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 magentic 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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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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