Forget the “Habitable Zone,” Think the “Biogenic Zone”

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An eruption on April 16, 2012 was captured here by NASA's Solar Dynamics Observatory in the 304 Angstrom wavelength, which is typically colored in red. Credit: NASA/SDO/AIA
A highly-energetic coronal mass ejection coming off the sun in 2012 was captured here by NASA’s Solar Dynamics Observatory.  Increasingly, the study of exoplanets and their potential habitability is focusing on the nature and dynamics of host stars.  (NASA/SDO/AIA)

 

It is hardly surprising that in this burgeoning exoplanet era of ours, those hitherto unknown planets get most of the attention when it comes to exo-solar systems.  What are the planet masses?  Their orbits?  The chemical makeup of their atmospheres? Their potential capacity to hold liquid surface water and thereby become “habitable.”

Less frequently highlighted in this exoplanet scenario are the host stars around which the planets orbit.  We’ve known for a long time, after all, that there are billions and billions of stars out there, and have only known for sure that there are planets for 20 years.  So the stars hosting exoplanets have largely played a background role focused on detection:  Does the light curve of a star show the tiny dips that tell of a transiting planet?  Does a star “wobble” every so slightly due to the gravitational forces or orbiting planets.

Gradually, however, that backseat role for stars in the exoplanet story is starting to change, especially as the key question moves from whether new exoplanets have been found to whether they hold the potential to support life.

And a growing number of scientists — and especially those specializing in stars — argue that central to that latter question are understanding the make-up and dynamics of the host stars.

Vladimir Airapetian, a research heliophysicist and astrophysicist at NASA’s Goddard Space Flight Center, has been a leader in this emphasis on the stellar side of the exoplanet story.  And now, he has proposed a re-conceiving  and re-naming of that area around stars where planets could potentially host liquid water and support life — the so-called “Goldilocks” or habitable zone.

His alternative:  the “biogenic zone.”

“Liquid water is undeniably important for possible life on a planet, but it is not sufficient,” he told me.  “I believe that equally important is the amount of  energy coming from the host star.

“The last twenty years has seen a huge increase in knowledge about our own sun, and the lessons learned are now being used on exoplanet-host star systems.  This is essential because without an understanding of the energy arriving at a planet from a star, it’s really impossible to assess its potential to support life.”

 

The power and dynamics of a host star plays an enormous --and increasingly studied -- role in assessing whether an exoplanet is potentially habitable. Artist rendering of planet transiting a xxx.
The power and dynamics of host stars play an enormous –and increasingly studied — role in determining whether an exoplanet is potentially habitable. Artist rendering of the transiting a “hot Jupiter” planet.  (NASA).

 

Airapetian made something of a splash last month with the release of a paper in Nature Geoscience that both potentially helps explain the conditions that allowed life to form on Earth (and possibly Mars), while also presenting a intriguing theory on how parallel conditions could be present on many exoplanets.

And it all has to do with the dynamics of our sun — in particular, the coronal mass ejections (CMEs),  the counterparts of  “super-flares” that can send vast amounts of energy out into the solar system.  They are the result of huge stellar explosions of magnetic field and of plasma, the fourth form of matter in the universe. Plasma in stars is mostly made up of hydrogen and helium atoms stripped of their electrons, and consequently they are highly energetic.

So how to CMEs play into the origin of life story?  Please bare with me a bit.

One of the more vexing questions about early Earth (and early Mars) is that the sun they orbited sent out measurably less energy than the sun does now, about 70 percent of today’s power.  This would be consistent with the known evolution of all stars like ours, which gain strength over time.

This “faint young sun” problem greatly complicates hypotheses for how and when life began in our solar system because it seems to preclude the pooling of liquid water on Earth, and certainly Mars, for much of the early epoch.  That includes the period when life is believed to have begun on Earth (somewhere between 3.5 and 3.8 billion years ago) and also the period on Mars when, thanks to the Curiosity rover, we know for sure now that water ran and pooled.  Especially on more distant Mars, where few remnants of potentially warming greenhouse gases have been found, the faint young sun should have produced an absolutely frigid planet.

Actually, Carl Sagan and colleague George Muller argued the same about Earth back in the 1970s.  But geological and paleontological evidence convincingly showed otherwise.

