A New Way to Find Signals of Habitable Exoplanets?

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Scientists propose a new and more indirect way of determining whether an exoplanet has a good, bad or unknowable chance of being habitable.  (NASA’s Goddard Space Flight Center/Mary Pat Hrybyk)

The search for biosignatures in the atmospheres of distant exoplanets is extremely difficult and time-consuming work.  The telescopes that can potentially take the measurements required are few and more will come only slowly.  And for the current and next generation of observatories, staring at a single exoplanet long enough to get a measurement of the compounds in its atmosphere will be a time-consuming and expensive process — and thus a relatively infrequent one.

As a way to potentially improve the chances of finding habitable conditions on those exoplanets that are observed, a new approach has been proposed by a group of NASA scientists.

The novel technique takes advantage of the frequent stellar storms emanating from cool, young dwarf stars. These storms throw huge clouds of stellar material and radiation into space – traveling near the speed of light — and the high energy particles then interact with exoplanet atmospheres and produce chemical biosignatures that can be detected.

The study, titled “Atmospheric Beacons of Life from Exoplanets Around G and K Stars“, recently appeared in Nature Scientific Reports. 

“We’re in search of molecules formed from fundamental prerequisites to life — specifically molecular nitrogen, which is 78 percent of our atmosphere,” said Airapetian, who is a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and at American University in Washington, D.C. “These are basic molecules that are biologically friendly and have strong infrared emitting power, increasing our chance of detecting them.”

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

So this technique, called a search for  “Beacons of Life,” would not detect signs of life per se, but would detect secondary or tertiary signals that would, in effect, tell observers to “look here.”

The scientific logic is as follows:

When high-energy particles from a stellar storm reach an exoplanet, they break the nitrogen, oxygen and water molecules that may be in the atmosphere into their individual components.

Water molecules become hydroxyl — one atom each of oxygen and hydrogen, bound together. This sparks a cascade of chemical reactions that ultimately produce what the scientists call the atmospheric beacons of hydroxyl, more molecular oxygen, and nitric oxide.

For researchers, these chemical reactions are very useful guides. When starlight strikes the atmosphere, spring-like bonds within the beacon molecules absorb the energy and vibrate, sending that energy back into space as heat, or infrared radiation. Scientists know which gases emit radiation at particular wavelengths of light.  So by looking at all the radiation coming from the that planet’s atmosphere, it’s possible to get a sense of what chemicals are present and roughly in what amounts..

Forming a detectable amount of these beacons requires a large quantity of molecular oxygen and nitrogen.  As a result, if detected these compounds would suggest the planet has an atmosphere filled with biologically friendly chemistry as well as Earth-like atmospheric pressure.  The odds of the planet being a habitable world remain small, but those odds do grow.

“These conditions are not life, but are fundamental prerequisites for life and are comparable to our Earth’s atmosphere,” Airapetian wrote in an email.

Stellar storms and related coronal mass ejections are thought to burst into space when magnetic reconnections in various regions of the star.  For stars like our sun,  the storms become less frequent within a relatively short period, astronomically speaking.  Smaller and less luminous red dwarf stars, which are the most common in the universe, continue to send out intense stellar flares for a much longer time.

Vladimir Airapetian is a senior researcher
at NASA Goddard and a member of NASA’s  Nexus for Exoplanet System Science (NExSS) initiative.

The effect of stellar weather on planets orbiting young stars, including our own four billion years ago, has been a focus of Airapetian’s work for some time.

For instance, Airapetian and Goddard colleague William Danchi published a paper in the journal Nature last year proposing that solar flares warmed the early Earth to make it habitable.  They concluded that the high-energy particles also provided the vast amounts of energy needed to combine evenly scattered simple molecules into the kind of complex molecules that could keep the planet warm and form some of the chemical building blocks of life.

In other words, they argue, the solar flares were an essential part of the process that led to us.

What Airapetian is proposing now is to look at the chemical results of stellar flares hitting exoplanet atmospheres to see if they might be an essential part of a life-producing process as well, or of a process that creates a potentially habitable planet.

