent of the power.
  • These less powerful red dwarf stars often have planets orbiting much closer to them than what’s found in solar systems like our own.   Proxima b, for instance, circles the star in 11.3 days.
  • A consequence of this proximity is that the planet is most likely tidally locked by the gravitational forces of the star — meaning that the planet does not rotate like Earth does but rather has a daytime and nighttime side like our moon.  Some now argue that a tidally locked planet could theoretically be habitable,  but the consensus seems to be that it is an obstacle to habitability rather than a benefit.
  • The authors of the Proxima b paper make clear that evidence that the planet is rocky (as opposed to gaseous) is limited, and that’s why they label it as a “candidate terrestrial planet.”

So to describe Proxima b as “Earth-like” seems unfortunate to me, and prone to giving the public the misguided impression of a planet with blue skies, oceans, and fish swimming in them.  Proxima b may have some very broadly defined characteristics that parallel Earth, but so do many other exoplanets that are definitely not habitable.  And therein lies the really interesting part.

Before getting into that area, it should be made clear that the Proxima b detection was a game-changer, a discovery of historic proportions.  It was and will be for a long time.

The detection, after all, provides the nearest opportunity possible for beginning to understand the extraordinary complexities of what makes a planet truly habitable — something that is far from understood today.  And later, Proxima b may become a petri dish of sorts for identifying and measuring chemical signatures that could be a byproduct of some kind of life.

These are difficult tasks, to say the least, and will take an army of scientists years to come up with answers.  But the good news is that the exoplanet field has already begun publishing papers on the dynamics and possible habitability of Proxima b, and they provide beginning insights into the issues, the excitement and the untold difficulties associated with this grandest of scientific chases.

The detection of Proxima b has been met with enormous enthusiasm in the exoplanet community. Some call it the biggest discovery since the detection of 51 Pegasi a, the first exoplanet to be positively identified. Detecting a planet, however, is just the beginning of the still unsettled process of determining its history and current makeup, and whether or not it might be habitable.
The detection of Proxima b has been met with enormous enthusiasm in the exoplanet community. Some call it the biggest discovery since the detection of 51 Pegasi a, the first exoplanet to be positively identified. Detecting a planet, however, is just the beginning of the still unsettled process of determining its history and current makeup, and whether or not it might be habitable. (ESO/L.Calçada/Nick Risinger)

Two Proxima papers that appeared soon after the August 24 announcement came from the University of Washington’s Virtual Planetary Laboratory. Supported by the NASA Astrobiology Institute since 2001, it is a leader in exoplanet research, modelling and habitability  The team, directed by Victoria Meadows, received the Proxima detection paper before it was released because of longstanding relationships with the lead author, Guillem Anglada-Escude.

The two papers from the VPL team — one with Meadows in the lead and the other organized by University of Washington astronomer Rory Barnes — take broad, interdisciplinary approaches to the planet.

“Part one is what happened to this planet over time — what can we learn about its history, its evolution?” Meadows said.  “Part two is what does the history mean for the current environment right now? We need photo-chemical models of what might be present, and then we have to look at what instruments we would need to detect what is decided we should be looking for.”

Victoria Meadows is an astrobiologist and planetary astronomer whose research interests focus on acquisition and analysis of remote-sensing observations of planetary atmospheres and surfaces. In addition to studying planets within our own Solar System, she is interested in exoplanets, planetary habitability and biosignatures. Since 2000, she has been the Principal Investigator for the Virtual Planetary Laboratory Lead Team of the NASA Astrobiology Institute. Her NAI team uses models of planets, including planet-star interactions, to generate plausible planetary environments and spectra for extrasolar terrestrial planets and the early Earth. This research is being used to help define signs of habitability and life for future extrasolar terrestrial planet detection and characterization missions.
Victoria Meadows is an astrobiologist and planetary astronomer at the University of Washington. Since 2000, she has been the Principal Investigator for the Virtual Planetary Laboratory Lead Team of the NASA Astrobiology Institute. Her NAI team uses models of planets, including planet-star interactions, to generate plausible planetary environments and spectra for extrasolar terrestrial planets and the early Earth.

The Barnes paper is here and the Meadows paper is here.