 

According to the stellar evolution theory, the young Sun radiated much less energy than it does today. It was only about one billion years ago that it warmed the Earth to above the freezing point of water. The Cambrian explosion followed 1/2 billion years later to initiate the diversification of multicellular life. However geological evidence has shown that unicellular organisms existed between 3.5 to 3.8 billion years ago even when there was not enough solar energy to liquefy the water. This is known as the “faint young sun paradox"
According to the stellar evolution theory, the young Sun radiated much less energy than it does today. It was only about one billion years ago that it warmed the Earth to above the freezing point of water. The Cambrian explosion followed 1/2 billion years later to initiate the diversification of multicellular life. However geological evidence has shown that unicellular organisms existed between 3.5 to 3.8 billion years ago even when there was not enough solar energy to liquefy the water. This is known as the “faint young sun paradox”

 

In his Nature paper, Airapetian proposes a solution, one based on valuable data collected by the Kepler Space Telescope, but little known to the public. What Kepler found was that many young stars experience “super-flares,” and they occur not infrequently.

“These huge events were detected in the optical band, and were generally most powerful on young stars,” he said.  “Scaling that information gave us a sense of super-flare frequency on our young sun, and it was roughly once every ten days.  These powerful flares usually come with energetic CMEs that may last for several days, so that’s a huge amount of energy hitting the early Earth.”

As a result, the star’s overall radiation output might be significantly lower than it would be as an older star, but these extraordinary bursts of energy could serve as both a source of consistent warming and could start a chemical cascade that would produce a lesser known but powerful greenhouse gas, nitrous oxide.

Not only would the regularity of the super-flares potentially keep a planet like early Earth warm enough to be wet, but their great energy could spark and set into motion other chemical processes that could, or would, produce some of the chemical building blocks of life. In a planet scattered evenly with simple molecules, as Earth was in its early days, it would take a huge amount of incoming energy to create the complex molecules such as RNA and DNA that eventually seeded life.

Here is a NASA video about this proposed process:

 

It should be noted that while the super-flare CMEs identified by Kepler could have played an important role in keeping Earth (and maybe Mars) warm and energetic enough for life to begin, they could also wreck havoc with the atmosphere of a planet, especially if its surround magnetic fields are not strong and protective.  A pounding from highly-energized and magnetized stellar clouds could rip off a planet’s atmosphere if that magnetosphere is weak. A better understanding of super-flare/atmosphere dynamics would help scientists determine what kinds of stars and what kinds of planets could be hospitable for life.

William Danchi, principal investigator of the project at Goddard and a co-author on the paper, put the results into this larger context:

“We want to gather all this information together, how close a planet is to the star, how energetic the star is, how strong the planet’s magnetosphere is in order to help search for habitable planets around stars near our own and throughout the galaxy.  This work includes scientists from many fields — those who study the sun, the stars, the planets, chemistry and biology. Working together we can create a robust description of what the early days of our home planet looked like – and where life might exist elsewhere.”

In keeping with this interdisciplinary approach, NASA’s Nexus for Exoplanet System Science (NExSS) initiative will hold a workshop in in New Orleans on November 29 to December 4 to delve into the star-exoplanet relationship. 

Host stars have additional characteristics that can help inform the search for habitable exoplanets.  For instance, metallicity — the presence of elements heavier than hydrogen and helium in the stars — determines what compounds will later be present in the proto-planetary cloud formed with the star.  That cloud in turn becomes the disk that provides material for planets and needs to deliver essential components for life such as carbon, oxygen, sulfur iron, magnesium and copper.

Vladimir Airapetian, research scientist at NASA's Goddard Space Flight Center.
Vladimir Airapetian, research scientist at NASA’s Goddard Space Flight Center.

The type and size of star is also key.  More than 70 percent of the stars in our vicinity of sky are M dwarfs, stars with substantially less luminosity than a star like our sun.  Airapetian said the limited energy from these stars would, in his model, make it far more difficult to form the molecules needed for life, although the presence of volcanoes on the planets would help.

While an exoplanet orbiting an M dwarf star might be in a “habitable zone,” he says, it may be entirely inhabitable because it is not in a “biogenic zone” — with the requisite water plus the radiation needed to form bio-molecules.

