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SETI Institute Press Release: June 12, 2019

DSC_0304_Telescope_at_night_2014March23

GPIES Video: https://vimeo.com/341427762

Gemini Planet Imager Planet Search Shows Giant Planets Orbiting Sun-like Stars May Be Rare 

June 12, 2019, Mountain View, CA — The Gemini Planet Imager (GPI), a dedicated planet-finding instrument at the Gemini South telescope in Chile, is concluding a 4-year survey – the GPI Exoplanet Survey (GPIES) – of 531 young, nearby stars searching for giant planets. Analysis of half of the survey data to be published in The Astronomical Journal suggests that giant planets orbiting Sun-like stars, slightly more massive than Jupiter in our solar system, may be rare. GPIES is led by Stanford astronomers and includes an international team

“We suspect that in our own solar system Jupiter and Saturn sculpted its final architecture that influences the properties of terrestrial planets like Mars and Earth, including basic elements for life like the delivery of water, and the impact rates,” said Franck Marchis, Senior researcher at the SETI Institute and a co-author of the paper. “A planetary system with only terrestrial planets and no giant planets will probably be very different to ours, and this could have consequences on the possibility for the existence of life elsewhere in our galaxy.”

The way GPI searches for exoplanets is different from the methods used in other exoplanet research. Most exoplanets discovered thus far, including those found by NASA’s Kepler spacecraft, are found via indirect methods, such as observing a dimming in the star’s light as an orbiting planet eclipses its parent star (the transit method), or by observing the star’s wobble as the planet’s gravity tugs on the star (the radial-velocity method). These methods have been incredibly successful, but mostly probe the central regions of planetary systems. Those regions outside the orbit of Jupiter, where the giant planets are located in our solar system, are usually out of their reach. GPI, however, endeavors to directly detect planets in this area of space by taking a picture of them alongside their parent stars.

Imaging a planet around another star is a difficult technical challenge reserved to a few instruments, including GPI. Planets are small, faint and very close to their host star. Distinguishing an orbiting planet from its star is like resolving the width of a dime from miles away, and even the brightest planets are ten thousand times fainter, and GPI can see planets a million times fainter than the stars they orbit. Searching for planets this way is also time consuming.

“GPI was designed and built specifically to overcome this challenge, and images from the survey are much more sensitive than those from previous generations of planet-imaging instruments,” said Bruce Macintosh, principal investigator of the project and professor at Stanford University. “The Gemini Observatory gave us time to do a careful, systematic survey. This analysis of the first 300 stars observed by GPIES represents the largest, most sensitive direct imaging survey for giant planets published to date.”

GPI Uncovers a Previously-Hidden Planet and Brown Dwarf 

An early success of GPIES was the discovery of 51 Eridani b in December 2014, a planet about two-and-a-half times more massive than Jupiter, and orbiting just beyond the distance Saturn orbits our own Sun. The host star 51 Eridani is just 97 light-years away and is only 26 million years old (nearby and young, by astronomy standards). It had been observed by multiple planet-imaging surveys with a variety of telescopes and instruments, but its planet was not detected until GPI’s superior instrumentation was able to suppress the starlight enough for the planet to be visible.

GPIES also discovered the brown dwarf HR 2562 B, which is at a distance similar to that between the Sun and Uranus and is 30 times more massive than Jupiter. Brown dwarfs are objects that are more massive than planets, but not massive enough to fuse hydrogen into helium like stars. A longstanding question is whether these intermediate-mass objects are born more like stars or planets. Stars form from the top down, by the gravitational collapse of large primordial clouds of gas and dust, while planets are thought—but have not been confirmed—to form from the bottom up, by the assembly of small rocky bodies that then grow into larger ones, a process also termed “core accretion”.

The discoveries made by GPIES, together with its confirmation of an additional five planets and two brown dwarfs that had been detected by earlier generations of instruments, have now shed important new light on questions of formation.

“With six detected planets and three detected brown dwarfs, along with unprecedented sensitivity to planets a few times the mass of Jupiter at orbital distances beyond Saturn’s, our sample can directly answer key questions about wide-separation giant planets and brown dwarfs,” said Eric Nielsen, researcher at Stanford University, previously a postdoctoral fellow at the SETI Institute and lead author of the paper.

“What the GPIES Team’s analysis shows is that the properties of brown dwarfs and giant planets run completely counter to each other,” said Eugene Chiang, professor of astronomy at UC Berkeley and a co-author of the paper. “Whereas more massive brown dwarfs outnumber less massive brown dwarfs, for giant planets the trend is reversed: less massive planets outnumber more massive ones. Moreover, brown dwarfs tend to be found far from their host stars, while giant planets concentrate closer in. These opposing trends point to brown dwarfs forming top-down, and giant planets forming bottom-up.”

More Massive Stars are Over-represented as Hosts of Detected Planets 

Out of 300 stars, 123 are more than one-and-a-half times more massive than our Sun. One of the most striking results of the GPI survey is that all hosts of detected planets are among these higher-mass stars, even though it is easier to see a giant planet orbiting a fainter, more Sun-like star.

The relationship between the mass of the star and the giant planet frequency suspected for years has been unambiguously confirmed by this study. The finding also appears to be consistent with the bottom-up formation scenario for planets.

Where are the Jupiters? 

