Storming the One-Meter-Per-Second Barrier

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Kitt Peak National Observatory mountain top at Dusk looking north. Visible in the picture are the NOAO 4-meter Mayall, the Steward Observatory 90-inch, the University of Arizona Lunar and Planetary Laboratory Spacewatch Telescopes, LOTIS, 0.4-meter Visitor Center Telescope, Case Western Reserve University Observatory and the SARA Observatory. Credit: P. Marenfeld (NOAO/AURA/NSF)
The Kitt Peak National Observatory, on the Tohono O’odham reservation outside Tucson, will be home to a next-generation spectrometer and related system which will allow astronomers to detect much smaller exoplanets through the radial velocity method.  P. Marenfeld (NOAO/AURA/NSF)

When the first exoplanet was identified via the radial velocity method, the Swiss team was able to detect a wobble in the star 51 Pegasi at a rate of 50 meters per second.   The wobble is the star’s movement back and forth caused by the gravitational pull of the planet, and in that first case it was dramatic — the effects of a giant Jupiter-sized planet orbiting extremely close to the star.

Many of the early exoplanet discoveries were of similarly large planets close to their host stars, but it wasn’t because there are so many of them in the cosmos.  Rather, it was a function of the capabilities of the spectrographs and other instruments used to view the star.  They were pioneering breakthroughs, but they didn’t have the precision needed to measure wobbles other than the large, dramatic ones caused by a close-in, huge planet.

That was the mid 1990s, and radial velocity astronomers have worked tirelessly since to “beat down” that 50 meters per second number.  And twenty years later, RV astronomers using far more precise instruments and more refined techniques have succeeded substantially:  1 meter per second of wobble is now achieved for the quietest stars.  That has vastly improved their ability to find smaller exoplanets further from their stars and is a major achievement.  But it has nonetheless been a major frustration for astronomers because to detect terrestrial exoplanets in the Earth-sized range, they have to get much more precise  — in the range of tens of centimeters per second.

A number of efforts to build systems that can get that low are underway, most notably the ESPRESSO spectrograph scheduled to begin work on the High Accuracy Radial Vlocity Planet Searcher (HARPS) in Chile next year. Then earlier this month an ambitious NASA-National Science Foundation project was awarded to Penn State University to join the race.  The next-generation spectrograph is scheduled to be finished in 2019 and installed at the Kitt Peak National Observatory in Arizona, and its stated goal is to reach the 20 to 30 centimeters per second range.

Suvrath Mahadevan, an assistant professor at Penn State, is principal investigator for the project.  It is called NEID, which means ‘to see’ in the language of the Tohono O’odham, on whose land the Kitt Peak observatory is located.

“For many reasons, the (radial velocity) community has been desperate for an instrument that would allow for detections of smaller planets, and ones in habitable zones,” he said.  “We’re confident that the instrument we’re building will — in time — provide that capability.”

Las Cumbres Observatory Global Telescope Network.
A illustration of how the radial velocity method of planet hunting works.  The wobble of the stars is far away miniscule in galactic terms, making extreme precision essential in measuring the movement. (Las Cumbres Observatory Global Telescope Network)

Project scientist Jason Wright, associate professor of astronomy and astrophysics at Penn State, put it this way:  “NEID will be more stable than any existing spectrograph, allowing astronomers around the world to make the precise measurements of the motions of nearby, Sun-like stars.”  He said his Penn State team will use the instrument “to discover and measure the orbits of rocky planets at the right distances from their stars to host liquid water on their surfaces.”

NASA and the NSF wanted the new spectrograph built on an aggressive timetable to meet major coming opportunities and needs, Mahadevan said.

The speedy three-year finish date is a function of the role that radial velocity detection plays in exoplanet research.  While many planets have been, and will be, first detected through the technique, it is also essential in the confirming of candidate planets identified by NASA space telescopes such as Kepler, the soon-to-be launched TESS (the Transiting Exoplanet Survey Satellite) and others into the future.  There is a huge backlog of planets to be confirmed, and many more expected in the relatively near future.

What’s more, as Mahadevan explained, an instrument like NEID could significantly help NASA’s planning for a possible 2030s Flagship space telescope mission focused on exoplanets.  Two of the four NASA contenders under study are in that category — LUVOIR (Large Ultraviolet Visible Infrared) Surveyor and Hab-Ex — and their capabilities, technologies, timetables and cost are all now under consideration.