Airapetian said that he is again working with Danchi, a Goddard astrophysicist, and the team from heliophysics to propose a NASA mission that would use some of their solar and stellar flare findings.  The mission being conceived, the Exo Life Beacon Space Telescope (ELBST),  would measure infrared emissions of an exoplanet atmosphere using direct imaging observations, along with technology to block the infrared emissions of the host star.

For this latest paper, Airapetian and colleagues used a computer simulation to study the interaction between the atmosphere and high-energy space weather around a cool, active star. They found that ozone drops to a minimum and that the decline reflects the production of atmospheric beacons.

They then used a model to calculate just how much nitric oxide and hydroxyl would form and how much ozone would be destroyed in an Earth-like atmosphere around an active star. Earth scientists have used this model for decades to study how ozone — which forms naturally when sunlight strikes oxygenin t he upper atmosphere — responds to solar storms.  But the ozone reactions found a new application in this study; Earth is, after all, the best case study in the search for habitable planets and life.

Will this new approach to searching for habitable planets out?

“This is an exciting new proposed way to look for life,” said Shawn Domagal-Goldman, a Goddard astrobiologist not connected with the study. “But as with all signs of life, the exoplanet community needs to think hard about context. What are the ways non-biological processes could mimic this signature?”

 

A 2012 coronal mass ejection from the sun. Earth is placed into the image to give a sense of the size of the solar flare, but our planet is of course nowhere near the sun. (NASA, Goddard Media Studios)

Today, Earth enjoys a layer of protection from the high-energy particles of solar storms due to its strong magnetic field.  However, some particularly strong solar events can still interact with the magnetosphere and potentially wreak havoc on certain technology on Earth.

The National Oceanic and Atmospheric Administration classifies solar storms on a scale of one to five (one being the weakest; five being the most severe). For instance, a storm forecast to be a G3 event means it could have the strength to cause fluctuations in some power grids, intermittent radio blackouts in higher latitudes and possible GPS issues.

This is what can happen to a planet with a strong magnetic field and a sun that is no longer prone to sending out frequent solar flares.  Imagine what stellar storms can do when the star is younger and more prone to powerful flaring, and the planet less protected.

Exoplanet scientists often talk of the possibility that a particular planet was “sterilized” by the high-energy storms, and so could never be habitable.  But this new research suggests that some stellar storms could have just the opposite effect — making the planet more habitable.

 

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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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Coming to Terms With Biosignatures

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Exoplanets are much too far away for missions to visit and explore, so scientists are learning about them remotely.  That includes the question of whether they might support life — an aspect of exoplanet science that is getting  new attention. This is artist Ron Miller’s impression of an exoplanet.
Exoplanets are much too far away for missions to visit and explore, so scientists are learning about them remotely. That includes the question of whether they might support life — an aspect of exoplanet science that is getting new attention. This is artist Ron Miller’s impression of an exoplanet.

The search for life beyond our solar system has focused largely on the detection of an ever-increasing number of exoplanets, determinations of whether the planets are in a habitable zone, and what the atmospheres of those planets might look like.  It is a sign of how far the field has progressed that scientists are now turning with renewed energy to the question of what might, and what might not, constitute a sign that a planet actually harbors life.

The field of “remote biosignatures” is still in its early stages, but a NASA-sponsored workshop underway in Seattle has brought together dozens of researchers from diverse fields to dig aggressively into the science and ultimately convey its conclusions back to the exoplanet community and then to the agency.

While a similar NASA-sponsored biosignatures workshop put together a report in 2002, much has changed since then in terms of understanding the substantial complexities and possibilities of the endeavor.  There is also a new sense of urgency based on the observing capabilities of some of the space and ground telescopes scheduled to begin operations in the next decade, and the related need to know with greater specificity what to look for.

“The astrobiology community has been thinking a lot more about what it means to be a biosignature,” said Shawn Domogal-Goldman of the Goddard Space Flight Center, one of the conveners of the meeting.  Some of the reason why is to give advice to those scientists and engineers putting together space telescope missions, but some is the pressing need to maintain scientific rigor for the good of one of humankind’s greatest challenges.

