The Stellar Side of The Exoplanet Story

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K2-33b, shown in this illustration, is one of the youngest exoplanets detected to date. It makes a complete orbit around its star in about five days. Credits: NASA/JPL-Caltech
K2-33b, shown in this illustration, is one of the youngest exoplanets detected to date. It makes a complete orbit around its star in about five days, and as a result its characteristics are very much determined by its host. (NASA/JPL-Caltech)

 

When it comes to the study of exoplanets, it’s common knowledge that the host stars don’t get much respect.

Yes, everyone knows that there wouldn’t be exoplanets without stars, and that they serve as the essential background for exoplanet transit observations and as the wobbling object that allows for radial velocity measurements that lead to new exoplanets discoveries.

But stars in general have been seen and studied for ever, while the first exoplanet was identified only 20-plus years ago.  So it’s inevitable that host stars have generally take a back seat to the compelling newly-found exoplanets that orbit them.

As the field of exoplanet studies moves forward, however, and tries to answer questions about the characteristics of the planets and their odds of being habitable, the perceived importance of the host stars is on the rise.

The logic:  Stars control space weather, and those conditions produce a space climate that is conducive or not so conducive to habitability and life.

Space weather consists of a variety of enormously energetic events ranging from solar wind to solar flares and coronal mass ejections, and their characteristics are defined by the size, variety and age of the star.  It is often said that an exoplanet lies in a “habitable zone” if it can support some liquid water on its surface, but absent some protection from space weather it will surely be habitable in name only.

A recognition of this missing (or at least less well explored) side of the exoplanet story led to the convening of a workshop this week in New Orleans on “The Impact of Exoplanetary Space Weather On Climate and Habitability.”

“We’re really just starting to detect and understand the secret lives of stars,”  said Vladimir Airapetian, a senior scientist at the Goddard Space Flight Center.  He organized the highly interdisciplinary workshop for the Nexus for Exoplanet Space Studies (NExSS,) a NASA initiative.

“What has become clear is that a star affects and actually defines the character of a planet orbiting around it,” he said.  “And now we want to look at that from the point of view of astrophysicists, heliophysicists, planetary scientists and astrobiologists.”

William Moore, principal investigator for a NASA-funded team also studying how host stars affect their exoplanets, said the field was changing fast and that “trying to understand those (space weather) impacts has become an essential task in the search for habitable planets.”

 The newly discovered giant planet orbits around its young and active host star inside the inner hole of a dusty circumstellar disk (artist view). Credit: Max Planck Institute for Astronomy. The newly discovered giant planet orbits around its young and active host star inside the inner hole of a dusty circumstellar disk (artist view). Credit: Max Planck Institute for Astronomy.
The newly discovered giant planet orbits around its young and active host star inside the inner hole of a dusty circumstellar disk (artist view). Credit: Max Planck Institute for Astronomy.

So with space weather in the forefront, the workshop grappled with issues not frequently on the exoplanet agenda including the formation and protective effects of magnetic fields around planets; how, why and at what rate potentially life-supporting elements and compounds are likely to “escape” from bombarded exoplanets; and the extent to which those solar winds in particular speed that escape process.

Direct exoplanet measurements of these sun/planet dynamics remains sparse to entirely absent. That’s why much of the workshop discussion has centered around what’s known about our solar system — the workings of our sun and the way our solar dynamics impact planets.

This quite mature science and provides an exoplanet road map of sorts, though one always used with the caveat that what happens in and around our sun might be quite different than what’s happening around a sun many light years away.

We’ll return to the workshop, but first a little stellar science:

Our sun is not only a nuclear reactor producing enormous heat, but also has massive and very active electromagnetic fields in its outer corona.

When oppositely directed magnetic fields meet and become “reconnected,”  an intense flare of high-energy photons can shoot out at a speed that will bring them to Earth in 20 minutes to several hours.  Often, a coronal mass ejection will accompany the flare.   These vast CMEs consist of bubbles of magnetic field and billions of tons of of super-heated plasma (protons and neutrons), and they will arrive on Earth in one to three days.

