Planets Still Forming Detected in a Protoplanetary Disk

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.



What Astrochemistry is Telling Us

This image shows the Rho Ophiuchi region of star formation where methyl isocyanate was detected.  The insert shows the molecular structure of this chemical, an important precursor for life’s chemical building blocks. ESO/Digitized Sky Survey 2/L. Calçada

Sometimes lost in the discussion of exoplanets and habitability is where the potential building blocks of life might come from and how they got there.

Yes, hydrogen and water and methane and carbon and nitrogen have been found in abundance around the cosmos, but how about the larger and more esoteric compounds needed for life to emerge?  The precursor compounds to amino acids and nucleobases, for instance. Are they formed in space, too.

Some have indeed been identified around young stars or in star-formation regions, but much of what we know about complex molecules in space comes via meteorites and comets.

The Philae lander, for instance, identified 16 organic compounds on the Churyumov-Gerasimenko comet in 2015, including four never-before detected on comets. Some of these compounds play a key role in the prebiotic synthesis of amino acids, sugars and nucleobases — the ingredients for life.

Now an additional and significant precursor compound has been detected around sun-like stars in the very early stage of their formation.  The chemical is methyl isocyanate, and it is an important building block of life.

The detection was made by two teams at the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope, high in the Chilean desert.  The researchers described their detection as the first one of this prebiotic molecule around a solar-type protostar, the type from which our solar system evolved.

“We are particularly excited about the result because these protostars are very similar to the Sun at the beginning of its lifetime, with the sort of conditions that are well suited for Earth-sized planets to form,” said Rafael Martín-Doménech of the Centro de Astrobiología in Madrid and Víctor M. Rivilla of the Osservatorio Astrofisico di Arcetri in Florence. They were lead authors of one of the two papers published on the subject by the Royal Astronomical Society.

“By finding prebiotic molecules in this study, we may now have another piece of the puzzle in understanding how life came about on our planet.”

The Atacama Large Millimeter/submillimeter Array (ALMA) is a partnership between nations in Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is the largest ground-based astronomical observatory in existence, and it is located on one of the driest spots on Earth. ALMA (ESO/NAOJ/NRAO)

The precursor compound was detected around IRAS 16293-2422, a triple protostar system consisting of a binary star (A1/A2) separated by a distance 47 times the distance from Earth to our Sun.  The far removed third star (B) is 750 times that Earth-Sun distance. IRAS 16293-2422 is around 400 light-years away in a large star-forming region called Rho Ophiuchi in the constellation of Ophiuchus.

Of paramount importance to the researchers is the finding that all of three IRAS stars have masses similar to that of the sun.  And all three were found to have the methyl isocyanate around them.

What’s more, astronomers using the ALMA array found also glycolaldehyde — a simple form of sugar — in the gas surrounding the same stars in 2012.

This discovery was the first time that a sugar had been found in space around a solar-type star in the region where a planet-forming disk is expected to arise — roughly corresponding to the distance between the Sun and Uranus.

Both the discovery of the sugar, and now of the methyl isocyanate,  place these chemical building blocks of life in the right place and at the right time to become part of planets that might be forming around the stars.

Earth and the other planets form from the material left over after the formation of the their host star. So studying solar-type protostars can therefore open a window to the past for astronomers and allow them to observe conditions similar to those that led to the formation of our solar system over 4.5 billion years ago.

Authors Niels Ligterink of Leiden Observatory and Audrey Coutens of University college of London had this to say about the discoveries:  “This star system seems to keep on giving!  Following the discovery of sugars, we’ve now found methyl isocyanate.

This family of organic molecules is involved in the synthesis of peptides  and amino acids which, in the form of proteins, are the biological basis for life as we know it.”

Ironically, while the compound can be an important precursor for life, it is also a very toxic substance.  Indeed, it was the main cause of death following the Bhopal industrial accident  in 1984.

In astrochemistry, a complex organic molecule is defined as consisting of six or more atoms, where at least one of the atoms is carbon. Methyl isocyanate contains carbon, hydrogen, nitrogen and oxygen atoms in the chemical configuration CH3NCO. Artist rendering by L. Calçada.

As described in a European Southern Observatory release about the papers, “ALMA’s capabilities allowed both teams to observe the molecule at several different and characteristic wavelengths across the radio spectrum. They found the unique chemical fingerprints located in the warm, dense inner regions of the cocoon of dust and gas surrounding young stars in their earliest stages of evolution.

“Each team identified and isolated the signatures of the complex organic molecule methyl isocyanate. They then followed this up with computer chemical modeling and laboratory experiments to refine their understanding of the molecule’s origin.”

Having some of the chemical ingredients and precursor ingredients of life present as planets are formed certainly doesn’t mean that life necessarily emerged there.  The same is true if those ingredients are delivered right to the planet surface via meteorite, comet or interstellar dust.

But as scientists work to put together an understanding of how life started on Earth and whether it might exist elsewhere, having some of the same important-for-life compounds present here and in areas where exoplanets form is intriguing for sure.