The two take on different tasks, but really are one. Both start with an appreciative nod to what will quickly become the planet that scientists want to study, and then they go into the extraordinary complexity of the task ahead.

As Barnes and his team concluded, a major obstacle to habitability on Proxima b is the well documented evolution of red dwarf stars.

While their energy output is relatively low when mature, they go through early phases when they are much brighter and send out enormous solar flares that can double their brightness in a matter of minutes.  Barnes said these intense phases could easily sterilize a close-in planet, leaving it incapable of evolving into a potentially living world even if, a billion years later, conditions for life were much more favorable.

“The planet is in a habitable zone and so could have had liquid water,  but my biggest concern is the retention of that water,” Barnes said of Proxima b.  “If the planet was formed in its current orbit, then it was baked enough for 100, 200 million years to form a runaway greenhouse effect.”  A different dynamic may have resulted in the same results on Venus, which once was wet but now is super-hot and parched.

This leads to the next big question:  Could Proxima b have been formed elsewhere, and was later pushed or pulled to its current location?  Barnes said it is certainly possible that the planet spent its early years much further from Proxima Centauri, and he said that the (relatively) nearby presence of much larger star Alpha Centauri A and B certainly could have had dramatic effects on the locations and evolution of the planet.

For these reasons and many more, Barnes said, there is absolutely no way to conclude now that Proxima b either is, or is not, potentially habitable.

This artist's impression shows a view of the surface of the planet Proxima b orbiting t he red dwarf star Proxima Centauri, the closest star to the Solar System. The double star A lpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, wh ere the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser
This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red
dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB
also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than
the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature issuitable for liquid water to exist on its surface.
(ESO/M. Kornmesser)

The VPL group specializes in modeling possible planetary scenarios based on particular proposed condition.  The team gets an idea of whether a planet might be habitable based on the history and potential current atmospheric and surface characteristics introduced into the model.

In the paper she led, Meadows and her team reported:

“We used coupled climate-photochemistry models to simulate several plausible states for the current environment of Proxima Cen b, for those various evolutionary scenarios. We find several post-runaway {greenhouse} states that are uninhabitable either due to extreme water loss or inclement surface temperatures. In particular, a dense Venus-like CO2 atmosphere will result in extremely high surface temperatures at Proxima Cen’s current semi-major axis.

“However, several evolutionary scenarios may lead to possibly habitable planetary environments, including O2-rich atmospheres that retain a remnant ocean after extreme water loss.”

So the conclusion of the VPL effort is that we really have no idea now whether Proxima b might be habitable, but the joined papers move the discussion significantly further by describing scenarios where the planet definitely would not be habitable, and some where it just might be.

These are essential guidepost for astronomers to know when actually observing Proxima b, which will surely become a target for many a telescope.  To make an important discovery,  it’s definitely useful  to know what you’re looking for.

Meadows also looked into which telescopes have capacities to make the needed observations, and concluded that large ground-based telescopes have a role to play, though with current technology it will be a very challenging one.  But given the possible results, she said, “I can’t image they wouldn’t make the upgrades happen as fast a possible.”

And then there’s the James Webb Space Telescope (JWST), due to launch in 2018.  Because it observes in the infrared section of the spectrum, it is able to measure heat signatures with precision.  And that opens some exciting possibilities.

Caption: This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lowe r-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope. Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani
This picture combines a view of the southern skies over the European Southern Observatory’s  3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discoveredusing the HARPS instrument on the ESO 3.6-metre telescope.
 (Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani)

Meadows and her team laid them out, and so did Harvard astronomer Laura Kreidberg, in a paper for The Astrophysical Journal with Harvard-Smithsonian Center for Astrophysics theoretical physicist and cosmologist  Abraham Loeb.

As Loeb explained in an email:  “As the planet orbits around the star, we should see a changing fraction of its day side, similarly to the phases of the Earth’s moon. The changing  color of the planet  as it orbits the star provides evidence for the temperature contrast between its day and night sides.

“This contrast has an extreme value for bare rock, but is moderated by an atmosphere or an ocean that transfers heat across the planet’s surface. Our paper shows that JWST will be able to distinguish between these cases with high significance after observing the planet for a full orbital time of 11 days.”

In other words, the temperature difference between the planet’s day side and its night side will be larger than expected if there is no atmosphere., and lower than expected if there is.