If NEID can identify some clearly Earth-sized planets in habitable zones, he said, then the planning for LUVOIR or Hab-Ex could be more focused (and the proposal potentially less costly.)  This is because the observatory could be designed to look at a limited number of exoplanets and their host stars, rather than scanning the skies for a clearly Earth-like planet.

“Right now we have no definite Earth-sized planets in a habitable zone, so a LUVOIR or Hab_ex design would have to include a blind search.  But if we know of maybe 15 planets we’re pretty sure are in their habitable zones, the targets get more limited and the project becomes a lot cheaper.”

Suvrath Mahadevan, assistant professor of Astronomy and Astrophysics at Penn State, and principal investigator for a new-generation high precision spectrometer. (Penn State)
Suvrath Mahadevan, assistant professor of Astronomy and Astrophysics at Penn State, and principal investigator for a new-generation high precision spectrometer. (Penn State)

These possibilities, however, are for the future.  Now, Mahadevan said, the Penn State team has to build a re-considered spectrograph, a significant advance on what has come before.  With its track record of approaching their work through interdisciplinary collaboration, the Penn State team will be joined by collaborators from NASA Goddard Space Flight Center, University of Colorado, National Institute of Standards and Technology, Macquarie University in Australia, Australian Astronomical Observatory, and Physical Research Laboratory in India.  Much of the work will be done over the next three years at Penn State, but some at the partner institutions as well.

Key to their assembly approach is that the instrument will be put together in vacuum-sealed environment and will have no vibrating or moving parts.  This design stability will prevent, or minimize, instrument-based misreadings of the very distant starlight being analyzed.

A major issue confronting radial velocity astronomers is that light from stars can fluctuate for many reasons other than a nearby planet — from sunspots, storms, and other magnetic phenomena.  The NEID instrument will try to minimize these stellar disruptors by providing the broadest wavelength coverage so far in an exoplanet spectrograph, Mahadevan said, collecting light from well into the blue range of the spectrum to almost the end of the red.

“We’re not really building a spectorograph but a radial velocity system, he said.  That includes upgrades to the telescope port, the data pipeline and more.

This is how Lori Allen, Associate Director for Kitt Peak, described that new “system”: “The extreme precision (of NEID) results from numerous design factors including the extreme stability of the spectrometer environment, image stabilization at the telescope, innovative fiber optic design, as well as state-of-the-art calibration and data reduction techniques”.

 

The new generation spectrograph will be installed on the 3.5 meter WYN telescope at Kitt Peak. Operated by National Optical Astronomy Observatory, the $10 million project is a collaboration of NASA and the National Science Foundation.
The new generation spectrograph will be installed on the 3.5 meter WYN telescope at Kitt Peak. The site is managed by the National Optical Astronomy Observatory, and $10 million spectrograph project is a collaboration of NASA and the National Science Foundation.

Sixteen teams ultimately competed to build the spectrograph, and the final two contenders were Penn State and MIT.  Mahadevan said that, in addition to its spectrograph design, he believed several factors helped the Penn State proposal prevail.

His team has worked for several years on another advanced spectrograph for the Hobby-Eberly Telescope in Texas, one that required complex vacuum-sealed and very cold temperature construction.  Although the challenges slowed the design, the team ultimately succeeded in demonstrating the environmental stability in the lab.  So Penn State had a track record. What’s more, the school and its Center for Exoplanets and Habitable Worlds have a history of working in an interdisciplinary manner, and have been part of several NASA Astrobiology Institute projects. (The instrument has a blog of its own: NEID.)

The Kitt Peak observatory, which saw first light in 1994, has been the sight of many discoveries, but in recent years has faced cutbacks in NSF funding.  There was some discussion of reducing its use, and the NASA-NSF decision t0 upgrade the spectrograph was in part an effort to make it highly relevant again.  And given the scientific need to confirm so many planets — a need that will grow substantially after TESS launches in 2017 or 2018 and begins sending back information on thousands of additional transiting exoplanets — enhancing the capabilities of the Kitt Peak 3.5 meter telescope made sense.