Elegant Image of Icy Disk Around The Young Fomalhaut System

Composite image of the Fomalhaut star system. The ALMA data, shown in orange, reveal the distant and eccentric debris disk in never-before-seen detail. The central dot is the unresolved emission from the star, which is about twice the mass of our sun. Optical data from the Hubble Space Telescope is in blue; the dark region was a blocked by an internal coronagraph which filtered out the otherwise overwhelming light of the central star.  ALMA (ESO/NAOJ/NRAO), M. MacGregor; NASA/ESA Hubble, P. Kalas; B. Saxton (NRAO/AUI/NSF)

An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) has made the first complete millimeter-wavelength image of the ring of dusty debris surrounding the young star Fomalhaut. This well-defined band of rubble and gas is likely the result of comets smashing together near the outer edges of a planetary system 25 light-years from Earth.

Earlier ALMA observations of Fomalhaut — taken in 2012 when the telescope was still under construction – revealed only about one half of the debris disk. Though this first image was merely a test of ALMA’s initial capabilities, it nonetheless provided tantalizing hints about the nature and possible origin of the disk.

The new ALMA observations offer a complete view of this glowing band of debris and also suggest that there are chemical similarities between its icy contents and comets in our own solar system.

“ALMA has given us this staggeringly clear image of a fully formed debris disk,” said Meredith MacGregor, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and lead author on one of two papers accepted for publication in the Astrophysical Journal describing these observations.

“We can finally see the well-defined shape of the disk, which may tell us a great deal about the underlying planetary system responsible for its highly distinctive appearance.”

Fomalhaut is a relatively nearby star system with harbors of the first planets to be directly imaged by a space telescope.  In all, about 20 star systems have exoplanets that have been imaged directly.

The entire Formalhaut system is approximately 440 million years old, or about one-tenth the age of our solar system.

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.  NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute)

As revealed in the new ALMA image, a brilliant band of icy dust about 2 billion kilometers wide has formed approximately 20 billion kilometers from the star.

Debris disks are common features around young stars and represent a very dynamic and chaotic period in the history of a solar system. Astronomers believe they are formed by the ongoing collisions of comets and other planetesimals in the outer reaches of a recently formed planetary system. The leftover debris from these collisions absorbs light from its central star and reradiates that energy as a faint millimeter-wavelength glow that can be studied with ALMA.

Using the new ALMA data and detailed computer modeling, the researchers were able to calculate the precise location, width, and geometry of the disk. These parameters confirm that such a narrow ring is likely produced through the gravitational influence of planets in the system, noted MacGregor.

Paul Kalas, an astronomer at the University of California, Berkeley, has been principal investigator for the campaign to directly image the Formalhaut system, with its three stars, at least one planet large debris disk. (U.C., Berkeley)

The new ALMA observations are also the first to definitively show “apocenter glow,” a phenomenon predicted in a 2016 paper by Margaret Pan, a scientist at the Massachusetts Institute of Technology in Cambridge, who is also a co-author on the new ALMA papers.

Like all objects with elongated orbits, the dusty material in the Fomalhaut disk travels more slowly when it is farthest from the star. As the dust slows down, it piles up, forming denser concentrations in the more distant portions of the disk. These dense regions can be seen by ALMA as brighter millimeter-wavelength emission.

Using the same ALMA data, but focusing on distinct millimeter-wavelength signals naturally emitted by molecules in space, the researchers also detected enormous stores of carbon monoxide gas in precisely the same location as the debris disk.

“These data allowed us to determine that the relative abundance of carbon monoxide plus carbon dioxide around Fomalhaut is about the same as found in comets in our own solar system,” said Luca Matrà with the University of Cambridge, UK, and lead author on the team’s second paper. “This chemical kinship may indicate a similarity in comet formation conditions between the outer reaches of this system and our own.”

ALMA image of the debris disk in the Fomalhaut star system. The ring is approximately 20 billion kilometers from the central star and about 2 billion kilometers wide. The yellow dot is the central star, about twice the mass of our sun.

Matrà and his colleagues believe this gas is either released from continuous comet collisions or the result of a single, large impact between super-comets hundreds of times more massive than Hale-Bopp.

The presence of this well-defined debris disk around Fomalhaut, along with its curiously familiar chemistry, may indicate that this system is undergoing its own version of the Late Heavy Bombardment.  In our solar system, that was a period approximately 4 billion years ago when the Earth and other planets were routinely struck by asteroids and comets left over from the formation of our solar system.

“Twenty years ago, the best millimeter-wavelength telescopes gave the first fuzzy maps of sand grains orbiting Fomalhaut. Now with ALMA’s full capabilities the entire ring of material has been imaged,” concluded Paul Kalas, an astronomer at the University of California at Berkeley and principal investigator on these observations. “One day we hope to detect the planets that influence the orbits of these grains.”

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan, in cooperation with the Republic of Chile.


This article is based on a release from the National Radio Astronomy Observatory, with some modifications and additions.