One of the greatest surprises that emerged in exoplanet studies has been how different other planetary systems are from our own. While our solar system has small, rocky planets in the inner parts and giant gas planets in the outer parts, we have learned from all exoplanet surveys how intrinsically uncommon giant planets seem to be around Sun-like stars, and how different other solar systems are. Extrapolation of simple models made us believe that GPI would find a dozen giant planets or more, but only saw 6. Putting it all together, giant planets may be present around only a minority of stars like our own. Although GPIES was not sensitive to low-mass planets such as Jupiter, the trends established for higher-mass planets, and in particular, their strong preference to be hosted by stars more massive than the Sun, are clues that one way in which our solar system may be atypical is Jupiter’s presence.

“If this finding is confirmed after analyzing the rest of the survey and more surveys from ground and space-based telescopes to come, it will have an impact on our understanding of the existence of life on terrestrial planets” said Marchis, “and that’s ultimately the raison d’etre of those surveys to understand how planetary system formed and what kind of life could exist elsewhere.”

GPIES observed its 531st, and final, new star in January 2019, and the team is currently following up the remaining candidates to determine which are truly planets and which are distant background stars impersonating giant planets. As this follow-up concludes, the team will move on to publishing the analysis of the entire survey and will begin an upgrade process to make GPI even more sensitive to smaller-mass, closer-in planets as it moves from Chile to Hawaii to begin a new search for planets at the Gemini-North telescope.

IMG_4796_Chilcote_Nov_17_2014

 

Gemini PR: Gemini: https://www.gemini.edu/node/21206

SETI Institute PR: https://www.seti.org/gemini-planet-imager-planet-search-shows-giant-planets-orbiting-sun-stars-may-be-rare

Stanford PR: https://news.stanford.edu/press-releases/2019/06/12/gemini-planet-imalyzes-300-stars/

UC Berkeley PR: https://news.berkeley.edu/2019/06/12/jupiter-like-exoplanets-found-in-sweet-spot-in-most-planetary-systems/

 


 

This research was based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership. It was funded by the National Science Foundation, the National Aeronautics and Space Administration, the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada, Fonds de Recherche du Québec, the Heising-Simons Foundation, Lawrence Livermore National Laboratory, the Center for Exoplanets and Habitable Worlds.

About the SETI Institute 

Founded in 1984, the SETI Institute is a non-profit, multi-disciplinary research and education organization whose mission is to explore, understand, and explain the origin and nature of life in the universe and the evolution of intelligence. Our research encompasses the physical and biological sciences and leverages expertise in data analytics, machine learning and advanced signal detection technologies. The SETI Institute is a distinguished research partner for industry, academia and government agencies, including NASA and NSF.

Contact information
Rebecca McDonald
Director of Communications, SETI Institute
189 Bernardo Ave. Suite 200
Mountain View, CA 94043
rmcdonald@seti.org
www.seti.org

 

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GPI at 2016 SPIE Astronomical Telescopes and Instrumentation

GPI is here at the SPIE Astronomical Telescopes and Instrumentation conference in Edinburgh, Scotland to talk exoplanets and engineering with our colleagues around the world. We have had talks and posters by GPIES team members on topics from AO and instruments to computing and pipelines. This week has also been a great opportunity to meet face-to-face and discuss some of the finer points of GPI technical challenges and plans going forward. Here are some pictures from the week:

Edinburgh, Scotland

Bruce gave a great talk on the GPI instrument and GPIES campaign in the instruments session:

Bruce’s talk about the GPI instrument

Vanessa gave a great talk about GPI’s AO performance in the AO session:

Vanessa’s talk about GPI AO

In her talk, I was especially struck by how well the GPI team has continued to monitor and improve the performance of the instrument, throughout the GPIES campaign.

Franck’s poster was about the GPIES project from a systems level:

Franck’s poster about the GPIES project

Marshall’s poster was about the GPIES pipeline and calibration, critical for extracting accurate exoplanet info from the raw data:

Marshall’s poster about the data pipeline and calibrations

Ben’s poster was about an improved algorithm for finding exoplanets particularly close to the star:

Ben’s poster about detecting exoplanets extremely close to their stars

There were a few more GPIES posters, and I will update the post if I get pictures of more of them soon.

We also took the opportunity to gather 17 of the attending GPIES members for a delicious Indian dinner:

GPIES dinner on Thursday night at the conference

GPIES dinner from Franck’s perspective

I had to add this picture because Dmitry somehow managed to hide in all the dinner pictures

Unfortunately Patrick and Lyra were not able to attend the dinner, and Stephen was only able to stop by to say hi.

After a conference like this, I always leave full of new ideas and a renewed energy for my work. Thanks GPI and SPIE for a great SPIE!

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GPIES May 2016 Observing Run: Women in Astronomy

Hello GPI fans! We are just wrapping up our cloudy, snowy May 2016 GPIES observing run. While the weather wasn’t the best, we accomplished what we could in between the clouds. We also enjoyed the fact that this was the first all-woman run that any of the 5 of us had ever been on. It was a celebration of women in astronomy!

Winter in Chile means snow in the high mountains to the East, as well as a chance of snow at the slightly lower telescope mountains. Here is a wintery view out my window on the plane ride in:

View out the airplane window from Santiago to La Serena

When we arrived at Gemini we were happy to see the cute little zorro that hangs out around the dorms:

Here is a cute little zorro that hangs out by the dorms. Great picture by Kate.

