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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.”… Read more

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.”… Read more

Many Worlds

Tag: coronal mass ejections

The Stellar Side of The Exoplanet Story

Forget the "Habitable Zone," Think the "Biogenic Zone"

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