“We don’t want to spend 20 years of our lives and billions in taxpayer money working for a mission to find evidence of life, and learn too late that our colleagues don’t accept our conclusions,” he told me.  “So we’re bringing them all together now so we can all learn from each other about what would be, and what would not be, a real biosignature.”

 

How to measure the chemical signatures in the atmosphere of a transiting exoplanet. The total light measured off-transit (B in the lower left figure) decreases during the transit, when only the light from the star is measured (A). By subtracting A from B, we get the planet counterpart, and from this the “chemical fingerprints” of the planet atmosphere can be revealed. Credits: NASA/JPL-Caltech.
How to measure the chemical signatures in the atmosphere of a transiting exoplanet. The total light measured off-transit (B in the lower left figure) decreases during the transit, when only the light from the star is measured (A). By subtracting A from B, we get the planet counterpart, and from this the “chemical fingerprints” of the planet atmosphere can be revealed. ( NASA/JPL-Caltech)

The three-day workshop is bringing together some 50 scientists ranging from astronomers, astrobiologists and planetary scientists to microbiologists and specialists in photosynthesis.  Organized by NASA’s Nexus for Exoplanet System Science (NExSS) — an initiative created to encourage interdisciplinary collaboration — it has been tasked with putting together a report for the larger exoplanet community and ultimately for NASA.

The first day of the workshop featured a review of previous work on biosignatures, which initially put forward the presence of oxygen in an exoplanet atmosphere as a strong and almost certain sign that biology was at work below. This is because oxygen, which is a byproduct of much life, bonds quickly with other molecules and so would be undetectable unless it was continuously replenished.

But as outlined by Victoria Meadows, director of the Virtual Planet Laboratory at the University of Washington, more recent research has shown large amounts of oxygen can be produced without biology under a number of (usually extreme)  conditions.  There has been a resulting focus on potential false positive signals regarding oxygen and other molecules.

From another perspective, Tim Lyons, a biogeochemist from the University of California, Riverside, used the early and middle Earth as an example how easy it is to arrive at a false negative result.

He said that current thinking is that for as long as two billion years, Earth was inhabited but the lifeforms produced little oxygen.  If analyzed from afar for all those years, the result would be a complete misreading of life on Earth.

With these kinds of false positives and negatives in mind, Meadows said that the current approach to understanding biosignatures is to look beyond a single molecule to the broader planetary and solar environment.

“We have to look not just at single biosignatures, but at their their context on the planet. How might life have modified an environment in a potentially detectable way?  And having stepped back a bit, does the biosignature make sense?”

As one example, while oxygen alone is no longer considered a sure biosignature, oxygen in an atmosphere in the presence of methane would be convincing because of the known results of the chemical interactions of the two.

 

Schematic for the concept of considering all small molecules in the search for biosignature gases. The goal is to start with chemistry and generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. Further investigation relates to the detectability: the sources and sinks that ultimately control the molecules’ accumulation in a planetary atmosphere of specific conditions as well as its spectral line characteristics. Geophysically or otherwise generated false positives must also be considered. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. Figure credit: S. Seager and D. Beckner.
Schematic for the concept of considering all small molecules in the search for biosignature gases.
The goal is to start with chemistry and generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. (S. Seager and D. Beckner)

 

In part because of the false positive/false negative issues involving oxygen, some have begun a concerted effort to produce a list of additional possible biosignatures.  William Bains, a member of Sara Seager’s team at the Massachusetts Institute of Technology, described the blunderbuss approach they have adopted:  examining some 14,000 compounds simple (fewer than six non-hydrogen atoms) and stable enough to exist in the atmosphere of an exoplanet.

In their Astrobiology Journal article, Seager, Bains and colleagues wrote that “To maximize our chances of recognizing biosignature gases, we promote the concept that all stable and potentially volatile molecules should initially be considered as viable biosignature gases.”