However, given what is known about the stages of growth of larger stars, he is quite sanguine about the possibilities for extra-solar life.  Because of the widespread presence of those super-flares in the early stages of larger star formation, he said, the sparks needed to create necessary molecules for life will be commonly present and so “life should be abundant.”

 

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A Flood of Newly Confirmed Exoplanets

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Artist renderings of exoplanets previously detected by the Kepler Space Telescope (NASA)
Artist renderings of exoplanets previously detected by the Kepler Space Telescope (NASA)

In the biggest haul ever of new exoplanets, scientists with NASA’s Kepler mission announced the confirmation of 1,284 additional planets outside our solar system — including nine that are relatively small and within the habitable zones of their host stars.  That almost doubles the number of these treasured rocky planets that orbit their stars at distances that could potentially support liquid water and potentially life.

Prior to today’s announcement, scientists using Kepler and all other exoplanet detection approaches had confirmed some 2,100 planets in 1,300 planetary systems.  So this is a major addition to the exoplanets known to exist and that are now available for further study by scientists.

These detections comes via the Kepler Space Telescope, which collected data on tiny decreases in the output of light from distant stars during its observing period between 2009 and 2013.  Those dips in light were determined by the Kepler team to be planets crossing in front of the stars rather than impostors to a 99 percent-plus probability.

As Ellen Stofan, chief scientist at NASA Headquarters put it,  “This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth.”

he histogram shows the number of planet discoveries by year for more than the past two decades of the exoplanet search. The blue bar shows previous non-Kepler planet discoveries, the light blue bar shows previous Kepler planet discoveries, the orange bar displays the 1,284 new validated planets. (NASA Ames/W. Stenzel; Princeton University/T. Morton)
The histogram shows the number of planet discoveries by year for more than the past two decades of the exoplanet search. The blue bar shows previous non-Kepler planet discoveries, the light blue bar shows previous Kepler planet discoveries, the orange bar displays the 1,284 new validated planets.
(NASA Ames/W. Stenzel; Princeton University/T. Morton)

The primary goals of the Kepler mission are to determine the demographics of exoplanets in the galaxy, and more specifically to determine the population of small, rocky planets (less than 1.6 times the size of Earth) in the habitable zones of their stars.  While orbiting in such a zone by no means assures that life is, or was, ever present, it is considered to be one of the most important criteria.

The final Kepler accounting of how likely it is for a star to host such an exoplanet in its habitable zone won’t come out until next year.  But by all estimations, Kepler has already jump-started the process and given a pretty clear sense of just how ubiquitous exoplanets, and even potentially habitable exoplanets, appear to be.

“They say not to count our chickens before they’re hatched, but that’s exactly what these results allow us to do based on probabilities that each egg (candidate) will hatch into a chick (bona fide planet),” said Natalie Batalha, co-author of the paper in the Astrophysical Journal and the Kepler mission scientist at NASA’s Ames Research Center.

“This work will help Kepler reach its full potential by yielding a deeper understanding of the number of stars that harbor potentially habitable, Earth-size planets — a number that’s needed to design future missions to search for habitable environments and living worlds.”

Batalha said that based on observations and statistics the Kepler mission has produced so far, we can expect that there are some 10 billion relatively small, rocky  (and potentially habitable) planets in our galaxy.  And with those numbers in mind, she said, the closest is likely to be in the range of 11 light years away.

She said that all of the exoplanets found in habitable zones are in the “exoplanet Hall of Fame.”  But she said two of the newly announced planets in habitable zones, Kepler 1286b and Kepler 1628b, joined two previous exoplanets of particular interest either because of their size (close to that of Earth) or their Earth-like distance from suns rather like ours.

Batalha said a new and finely-tuned software pipeline has been developed to better analyze the data collected during those four years of Kepler observations.  Asked if the final Kepler catalogue of exoplanets, expected to be finished next summer, would increase the current totals of exoplanets found, she replied:  “It wouldn’t surprise me if we had hundreds more to add.”