Kitt Peak is unusual in being open to all comers with a great proposal, whether they’re from the U.S. or abroad.  The Penn State team and partners will get a certain number of dedicated night to observe, but many others will be allocated through competitive reviews.  And so when NEID is completed, astronomers from around will have a shot at using this state-of-the-art planet finder.

 

 

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The Habitable Zone Gets Poked, Tweaked and Stretched to the Limits

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To find another planet like Earth, astronomers are focusing on the "Goldilocks" or habitable zone around stars--where it's not too hot and not too cold for liquid water to exist on the surface. (NASA)
To find another planet like Earth, astronomers are focusing on the “Goldilocks” or habitable zone around stars–where it’s not too hot and not too cold for liquid water to exist on the surface. (NASA)

For more than 20 years now — even before the first detection of an extra-solar planet — scientists have posited, defined and then debated the existence and nature of a habitable zone.  It’s without a doubt a central scientific concept, and  the idea has caught on with the public (and the media) too.  The discovery of “habitable zone planets” has become something of a staple of astronomy and astrophysics.

But beneath the surface of this success is a seemingly growing discomfort about how the term is used. Not only do scientists and the general public have dissimilar understandings of what a habitable zone entails, but scientists have increasingly divergent views among themselves as well.

And all this is coming to the fore at a time when a working definition of the habitable zone is absolutely essential to planning for what scientists and enthusiasts hope will be a long-awaited major space telescope focused first and foremost on exoplanets.  If selected by NASA as a flagship mission for the 2030s, how such a telescope is designed and built will be guided by where scientists determine they have the best chance of finding signs of extraterrestrial life — a task that has ironically grown increasingly difficult as more is learned about those distant solar systems and planets.

Most broadly, the habitable zone is the area around a star where orbiting planets could have conditions conducive to life.  Traditionally, that has mean most importantly orbiting far enough from a star that it doesn’t become a desiccated wasteland and close enough that it is not forever frozen.  In this broad definition, the sometimes presence of liquid water on the surface of a planet is the paramount issue in terms of possible extraterrestrial life.

 The estimated habitable zones of A stars, G stars and M stars are compared in this diagram. More refinement is needed to better understand the size of these zones. Image credit: NASA/JPL-Caltech/MSSS.

The estimated habitable zones of A stars, G stars and M stars are compared in this diagram. More refinement is needed to better understand the size of these zones. Image credit: NASA/JPL-Caltech/MSSS.

It was James Kasting of Penn State University, Daniel Whitmire, then of Louisiana State University, and Ray Reynolds of NASA’s Ames Research Center who defined the modern outlines of a habitable zone, though others had weighed in earlier.  But Kasting and the others wrote with greater detail and proposed a model that took into account not only distance from the host star, but also the presence of planetary systems that could maintain relatively stable climates by cycling essential compounds.

Their concept became something of a consensus model, and remains an often-used working definition.

But with the detection now of thousands of exoplanets, as well as a better understanding of potential habitability in our solar system and the workings of atmospheric gases around planets, some scientists argue the model is getting outdated.  Not wrong, per se, but perhaps not broad enough to account for the flood of planetary and exoplanetary research and discovery since the early 1990s.

Consider, first our own habitable zone:  Two bodies often discussed as potentially habitable are the moons Europa and Enceladus. Both are far from the solar system’s traditional habitable zone, and are heated by gravitational forces from Jupiter and Saturn.

And then there’s the Mars conundrum.  The planet, now viewed as unable to support life on the surface, is currently within the range of our sun’s habitable zone.  Yet when Mars was likely quite wet and warmer and “habitable” some 3.5 billion years ago — as determined by the Curiosity rover team — it was outside the traditional habitable zone because the sun was less luminous and so Mars would ostensibly be frozen.

Remnants of an ancient alluvial fan have been found at Gale Crater, Mars, indicating that water flowed there for long periods of time billions of years ago.
Remnants of an ancient alluvial fan have been found at Gale Crater, Mars, indicating that water flowed there for long periods of time billions of years ago. Traditional habitable zone models cannot account for this wet and warm period on ancient Mars.  (NASA/JPL-Caltech)

Just as the source of heat keeping water on the moons liquid is not the sun, scientists have also proposed that even giant and distant planets with thick atmospheres of molecular hydrogen, a powerful greenhouse gas, could maintain liquid water on their surfaces.  Some have suggested that a hydrogen-rich atmosphere could keep a planet ten times further from the sun than Earth warm enough for possible life.