We got a tour of the telescope. It was the first time GPIES student Sarah Blunt had been to Chile, and the rest of us enjoy seeing Gemini again anyway.

Here we are dwarfed by the telescope. From left to right: Katie, Sarah, Jenny, Kim

Gaetano gave us a tour of the telescope. Here we are going up the stairs from the dome floor to the platform.

The observing team for the May 2016 GPIES run. From left: Kate Follette, Kim Ward-Duong, Sarah Blunt, Jenny Patience, and Katie Morzinski

Jenny and Kate led the team and the run, while Kim and Sarah and I were there to help out. We also had assistance on-site from Gemini scientist and GPIES team member Fredrik Rantakyro, as well as other Gemini staff. Remote support was provided by GPIES team members Vanessa Bailey, Rob de Rosa, Jeff Chilcote, and Bruce Macintosh on the Polycom, and others by email and Slack. The reason for such a large complex team is that we optimize the observing campaign to maximize the chance of finding planets.

What does observing for the GPIES campaign involve? Well, 1 person selects targets and directs the night (this was Kate and Jenny), 1 person runs GPI and takes the data (this was Kate and me), 1 person records the log and updates the team (this was Sarah and Kim and me), 1 person runs the pipeline and evaluates the quality of the data (this was Kim and Kate), and 1 person synthesizes and analyzes the progress and makes the big decisions about how best to optimize our time (this was mostly Jenny and a bit Kate).

What are these big decisions about how to best optimize our time? For example, one night the clouds had cleared up a bit but the seeing wasn’t that great, so I took a few data sets while Kim, Kate, and Sarah evaluated the data quality. Then Jenny was comparing our data quality to histograms made by GPIES team members Rob de Rosa and Abhi Rajan to determine whether our data quality was good enough to improve the chances of detecting a planet around the star in question. It was not, so we decided to hand the next hour of observing over to Fredrik to run a GPI queue program. This optimizes use of the telescope for everyone, because we can try to observe that particular star on a different night, while Gemini can get some good science out of its poorer weather by observing targets for a program that doesn’t require the best conditions.

Jenny directs the May 2016 GPIES observing team

The May 2016 GPIES observing team at work

Well, in astronomy as in life, you never know what Mother Earth is going to throw at you. Unfortunately, this run we had a lot of weather, meaning clouds that make for beautiful photos but poor observing:

Sunset

A beautiful backdrop is bad news for astronomers

Virgas* streaming down from the clouds (*A virga is an observable streak or shaft of precipitation, whereas a viga is a wooden beam characteristic of adobe buildings of the southwestern United States and northern Mexico.)

Sunset through the louvers from inside the Gemini dome.

Finally, on the second-to-last day it started snowing, so we had to write off our last night, and chose to head down the mountain a day early to ensure we would catch our flights, since the snow meant the dome would be covered and would not be able to open that night:

Snow on the hillside

Snow at Gemini

The weather was disappointing, but I did have a good time helping out run GPI and discussing the finer points of AO with Vanessa and planet-finding with Jenny, Kate, Kim, and Sarah. The complex GPI instrument and GPIES campaign are made possible by a great collaboration that communicates well to make the best use of our time and observing conditions. Better luck next run, GPIES!

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Observing planet formation at close range: Gemini Planet Imager’s view of the TW Hya disk

Investigations of star and planet formation have long focused on the rich stellar nurseries of Taurus, Ophiuchus, Chamaeleon, and a handful of similarly nearby (but lower mass) molecular clouds. These regions, which lie just beyond 100 pc, are collectively host to hundreds of low-mass, pre-main sequence (T Tauri) stars with ages of a few million years and less. They hence provide large samples of stars with orbiting circumstellar disks that span a wide range of evolutionary stages.

Examples of protoplanetary disks that lie closer than ~100 pc to Earth are far fewer and farther between. However — because their proximity affords the maximum possible linear spatial resolution — these nearby disks provide unique opportunities to test theories describing the planet formation process (see http://cosmicdiary.org/geminiplanetimager/2015/09/16/what-do-we-know-about-planet-formation/). Furthermore, the T Tauri star-disk systems within 100 pc of the Sun tend to be older, on average, than the large numbers of star-disk systems that are still found in or near their natal dark clouds. Hence, circumstellar disks orbiting the nearest known young stars are particularly informative about the late stages of planet formation, as disks disperse and any planets born therein are reaching their final masses (for a brief overview of the study of nearby young stars, see 2015arXiv151000741K).

TW Hydrae was the first of these nearby T Tauri stars to be identified, and remains the best-studied such system. At just 54 pc from Earth and a ripe young age of roughly 8 million years, this nearly solar-mass star and its orbiting, circumstellar disk of dust and gas has become a “go-to” target for new imaging facilities seeking to demonstrate their capabilities. For example, TW Hya has already been the subject of a significant number of ALMA First Light and Early Science programs aimed at investigating the chemistry and structure of its 200-AU-diameter disk.

