Planets Still Forming Detected in a Protoplanetary Disk

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An artist rendering of infant star HD 163296 with three protoplanets forming in its disk  The planets were discovered using a new mode of detection — identifying unusual patterns in the flow of gas within a protoplanetary disk. (NRAO/AUI/NSF; S. Dagnello)

Just as the number of planets discovered outside our solar system is large and growing — more than 3,700 confirmed at last count — so too is the number of ingenious ways to find exoplanets ever on the rise.

The first exoplanets were found by measuring the “wobble” in their host stars caused by the gravitational pull of the planets, then came the transit technique that measured dips in the light from stars as planets passed in front of them, followed by the direct imaging of moving objects deemed to be planets, and numerous more.

A new technique can now be added to the toolkit, one that is useful only in specific galactic circumstances but is nonetheless ingenious and intriguing.

By detecting unusual patterns in the flow of gas within the protoplanetary disk of a young star, two teams of astronomers have confirmed the distinct, telltale hallmarks of newly formed planets orbiting the infant star.

In other words, the astronomers found planets in the process of being formed, circling a star very early in its life cycle.

These results came thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), and are presented in a pair of papers appearing in the Astrophysical Journal Letters.

Richard Teague, an astronomer at the University of Michigan and principal author on one of the papers, said that his team looked at “the localized, small-scale motion of gas in a star’s protoplanetary disk. This entirely new approach could uncover some of the youngest planets in our galaxy, all thanks to the high-resolution images coming from ALMA.”

ALMA image of the protoplanetary disk surrounding the young star HD 163296 as seen in dust. ( ALMA: ESO/NAOJ/NRAO; A. Isella; B. Saxton NRAO/AUI/NSF.

To make their respective discoveries, each team analyzed the data from various ALMA observations of the young star HD 163296, which is about 4 million years old and located about 330 light-years from Earth in the direction of the constellation Sagittarius.

Rather than focusing on the dust within the disk, which was clearly imaged in an earlier ALMA observation, the astronomers instead studied the distribution and motion of carbon monoxide (CO) gas throughout the disk.

As explained in a release from the National Radio Astronomy Observatory, which manages the American operations of the multi-national ALMA, molecules of carbon monoxide naturally emit a very distinctive millimeter-wavelength light that ALMA can observe. Subtle changes in the wavelength of this light due to the Doppler effect provide a glimpse into the motion of the gas in the disk.

If there were no planets, gas would move around a star in a very simple, predictable pattern known as Keplerian rotation.

“It would take a relatively massive object, like a planet, to create localized disturbances in this otherwise orderly motion,” said Christophe Pinte of Monash University in Australia and lead author on the other of the two papers. 

And that’s what both teams found.

ALMA is a radio astronomy array located in Chile and set 16,000 feet above sea level. It’s a partnership between the European Southern Observatory (ESO), the National Science Foundation (NSF) of the United States and the National Institutes of Natural Sciences (NINS) of Japan in collaboration with the Republic of Chile. ALMA, which began operations in 2013, is used to observe light from space in comparatively long radio wavelengths. ((ESO/José Francisco Salgado )

Detecting planets within a protoplanetary disk — or finding theorized planets within those disks — is a big deal. 

That’s because information about the characteristics of very young planets orbiting young stars can potentially add substantially to one of the long-debated questions of planetary science:  How exactly did those billions upon billions of planets out there form?

The leading theory of planet formation, the “core accretion model,” has planets forming slowly — with dust, small objects and then planetesimals smashing into a rocky core and leaving matter behind.  In this model, the planet building takes place in a region close to the protoplanet’s stars.

Another theory looks to gravitational instabilities in the disk, arguing that giant planets can form quickly and far from their host stars.

The distribution of current solar system planets and beyond can give some clues based on the size, type and distribution of those planets.  But planets migrate and evolve, and they have never been studied before they had a chance to do much of either.

The techniques currently used for finding exoplanets in fully formed planetary systems — such as measuring the wobble of a star or how a transiting planet dims starlight — don’t lend themselves to detecting protoplanets.

With this new method for looking into those early protoplanetary disks, the hunt for infant planets becomes possible.  And the results in terms of understanding planet formation look to be very promising.

“Though thousands of exoplanets have been discovered in the last few decades, detecting protoplanets is at the frontier of science,” said Pinte.

 

These earlier images from ALMA reveal details in the planet-forming disk around a nearby sun-like star, TW Hydrae, including an intriguing gap at the same distance from the star as the Earth is from the sun. This structure may mean that an infant version of our home planet is beginning to form there, although these dust gaps are considered to be suggestive rather than conclusive. ( S. Andrews; Harvard-Smithsonian CfA, ALMA (ESO/NAOJ/NRAO)}ALMA

This is not the first time that ALMA images of protoplanetary disks have been used to identify what seem to be protoplanets.

