A Four Planet System in Orbit, Directly Imaged and Remarkable

Facebooktwittergoogle_plusredditpinterestlinkedinmail

Now on Facebook:  http://facebook.com/nexssmanyworlds/

The era of directly imaging exoplanets has only just begun, but the science and viewing pleasures to come are appealingly apparent.

This evocative movie of four planets more massive than Jupiter orbiting the young star HR 8799 is a composite of sorts, including images taken over seven years at the W.M. Keck observatory in Hawaii.

The movie clearly doesn’t show full orbits, which will take many more years to collect. The closest-in planet circles the star in around 40 years; the furthest takes more than 400 years.

But as described by Jason Wang,  an astronomy graduate student at the University of California, Berkeley, researchers think that the four planets may well be in resonance with each other.

In this case it’s a one-two-four-eight resonance, meaning that each planet has an orbital period in nearly precise ratio with the others in the system.

The black circle in the center of the image is part of the observing and analyzing effort to block the blinding light of the star, and thus make the planets visible.

The images were initially captured by a team of astronomers including Christian Marois of the National Research Council of Canada’s Herzberg Institute of Astrophysics, who analyzed the data.  The movie animation was put together by Wang, who is part of the Berkeley arm of the Nexus for Exoplanet System Science (NExSS), a NASA-sponsored group formed to encourage interdisciplinary exoplanet science.

The star HR 8799 has already played a pioneering role in the evolution of direct imaging of exoplanets.  In 2008, the Marois group announced discovery of three of the four HR 8799 planets using direct imaging for the first time. On the same day that a different team announced the direct imaging of a planet orbiting the star Fomalhaut.


This false-color composite image traces the motion of the planet Fomalhaut b, a world captured by direct imaging. (NASA, ESA, and P. Kalas, University of California, Berkeley and SETI Institute)

HR 8799 is 129 light years away in the constellation of Pegasus.  By coincidence, it is quite close to the star 51 Pegasi, where the first exoplanet was detected in 1995.  It is less than 60 million years old, Wang said, and is almost five times brighter than the sun.

Wang said that the animation is based on eight observations of the planets since 2009.  He then used a motion interpolation algorithm to draw the orbit between those points.

Much can be learned from the motion of the planets, however long it may take for them to circle their sun.  Based on the Keck observations, astronomers have concluded that the four planets orbit in roughly Keplerian motion around the star — almost circular, but not entirely.

Jason Wang is a graduate student in astronomy at the University of California, Berkeley.

The planets are quite far from each other, which is to be expected due to their enormous size.   Because of those large separations, Wang said astronomers will be watching to see if the system is stable or if some of the planets may be ejected from the system.

Although the first three HR 8799 planets were officially discovered in 2008,  researchers learned afterwards that the planets had actually already been observed.  The “precovery” had been made in 1998 by the NICMOS instrument on the Hubble Space Telescope, but was teased out only after a newly developed image-processing technique was installed.

Christian Marois was part of the team that discovered HR 8799 using direct imaging. He is also on the engineering and science teams of the Gemini Planet Imager, which he helped design and build.

The fourth HR 8799 planet was found after further observations in 2009–2010.  That planet orbits inside the first three planets, but is still fifteen times the distance from its sun than Earth to our sun.  (The team working with Marois included Quinn Konopacky of the University of California, San Diego, Bruce Macintosh of Stanford University, Travis Barman of the University of Arizona and Ben Zuckerman of UCLA.)

James Graham is leader of the Berkeley NExSS group, and he was struck by some of the connections between what has been found around HR 8799 and what exists in our own solar system.

For instance, he said that “it’s delightful that these recently discovered planets exhibit the same type of harmony exhibited by the Galilean moons, Io, Europa, and Ganymede (1:2:4) and illustrating some of the connections between our own solar system and those orbiting other stars. ”

The outer planet orbits inside a dusty disk like our Kuiper Belt. It is one of the most massive disks known around any star within 300 light years of Earth, and there is room in the inner system for rocky planets.