Hence, when Gemini Planet Imager (GPI) became available for Early Science observations last year, TW Hya beckoned. Given GPI’s potential to perform diffraction-limited, coronagraphic near-infrared imaging on the 8-meter Gemini South telescope, GPI imaging of TW Hya offered the chance to image a protoplanetary disk in its giant planet and Kuiper Belt formation (~10-50 AU) regions at a jaw-dropping ~1.5 AU resolution. In combination with GPI’s polarimetric capability, such observations can tease out the faint signature of starlight scattered off circumstellar dust, potentially yielding an unprecedently detailed view of the surface of the nearly face-on disk.

Our team’s observations of TW Hya were challenging for GPI; the star lies at the faint end of the useful range of its adaptive optics (AO) unit. But our team had successfully imaged the circumbinary disk orbiting the close binary T Tauri system V4046 Sgr with GPI (Rapson et al. 2015ApJ…803L..10R), a system very similar to TW Hya in many respects (including its I magnitude). So we had hope for TW Hya as well.

TWHya_GPIpubImage

The GPI observations of TW Hya did not disappoint. These new GPI coronagraphic/polarimetric AO images confirm the presence of a dark gap in the TW Hya disk at 23 AU that was previously tentatively identified via near-infrared imaging with the Subaru telescope (Akiyama et al. 2015ApJ…802L..17A). The GPI imaging furthermore clearly resolve the disk gap, allowing us to measure its width (~5 AU) and depth (~50%) and thereby facilitating direct comparison with detailed numerical simulations of planets forming in circumstellar disks. The comparisons we have carried out thus far (see above) indicate that the 5-AU-wide gap’s observed structure could be generated by a sub-Jupiter-mass planet orbiting within the disk at a position roughly equivalent to that of Uranus in our solar system. For the gory details, see Rapson et al. (2015ApJ…815L..26).

Further scrutiny of the TW Hya disk with GPI and SPHERE in their differential coronagraphic imaging modes may yield direct detection of the planet(s) that appears to be actively carving a gap in the TW Hya disk — especially if the putative planet is still actively accreting gas from the disk. There are other possible explanations for the formation of gaps and rings in disks, however. In particular, dust grain fragmentation and ice condensation rates may change rapidly with disk radius, yielding sharp variations in small grain surface densities and/or reflective properties that can produce the appearance of disk gaps when imaged in scattered starlight. Or the inner regions of the disk may be partially shadowing exterior regions. ALMA imaging of the TW Hya disk should provide definitive tests of these alternative scenarios for the gap at 23 AU seen in our GPI imaging.

-Joel Kastner (Center for Imaging Science and School of Physics & Astronomy, Rochester Institute of Technology)

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Extreme Solar Systems Featured in Online Press Conference

Announcement from the AAS

The American Astronomical Society (AAS) will convene an online press conference on Tuesday, 1 December, featuring exciting new results on exoplanets from Extreme Solar Systems III, a conference taking place from 29 November through 4 December 2015 at the Waikoloa Beach Marriott Resort & Spa on Hawaii Island.

ExSS III is the third in a series of conferences that began with Extreme Solar Systems in 2007 in Santorini, Greece, and was followed by Extreme Solar Systems II in 2011 in Jackson Hole, Wyoming. Next week’s conference, like the previous two, will cover all aspects of research on exoplanets. Some 350 researchers from all over the world are registered for the meeting.

ExSS III isn’t set up for the types of press conferences held during regular AAS meetings, but because there are so many potentially newsworthy presentations, the organizers and the AAS Press Office have come up with a plan for an online briefing to be held beginning at 7 am HAST on Tuesday, 1 December. That’s 9 am PST on the US West Coast, 12 pm EST on the East Coast, 17:00 UTC/GMT, and 6:00 pm CET in Central Europe. The briefing will last no more than one hour and will be conducted as a Google+ Hangout On Air; viewers without Google+ accounts may participate via YouTube.

Presenters and onsite reporters will gather in the Paniolo room at the Marriott — though, strictly speaking, this isn’t necessary, as all participants will be using their own computers to connect to the briefing. Here’s the lineup:

Rick Fienberg (American Astronomical Society)
Host & Moderator

Paul Kalas (Univ. of California, Berkeley) & Abhijith Rajan (Arizona State Univ.)
Resolving the HD 106906 Disk with the Gemini Planet Imager

Thayne Currie (National Astronomical Observatory of Japan)
Extreme Exoplanet Direct Imaging: New Results and the Path to Imaging Another Earth

Jason Steffen (University of Nevada, Las Vegas)
Dynamical Considerations for Life in Multihabitable Planetary Systems

Lisa Kaltenegger (Cornell University)
Independent Commentary

The briefing will last no more than one hour and will be conducted as a Google+ Hangout On Air, with journalists and the public invited to participate via the associated YouTube or Google+ pages:

YouTube: https://www.youtube.com/watch?v=HXfbVxbqWKU

Google+: https://plus.google.com/events/cq682ju6jeetqmsosst1t0vk3eg

Those watching on YouTube should be able to ask questions via text chat, while those on Google+ should be able to ask questions via the built-in Q&A app. In case either or both of those features don’t work, as a backup journalists may send questions by email to press@aas.org.

Links to the briefing will be provided in a subsequent advisory, on Monday, 30 November.

Contact:

Rick Fienberg
AAS Press Officer
+1 202-328-2010 x116
rick.fienberg@aas.org

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What do we know about planet formation?

Understanding how planets form in the Universe is one of the main motivations for GPI. Thanks to its advanced design, GPI specializes in finding and studying giant planets that are similar to Jupiter in our solar system. These are the kind of planets whose origin we hope to understand much better after our survey is complete.