In addition, the million degree heat of the sun’s outer corona produces a solar wind that also sends high-energy particles into space.  Unlike flares and CMEs, the solar wind is always blowing.

These phenomena and more would fry Earth were it not for our own protective magnetic field.  But this space weather can wreck havoc with satellites, GPS and electric power grids.  And it can potentially harm unprotected astronauts in space.  Not surprisingly, the study of space weather is a hot subject now.

 

Illustration of solar wind arriving at Earth's magnetosphere
Illustration of solar wind arriving at Earth’s magnetosphere

The same or similar space weather is inferred to exist in other solar systems as well.  Flares have been actually detected, but workshop scientists said the CMEs have not been measured so far on the host stars of exoplanets.

The effects of space weather are especially important when it comes to red dwarfs — smaller and cooler stars that make up some 75 percent of the stars out there.

These smaller stars generally form exoplanets that orbit quite close in, leaving them in danger of a complete sterilizing from solar wind or other space weather.  Adding to the risk, red dwarfs  are generally very active in their early lives, throwing out large and powerful flares and more.  Only later do they become far more sedentary, long-lived and seemingly good targets for habitable exoplanets.

But while an exoplanet of a red dwarf might orbit in a habitable zone later in its life and have other characteristics of habitability, the planet is considered unlikely to ever recover if it was sterilized eons before by a solar flare.

Space weather is often discussed in terms of the damage it can do, but the same high energy protons that can sterilize one planet may be able, through photochemistry,  to create some of the chemical building blocks of life.

Airapetian and Goddard colleague William Danchi published a paper in the journal Nature in June proposing that solar super-flares not only warmed the early Earth to make it habitable, but 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.

What’s more, Gregg Hallinan of Caltech proposed future searches for protective magnetic fields as way to identify potentially habitable exoplanets. He said that as techniques improve for detecting stellar flares, they should as well for observing stellar CMEs and ultimately planetary magnetic fields.

 

NASA's Swift mission detected a record-setting series of X-ray flares unleashed by DG CVn, a nearby binary consisting of two red dwarf stars, illustrated here. At its peak, the initial flare was brighter in X-rays than the combined light from both stars at all wavelengths under normal conditions. Credit: NASA's Goddard Space Flight Center/S. Wiessinger
NASA’s Swift mission detected a record-setting series of X-ray flares unleashed by DG CVn, a nearby binary consisting of two red dwarf stars, illustrated here. At its peak, the initial flare was brighter in X-rays than the combined light from both stars at all wavelengths under normal conditions. (NASA’s Goddard Space Flight Center/S. Wiessinger)

Bill Moore’s “Living, Breathing Planet” team was well represented at the workshop.   While Moore is a professor at Virginia’s Hampton University’s Department of Atmospheric and Planetary Sciences, his NASA-sponsored team includes scientists from six other institutions along the East Coast.

The talks by team members focused on how and why material escapes from planetary atmospheres, and the implications of that escape.  On Mars, for instance, hydrogen escape due to solar winds is a major factor as water and other molecules break apart and send that lightest element into space.

One result that exoplanet scientists worry about is that the escaping hydrogen can leave behind reservoirs of oxygen that might lead to misleading conclusions.  An atmosphere filled with oxygen has long been seen as a promising one in terms of extraterrestrial life; indeed, oxygen and ozone are considered essential biosignatures of life.

But if oxygen can also be left behind when an atmosphere is stripped of hydrogen, then that clearly must be taken into account.  So models for detecting actual biosignatures on exoplanets now include oxygen and other compounds together, rather than oxygen alone.

William Moore of Hampton University, and principal investigator of the Living, Breathing Planet team.
William Moore of Hampton University, and principal investigator of the Living, Breathing Planet team.

“As the field turned to habitability on exoplanets rather than solely detection, we had to start worrying more about the host star.  The issue became not detection but how to live around that star, which we’re finding is, not surprisingly, a very complex question.”

Inevitably, that question involves not only current space conditions, but the evolution of the exoplanet and its solar system.  In particular, that requires an understanding of the stellar radiation environment that the planet formed in and has lived in, as well as what other stars might be close by.