Both Wang and Marois are also on the team operating the Gemini Planet Imager, a cutting-edge addition to the Gemini South telescope in Atacama Desert of Chile.

The GPI includes a next-generation adaptive optics instrument that allows for much clearer seeing through the Earth’s atmosphere by correcting for turbulence.  The result is better direct imaging.   A key goal of the GPI project is to image large extrasolar planets orbiting at distances from their host stars similar to, or greater than, between Jupiter and our sun.

This looping animation of a series of images taken between November 2013 and April 2015 with the Gemini Planet Imager (GPI) on the Gemini South telescope in Chile shows the exoplanet Beta Pictoris b orbiting the star Beta Pictoris. In the images, the star is at the center of the left-hand edge of the frame; it is hidden by the Gemini Planet Imager’s coronagraph. We are looking at the planet’s orbit almost edge-on. (M. Millar-Blanchaer, University of Toronto; F. Marchis, SETI Institute)

The idea for the HR 8799 movie came from a similar, but less elaborate, orbital animation of a planet detected by GPI circling the star Beta Pictoris.

It was initially thought (and hoped) that the planet might transit in front of Beta Pictoris,  providing a unique opportunity to learn the radius of the planet and thus understand the size of the atmosphere.  Unfortunately, the geometry of the planet’s orbit doesn’t quite line up in a way that would have the planet pass in front of the star from our point of view.

However, although the planet doesn’t transit, what is called its Hill sphere does. The Hill sphere is the region surrounding the planet where its gravitational influence dominates over the gravitational influence of the star. As a result, the remnants of the disk left over from planet formation, planetary rings and moons could transit the star later this year and may be detectable.

Those smaller bodies are unlikely to be the subject of any evocative movie animations, but direct imaging will be bringing many more of them to us in the days ahead.

“The Beta Pic animation looked so cool that we’ve wanted to do more,” Wang said, explaining why the HR 8799 movie was made.  “We wanted to make one that was even more impactful for the audience and could begin to show what one of these systems looks like.”

I think they succeeded.

 

 

 

 

Facebooktwittergoogle_plusredditpinterestlinkedinmail

Three Star Ballet, With Exoplanet

Facebooktwittergoogle_plusredditpinterestlinkedinmail
This artist's impression shows a view of the triple-star system HD 131399 from close to the giant planet orbiting in the system. The planet is known as HD 131399Ab and appears at the lower-left of the picture. Located about 320 light-years from Earth in the constellation of Centaurus (The Centaur), HD 131399Ab is about 16 million years old, making it also one of the youngest exoplanets discovered to date, and one of very few directly imaged planets. With a temperature of around 580 degrees Celsius and an estimated mass of four Jupiter masses, it is also one of the coldest and least massive directly-imaged exoplanets.
An artist’s impression of the triple-star system HD 131399 from close to the giant planet orbiting in the system. The planet is known as HD 131399Ab and appears at the lower-left of the picture. Located about 320 light-years from Earth in the constellation of Centaurus (The Centaur), HD 131399Ab is about 16 million years old, making it also one of the youngest exoplanets discovered to date, and one of very few directly imaged planets. (ESO/Luis Calcada)

It hardly seems possible, but researchers have detected a planet in apparently stable orbit within a three star system — a configuration now known as a trinary.

The ubiquity of binary stars has been understood for some time, and the presence of exoplanets orbiting around and within them is no longer a surprise.  But this newest planet detected — four times the mass of Jupiter — is most unusual because trinary systems are not known to be particularly conducive to keeping planets in orbit, and especially not a planet in an extremely wide (i.e., 550 year) orbit.

Yet this planet has found the sweet spot between the stars where it balances the gravitational pulls of the three.  The system is a relative toddler at 16 million years old, and so the researchers involved in its detection say it may later be ejected from the system.  But for now, it is the only known planet of its kind.

The discovery, reported in the journal Science, was made using the European Southern Observatory’s Very Large Telescope (VLT) in Chile’s Atacama desert.  The team was from the University of Arizona in Tucson and was led by Daniel Apai, an assistant professor of Astronomy and Planetary Sciences who leads a planet finding and observing group.  That team includes research doctoral student Kevin Wagner, the first author on the paper.