Elaborating during the workshop, Bains asked:  “Why does life produce the gases that it does? We really don’t know, so we’re bringing in everything as a possibility.”   Not surprisingly, he said, “The more you search, the more you find.”

And as for the possibility of life existing in extreme environments, Bains referred to the microbes known to live in radioactive environments, in plastic, and virtually everywhere else on Earth.

Because the science of remote biosignatures is still in its early stages, the unknowns can seem to overwhelm the knowns, making the whole endeavor seem near impossible.  After all, it’s proven extremely difficult to determine whether there was ever life on “nearby” Mars, and scientists have Martian meteorites to study and rovers sending back information about the geology, the geochemistry, the weather, the atmospheric conditions and the composition of the planet.

By comparison, learning how to probe the atmospheres of faraway exoplanets and assess what might or might not be a biosignature will have to be done entirely with next generation space telescopes and the massive ground telescopes in development.  The information in the photons they collect will tell scientists what compounds are present, whether liquid water is present on the surface, and potentially whether the surface is changing with seasons.  And then the interpretation begins.

That’s why Mary Voytek, the originator of NExSS and the head of the NASA astrobiology program, said at the workshop that the goal was to test and ultimately provide as many biosignatures as possible.  She wants many molecules potentially associated with life to be identified and then studied and restudied in the same critical way that oxygen has been — embraced for the biosignature possibilities it offers, and understood for the false positives and false negatives that might mislead.

“What we need is an arsenal,” she said, as many ways to sniff out the byproducts of exoplanet life as that daunting task demands.

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Rocky, Close and Potentially Habitable Planets Around a Dwarf Star

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This artist’s impression shows an imagined view from the surface one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth that were discovered using the TRAPPIST telescope at ESO’s La Silla Observatory. (M. Kornmesser/ESO)
This artist’s impression shows an imagined view from the surface one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth that were discovered using the TRAPPIST telescope at ESO’s La Silla Observatory. (M. Kornmesser/ESO)

Forty light-years away is no small distance. But an announcement of the discovery of two planets at that separation that have been determined to be rocky and Earth-sized adds a significant new twist to the ever-growing collection of relatively close-by exoplanets that just might be habitable.

The two planets in the TRAPPIST-1 system orbit what is known as a red dwarf star, a type of star that is typically much cooler than the sun, emitting radiation in the infrared rather than the visible spectrum.  While there has been much debate about whether an exoplanet around a dwarf can be deemed habitable, especially since they are all believed to be tidally locked and so only one side faces the star, a consensus appears to be growing that dwarf stars could host habitable planets.

The two new rocky exoplanets were detected using the Hubble Space Telescope and were deemed most likely rocky by the compact sizes of their atmospheres — which were not large and diffuse hydrogen/helium envelopes (like that of the Jupiter) but instead more tightly packed, more like the atmospheres of Earth, Venus, and Mars.  It was the first time scientists have been able to search for and at least partially characterize of atmospheres around a temperate, Earth-sized planet.

Having determined that the planets are rocky, principal investigator Julien de Wit of M.I.T’s Department of Earth, Atmospheric and Planetary Sciences, said the goal now is to characterize their atmospheres.

“Now the question is, what kind of atmosphere do they have?” de Wit said. “The plausible scenarios include something like Venus, where the atmosphere is dominated by carbon dioxide, or an Earth-like atmosphere with heavy clouds, or even something like Mars with a depleted atmosphere. The next step is tomtry to disentangle all these possible scenarios that exist for these terrestrial planets.”

Artist's impression of the two planets in the Trappist-1 solar system. These worlds have sizes, temperatures and potentially atmospheres similar to those of Venus and Earth. Some believe they may be 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. (Nasa/ESA/STScI)
Artist’s impression of the two planets in the Trappist-1 solar system. These worlds have sizes, temperatures and potentially atmospheres similar to those of Venus and Earth. Some believe they may be 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. (Nasa/ESA/STScI)

 

Host stars with exoplanets that are (very relatively) close to us are highly valued because they are potentially easier to observe and characterize.