He offered as an example the exciting discovery of a planet in the habitable zone around the star Proxima Centauri, the closest star to our own.  Teams around the world are now studying the planet for potential habitability, and they may get some promising results.

But Moore’s group is looking at the Proxima Centauri planet from the perspective of its environment, and that it’s located in what is essentially a three-star cluster.  That means the planet has potentially been exposed to the forces of all three stars.

“We see a planet sitting out there on its own, and it seems to be a closed system.  But that’s not true; it’s related to the stars.  The Proxima planet is a case in point.  We’ve done some work and have found a very complicated environment to live in — to say the least.”

 

Also coming next week from the New Orleans conference:  Using host stars to find the most tempting targets for observation.

 

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Movement in The Search For ExoLife

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A notional version of an observatory for the 2030s that could provide revolutionary direct imaging of exoplanets. GSFC/JPL/STScI
A notional version of an observatory for the 2030s that could provide revolutionary direct imaging of exoplanets. GSFC/JPL/STScI

Assuming for a moment that life exists on some exoplanets, how might researchers detect it?

This is hardly a new question.  More than ten years ago, competing teams of exo-scientists and engineers came up with proposals for a NASA flagship space observatory capable of identifying possible biosignatures on distant planets. No consensus was reached, however, and no mission was developed.

But early this year, NASA Astrophysics Division Director Paul Hertz announced the formation of four formal Science and Technology Definition Teams to analyze proposals for a grand space observatory for the 2030s.  Two of them in particular would make possible the kind of super-high resolution viewing needed to understand the essential characteristics of exoplanets.  As now conceived, that would include a capability to detect molecules in distant atmospheres that are associated with living things.

These two exo-friendly missions are the Large Ultraviolet/Optical/Infrared (LUVOIR) Surveyor and the Habitable Exoplanet (HabEx) Imaging Mission.   Both would be on the scale of, and in the tradition of, scientifically and technically ground-breaking space observatories such as the Hubble and the James Webb Space Telescope, scheduled to launch in 2018.  These flagship missions provide once in a decade opportunities to move space science dramatically forward, and not-surprisingly at a generally steep cost.

 

A simulated spiral galaxy as viewed by Hubble, and the proposed High Definition Space Telescope (HDST) at a lookback time of approximately 10 billion years (z = 2) The renderings show a one-hour observation for each space observatory. Hubble detects the bulge and disk, but only the high image quality of HDST resolves the galaxy’s star-forming regions and its dwarf satellite. The zoom shows the inner disk region, where only HDST can resolve the star-forming regions and separate them from the redder, more distributed old stellar population. Image credit: D. Ceverino, C. Moody, G. Snyder, and Z. Levay (STScI)500 light years away, as imaged by Hubble and potential of the kind of telescope the exoplanet community is working towards.
A simulated spiral galaxy as viewed by Hubble, and as viewed by the kind of high definition space telescope now under study.   Hubble detects the bulge and disk, but only the high definition image resolves the galaxy’s star-forming regions and its dwarf satellite. The zoom shows the inner disk region, where only high definition can resolve the star-forming regions and separate them from the redder, more distributed old stellar population. (D. Ceverino, C. Moody, G. Snyder, and Z. Levay (STScI)

 

Because the stakes are so high, planning and development takes place over decades — twenty years is the typical time elapsed between the conception of a grand flagship mission and its launch.  So while what is happening now with the science and technology definition teams  is only a beginning — albeit one with quite a heritage already — it’s an essential, significant and broadly-supported start.  Over the next three years, the teams will undertake deep dives into the possibilities and pitfalls of LUVOIR and HabEx, as well as the two other proposals.  There’s a decent chance that a version of one of the four will become a reality.

Aki Roberge, an astrophysicist at the Goddard Space Flight Center and staff scientist of the LUVOIR study, said that the explicit charge to the teams is to cooperate rather than compete.  Any of the four observatories under consideration, she said, would enable transformative science. But from an exoplanet perspective, the possibilities she described are pretty remarkable.