“It is not clear how this planet ended up on its wide orbit in this extreme system — and we can’t say yet what this means for our broader understanding of the types of planetary systems — but it shows that there is more variety out there than many would have deemed possible,” Wagner said.

This new planet is a gas giant and definitely not habitable, but the possible universe of exoplanets that just might meet some of the basic criteria for habitability may well have grown.

“What we do know is that planets in multi-star systems have been studied far less often, but are potentially just as numerous as planets in single-star systems,” Wagner said.

An artist's rendering of the Kepler-16 system, showing the binary star being orbited by Kepler-16b
An artist’s rendering of the Kepler-16 system, showing the binary star being orbited by Kepler-16b.  The planet was the first confirmed to have a circumbinary 0rbit — traveling around both stars in 229 days.  (NASA)

Astronomers estimate that about half of the stars we see in the sky are actually two stars or more.  Stars are created when massive clouds of gas and dust collapse in on themselves, and sometimes that results in a fragmenting into multiple stars.

Two of the stars in the HD 131399 system are close together, twirling around each other like a spinning dumbbell.  The third and far brightest star, which is located about 300 times further away than the distance between Earth and the sun,  is orbited by the new gas giant planet HD 131399Ab. With a temperature of around 580 degrees Celsius and an estimated mass of four Jupiter masses, it is one of the coldest and least massive directly-imaged exoplanets.

The three-star system and the planet were found using direct imagining of thermal emissions, as opposed to the traditional techniques of searching for the effects of an exoplanet on the host sun and other planets.

This planet discovery was a first for SPHERE, the Spectro-Polarimetric High-Contrast Exoplanet Research Instrument, which took a decade to build.  The instrument is sensitive to infrared light and is capable of detecting the heat signatures of young planets picked up by the Very Large Telescope’s mirrors.  SPHERE has a coronograph to block out the otherwise blinding light of the host star, and new capabilities to  correct for disturbances caused by features of our atmosphere.

Apai said that detecting a planet in such a triple system was both surprising and “really cool.”

“We’re now are going back and take a careful look at all the other triple systems that haven’t been observed because we didn’t think planets could be there. I’m very curious to continue to study this system to figure out whether the planet formed in that odd orbit or if it moved there after encountering another planet or one of the double stars.”

Here is a wonderful video animation of the choreography of the stars and planet, the work of the ESO’s Luis Calçada and Martin Kornmesser:

As is so often the case, the discovery involved substantial serendipity;  it was not at all what the group was looking for.

Rather, Apai’s team has been looking to prove, or disprove, a confounding pattern in exoplanet discoveries:  most of the (very few) planets discovered via direct imaging were found around stars with masses about twice of the sun’s mass, while only one planet has been found around a sun-like star.  Most were also short orbital period planets, unlike the one with a 550-year orbit just discovered. If this pattern of detections were found to be a feature of the galaxy as a whole, it would challenge some components of the basic planet formation model.

“Strangely, most of the eight or so planets discovered via direct imaging were found around stars with masses about twice of the Sun’s mass, while – up to now – around Sun-like stars only one Jovian planet was discovered,” he said in an email.

So the group set out to study about 100 stars more massive than the Sun to determine how many of them have giant planets that can be imaged — with the goal of seeing whether they are really more common than around Sun-like stars, or if it was just a coincidence.

While that campaign will continue for another year finding the three star exoplanet may well become their most significant finding.

Wagner, who first saw the presence of the exoplanet in the SPHERE data, confirmed the finding during observations a half year later.

“For much of the planet’s year the stars appear close together, giving it a familiar night-side and day-side with a unique triple-sunset and sunrise each day,” he said. “As the planet orbits and the stars grow further apart each day, they reach a point where the setting of one coincides with the rising of the other – at which point the planet is in near-constant daytime for about one-quarter of its orbit, or roughly 140 Earth-years.”