Since Kepler launched in 2009, 21 planets less than twice the size of Earth have been discovered in the habitable zones of their stars. The orange spheres represent the nine newly validated planets announcement on May 10, 2016. The blue disks represent the 12 previous known planets. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. (NASA Ames/N. Batalha and W. Stenzel)
Since Kepler launched in 2009, 21 planets less than twice the size of Earth have been discovered in the habitable zones of their stars. The orange spheres represent the nine newly validated planets announcement on May 10, 2016. The blue disks represent the 12 previous known planets. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. (NASA Ames/N. Batalha and W. Stenzel)

Once the Kepler exoplanet list is updated, scientists around the world will begin to study some of the most surprising, enticing, and significant finds.  Kepler can tell scientists only the location of a planet, its mass and its distance from the host star.  So the job of further characterizing the planets — and ultimately determining if any are indeed potentially habitable — requires other telescopes and techniques.

Nonetheless, Kepler’s ability to give scientists a broad picture of the distribution of exoplanets — to find large numbers of them rather than, as pre-Kepler, one or two at a time — has been revolutionary.  It has also been remarkably speedy, thanks in large part to an automated system of analyzing transit data devised by Tim Morton, a research scientist at Princeton University,

“Planet candidates can be thought of like bread crumbs,” Morton said in a NASA teleconference. “If you drop a few large crumbs on the floor, you can pick them up one by one. But, if you spill a whole bag of tiny crumbs, you’re going to need a broom. This statistical analysis is our broom.”

Kepler identified another 1,327 candidates that are very likely to be exoplanets, but didn’t meet the 99 percent certainty level required to be deemed an exoplanet.

A large percentage of the newly confirmed planets are either “super-Earths” or “sub-Neptunes” — planets in a size range absent in our solar system.  Initially, the widespread presence of exoplanets of these dimensions was a puzzle to the exoplanet community,  but now the puzzle is more why they are absent in our system.

Despite the abundance of these exoplanets — which are believed to be mostly gas or ice giants — scientists are convinced there are considerably more rocky, even Earth-sized planets that current telescopes can’t detect.

 The size distribution of discovered exoplanet has been a surprise to scientists. The blue bars on the histogram represent all previously verified exoplanets by size. The orange bars on the histogram represent Kepler's 1,284 newly validated planets. (NASA Ames/W. Stenzel)
The size distribution of discovered exoplanet has been a surprise to scientists. The blue bars on the histogram represent all previously verified exoplanets by size. The orange bars on the histogram represent Kepler’s 1,284 newly validated planets. (NASA Ames/W. Stenzel)

 

The primary Kepler mission focused on one small piece of the sky — about 0.25 percent of it — and a distant part at that. It watched nonstop for transiting planets in that space for four years, watching unblinkingly at some 150.000 stars. The result has been a treasure trove of data that can then be broadened statistically to tell us about the entire galaxy.

So Kepler has revolutionized our understanding of the galaxy and what’s in it, and has proven once and for all that exoplanets are common.  But the individual planets that it has detected are unlikely to be the ones that allow for breakthroughs in terms of sniffing out what chemicals are in their atmospheres — an essential process for determining if a potentially habitable planet actually has some of the ingredients for life.

This is because Kepler was looking far into the cosmos, between 600 and 3,000 light years from our sun.  While the telescope identified almost 5,000 “candidate planets” during its four years of primary operation — and now more than 2,200 confirmed planets — the planets are generally considered too distant for the more precise follow-up observing needed to understand their atmospheres and chemical make-ups.

This work will fall to ground-based telescopes looking at nearer stars, and to future generations of American and European space telescopes using the transit method of detection pioneered by Kepler. (See graphic above.)  The next space satellite in line is NASA’s Transiting Exoplanet Survey Satellite  (TESS), which is scheduled to launch in 2017 and will focus on planets orbiting much closer and brighter stars.  The long-awaited James Webb Space Telescope, due to launch in 2018, also has the potential to study exoplanets with a precision, and in wavelengths, not available before.

NASA has begun development of the more sophisticated Wide Field Infrared Survey Satellite (WFIRST) to further exoplanet research in the 2020s,  and has set up formal science and technology definition teams to plan for a possible flagship exoplanet mission for the 2030s.  That mission would potentially have the power and techniques to determine whether an exoplanet actually has the components, or the presence, of life.