Fig1

This artist’s impression shows the formation of a gas giant planet around a young star. Credit: ESO/L. Calçada

We know that planets form within protoplanetary disks that orbit young stars, and gas giants need to be fully formed within 3-10 million years of the formation of their parent star as the gaseous nebula dissipates past this point. This very important time constraint is based on statistics of observed protoplanetary disks in nearby young stellar associations such as Taurus. At present there are two main rival theories of giant planet formation — core accretion and gravitational instability, which effectively represent the “bottom up” and “top down” routes to planetary genesis.

Core accretion relies on the formation of a planetary core — a compact, massive object composed of refractory elements, similar to a terrestrial planet like Earth but typically more massive. Cores form by assembly of a large number of planetesimals — smaller asteroid-like bodies composed of rock and ice that collide with each other, merge and grow in size. This process is thought to be rather slow, especially if it happens far from the star, where the characteristic timescales, which are determined by the local orbital period, become very long. As protoplanetary cores increase their mass, their gravitational pull attracts gas from the surrounding nebula, forming atmospheres around them. Atmospheric mass increases quite rapidly, and at some point the whole gaseous envelope becomes self-gravitating. In theoretical models, this transition typically occurs when the core mass exceeds 10-20 Earth masses. Beyond this point rapid gas accretion ensues, turning the core into a giant planet in a relatively short period of time. This process is accompanied by a brief phase of high luminosity as the gravitational energy of accreted gas is radiated away. The final mass of the planet is likely to be set by how much nebular gas is available for accretion, which may be limited by the formation of a gap around the planetary orbit, or by the dispersal of the protoplanetary disk.

Gravitational instability may operate in cold, massive disks in which random gas overdensities start growing under their own self-gravity, which neither pressure nor rotational support can initially resist. If this collapse of dense regions can continue deeply into the nonlinear regime, and their density come to far exceed the nebula’s, such clumps become self-gravitating objects — and, over time, contract, cool, and look like giant planets. Theoretical arguments and numerical simulations suggest that this is possible only when the collapsing gas is able to cool rapidly. Otherwise, pressure inside the contracting clump would increase so fast that it could stall collapse. Detailed analysis shows that in gravitationally unstable disks, conditions for such rapid cooling are realized only far from the star, beyond approximately 50 AU, which is in the range of separations probed by GPI.

Fig2

A schematic illustration of the two main theoretical channels of the giant planet formation: core accretion (on the left) and gravitational instability (on the right). Credit: NASA and A. Feild (STScI)

Both theories have their own virtues and problems. Core accretion is natural in the sense that it represents a culminating step in the formation of terrestrial planets (or massive cores). It nicely explains the overabundance of refractory elements and the presence of cores for giant planets in the solar system (although the latter is not so obvious in the case of Jupiter). It also naturally accounts for the so-called metallicity correlation — the tendency of more metal-rich stars to host a giant planet: higher abundance of metals in the nebula increases the availability of solids and speeds up core growth, facilitating core accretion. However, formation of the core is the Achilles’ heel of core accretion because it typically takes a long time. Standard theory predicts that far from the star, the core buildup by planetesimal coagulation should take much longer than the lifespan of a protoplanetary disk, which is several million years. This makes it problematic to explain the origin of directly imaged planets by core accretion at tens of AU from their stars. That’s why a number of ideas have been proposed recently for speeding up core formation, by efficient accretion of either cm- or mm-sized “pebbles” early on, or small fragments and debris resulting from planetesimal collisions at later stages.

Gravitational instability nicely bypasses the nebula-lifetime constraint as it should operate on a relatively short dynamical timescale, which is about thousands of years at 100 AU distances. However, as mentioned above, conditions for this formation channel can exist only far from the star. It also requires very massive protoplanetary disks, typically tens of percent of the stellar mass, which may not be unusual early on but is not very common in later stages. High disk mass raises a number of important issues for gravitational instability. Bound objects produced by gravitational instability are expected to have rather high initial masses (of order 10 Jupiter masses), and can easily grow from planetary mass into the brown dwarf regime by accreting from the surrounding dense nebula. Tidal coupling to a massive disk may also lead to fast migration of forming protoplanets towards the star, where they can be efficiently destroyed.

Fig3

A series of snapshots from a simulation of a gravitationally unstable protoplanetary disk. Bright objects seen at late stages are the self-gravitating clumps forming as a result of gravitational instability that may subsequently turn into giant planets. Credit: G. Lufkin et al (University of Washington)

Discoveries of planets such as 51 Eri b are very important for understanding the efficiency of each of these two channels (their “branching ratios”) in producing the planets we have observed. With its small projected separation of only 13 AU and relatively low mass (possibly as low as two Jupiter masses) this object would have little problem forming by core accretion even at its current location, which is close to the orbit of Saturn in our solar system. At this stage, gravitational instability appears a more unlikely scenario for 51 Eri b. GPI and similar surveys will provide better statistics for directly imaged planets at different separations, and give us a much better understanding of how the majority of giant planets form in the Universe.

Roman Rafikov (IAS)

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What Self-Luminous Planets are Like

The planets that we are familiar with in our own solar system have evolved, aged, and cooled, for over 4.5 billion years since the Sun and planets formed. What do planets look like at younger ages? Can we use the light that a planet emits to understand its past history?