The precise orbit of the planet has actually not yet been determined, and doing so is one of Wagner’s next tasks.  The team knows that the planet is 80 times further from the A star than the Earth is from our sun, and that the A and BC stars are 300 times further away from each other. Asked if that orbit could change dramatically, even becoming circumtrinary around all three stars, he said it was quite unlikely but that it couldn’t be ruled out.

Wagner used an apt quote to describe his reaction to the discovery, one attributed to many great astrophysicists but originally probably from Arthur Stanley Eddington,  who helped confirm Einstein’s General Theory of Relativity in the early 1900s.

“Not only is the universe stranger than we imagine,” he said, “it is stranger than we can imagine.”

This annotated composite image shows the newly discovered exoplanet HD 131399Ab in the triple-star system HD 131399. The image of the planet was obtained with the SPHERE imager on the ESO Very Large Telescope in Chile. This is the first exoplanet to be discovered by SPHERE and one of very few directly-imaged planets. With a temperature of around 580 degrees Celsius and an estimated mass of four Jupiter masses, it is also one of the coldest and least massive directly-imaged exoplanets. This picture was created from two separate SPHERE observations: one to image the three stars and one to detect the faint planet. The planet appears vastly brighter in this image than in would in reality in comparison to the stars.
This picture was created from two separate SPHERE observations: one to image the three stars and one to detect the faint planet. The planet appears vastly brighter in this image than in would in reality in comparison to the stars. (ESO)
Facebooktwittergoogle_plusredditpinterestlinkedinmail

Movement in The Search For ExoLife

Facebooktwittergoogle_plusredditpinterestlinkedinmail
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.

Facebooktwittergoogle_plusredditpinterestlinkedinmail

How Will We Know What Exoplanets Look Like, and When?

Facebooktwittergoogle_plusredditpinterestlinkedinmail

An earlier version of this article was accidently published last week before it was completed.  This is the finished version, with information from this week’s AAS annual conference.

This image of a pair of interacting galaxies called Arp 273 was released to celebrate the 21st anniversary of the launch of the NASA/ESA Hubble Space Telescope. The distorted shape of the larger of the two galaxies shows signs of tidal interactions with the smaller of the two. It is thought that the smaller galaxy has actually passed through the larger one.
This image of a pair of interacting galaxies called Arp 273 was released to celebrate the 21st anniversary of the launch of the NASA/ESA Hubble Space Telescope. The distorted shape of the larger of the two galaxies shows signs of tidal interactions with the smaller of the two. It is thought that the smaller galaxy has actually passed through the larger one.

Let’s face it:  the field of exoplanets has a significant deficit when it comes to producing drop-dead beautiful pictures.

We all know why.  Exoplanets are just too small to directly image, other than as a miniscule fraction of a pixel, or perhaps some day as a full pixel.  That leaves it up to artists, modelers and the travel poster-makers of the Jet Propulsion Lab to help the public to visualize what exoplanets might be like.  Given the dramatic successes of the Hubble Space Telescope in imaging distant galaxies, and of telescopes like those on the Cassini mission to Saturn and the Mars Reconnaissance Orbiter, this is no small competitive disadvantage.  And this explains why the first picture of this column has nothing to do with exoplanets (though billions of them are no doubt hidden in the image somewhere.)

The problem is all too apparent in these two images of Pluto — one taken by the Hubble and the other by New Horizons telescope as the satellite zipped by.

image

Pluto image taken by Hubble Space Telescope (above) and close up taken by New Horizons in 2015. (NASA)
Pluto image taken by Hubble Space Telescope (above) and close up taken by New Horizons in 2015. (NASA)

Pluto is about 4.7 billion miles away.  The nearest star, and as a result the nearest possible planet, is 25 trillion miles  away.  Putting aside for a minute the very difficult problem of blocking out the overwhelming luminosity of a star being cross by the orbiting planet you want to image,  you still have an enormous challenge in terms of resolving an image from that far away.