There are 24 known exoplanets within 40 light-years, 14 are within 30 light-years, and only six are within 20 light-years. The closest exoplanet considered confirmed by NASA is Epsilon Eridani b, 10.5 light-years away from our solar system, while the closest known rocky planet is HD 219134 b, which is 21 light-years away..  Planetary companions have been suggested to exist in some of the nine star systems located within 10 light-years away, including in the closest system, Alpha Centauri (4.1 light-years away).

TRAPPIST-1 (planets b and c) are among the closest orbiting a red dwarf star, and they provided an unusual double transit to observe.

“The two planets actually transited their star just 12 minutes apart so we got two planets for the price of one,” said co-author Hannah Wakeford of NASA’s Goddard Space Flight Center.  “This is the first time two planets have been characterized with Hubble at the same time on purpose, and the first time such small (Earth-sized) planets have had atmospheric follow-up done.”

The researchers hope to use Hubble to conduct follow-up observations to search for thinner atmospheres, composed of elements heavier than hydrogen, like those of Earth and Venus.

“With more data, we could perhaps detect methane or see water features in the atmospheres, which would give us estimates of the depth of the atmospheres,” she said.

The results were reported in the journal Nature.

Hubble/WFC3 white-light curve for the TRAPPIST-1b and TRAPPIST-1c double transit of 4 May 2016. (NASA/SScI)
Hubble/WFC3 white-light curve for the TRAPPIST-1b and
TRAPPIST-1c double transit of 4 May 2016. (NASA/STScI)

There’s an interesting story behind their Hubble observation of the two transits.  Using their relatively small telescope at the European Southern Observatory’s La Silla facility in Chile, the TRAPPIST-1 team detected the unusual three-planet system around the small, cool star and published their discovery a little more than two months ago.

Within days, they realized that planets b and c would be orbiting the star at almost exactly the same time — an unexpected and quite valuable occurrence.  (Information about that double transit was provided via the Spitzer Space Telescope, which had also been studying the orbits of planets in the TRAPPIST-1 system.)

The upcoming double transit was confirmed but two weeks before the event. The team requested Hubble time for a quick observation, and it was granted.  The successful observation soon followed.

DeWit said that planets with the sizes and equilibrium temperatures of TRAPPIST-1b and TRAPPIST-1c could possess relatively thick atmospheres with water, carbon dioxide, nitrogen and oxygen.

The TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) project is the creation in large part of the Origins in Cosmology and Astrophysics group of the University of Liege in Belgium.

The TRAPPIST instrument is new kind of ground telescope designed to survey the sky in infrared. TRAPPIST was built as a 60-centimeter prototype to monitor the 70 brightest dwarf stars in the southern sky. Now, the researchers have formed a consortium, called SPECULOOS (Search for habitable Planets Eclipsing ULtra-cOOl Stars), and are building four larger versions of the telescope in Chile, to focus on the brightest ultracool dwarf stars in the skies over the southern hemisphere. The researchers are also trying to raise money to build telescopes in the northern sky.

The 60cm telescope is devoted to the detection and characterization of planets located outside our Solar System and to the study of comets and other small bodies in our solar system. (Trappist/ESO)
The 60cm telescope is devoted to the detection and characterization of planets located outside our Solar System and to the study of comets and other small bodies in our solar system. (Trappist/ESO)

According to De Wit, he TRAPPIST telescopes are inexpensive compared with their peer — about $400,000 per instrument.

He is pushing to make them a relatively affordable “prescreening tool” that scientists can use to identify planets that are potentially habitable.  The TRAPPIST observations would then be followed up by my detailed study using powerful telescopes such as Hubble and NASA’s James Webb Telescope, which is scheduled to launch in October 2018.

“With more observations using Hubble, and further down the road with James Webb, we can know not only what kind of atmosphere planets like TRAPPIST-1 have, but also what is within these atmospheres,” de Wit says. “And that’s very exciting.”