Determining that there is an atmosphere present, Loeb said, would substantially increase the chances that Proxima b is, or once was, habitable.  The first order would be to look for things like oxygen, water vapor, and methane, which could indicate habitable conditions if not active biological processes.

This is a very difficult task because it requires the ability to catch starlight as it bounces off or filters through the planet’s atmosphere.  He said that while the JWST might be able to detect a few compounds including ozone, full atmospheric analysis will have to wait for future ground-based observatories like the European Extremely Large Telescope, which is expected to see first light in the mid-2020s. Ultimately, it will take a direct imaging space telescope like the one being proposed for a launch in the 2030s to answer many of the important questions.

So the process of getting to really know Proxima b, of learning more than its promising but less-than-revealing location and mass is about to begin.

It’s exciting for sure, and not because Proxima b is “Earth-like.”  Rather, there’s the real possibility of finding a habitable planet that — except in some grand-scale structural ways — is really not so “Earth-like” at all.

 

 

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Ranking Exoplanet Habitability

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The Virtual Planetary Lab at the University of Washington has been working to rank exoplanets (or exoplanet candidates) by how likely they are to be habitable. (Rory Barnes)
The Virtual Planetary Lab at the University of Washington has been working to rank exoplanets (or exoplanet candidates) by how likely they are to be habitable. (Rory Barnes)

 

Now that we know that there are billions and billions of planets beyond our solar system, and we even know where thousands of confirmed and candidate planets are located, where should we be looking for those planets that could in theory support extraterrestrial life, and might just possibly support it now?

The first order answer is, of course, the habitable zone — that region around a host star that would allow orbiting planets to have liquid water on the surface at least some of the time.

That assertion is by definition a theoretical one — at this point we have no detection of an exoplanet with liquid water orbiting a distant star — and it is actually a rather long-held view.

For instance, this is what William Whewell, the prominent British natural philosopher-scientist-theologian (and Master of Trinity College at Cambridge) wrote in 1853:

William Whewell was
William Whewell was an early proponent of a region akin to a habitable zone.  He also coined the words “scientist” and “physicist.”

“The Earth is really the domestic hearth of this solar system; adjusted between the hot and fiery haze on one side, the cold and watery vapour on the other.  This region is fit to be the seat of habitation; and in this region is placed the largest solid globe of our system; and on this globe, by a series of creative operations…has been established, in succession, plants, and animals, and man…The Earth alone has become a World.”

Whewell wrongly limited his analysis to our solar system, but he was pretty much on target regarding the crude basics of a habitable zone. His was followed over the decades by other related theoretical assessments, including in more modern times Steven Dole for the Rand Corporation in 1964 and NASA’s Michael Hart in 1979.  All pretty much based on an Earth-centric view of habitable zones throughout the cosmos.

It was this approach, even in its far more sophisticated modern versions, that got some of the scientists at the University of Washington’s Virtual Planetary Laboratory thinking three years ago about how they might do better.  What they wanted to do was to join the theory of the habitable (or more colloquially, the “Goldilocks zone”) with actual data now coming in from measurements of transiting exoplanets.

Although the measurements remain pretty limited, the group was convinced that the process could come up with the beginnings of a “Habitability Index” that would rate — based on evidence-based calculations and models — which exoplanets had the best chance of being able to support life.

“We certainly are constrained by the observations being made, but we do have some important physical measurements to work with,” said Rory Barnes, a astrophysical theorist with the VPL.  “And what we’ve done is to connect the possibility of life with the fundamental observables we do have….This really hasn’t been done before.”