While current detection methods have been successful in confirming more than 2,000 exoplanets in the past 20 years (with another 2,000-plus candidates awaiting confirmation or rejection),  they have been extremely limited in terms of actually producing images of those planetary fireflies in very distant headlights.  And absent direct images — or more precisely, light from those planets — the amount of information gleaned about the chemical makeup of their atmospheres  as been limited, too.

But despite the enormous difficulties, astronomers and astrophysicist are making some progress in their quest to do what was considered impossible not that long ago, and directly image exoplanets.

In fact, that direct imaging — capturing light coming directly from the sources — is pretty uniformly embraced as the essential key to understanding the compositions and dynamics of exoplanets.  That direct light may not produce a picture of even a very fuzzy exoplanet for a very long time to come, but it will definitely provide spectra that scientists can read to learn what molecules are present in the atmospheres, what might be on the surfaces and as a result if there might be signs of life.

This diagram illustrates how astronomers using NASA's Spitzer Space Telescope can capture the elusive spectra of hot-Jupiter planets. Spectra are an object's light spread apart into its basic components, or wavelengths. By dissecting light in this way, scientists can sort through it and uncover clues about the composition of the object giving off the light. To obtain a spectrum for an object, one first needs to capture its light. Hot-Jupiter planets are so close to their stars that even the most powerful telescopes can't distinguish their light from the light of their much brighter stars. But, there are a few planetary systems that allow astronomers to measure the light from just the planet by using a clever technique. Such "transiting" systems are oriented in such a way that, from our vantage point, the planets' orbits are seen edge-on and cross directly in front of and behind their stars. In this technique, known as the secondary eclipse method, changes in the total infrared light from a star system are measured as its planet transits behind the star, vanishing from our Earthly point of view. The dip in observed light can then be attributed to the planet alone. To capture a spectrum of the planet, Spitzer must observe the system twice. It takes a spectrum of the star together with the planet (first panel), then, as the planet disappears from view, a spectrum of just the star (second panel). By subtracting the star's spectrum from the combined spectrum of the star plus the planet, it is able to get the spectrum for just the planet (third panel). This ground-breaking technique was used by Spitzer to obtain the first-ever spectra of two planets beyond our solar system, HD 209458b and HD 189733b. The results suggest that the hot planets are socked in with dry clouds high up in the planet's stratospheres. In addition, HD 209458b showed hints of silicates, indicating those high clouds might be made of very fine sand-like particles.
This diagram illustrates how astronomers using NASA’s Spitzer Space Telescope can capture the elusive spectra of hot-Jupiter planets. Spectra are an object’s light spread apart into its basic components, or wavelengths. By dissecting light in this way, scientists can sort through it and uncover clues about the composition of the object giving off the light. (NASA/JPL-Caltech)

There has been lots of technical and scientific debate about how to capture that light, as well as debate about how to convince Congress and NASA to fund the search.  What’s more, the exoplanet community has a history of fractious internal debate and competition that has at times undermined its goals and efforts, and that has been another hotly discussed subject.  (The image of a circular firing squad used to be a pretty common one for the community.)

But a seemingly much more orderly strategy has been developed in recently years and was on display at the just-completed American Astronomical Society annual meeting in Florida.  The most significant breaking news was probably that NASA has gotten additional funds to support a major exoplanet direct imaging mission in the 2020s, the Wide Field Infrared Survey Telescope (WFIRST), and that the agency is moving ahead with a competition between four even bigger exoplanet or astrophysical missions for the 2030s. The director of NASA Astrophysics, Paul Hertz, made the formal announcements during the conference, when he called for the formation of four Science and Technology Definition Teams to assess in great detail the potentials and plausibilities of the four possibilities.

Paul Hertz, Director of the Astrophysics Division of NASA's Science Mission Directorate.
Paul Hertz, Director of the Astrophysics Division of NASA’s Science Mission Directorate.

Putting it into a broader perspective, astronomer Natalie Batalha, science lead for the Kepler Space Telescope, told a conference session on next-generation direct imaging that  “with modern technology, we don’t have the capability to image a solar system analog.”  But, she said, “that’s where we want to go.”