In 2016, a team led by Andrea Isella of Rice University reported the possible detection of two planets, each the size of Saturn, orbiting the same star that is the subject of this week’s report, HD 163296.

These possible planets, which are not yet fully formed, revealed themselves by the dual imprint they left in both the dust and the gas portions of the star’s protoplanetary disk.

But at the time that paper was published, in Physical Review Letters, Isella said the team was focused primarily on the dust in the disks and the gaps they created, and as a result they could not be certain that the features they found were created by a protoplanet.

Teague’s team also studied the dust gaps in the disk of HD 163296, and concluded they provided only  circumstantial evidence of the presence of protoplanets.  What’s more, that kind of detection could not be used to accurately estimate the masses of the planets.

“Since other mechanisms can also produce ringed gaps in a protoplanetary disk,” he said, “it is impossible to say conclusively that planets are there by merely looking at the overall structure of the disk.”

But studying the behavior of the gas allowed for a much greater degree of confidence.

 

Composite image of the protoplanetary disk surrounding the young star HD 163296. The inner red area shows the dust of the protoplanetary disk. The broader blue disk is the carbon monoxide gas in the system. ALMA observed dips in the concentration and behavior of carbon monoxide in outer portions of the disk, strongly suggesting the presence of planets being formed. ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)

The team led by Teague identified two distinctive planet-like patterns in the disk, one at approximately 80 astronomical units (AU) from the star and the other at 140 AU. (An astronomical unit is the average distance from the Earth to the sun.)  The other team, led by Pinte, identified the third at about 260 AU from the star. The astronomers calculate that all three planets are similar in mass to Jupiter.

The two teams used variations on the same technique, which looked at anomalies in the flow of the gas – as seen in the shifting wavelengths of the CO emission — that would indicate it was interacting with a massive object.

Teague and his team measured variations in the gas’s velocity. This revealed the impact of several planets on the gas motion nearer to the star.

Pinte and his team more directly measured the gas’s actual velocity, which is better precise method when studying the outer portion of the disk and can more accurately pinpoint the location of a potential planet.

“Although dust plays an important role in planet formation and provides invaluable information, gas accounts for 99 percent of a protoplanetary disks’ mass,” said coauthor Jaehan Bae of the Carnegie Institute for Science.

So while those images of patterns within the concentric rings of a protoplanetary disk are compelling and seem to be telling an important story, it’s actually the gas that is the key.

This is all an important coup for ALMA, which saw its first light in 2013.  The observatory was not designed with protoplanet detection and characterization as a primary goal, but it is now front and center.

Coauthor Til Birnstiel of the University Observatory of Munich said the precision provided by ALMA is “mind boggling.” In a system where gas rotates at about 5 kilometers per second, he said,  ALMA detected velocity changes as small as a few meters per second.

“Oftentimes in science, ideas turn out not to work or assumptions turn out to be wrong,” he said. “This is one of the cases where the results are much more exciting than what I had imagined.

 

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France and Exoplanets

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Artist impression of the CoRot (COnvection ROtation and planetary Transits) space telescope. A French-led mission, it launched in 2006 and send back images until 2012. (ESA)
Artist impression of the CoRot (COnvection ROtation and planetary Transits) space telescope. A French-led mission, it launched in 2006 and send back images until 2012. (ESA)

For reasons all too regrettable, it seems appropriate today to highlight the extensive work being done in France and by French scientists to move forward the science of exoplanets.

The American public tends to view space observatories and exoplanet research as largely the domain of NASA and our nation. While we are leaders for sure, others are adding considerably to the field and to the world’s knowledge. And few nations have been as involved in the search for faraway worlds and efforts to understand them than France, and the work there will clearly continue.

“The French have been major contributors in the exoplanet field,” said David Latham, a senior astronomer at the Harvard-Smithsonian Center for Astrophysics, who has worked a lot with French colleagues.  “It’s spanned several decades now — from the detection of our first exoplanet (51 Pegasi b) on a French telescope to the large number of talented young French exoplanet researchers you see everywhere today.”

Probably the greatest single French contribution to exoplanet study has been the five-year run of the CoRoT (Convection, Rotation, and planetary Transits) spacecraft. Launched in 2006 – three years before our Kepler Space Telescope – it pioneered the space-borne detection and initial characterizing of exoplanets using the transit method. CoRoT was not as powerful as Kepler in its ability to see into space, but it did end up finding more than 30 exoplanets using the same transit method of detection, with more than 100 additional planet candidates awaiting confirmation.