Of the 1,030 confirmed planets from Kepler, a dozen are less than twice the size of Earth and reside in the habitable zone of their host star. The sizes of the exoplanets are represented by the size of each sphere. These are arranged by size from left to right, and by the type of star they orbit, from the M stars that are significantly cooler and smaller than the sun, to the K stars that are somewhat cooler and smaller than the sun, to the G stars that include the sun. The sizes of the planets are enlarged by 25X compared to the stars. The Earth is shown for reference. NASA Ames/JPL-CalTech/R. Hurt
Of the 1,030 confirmed planets from Kepler, a dozen are less than twice the size of Earth and reside in the habitable zone of their host star. They are arranged by by size and by the type of star they orbit — from the M stars that are significantly cooler and smaller than the sun, to the K stars that are somewhat cooler and smaller than the sun, to the G stars that include the sun. The sizes of the planets are enlarged by 25 times compared to the stars. The Earth is shown for reference. (NASA Ames/JPL-CalTech/R. Hurt)

The result was a detailed paper in the Astrophysical Journal that showed observations and modeling that can be harnessed together to come up with a list of the 10 exo-objects most likely to support life.   I specifically didn’t write “exoplanets” because nine of the ten remain  “candidate” planets detected by the Kepler Space Telescope as transiting objects that block out a small bit of light from the host star.  But they have not yet been confirmed through other detection techniques.

And why do the hard work of teasing out the potentially most habitable planets (objects) from the many thousands of others identified?  Clearly, it’s not because the data will point to some planet/objects that have a very good chance of being habitable.  The information available just won’t allow for that.

Rather, the next-generation James Webb Space Telescope is scheduled to launch in 2018, and it will be able to measure the components of exoplanets and their atmospheres in a whole new way.But access to a telescope like the JWST is costly and the observing and analyzing is and time-consuming.  And so the Virtual Planetary Laboratory’s index is designed to help fellow astronomers identify which worlds might have the best chance of hosting life, and so are worthy of all the necessary time and money.

Is the Habitability Index that much more useful than the more traditional habitable zone assessments based on a planet’s proximity to a particular star of a particular strength?  And is it more predictive than some related assessments such as the Earth Similarity Index, created by Abel Mendez at the University of Puerto Rico at Arecibo.

Because it takes into account so much more information, it certainly seems likely that it is more predictive, especially as new and better information is added to the system.  While the traditional habitable zone points to a locations, the Habitability Index identifies distinctions within a habitable zone that would make an exoplanet more or less likely to support life.

rory

Rory Barnes is a theorist in the Virtual Planetary Laboratory primarily interested in the formation and evolution of habitable planets.
Rory Barnes is a theorist in the Virtual Planetary Laboratory primarily interested in the formation and evolution of habitable planets.

The new index is more nuanced, producing a continuum of values that astronomers can punch into a Virtual Planetary Laboratory Web form to arrive at the single-number habitability index.

In creating the index, the researchers factored in estimates of a planet’s rockiness, rocky planets being the more Earth-like. They also accounted for a phenomenon called “eccentricity-albedo degeneracy,” which comments on a sort of balancing act between the a planet’s albedo — the energy reflected back to space from its surface — and the circularity of its orbit, which affects how much energy it receives from its host star.

The two counteract each other. The higher a planet’s albedo, the more light and energy are reflected off to space, leaving less at the surface to warm the world and aid possible life. But the more non-circular or eccentric a planet’s orbit, the more intense is the energy it gets when passing close to its star in its elliptic journey.

A life-friendly energy equilibrium for a planet near the inner edge of the habitable zone — in danger of being too hot for life — Barnes said, would be a higher albedo, to cool the world by reflecting some of that heat into space. Conversely, a planet near the cool outer edge of the habitable zone would perhaps need a higher level of orbital eccentricity to provide the energy needed for life.

These are the kinds of measurements being analyzed as well by the NASA’s Kepler Habitable Zone Working Group, a collection of scientists within the Kepler team with the task of identifying some of the most promising targets for future observation.

Stephen Kane is leading the group, and expects to come out with an assessment this summer.

Barnes, Meadows and Evans ranked in this way planets so far found by the Kepler Space Telescope, in its original mission as well as its “K2” follow-up mission. They found that the best candidates for habitability and life are those planets that get about 60 percent to 90 percent of the solar radiation that the Earth receives from the sun, which is in keeping with current thinking about a star’s habitable zone.

The research is part of the ongoing work of the Virtual Planetary Laboratory to study faraway planets in the ongoing search for life, and was funded by the NASA Astrobiology Institute.

“This innovative step allows us to move beyond the two-dimensional habitable zone concept to generate a flexible framework for prioritization that can include multiple observable characteristics and factors that affect planetary habitability,” said Meadows.

“The power of the habitability index will grow as we learn more about exoplanets from both observations and theory.”

 

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