And the road to discovering exoplanets that might actually sustain life may well require a space-based telescope in the range of eight to twelve meters in radius, she and others are convinced.  Considering that a very big challenge faced by the engineers of the James Webb Space Telescope (scheduled to launch in 2018) was how to send a 6.5 meter-wide mirror into space, the challenges (and the costs) for a substantially larger space telescope will be enormous.

We will come back in following post to some of these plans for exoplanet missions in the decades ahead, but first let’s look at a sample of the related work done in recent years and what might become possible before the 2020s.  And since direct imaging is all about “seeing” a planet — rather than inferring its existence through dips in starlight when an exoplanet transits, or the wobble of a sun caused by the presence of  an orbiting ball of rock (or gas)  — showing some of the images produced so far seems appropriate.  They may not be breath-taking aesthetically, but they are remarkable.

There is some debate and controversy over which planets were the first to be directly imaged. But all agree that a major breakthrough came in 2008 with the imaging of the HR8799 system via ground-based observations.

NASA/JPL-Caltech/Palomar Observatory - http://www.nasa.gov/topics/universe/features/exoplanet20100414-a.html This image shows the light from three planets orbiting a star 120 light-years away. The planets' star, called HR8799, is located at the spot marked with an "X." This picture was taken using a small, 1.5-meter (4.9-foot) portion of the Palomar Observatory's Hale Telescope, north of San Diego, Calif. This is the first time a picture of planets beyond our solar system has been captured using a telescope with a modest-sized mirror -- previous images were taken using larger telescopes. The three planets, called HR8799b, c and d, are thought to be gas giants like Jupiter, but more massive. They orbit their host star at roughly 24, 38 and 68 times the distance between our Earth and sun, respectively (Jupiter resides at about 5 times the Earth-sun distance).
This 2010 image shows the light from three planets orbiting HR8799, 120 light-years away.  The three planets, called HR8799b, c and d, are thought to be gas giants like Jupiter, but more massive. (NASA/JPL-Caltech/Palomar Observatory)

First, three Jupiter-plus gas giants were identified using the powerful Keck and Gemini North infrared telescopes on Mauna Kea in Hawaii by a team led by Christian Marois of the National Research Council of Canada’s Herzberg Institute of Astrophysics. That detection was followed several years later the discovery of a fourth planet and then by the release of the surprising image above, produced with the quite small (4.9 foot) Hale telescope at the Palomar Observatory outside of San Diego.

As is the case for all planets directly imaged, the “pictures” were not taken as we would with our own cameras, but rather represent images produced with information that is crunched in a variety of necessary technical ways before their release.  Nonetheless, they are images in a way similar the iconic Hubble images that also need a number of translating steps to come alive.

Because light from the host star has to be blocked out for direct imaging to work, the technique now identifies only planets with very long orbits. In the case of HR8799, the planets orbit respectively at roughly 24, 38 and 68 times the distance between our Earth and sun.  Jupiter orbits at about 5 times the Earth-sun distance.

In the same month as the HR8799 announcement, another milestone was made public with the detection of a planet orbiting the star Formalhaut.  That, too, was done via direct imagining, this time with the Hubble Space Telescope.

The Hubble images were taken with the Space Telescope Imaging Spectrograph in 2010 and 2012. This false-color composite image, taken with the Hubble Space Telescope, reveals the orbital motion of the planet Fomalhaut b. Based on these observations, astronomers calculated that the planet is in a 2,000-year-long, highly elliptical orbit. The planet will appear to cross a vast belt of debris around the star roughly 20 years from now. If the planet's orbit lies in the same plane with the belt, icy and rocky debris in the belt could crash into the planet's atmosphere and produce various phenomena. The black circle at the center of the image blocks out the light from the bright star, allowing reflected light from the belt and planet to be photographed. Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute)
The Hubble images the the star Formalhaut and planet Formalhaut b were taken with the Space Telescope Imaging Spectrograph in 2010 and 2012. This false-color composite image reveals the orbital motion of the Fomalhaut b. Based on these observations, astronomers calculated that the planet is in a 2,000-year-long, highly elliptical orbit. The black circle at the center of the image blocks out light from the very bright star, allowed reflected light from the belt and planet to be captured.  Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute)