When we look at a planet like Jupiter with our eyes, the light that we see is Sunlight that is reflected back to us at Earth, scattered by clouds and gasses in the planet’s atmosphere. But what would Jupiter look like if we instead could see only its thermal “heat” emission, far beyond the visible spectrum? We can use infrared light detectors on telescopes to see this thermal emission, which comes from the deep interior below the visible clouds. We can use this light to precisely “take Jupiter’s temperature” in a way that you can’t do with reflected Sunlight. It turns out that our 4.5 billion-year-old planets are cold enough that their thermal “heat” radiation is quite small. We can only detect it because we are so close by.

[left] Jupiter as it appears to our eyes, in reflected sunlight. [right] Jupiter as it appears at a wavelength of 5 microns.  These long wavelengths are far beyond visible light, in the thermal infrared.

[left] Jupiter as it appears to our eyes, in reflected sunlight. [right] Jupiter as it appears at a wavelength of 5 microns. These long wavelengths are far beyond visible light, in the thermal infrared.

But that wasn’t always the case. Planets start their lives hot. In our current understanding of planet formation, making a planet is a tremendously energetic event. For Jupiter-like giant planets, hundreds of Earth masses of hydrogen dominated gas is pulled via gravity from large distances within the planet-forming nebula, down onto a large “seed core” of ice and rock. The kinetic energy of this moving gas becomes trapped in the opaque planet interior and is converted into thermal (heat) energy within the planet. A young Jupiter, right after formation, might be 10 times hotter in temperature, and over 10,000 times brighter in infrared radiation. These young planets are hot, bright, and much easier to see.

But like an ember that cools after being ejected from the fire, so do planets cool off over time. Trying to understand the specifics of how fast this cooling occurs, and how that impacts the planetary properties that we see, is one of the main reasons that GPI was built.   The interior energy that slowly flows through the planet’s atmosphere, out into space as infrared light, wanes over time, so the atmosphere we see cools and changes.

The atoms and molecules we expect to see in these atmospheres depend strongly on temperature. Three of the most abundant molecules are water vapor (H2O), carbon monoxide (CO), and methane (CH4). These molecules all absorb light at infrared wavelengths, so they are easy for GPI to see. When atmospheres cool, they also form clouds. Earth’s clouds are made of water and Jupiter’s clouds are made of ammonia (NH3). But warmer gas giant planets can have much stranger clouds. The hottest planets will have clouds made of iron droplets and rock dust. Cooler planets, like a middle-aged Jupiter, should have clouds of salt (KCl) and sodium sulfide (Na2S). What we don’t yet know is how thick these clouds are, and if they form all over the visible atmosphere.

This rendering shows a possible view of a young Jupiter-like planet, 51 Eri b.  It is glowing in thermal emission and shown as being partly cloudy.

This rendering shows a possible view of a young Jupiter-like planet, 51 Eri b. It is glowing in thermal emission and shown as being partly cloudy.

 

We expect to see significant differences between very young planets, and those that are cooler and more middle-aged. As mentioned above, their temperatures and atmospheres change over time, but even the sizes of the planets changes. When their interiors cool off, the hydrogen gas contracts and the planets shrink. We’re using GPI to understand how this planetary cooling and shrinking process works, for planets at different ages.

Could we detect a young, hot rocky planet, like a young Earth? That will be possible in the future, but is beyond the reach of GPI. Such planets are much harder to see than young gas giants because of their much smaller sizes. They just don’t put out as much infrared light. It also might be difficult to recognize such a planet if we saw it, because we have little intuition for what such a planet might look like. Its atmosphere might be made of a mix of water vapor, CO2, and some gaseous components from vaporized rocks. Work for the future!

Jonathan Fortney

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How GPI Works to See Planets

I sometimes compare the challenge of directly detecting a Jupiter orbiting a nearby star to finding a glowing needle in a haystack.  Oh, and by the way, the haystack is on fire.

It’s about as hard as seeing a candle a foot away from a spotlight (1 million candlepower) at a distance of 100 miles.

Why is doing this so difficult?  There are three primary reasons:

1.  We observe through atmospheric turbulence, which blurs our view of the star-planet system and scatters starlight, obscuring the faint planet beneath.

2.  Light obeys a wave property called diffraction, which scatters light from a planet’s host star throughout the image, obscuring the planet.  This effect occurs even in near-perfect optical systems above the atmosphere, like the Hubble Space Telescope.

3.  The optics within telescopes and instruments are imperfect, which further distorts the optical wavefront and introduces speckles, or tiny images of the star that look like false planets, into the image.

The Gemini Planet Imager was designed to control and suppress these effects.  The four components of GPI that do this are:  an Adaptive Optics system, a coronagraph, a sensor that spectroscopically differentiates planets from starlight, and data post-processing techniques.

The Adaptive Optics (AO) system is the component of GPI that corrects for the effects of atmospheric turbulence and optical imperfections, allowing the telescope to observe stars as though the atmosphere were not present.  AO systems use flexible deformable mirrors to cancel the distortion induced by the atmosphere.  In GPI, a wavefront sensor updates 1000 times per second to measure the distortion at high precision.  A real-time control computer uses these measurements to estimate the deformable mirror shape needed to exactly cancel this turbulence.