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Out of the Stovepipes and Into the Galaxy

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This “Many Worlds” post is written by Andrew Rushby, a postdoctoral fellow from the United Kingdom who recently began working with NASA’s NExSS initiative. The column will hopefully serve to both introduce this new NExSS colleague and to let him share his thoughts about the initiative and what lies ahead.

NExSS encourages a "systems science" approach to understanding exoplanets, and especially whether they might be habitable. Systems science is inherently interdisciplinary, and so fields such as earth science and planetary science (and many more) provide needed insights into how exoplanets might be explored. (NASA)
NExSS encourages a “systems science” approach to understanding exoplanets, and especially whether they might be habitable. Systems science is inherently interdisciplinary, and so fields such as earth science and planetary science (and many more) provide needed insights into how exoplanets might be explored. (NASA)

I’m most excited to join NExSS at its one year anniversary, and hope that I can help the network as it advances into, and works to fashion, the exoplanetary future.

Coming in from the outside, the progress I already see in terms of bringing researchers together to work on interdisciplinary exoplanet science is impressive. But more generally, I see this as a significant juncture in the fast-expanding study of these distant worlds, with NExSS and its members poised to facilitate a potentially revolution in how we look at planets in this solar system and beyond.

The ‘systems science’ approach to understanding exoplanets is, I believe, the right framework for advancing our understanding.  Earth scientists and biogeochemists have been using systems science for some time now to build, test, and improve theories for how the Earth functions as an interconnected system of physical, chemical and biological components — all operating over eons in a complex and tangled evolutionary web that we are only now unraveling.

It is this method that allows us to better understand the respective roles of the atmosphere, ocean, biosphere, and geosphere in influencing the past and present climate of this planet. It allows us to clearly see the damage we are causing to these systems through the release of industry and transport-created greenhouse gases, and offers opportunities for mitigation. We know the systems science approach works for the Earth, and the time to make it work for exoplanets is now.

But as Marc pointed out in his previous post about the first year of NExSS, the opportunity to leverage this method for comparative planetology is a relatively new one. We just haven’t had the data for building exoplanet systems models and making  testable hypotheses.

Understanding a planetary system like this artist's view of an ocean world, scientists have learned, takes an interdisciplinary approach.
Understanding a planetary system like this artist’s view of an ocean world, scientists have learned, takes an interdisciplinary approach.

The work that NExSS is doing is extremely relevant to this effort because we recognize that faced with the gargantuan task of discerning how innumerable planets beyond our solar system can be found, formed, characterized and understood — we have to do something different. The decisions we make and the effort we invest at this still very early stage will determine the future of planet discovery, and build the foundations for how we come to understand the planet system and its evolution.

We are likely the first of many generations of interdisciplinary exoplanetologists. To build a sturdy foundation for this approach, we will need to further break down the often restrictive scientific “stovepipes” that can keep necessary data known to scientists in one field from others who need it to understand their own data.  After the stovepipes have been dismantled (or at least modified), then comes the process of together building the exoplanetary chimney.

This is not an effort that can be successfully undertaken by individuals alone, or even already formed cross-disciplinary groups.

By its very nature, exoplanet science needs the insights and energy of the entire community of astrophysicists, heliophysicists, Earth scientists and planetary scientists interested in how their particular field of study fits in to the grand picture beginning to take shape.

Planets are endlessly complex and dynamic islands at the confluence of the physical, chemical and biological realms, and therefore our approach to making sense of them must be interdisciplinary, inclusive and epistemologically unique.

Biogeochemistry postdoc Andrew Rushby arrived at Ames last month and will remain for two years, with some of his time dedicated to the NExSS program.
Biogeochemistry postdoc Andrew Rushby arrived at Ames last month and will remain for two years, with some of his time dedicated to the NExSS program.

I’ve always thought, naively perhaps, of the search for exoplanets and for other life in the universe as a proxy search for our own place.