“What we’re aiming for is the capability to really search for the true Earth analogues out there, the Earth-sized planets in the habitable zones of sun-like stars.  We need to understand their atmospheres, their climates, their compositions.  And ultimately, the goal is to search for life.”

The co-chair of the HabEx team, Bertrand Menneson of the Jet Propulsion Lab, said the goals are the same:  A major jump forward in our ability to understand exoplanets and a serious effort to find life.

 

actual image of venus crossing in front of the sun. Exoplanets will not be imaged like this in our lifetimes, but this is the goal.
Actual image of Venus crossing in front of the sun in 2012 taken by NASA’s Solar Dynamics Observatory. Exoplanets will not be imaged like this in our lifetimes, but this is the ultimate goal.

 

The field of exoplanet detection and research has exploded over the past two decades, with an essential boost from increasingly capable observatories on Earth and in space.  With at least three more major exoplanet-friendly space telescopes scheduled (or planned) for the next decade — as well as first light at several enormous ground-based mirrors — the brisk pace of discoveries is sure to continue.

So why are so many scientists in the field convinced that a grand, Flagship-class NASA space observatory is essential, and that it needs to be developed and built ground-up with exoplanet research in mind?  Can’t the instruments in use today, and planned for the next decade, provide the kind of observing power needed to continue making breakthroughs?

Well, no, they can’t and won’t.  That has been the conclusion of numerous studies over the years, and most recently an in-depth effort by the Association of Universities for Research in Astronomy (AURA,)   http://www.hdstvision.org/report which last summer called for development of a 12-meter (about 44 feet across) High Definition Space Telescope with the super high resolution needed to study exoplanets.  Generally speaking, a larger light-collecting mirror allows astronomers and astrophysicists to see further and better.

 

A direct, to-scale, comparison between the primary mirrors of the Hubble Space Telescope, James Webb Space Telescope, and the proposed High Definition Space Telescope (HDST). In this concept, the HDST primary is composed of 36 1.7 meter segments. Smaller segments could also be used. An 11 meter class aperture could be made from 54 1.3 meters segments. Image credit: C. Godfrey (STScI)
A direct, to-scale, comparison between the primary mirrors of the Hubble Space Telescope, James Webb Space Telescope, and the High Definition Space Telescope (HDST) proposed by the AURA group. In this concept, the HDST primary is composed of 36 1.7 meter segments.  The LUVOIR mirror under consideration is in the eight to twelve meters range. C. Godfrey (STScI)

 

The group, headed by Julianne Dalcanton of the University of Washington and Sara Seager of MIT, began with this overview of the state of play when it comes to exoplanets, instruments, and what is possible now and might be in the future:

While we now have a small sample of potentially habitable planets around other stars, our current telescopes lack the power to confirm that these alien worlds are truly able to nurture life. This small crop of worlds may have temperate, hospitable surface conditions, like Earth.

But they could instead be so aridly cold that all water is frozen, like on Mars, or so hot that all potential life would be suffocated under a massive blanket of clouds, like on Venus. Our current instruments cannot tell the difference for the few rocky planets known today, nor in general, for the larger samples to be collected in the future.

Without better tools, we simply cannot see their atmospheres and surfaces, so our knowledge is limited to only the most basic information about the planet’s mass and/ or size, and an estimate of the energy reaching the top of the planet’s atmosphere. But if we could directly observe exoplanet atmospheres, we could search for habitability indicators (such as water vapor from oceans) or for signs of an atmosphere that has been altered by the presence of life (by searching for oxygen, methane, and/or ozone).

A central goal for both LUVOIR and HabEx is to provide that “seeing” through much more sophisticated direct imaging — that is, capturing the actual reflected light from exoplanets rather than relying on indirect techniques and measurements.  The many indirect methods of finding and studying exoplanets have played and will continue to play an essential role.  But there is now a community consensus that next generation direct imaging from space is the gold standard.

 

Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars, by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)
There are more than 4,000 Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars.  This graphic shows their distribution by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)

That a major space observatory for the 2030s just might be exoplanet-focused reflects a definite maturing of the field.  From a science perspective, the discoveries of the Kepler mission in particular made clear that exoplanets are everywhere, and not infrequently orbiting in habitable zones.  The work of the Curiosity rover on Mars, and especially the conclusion that the planet once was wet and “habitable,” added to the general interest and excitement about possible life beyond Earth.