GPI’s deformable mirror is a 2 cm-square MEMS, or Micro-Electrical Mechanical System, with 4096 microactuators.  Manufactured by Boston Micromachines, MEMS devices are etched from silicon wafers lithographically in a method similar to semiconductor fabrication.  Once nothing more than a glint in an engineer’s eyes, MEMS deformable mirrors have now been demonstrated on multiple astronomical AO systems and have proven reliable.

GPI-MEMS3Courtesy Daren Dillon / UCSC.

coronagraph is an optical device used to block starlight and physically suppress the effects of diffraction.  GPI uses a Lyot coronagraph design, named after Bernard Lyot, who first used a coronagraph to block out the sun’s glare and reveal its corona.  GPI’s Lyot coronagraph introduces a hard edged stop where the star comes to focus, blocking most of its light, as well as an apodized optic in the Lyot plane, where the remaining diffracted light from the star is isolated and blocked.  Light from other objects in the field passes through the coronagraph unimpeded.

The Integral Field Spectrograph (IFS) in GPI breaks light into its constituent wavelengths, or a spectrum, just as a prism does.  The magic of the IFS is that it performs this task at every location in the image, so that the spectrum of a potential planet can be measured simultaneously with the starlight spectrum.  Since light emitted from the planet has different spectral features from the starlight (induced by chemical components in its atmosphere such as methane), this information can be used to differentiate a planet candidate from residual star speckles.  In addition, since star speckles are produced by diffraction, they appear to change position at different wavelengths, while true planets stay fixed at all wavelengths.

The post-processing analysis takes advantage of this differentiation to find fainter planets than would be visible otherwise in a process known as Spectral Differential Imaging (SDI).  A further step is Angular Differential Imaging (ADI), which takes advantage of the natural rotation of the sky, and thus the star-planet orientation, with respect to the telescope’s azimuth.   ADI algorithms essentially look for planets that rotate with the sky rather than with the starlight speckle pattern.

AOoffon_corona_GPI_arrows

GPI’s view of the Beta Pictoris star/planet system as each component is turned on.  Image credit:  GPIES team; Beta Pictoris:  C. Marois/NRC Canada, Gemini Observatory.

It is only the combination of these four approaches in tandem – and not any one of them – that allows GPI to detect planets successfully.  For example, the coronagraph, IFS, and post-processing components all rely critically on the AO system to remove the effects of the atmosphere first.  Similarly, if no post-processing were used, GPI’s performance would not be sufficient to detect planets as faint as it does.

So, quite literally, GPI is more than the sum of its parts.


The Gemini Planet Imager instrument and scientific data reduction pipeline were built by many scientists at the American Museum of Natural History, ASU, Univ. Toronto, Gemini Observatory, HIA, JPL, LLNL, Lowell Observatory, NASA Ames, SETI, STSci, Univ. Montreal, Berkeley, UCLA, UCSC, and UGA.  

 

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Debris Disks: Searching for Dust to Find Planets

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A star system where gas and dust have formed into a disk around a newly formed star. The leftover disk will most likely form planets, comets and asteroids. Credit: NASA

No one is ever excited when the topic of “dust” is brought up. Usually dust is a hindrance – something you sweep away during spring-cleaning, or an annoyance because your allergies can’t handle it. But for astronomers, finding dust around another star – i.e., circumstellar dust – is like finding the next piece of an interstellar puzzle. That’s because circumstellar dust holds clues to understanding not only the origins of planets outside of our solar system, but also gives us a leg up in figuring out our place in the Universe.

Before we can uncover the secret of how dust and exoplanets are linked, we need to understand what happens after a planetary system forms. Once a star is created, it leaves behind a large disk of gas and dust. The gas and dust start sticking together and coalescing into larger objects such as planets, asteroids and comets. The central star removes the remaining gas and dust either by accreting it, or by throwing it out of the system with its stellar wind and radiation pressure. About 10 million years or so after the star forms, all of the leftover dust and gas have either been ejected out of the system, eaten up by the star, or used to form planets, asteroids and comets. But here’s a mystery: astronomers have found stars much older than 10 million years with circumstellar dust.

So then where does all this dust come from if we know it shouldn’t be there?

One word – collisions.

GPI_BP1_Collisions_2

Large planets will gravitationally tug on smaller bodies such as asteroids and comets as they pass by. And if these asteroids and comets are pulled hard enough, they will eventually collide with one another! This can be catastrophic and destructive, breaking the original asteroid into smaller chunks. These chunks can then collide and become still smaller and smaller andGPI_BP1_signposts_3 smaller – grinding down the original body into dust. The dust then forms a disk around the star — what astronomers call a “debris disk.”

Had there not been large planets around these stars, there might not have been any destructive collisions, which in turn would not produce any dust for us to find. In other words, finding dust around a star is like seeing a large signpost saying “PLANETS! Come and get ‘em’!” Of course, initially we don’t actually see the planets. But knowing a debris disk exists around these stars tells us there is a good chance we’ll find them.