With our telescopes we peer ever outward, trying to find other worlds like our own to give meaning to that which we know best – this planet and its biosphere. It’s the next inevitably tumultuous battle in the long and sustained campaign for a greater perspective on ourselves, one that began when we first looked up at the night sky and wondered if what was out there was anything like what was down here.

It’s difficult, of course, to see how far you’ve already traveled on a journey of unknown length. But his field has come so far already and made so many strides in the last twenty years that analogies to its pace of discovery are difficult to come by.

As for me, I come from a biogeochemical background and was schooled at the University of East Anglia the UK. I spent my PhD years building models to investigate how the planetary evolution of Earth and Earth-like planets may affect their long-term habitability.  But I have also dedicated much time to telling just about anyone who would listen just how very cool exoplanets are. I hope to continue this work too.

Artist illustration of an exoplanet and a debris disc orbiting their host star. (NASA)
Artist illustration of an exoplanet and a debris disc orbiting their host star. (NASA)

During my exoplanet talks in the UK – at meetings, science cafes, outreach events, at the pub and online through my blog — I can honestly say I’ve never encountered anyone who didn’t find this science fascinating. Whatever my own speaking skills may be,  I know that it’s not difficult to enthuse people about the idea of exotic planets, alien life, and space missions. It’s almost cheating. There’s an optimism inherent in the field – one that says we can learn about these distant, shrouded and endlessly complex planetary systems through our own hard work, cleverness and technology. I think it’s an optimism that people relate to.

I hope therefore to make some small contribution to this effort through my time with NExSS by helping to build and maintain future and current collaborations among our existing members, growing our network, helping with our upcoming meetings and workshops, and facilitating communication from the network and its members.

I look forward to both embracing the differences, and identifying the similarities, between my training in the UK and the research culture here in USA. I think other countries would benefit greatly from the ‘NExSS approach’, and I look forward to welcoming the input of international colleagues in making NExSS a truly global enterprise. Many countries may be looking to NExSS’s example: let’s lead the way in showing the world how best to find and understand other worlds.

The 3-day in-person workshop will be coordinated with pre-workshop online activities to summarize the state of the science of exoplanet biosignatures. This review will provide background for the in-person workshop, which will focus on advancing the science of biosignatures, and understanding the technological needs and capabilities for their detection. This information will be exchanged with the Science Technology Definition Teams (STDTs) of upcoming planet-observing missions.
The 3-day in-person workshop will be coordinated with pre-workshop online activities to summarize the state of the science of exoplanet biosignatures. This review will provide background for the in-person workshop, which will focus on advancing the science of biosignatures, and understanding the technological needs and capabilities for their detection. This information will be exchanged with the Science Technology Definition Teams (STDTs) of upcoming planet-observing missions.

In addition to on-going PI collaborations,  NExSS has some exciting plans for the year ahead, as well as some tasks with significant and formal purposes.

First is the NExSS Exoplanet Biosignatures Workshop Without Walls in Seattle in July, which will have 30-35 on-site participants and an opportunity for many more to participate online.  Insights and conclusions from the Biosignatures Workshop will be exchanged with NASA’s Science Technology Definition Teams (STDTs) for upcoming planet-observing missions.

In addition, summary reports from the workshop will be circulated to the community for feedback. These reports will then be filed with a dedicated Exoplanet Biosignatures Study Analysis Group (SAG 16) of the Exoplanet Exploration Program Analysis Group (ExoPAG).

A heliophysics-based workshop (title and details tbc) will also be held in November, with title and details to be announced.

The second Face-to-Face meeting of all 17 NExSS PIs and associated team members, Co-leads, and NASA HQ representatives will be held at NASA Headquarters and the Carnegie Institution for Science in May. The two-day event offers an opportunity to discuss what worked and what didn’t work so well during the first year of NExSS, to hopefully come up with new collaborations, and to look ahead to the future of the network.

Over the next two years, the initiative will also continue to engage with the wider exoplanet community through workshops, meetings, and outreach activities in order to grow the network organically while ensuring inclusivity and representation from all of areas of this multidisciplinary field.

Per unitatem ad astra! (Through unity, to the stars!)

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