This artist’s impression shows the transiting exoplanet Corot-9b. Discovered by combining observations from the CoRoT satellite and the ESO HARPS instrument, Corot-9b is the first “normal” exoplanet that can be studied in great detail. This planet has the size of Jupiter and an orbit similar to that of Mercury. It orbits a star similar to the Sun located 1,500 light-years away from Earth towards the constellation of Serpens (the Snake). Corot-9b passes in front of its host star every 95 days, as seen from Earth. This “transit” lasts for about 8 hours. Like our own giant planets, Jupiter and Saturn, the planet is mostly made of hydrogen and helium, and it may contain up to 20 Earth masses of other elements, including water and rock at high temperatures and pressures.
This artist’s impression shows the transiting exoplanet Corot-9b. Discovered by combining observations from the CoRoT satellite and the European Southern Observatory HARPS instrument, Corot-9b was among the first “normal” exoplanets that could  be studied in great detail. Corot-9b passes in front of its host star every 95 days, as seen from Earth. This “transit” lasts for about 8 hours. Like our own giant planets, Jupiter and Saturn, the planet is mostly made of hydrogen and helium, and it may contain up to 20 Earth masses of other elements, including water and rock at high temperatures and pressures. (ESO)

One of its most significant contributions was the discovery of the first definitively rocky exoplanet, COROT-7b. Less than twice Earth’s size and roughly the same density, it orbits its Sun-like star every 20 hours and is so likely molten. It was a surprising discovery for a telescope designed to find Jupiter-size planets.

The satellite mission was a collaboration of the French space agency, the Centre National d’Etudes Spatiales (CNES) and European space agencies, but France paid much of the cost and French scientists and engineers designed and made most of the instruments.

The head of the CoRoT exoplanet program was Magali Deleuil from Laboratoire d’Astrophysique de Marseille. Her handling of the project was so strong that she was appointed to oversee two upcoming European exoplanet missions – Cheops (scheduled to launch in 2017, with Swiss leadership) and Plato (planned for 2024 and led by Germany.)

Magali Deleuil, head of exo-planet research for CoRot. Magali Deleuil, head of exo-planet research for CoRot
Magali Deleuil, head of exo-planet research for CoRot

Cheops (the CHaracterizing ExOPlanet Satellite) is designed primarily to further study exoplanets identified by CoRoT and Kepler and measure their radii, while the goal of the more sophisticated Plato (Planetary Transits and Oscillations of stars) mission is to find planets like Earth, in terms of both size and potential habitability. Plato will have a much larger field of view than Kepler, and so will be able to see far more stars and exoplanets.

The principal investigator for CoRoT was Annie Baglin, senior researcher at the Paris Observatory. Her main field of research is stellar physics and her major interest is now stellar seismology – the deep internal vibrations that regularly shake our sun and others. These dynamics of stars have become increasingly important as a method to understand what is going on in their interiors, and to tease out some aspects of their composition.

Annie Baglin giving the final command to CoRoT in 2014, sending it out of orbit. The pioneering telescope sent back planet transit images for six years. (CNES)

In 1964, France became one of the founding members of the European Southern Observatory, a consortium now of 17 nations that has together planned, funded and completed construction of four of the most important ground-based telescopes in the world. All are in Chile, and have made significant contributions to exoplanet research.  (That observatory where 51 Peg was discovered is the Haute-Provence Observatory, which opened more than seventy years  in southeastern France and is still operating.)

Another ESO telescope is scheduled to come on line in 2024, and it will have the largest mirror by far in the world – more than 39 meters across. The European Extremely Large Telescope has as one of its primary goals the detection and characterizing of exoplanets.

The European Southern Observatory’s Extremely Large Telescope compared in size with the ESO’s Very Large Telescope and the coliseum of Rome. (ESO)

France has also been the lead European nation in the development and operation of the Ariane family of rockets, and an Ariane 5 will launch the James Webb Space Telescope into space in 2018.

And there has also been a steady flow of French exoplanet researchers to American institutions, and vice versa. Francois Forget, for instance, has worked on climate modeling of exoplanet atmospheres at the NASA Ames Research Center in California, and is now a senior researcher at the Institut Pierre Simon Laplace Universite in Paris.   His work on exoplanet atmospheres is highly respected world.

Francois Forget, a highly-respected modeler of exoplanet atmospheres.
Francois Forget, a highly-respected modeler of exoplanet atmospheres.

American Sean Raymond of Laboratoire d’Astrophysique de Bordeaux in France, has seen that exoplanet focus in action.

“There is indeed a lot of exoplanet research in France,” he said.  “In some subjects such as planet formation, I would say that France is doing as much or more than any other country.”

Similarly, David Latham’s CfA published results last week about a planet with a radius only 1.2 times that of Earth, and both relatively close to our solar system and with a star that gives it good definition.  The planet, Gliese 1132b, is expected to be a prime candidate for study well into the future.

As Latham pointed out, five of the co-authors were from France.

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