Signs of the planet were first detected in 2004 and 2006 by a group headed by Paul Kalas at the University of California, Berkeley, and they made the announcement in 2008.  The discovery was confirmed several years later and tantalizing planetary dynamics began to emerge from the images (all in false color.) For instance, the planet appears to be on a path to cross a vast belt of debris around the star roughly 20 years from now.  If the planet’s orbit lies in the same plane with the belt, icy and rocky debris could crash into the planet’s atmosphere and cause interesting damage.

The region around Fomalhaut’s location is black because astronomers used a coronagraph to block out the star’s bright glare so that the dim planet could be seen. This is essential since Fomalhaut b is 1 billion times fainter than its star. The radial streaks are scattered starlight. Like all the planets detected so far using some form of direct imaging,  Fomalhaut b if far from its host star and completes an orbit every 872 years.

Adaptive optics of the Gemini Planet Imager, at the Gemini South Observatory in Chile, has been successful in imaging exoplanets as well.  The GPI grew out of a proposal by the Center for Adaptive Optics, now run by the University of California system, to inspire and see developed innovative optical technology.  Some of the same breakthroughs now used in treating human eyes found their place in exoplanet astronomy.

 

Discovery image of 51 Eri b with the Gemini Planet Imager taken in the near-infrared light on December 18, 2014. The bright central star has been mostly removed by a hardware and software mask to enable the detection of the exoplanet one million times fainter. Credits: J. Rameau (UdeM) and C. Marois (NRC Herzberg).
Discovery image of 51 Eri b with the Gemini Planet Imager taken in the near-infrared light on December 18, 2014. The bright central star has been mostly removed by a hardware and software mask to enable the detection of the exoplanet one million times fainter. Credits: J. Rameau (UdeM) and C. Marois (NRC Herzberg).

The Imager, which began operation in 2014, was specifically created to discern and evaluate dim, newer planets orbiting bright stars using a different kind of direct imaging. It is adept at detecting young planets, for instance, because they still retain heat from their formation, remain luminous and visible. Using the GPI to study the area around the y0ung (20-million-year-old) star 51 Eridiani, the team made their first exoplanet discovery in 2014.

By studying its thermal emissions, the team gained insights into the planet’s atmospheric composition and found that — much like Jupiter’s — it is dominated by methane.  To date, methane signatures have been weak or absent in directly imaged exoplanets.

James Graham, an astronomer at the University of California, Berkeley, is the project leader for a three-year GPI survey of 600 stars to find young gas giant planets, Jupiter-size and above.

“The key motivation for the experiment is that if you can detect heat from the planet, if you can directly image it, then by using basic science you can learn about formation processes for these planets.”  So by imaging the planets using these very sophisticated optical advances, scientists go well beyond detecting exoplanets to potentially unraveling deep mysteries (even if we still won’t know what the planets “look like” from an image-of-the-day perspective.

The GPI has also detected a second exoplanet, shown here at different stages of its orbit:

 

The animation is a series of images taken between November 2013 and April 2015 with the Gemini Planet Imager (GPI) on the Gemini South telescope in Chile, and shows the exoplanet β Pictoris b, which is more than 60 lightyears from Earth. The star is the black area on the left edge of the frame and is hidden by the Gemini Planet Imager’s coronagraph. We are looking at the planet’s orbit almost edge-on, with the planet closer to the Earth than the star. (M. Millar-Blanchaer, University of Toronto; F. Marchis, SETI Institute)
The animation is a series of images taken between November 2013 and April 2015 with the Gemini Planet Imager (GPI) on the Gemini South telescope in Chile, and shows the exoplanet β Pictoris b, which is more than 60 lightyears from Earth. The star is the black area on the left edge of the frame and is hidden by the Gemini Planet Imager’s coronagraph. (M. Millar-Blanchaer, University of Toronto; F. Marchis, SETI Institute)

A next big step in direct imaging of exoplanets will come with the launch of the James Webb Space Telescope in 2018.  While not initially designed to study exoplanets — in fact, exoplanets were just first getting discovered when the telescope was under early development — it does now include a coronagraph which will substantially increase its usefulness in imaging exoplanets.