And then there are the lessons learned from the earlier bruising battles among exoplanet scientists, who had developed a reputation for serious in-fighting.  THEIA, the Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy, was put forward as a flagship direct imaging mission in 2010, when the Astronomy and Astrophysics Decadal Survey that sets priorities for the field was being put together by the National Academy of Sciences.  But THEIA was not adopted.

A cartoon from Chas Beichman’s ExoPAG presentation illustrates the infighting within the exoplanet science community during the 2010 decadal survey, with cosmologists, represented by “dark energy” to the side, ready to reap the benefits of that debate.
A cartoon from a exoplanet science presentation illustrates the infighting within the exoplanet science community during the 2010 decadal survey, with cosmologists, represented by “dark energy” to the side, ready to reap the benefits of that debate. ( Chas Beichman)

With the 2020 Decadal Survey on the horizon, exoplanet scientists have tried to limit conflicts and to work with the larger astronomy community.  The formal NASA/community study group, the Exoplanet Exploration Program Analysis Group (ExoPAG), brought two related groups together and ultimately recommended the intensified study for LUVOIR, HabEx and the two other proposals —  which focus on black holes, ancient galaxy formation, and other aspects of the early cosmos.  https://exep.jpl.nasa.gov/files/exep/ExoPAG_Large_Missions.pdf

When completed, the studies will go to the National Academy of Sciences for further review, discussion, and ultimately a recommendation to NASA regarding which project should go forward.

The leader of the ExoPAG  group was astronomer Scott Gaudi of Ohio State University, who specializes in characterizing exoplanets but played no favorites in the ExoPAG report and recommendations.

“What we want is to set up a fair process of intense review so the most compelling science can be chosen to go forward.  At this point, we don’t know if the necessary technologies will be available in time, and we don’t know what the costs will be.  There’s only so much money that comes from NASA for our (astrophysics) community, and maybe a top choice will cost more than the community is willing to spend.  So there are so many factors to consider.”

(The LUVOIR mission is generally considered to be somewhat more ambitious than HabEx, and would require a larger telescope mirror — greater than 8 meters across –and more funding.  Flagship missions are expensive, as NASA learned once again with the James Webb telescope, which will have cost $8.8 billion by the time of its scheduled launch.)

I asked Gaudi if the seemingly substantial public interest in exoplanets could play any role in subsequent decision-making, and he replied that it possibly would.  “In the past five or ten years, exoplanets have become a prominent topic for sure.  And the public is clearly very, very interested in that topic.”  But that public interest, he said, won’t mean much if the science and technical feasibility isn’t there.

Scott Gaudi, chairman of ExoPAG in 2015.
Scott Gaudi, chairman of ExoPAG in 2015.

We won’t know for some years if the stars will align in a way that will lead to a major observatory with direct imaging and exoplanets at its center.  But for those active in the field, the opportunity to take part in a major effort to formally determine its scientific merit and feasibility is irresistible.

Shawn Domagal-Goldman, a research space scientist at Goddard, was selected to be a deputy on the LUVOIR science and technology team, which he sees as a much-anticipated “proof of concept” effort for the exoplanet research of the future.

Between 12 and 18 scientists and engineers will be selected by NASA headquarters for each team, and Domagal-Goldman said it’s essential that they make up a broad and inter-disciplinary group, including people from industry.  Scientists from abroad not associated with an American institution can’t be formal members, but they can observe and may become more involved if their national space agencies decide to join in the effort. He encourages researchers — from newly minted PhDs to career scientists — to nominate themselves to join.

“Nobody gets paid for this, it’s a labor of love,” he said.  “But what would be more satisfying than having some of your intellectual contribution go into the formulation of missions like these.

“Direct imaging of exoplanets is clearly a direction where the community is headed. These are the missions of the future in one form or another, and if you’re a PhD or postdoc who’s qualified, this could be your career.”

Of course, it just might make the greatest discovery of modern science — finding life beyond Earth.

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