It was Raymond Pierrehumbert  at University of Chicago and Eric Gaidos of the University of Hawaii who first proposed this possibility in 2011, but others have taken it further.  Perhaps most forcefully has been Sara Seager at MIT, who has argued that the exoplanet community’s definition of a habitable zone needs to be broadened to keep up with new thinking and discoveries.  This is what she wrote in an influential 2013 Science paper:

“Planet habitability is planet specific, even with the main imposed criterion that surface liquid water must be present. This is because the huge range of planet diversity in terms of masses, orbits, and star types should extend to planet atmospheres and interiors, based on the stochastic nature of planet formation and subsequent evolution. The diversity of planetary systems extends far beyond planets in our solar system. The habitable zone could exist from about 0.5 AU out to 10 AU (astronomical units, the distance from the sun to the Earth) for a solar-type star, or even beyond, depending on the planet’s interior and atmosphere characteristics. As such, there is no universal habitable zone applicable to all exoplanets.”

Seager even makes room for the many rogue planet floating unconnected to a solar system as possible candidates, with the same kind of warming deep hydrogen covering that Pierrehumbert proposed. Clearly, her goal is to add exoplanets that are far less like Earth to the possible habitable mix.

 

In this artist's concept shows "The Behemoth," an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. The hydrogen is evaporating from the planet due to extreme radiation from the star, but on many exoplanets it remains a thick covering. (NASA, ESA, and G. Bacon, STScI)
In this artist’s concept shows “The Behemoth,” an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. The hydrogen is evaporating from the planet due to extreme radiation from the star, but on many exoplanets it remains a thick covering. (NASA, ESA, and G. Bacon, STScI)

Meanwhile, scientists have been adding numerous conditions beyond liquid surface water to enable a planet to turn from a dead to a potentially habitable one.  Kasting and Whitmore did include some of these conditions in their initial 1993 paper, but the list is growing.  A long-term stable climate is considered key, for instance, and that in turn calls for the presence of features akin to plate tectonics, volcanoes, magnetic fields and cycling into the planet interior of carbon, silicates and more.  Needless to say, these are not planetary features scientists will be able to identify for a long time to come.

So the disconnect grows between how exoplanet hunters and researchers use the term “habitable zone” and how the public understands its meaning.  Scientists describe a myriad of conditions and add that they are “necessary but not sufficient.”  Meanwhile, many exoplanet enthusiasts in the public are understandably awaiting a seemingly imminent discovery of extraterrestrial life on one of the many habitable zone planets announced.  (In fairness, no Earth-sized planet orbiting a sun-like star has been identified so far.)

Kasting, for one, does not see all this questioning of the necessary qualities of a habitable zone as a problem.

“Push back is what scientists do; we’re brought up to question authority.  My initial work is over 20 years old and a lot has been learned since then.  Not all things that are written down are correct.”

James Kasting of Penn State University, a pioneer in defining a habitable zone.
James Kasting of Penn State University, a pioneer in defining a habitable zone.

But in this case, he says, a lot of the conventional habitable zone concept is pretty defensible.

What’s more, it’s practical and useful.  While not discounting the possibility of life on exo-moons, on giant planets surrounded by warming molecular hydrogen or other possibilities, he says that the technical challenges to making a telescope that could capture the light necessary to analyze these moons or far-from-their-star planets would be so faint as to be undetectable given today’s (or even tomorrow’s) technology.  With those two exoplanet-focused telescopes (LUVOIR and Hab-Ex) now under formal study for a possible mission in the 2030s, Kasting thinks it’s essential to think inside, rather than outside, the box.

“I think that when the teams sit down and think about the science and technology of those projects, our habitable zone is the only one that make sense.  If you design a telescope to capture possible evidence of life as far out as 10 AU, you give up capability to study with the greatest precision planets close in the traditional habitable zone.  That doesn’t mean the telescope can’t look for habitable worlds outside the traditional habitable zone, but but don’t design the telescope with that as a high priority.  Better to focus on what we know does exist.”

Coming soon:  The Habitability Inde

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