As explained by Joel Green, a project scientist for the Webb at the Space Telescope Science Institute in Baltimore, the new observatory will be able to capture light — in the form of infrared radiation– that will be coming from more distant and much colder environments than what Hubble can probe.

“It’s sensitive to dimmer things, smaller planets that are more earth-sized.  And because it can see fainter objects, it will be more help in understanding the demographics of exoplanets.  It uses the infrared region of the spectrum, and so it can look better into the cloud levels of the planets than any telescope so far and see deeper.”

James Webb Space Telescope mirror being inspected at Goddard Space Flight Center, as it nears completion.  The powerful, sophisticated and long-awaited telescope is scheduled to launch in 2018.
James Webb Space Telescope mirror being inspected at Goddard Space Flight Center, as it nears completion. The powerful, sophisticated and long-awaited telescope is scheduled to launch in 2018.

These capabilities and more are going to be a boon to exoplanet researchers and will no doubt advance the direct imaging effort and potentially change basic understandings about exoplanets.  But it is not expected produce gorgeous or bizarre exoplanet pictures for the public, as Hubble did for galaxies and nebulae.  Indeed, unlike the Hubble — which sees primarily in visible light —  Webb sees in what Green said is, in effect, night vision.   And so researchers are still working on how they will produce credible images using the information from Webb’s infrared cameras and translating them via a color scheme into pictures for scientists and the public.

Another compelling exoplanet-imaging technology under study by NASA is the starshade, or external occulter, a metal disk in the shape of a sunflower that might some day be used to block out light from host stars in order to get a look at faraway orbiting planets.  MIT’s Sara Seager led a NASA study team that reported back on the starshade last year in a report that concluded it was technologically possible to build and launch, and would be scientifically most useful.  If approved, the starshade — potentially 100 feet across — could be used with the WFIRST telescope in the 2020s.  The two components would fly far separately, as much as 35,000 miles away from each other, and together could produce breakthrough exoplanet direct images.

An artist's depiction of a sunflower-shaped starshade that could help space telescopes find and characterize alien planets. Credit: NASA/JPL/Caltec
An artist’s depiction of a sunflower-shaped starshade that could help space telescopes find and characterize alien planets.  Credit: NASA/JPL/Caltech

Here is a link to an animation of the starshade being deployed: http://planetquest.jpl.nasa.gov/video/15

The answer, then, to the question posed in the title to this post — “How Will We Know What Exoplanets Look Like, and When?”– is complex, evolving and involves a science-based definition of what “looking like” means.  It would be wonderful to have images of exoplanets that show cloud formations, dust and maybe some surface features, but “direct imaging” is really about something different.  It’s about getting light from exoplanets that can tell scientists about the make-up of those exoplanets and their atmospheres, and ultimately that’s a lot more significant than any stunning or eerie picture.

And with that difference between beauty and science in mind, this last image is one of the more striking ones I’ve seen in some time.

Moon glow over Las Campanas Observatory, run by the Carnegie Institution of Science, in Chile. (Yuri Beretsky)
Moon glow over Las Campanas Observatory, operated by the Carnegie Institution of Science, in Chile. (Yuri Beretsky)

It was taken at the Las Campanas Observatory in Chile last year, during a night of stargazing.  Although the observatory is in the Atacama Desert, enough moisture was present in the atmosphere to create this lovely moon-glow.

But working in the observatory that night was Carnegie’s pioneer planet hunter Paul Butler, who uses the radial velocity method to detect exoplanets.  But to do that he needs to capture light from those distant systems.  So the night — despite the beautiful moon-glow — was scientifically useless.

Facebooktwittergoogle_plusredditpinterestlinkedinmail