 

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A Dwarf Star, Trappist-1, Produces a Major Discovery

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his artist's illustration depicts an imagined view from the surface of one of the three newfound TRAPPIST-1 alien planets. The planets have sizes and temperatures similar to those of Venus and Earth, making them the best targets yet for life beyond our solar system, scientists say. Credit: ESO/M. Kornmesser
An imagined view from the surface of one of the three newfound TRAPPIST-1 exoplanets. The planets have sizes and temperatures similar to those of Venus and Earth, making them attractive scientific targets in the search for potentially habitable planets beyond our solar system.
(ESO/M. Kornmesser)

The detection of potentially habitable exoplanets is not the big news it once was — there have been so many identified already that the novelty has faded a bit.  But that hardly means surprising and potentially breakthrough discoveries aren’t being made.  They are, and one of them was just announced Monday.

This is how the European Southern Observatory, which hosts the telescope used to make the discoveries, introduced them:

Astronomers using the TRAPPIST telescope at ESO’s La Silla Observatory have discovered three planets orbiting an ultra-cool dwarf star just 40 light-years from Earth. These worlds have sizes and temperatures similar to those of Venus and Earth and are the best targets found so far for the search for life outside the Solar System. They are the first planets ever discovered around such a tiny and dim star.

A team of astronomers led by Michaël Gillon, of the Institut d’Astrophysique et Géophysique at the University of Liège in Belgium, have used the Belgian TRAPPIST telescope to observe the star, now known as TRAPPIST-1. They found that this dim and cool star faded slightly at regular intervals, indicating that several objects were passing between the star and the Earth. Detailed analysis showed that three planets with similar sizes to the Earth were present.

The discovery has much going for it — the relative closeness of the star system, the rocky nature of the planets, that they might be in habitable zones.  But of special importance is that the host star is so physically small and puts out a sufficiently small amount of radiation that the planets — which orbit the star in only days — could potentially be habitable even though they’re so close.  The luminosity (or power) of Trappist-1 is but 0.05 percent of what’s put out by our sun.

This is a very different kind of sun-and-exoplanet system than has generally been studied.  The broad quest for an Earth-sized planet in a habitable zone has focused on stars of the size and power of our sun.  But this one is 8 percent the mass of our sun —  not that much larger than Jupiter.

“This really is a paradigm shift with regards to the planet population and the path towards finding life in the universe,” study co-author Emmanuël Jehin, an astronomer at the University of Liège, said in a statement. “So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, but now we have not just one lonely planet around such a faint red star but a complete system of three planets!”

 

Our sun and the ultracool dwarf star TRAPPIST-1 to scale. The faint star has only 11% of the diameter of the sun and is much redder in colour. (ESO)
Our sun and the ultracool dwarf star TRAPPIST-1 to scale. The faint star has only 11% of the diameter of the sun and is much redder in colour. (ESO)

 

The TRAPPIST-1 star is very faint and was identified because a Belgian team built a telescope especially to look for stars, and exoplanets, like the ones they found.  TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) is tiny by today’s standards, but collects light at infrared wavelengths and that makes it well designed for the task.

The observations began only in September, 2015, and targeted a dwarf star well known to astronomers.  TRAPPIST spends much of its time monitoring the light from around 60 of the nearest ultracool dwarf stars and brown dwarfs (“stars” which are not quite massive enough to initiate sustained nuclear fusion in their cores), looking for evidence of planetary transits.

Because the star and planets are so relatively close, they offer an unusual opportunity to potentially characterize the atmospheres of the planets and determine what molecules are in the air.  These measurements are essential to learning whether a planet is indeed habitable (or even inhabited.)

TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) is a 60 cm telescope at La Silla devoted to the study of planetary systems and it follows two approaches: the detection and characterisation of exoplanets around other stars and the study of comets orbiting around the Sun. The robotic telescope is operated from a control room in Liège, Belgium. The project is led by the Department of Astrophysics, Geophysics and Oceanography of the University of Liège, in close collaboration with the Geneva Observatory (Switzerland). TRAPPIST is mostly funded by the Belgian Fund for Scientific Research with the participation of the Swiss National Science Foundation. The name TRAPPIST was given to the telescope to underline the Belgian origin of the project. Trappist beers are famous all around the world and most of them are Belgian.
TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) is a 60 cm telescope at La Silla devoted to the study of planetary systems and it follows two approaches: the detection and characterisation of exoplanets around other stars and the study of comets orbiting around the Sun. The robotic telescope is operated from a control room in Liège, Belgium. The project is led by the Department of Astrophysics, Geophysics and Oceanography of the University of Liège, in close collaboration with the Geneva Observatory (Switzerland). TRAPPIST is mostly funded by the Belgian Fund for Scientific Research with the participation of the Swiss National Science Foundation.