Detecting Dust

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Emission spectrum seen from a star that does and does not have dust around it. A star with dust will have excess infrared radiation compared to the emission from the star. Credit: NASA

Now, astronomers typically look for debris disks by measuring the infrared light coming from a star. Dust around a star will warm as it soaks up the light from the central star. As it heats up, it will start emitting its own light in the infrared – just like a stove-top coil will begin to feel hot before you see it glow red. The amount of infrared light the dust gives off, combined with what the star produces, will be more than the amount of infrared light produced only by the star. This excess light is what betrays the presence of dust in the system. This dust is usually 10 to 100 microns in size, or roughly the thickness of a human hair.

Astronomers have found infrared excess emission – most likely caused by debris disks – in over 1,000 star systems, all of which have the potential to host planetary systems.

But what about planets? How can we confirm that planets are responsible for the creation of the dust we’re seeing?

combinedimage

Left: Hubble image of the Fomalhaut dust ring. Credit: Kalas et al. 2012. Center: Beta pic debris disk by Smith and Terrile. Right: Beta pic b planet over plotted on image of the beta pic debris disk. The stars in each image are blocked out by a coronagraph.

In some cases, taking an image of a system that has a known debris disk reveals much more than one can discern from just measuring the amount of infrared light produced by a star. In 2005, GPIES’ own Dr. Paul Kalas and his team used the Hubble Space Telescope to image the debris belt of Fomalhaut – a star roughly 16 times brighter and almost four billion years younger than our sun. These images show a sharp, eccentrically misaligned ring, which hints that a large planet might be orbiting inside the ring. Follow-up observations inspired by this suspicion revealed a planet, Fomalhaut b, though not the one thought responsible for the shape of the debris disk.

Another star system whose imaged disk betrays the existence of a planet is “beta Pictoris” – or beta Pic for short. Beta Pic’s debris disc was the first to be imaged. In 1984, Dr. Brad Smith and Dr. Rich Terrile took an image of the beta Pic disk by blocking out the star’s light using a coronagraph, which revealed an edge-on disk. The disk of this 20-million- year-old star – much younger than our sun – is full of warps and substructures. Such structures in the disk led astronomers to believe that a large planet may be influencing its shape.

And then..

In 2008, astronomers finally discovered the faint signal of a planet eight times the mass of Jupiter, captured in images taken by the Very Large Telescope in 2003. These two systems are only the tip of the iceberg, as many more such stars have been studied, all of which show a wide variety of disk structures and hint at the presence of planets hidden behind the dust.

How does this all link back to us here on Earth?

Although the puzzle of each exosolar system becomes clearer when we study its debris disk, information gleaned from every system can be used to deduce whether a planetary system like ours can form elsewhere in the Universe. This is mainly because the interaction of our solar system’s planets and debris disk – which is made up of the asteroid belt between Mars and Jupiter, and the Kuiper belt way past Neptune – have heavily influenced the current structure of our solar system.GPI_BP1_LHB_8

Roughly 3.7 billion years ago, Jupiter and Saturn began a gravitational dance in which the orbital periods of the titans lined up, and for every orbit around the Sun Jupiter made, Saturn made two. This enhanced gravitational interaction forced Saturn to migrate slowly away from the Sun, pushing Uranus and Neptune further out in the solar system as well. Neptune fatefully crashed into the outer Kuiper belt, sending large ice and rock bodies hurling all over the solar system, and turning its inner region– and Earth – into an interplanetary shooting range. A large amount of dust would have been created during this time period, one quite noticeable to any alien observing our system.

Although this event made the primordial Earth a hellish place to live, there is some good news. Some scientists have theorized that a large portion of Earth’s oceans were fed by the transport of these water-rich comets and asteroids from the Kuiper belt during the “Late Heavy Bombardment.”. And so had it not been for the relationship between our planets and debris belts, Earth would probably not exist as we know it.

We have seen evidence of similar catastrophic events around the young “eta Corvi” system. Using spectroscopic data obtained in 2010 from the Spitzer Space Telescope, Dr. Carey Lisse’s team discovered that for some reason, large numbers of cometary bodies from the outer regions of this system were colliding with a planetary sized body in the inner regions, and releasing water ice dust whose total mass was about 0.1% of all the water in Earth’s oceans!

The similarity between eta Corvi and our early solar system is uncanny. Additionally, it’s exciting to think that the events that may have allowed for life to arise on Earth are currently going on around other stars – events that require a symbiotic relationship between planets and the remnant asteroid and comet population.

Events that we can witness by studying debris disks.

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Hello from AAS!

Happy new year, Internet! I’m starting off the year at the 225th meeting of the American Astronomical Society. It’s an annual conference where all the professional astronomers in the United States get together and talk about space! There’s been some really cool presentations, including the discovery of Earth-sized planets in possibly habitable orbits around other stars by Kepler. Sounds pretty cool right?

A subset of the GPI team was here for the AAS. We gave an update on the GPI Exoplanet Survey, presented posters on debris disks and exoplanets imaged by GPI, and even had a press conference on recent GPI results!

In addition to all the GPI results, the GPI team also had a team lunch to talk about starlight subtraction. Even with the star masked out, starlight still diffracts around the coronagraph and hides the faint exoplanets and debris disks that we are trying to see. As you might guess, starlight subtraction is a really important for GPI, especially with the kickoff of the GPI Exoplanet Survey just a couple of months ago. The content of meeting was a bit technical so I’ll spare you the summary here. It was a productive lunch though, and overall it’s been a great conference!

The only good picture is the one in which my eyes are closed.

The GPI Team in Seattle. Photo credit: Marshall Perrin

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