Co-author Julien de Wit, a postdoc in the Department of Earth, Atmospheric, and Planetary Sciences, said scientists will soon be able to study the planets’ atmospheric compositions quite soon.

“These planets are so close, and their star so small, we can study their atmosphere and composition, and further down the road, which is within our generation, assess if they are actually inhabited,” de Wit said. “All of these things are achievable, and within reach now. This is a jackpot for the field.”

Rory Barnes, a specialist in dwarf stars and their exoplanets at the University of Washington, agreed that the TRAPPIST-1 discovery was  both intriguing today and inviting of a lot more future study.  Indeed, he said that efforts to characterize exoplanet atmospheres will most likely focus for the next decade on the smaller stars in our galactic neighborhood — the ubiquitous M dwarfs.

“It’s just easier to find exoplanets around smaller stars because they block out a great percentage of the star’s light when they transit,” he said. “And with small stars, the planets are usually closer in, which also makes them easier to find.”

But there are also significant barriers to habitability in the TRAPPIST-1 system.  Because the planets are so close to their host star — the first has an orbit of 1.5 days, the second an orbit of 2.4 days and the third an ill-defined orbit of between 4.5 and 73 days — that means they are tidally-locked, as is our moon.  Not long ago, exoplanet scientists doubted that a planet that doesn’t rotate can be truly habitable since the extremes of hot and cold would be too great.  That view has changed with creation of models that suggest tidal locking is not necessarily fatal for habitability, but it most likely does make it more difficult to achieve.

A larger potential barriers is that the dwarf star once was quite different.  Jonathan Fortney, a University of California at Santa Clara specialist in dwarf stars and brown dwarfs (objects which are too large to be called planets and too small to be stars), focused on that stellar history:

“One thing to keep in mind is that this star was much much brighter in the past,” he said in an email. “M stars (like TRAPPIST-1) are hottest when they are young and take a long time to cool off and settle down.  Their energy comes from contraction at first.  A star like this takes 1 billion years to even settle onto the main sequence (where it starts burning hydrogen).”

Barnes also focused on the stellar evolution, which he said is always complex and pertinent when talking about dwarf stars and exoplanets.  A small dwarf star like TRAPPIST-1 — which the authors estimate is 500 million years old — would have spent a much longer time as a much hotter protostar, sending out intense heat from its formation process before it achieved fusion.  That means a planet in the star’s habitable zone now may well have been baked like Venus eons ago, Barnes said, and there is no known way to become habitable after that.

So the relatively benign conditions around TRAPPIST-1 now in terms of radiation and heat clearly have not always been present.

The study authors said — and other scientists agree — that the most likely planet in the system to be actually habitable is the one furthest out.  But the orbit of that third planet has not been well defined, as seen in the estimate that it orbits its star within somewhere between 4.5 and 73 days.

As it turns out, the follow-on Kepler mission (K2) will be observing in the area that includes TRAPPIST-1 from this coming December through March 2017.

Kepler Mission Scientist Natalie Batalha said that she hoped the team put in a proposal to observe TRAPPIST-1.  If they did, she said, the proposal will be peer reviewed this month and could be among those selected. Assuming the telescope is in good working order and operations continue to be funded come December, K2 observations could better define that third planet’s orbit.

The Trappist-1 system is at the edge of the field that will be observed starting in December. The graphic shows detector that Campaign 12 detector field. (NASA/ Natalie Batalha)
The TRAPPIST-1 system lies within the field that is planned for Campaign 12 starting in December. The graphic shows its predicted location at the edge of one of Kepler’s detectors. (NASA/ Natalie Batalha)

But whatever happens with K2, TRAPPIST-1 is now an astronomical “star” and will no doubt be getting scientific attention of all kinds.

 

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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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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 Index

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