It's a Planet: New Evidence of Baby Planet in the Making

Artist's illustration of a small Saturn-like planet discovered in the system LkCa 15. The planet resides within dense rings of dust and gas that surround a bright yellow star. Material accumulates in a clump and arc-shape, about 60 degrees away from the planet. Note: This illustration is not to scale. Credit: M.Weiss/Center for Astrophysics | Harvard & Smithsonian

 

Astronomers have developed a new technique to identify small planets hidden in protoplanetary disks.

Cambridge, MA – Astronomers agree that planets are born in protoplanetary disks — rings of dust and gas that surround young, newborn stars. While hundreds of these disks have been spotted throughout the universe, observations of actual planetary birth and formation have proved difficult within these environments.

Now, astronomers at the Center for Astrophysics | Harvard & Smithsonian have developed a new way to detect these elusive newborn planets — and with it, "smoking gun" evidence of a small Neptune or Saturn-like planet lurking in a disk. The results are described today in The Astrophysical Journal Letters.

"Directly detecting young planets is very challenging and has so far only been successful in one or two cases," says Feng Long, a postdoctoral fellow at the Center for Astrophysics who led the new study. "The planets are always too faint for us to see because they’re embedded in thick layers of gas and dust."

Scientists instead must hunt for clues to infer a planet is developing beneath the dust.

"In the past few years, we've seen many structures pop up on disks that we think are caused by a planet's presence, but it could be caused by something else, too" Long says. "We need new techniques to look at and support that a planet is there."

For her study, Long decided to re-examine a protoplanetary disk known as LkCa 15. Located 518 light years away, the disk sits in the Taurus constellation on the sky. Scientists previously reported evidence for planet formation in the disk using observations with the ALMA Observatory.

Long dove into new high-resolution ALMA data on LkCa 15, obtained primarily in 2019, and discovered two faint features that had not previously been detected.

About 42 astronomical units out from the star — or 42 times the distance Earth is from the Sun — Long discovered a dusty ring with two separate and bright bunches of material orbiting within it. The material took the shape of a small clump and a larger arc, and were separated by 120 degrees.

Long examined the scenario with computer models to figure out what was causing the buildup of material and learned that their size and locations matched the model for the presence of a planet.

"This arc and clump are separated by about 120 degrees," she says. "That degree of separation doesn’t just happen — it’s important mathematically."

Long points to positions in space known as Lagrange points, where two bodies in motion — such as a star and orbiting planet — produce enhanced regions of attraction around them where matter may accumulate.

"We're seeing that this material is not just floating around freely, it's stable and has a preference where it wants to be located based on physics and the objects involved," Long explains.

In this case, the arc and clump of material Long detected are located at the L4 and L5 Lagrange points. Hidden 60 degrees between them is a small planet causing the accumulation of dust at points L4 and L5.

The results show the planet is roughly the size of Neptune or Saturn, and around one to three million years old. (That's relatively young when it comes to planets.)

Directly imaging the small, newborn planet may not be possible any time soon due to technology constraints, but Long believes further ALMA observations of LkCa 15 can provide additional evidence supporting her planetary discovery.

She also hopes her new approach for detecting planets — with material preferentially accumulating at Lagrange points — will be utilized in the future by astronomers.

"I do hope this method can be widely adopted in the future," she says. "The only caveat is that this requires very deep data as the signal is weak."

Long recently completed her postdoctoral fellowship at the Center for Astrophysics and will join the University of Arizona as a NASA Hubble Fellow this September.

Co-authors on the study are Sean Andrews, Chunhua Qi, David Wilner and Karin Oberg of the CfA; Shangjia Zhang and Zhaohuan Zhu of the University of Nevada; Myriam Benisty of the University of Grenoble; Stefano Facchini of the University of Milan; Andrea Isella of Rice University; Jaehan Bae of the University of Florida; Jane Huang of the University of Michigan and Ryan Loomis of the National Radio Astronomy Observatory.

This study involved high resolution ALMA observations taken with Band 6 (1.3mm) and Band 7 (0.88mm) receivers.

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About the Center for Astrophysics | Harvard & Smithsonian

The Center for Astrophysics | Harvard & Smithsonian is a collaboration between Harvard and the Smithsonian designed to ask—and ultimately answer—humanity's greatest unresolved questions about the nature of the universe. The Center for Astrophysics is headquartered in Cambridge, MA, with research facilities across the U.S. and around the world.

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Media Contact:

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Center for Astrophysics | Harvard & Smithsonian
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Source: Harvard-Smithsonian Center for Astrophysics (CfA)

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News Maunakea Observatories Help Shed New Light on Obscured Infant Solar System

LkCa 15

An expanded view of the central part of the cleared region around LkCa 15, showing a composite of two reconstructed images (blue: 2.1 microns, from November 2010; red: 3.7 microns) for LkCa 15. The location of the central star is also marked. Credit:Kraus & Ireland, 2011

Figure 1 – Keck Observatory/NIRC2 image of the Sun-like star LkCa 15 obtained from data taken in November 2009 and retrieved from the Keck Observatory Archive (top left) and taken in December 2017 (bottom-left). Both images show two arcs of light consistent with two components of LkCa 15’s circumstellar disk. The right panels show the 2009 and 2017 images of the innermost arc of light were three planets around LkCa 15. North is up and east is left in the images. The star is about 500 light-years from Earth. Light around LkCa 15 can be seen as close as ~9 au from the star (dark-blue masked region in the upper-right panel; Sun-to-Saturn distance); the innermost arc of light extends out to ~0.2” or ~30 au (Sun-to-Pluto distance). While the combined light from the simulated planets is blended, the NIRC2 data would show evidence of their orbital motion if the planets were present in these data. Analysis of Keck/NIRC2 data shows that most of the light around LkCa 15 comes from disk material instead of from planets. 

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W. M. Keck Observatory and Subaru Telescope Data Taken Over Eight Years Solve Planet Formation Mystery

Maunakea, Hawaii – Astronomers using the combined power of two Hawaii telescopes have taken groundbreaking, sharp new images of a distant planetary system that likely resembles a baby version of our solar system.

Using Subaru Telescope and W. M. Keck Observatory, the team obtained and analyzed data for an infant Sun-like star named LkCa 15. Previous studies using an advanced interferometry method had inferred that three infant planets were orbiting this star. However, for this method, determining exactly how much light comes from a planet versus other sources like a disk can be particularly difficult. New Subaru and Keck Observatory data appear to solve this mystery; most of the light thought to come from the three candidate planets appears to originate from a disk of gas and dust.

“LkCa 15 is a highly complex system,” said Thayne Currie, lead author of the study and astrophysicist at NASA-Ames Research Center and the Subaru Telescope. “Prior to analyzing our Keck & Subaru data and given the same prior aperture masking data, we also would have concluded that LkCa 15 has three detected superjovian planets.”

The team’s results will soon be published in The Astrophysical Journal Letters; a preprint is available here: https://arxiv.org/abs/1905.04322 .

LkCa 15 is surrounded by a massive protoplanetary disk made of gas and dust, which are the building blocks of planets. Early analysis of this disk showed it has a large cavity depleted of dust – a tell-tale sign that much of the disk material has already been incorporated into massive, developing planetary embryos, or “protoplanets.” While the study rules out very bright superjovian planets, Currie says it is likely that fainter, less massive planets may be in the LkCa 15 system: perhaps those like Jupiter and Saturn.

“The planets in this infant solar system could actually be a lot more like our own solar system than previously thought. They are certainly there somewhere, possibly embedded in the disk. We will keep trying to find them,” said Currie.

Methodology

The findings were made using high-resolution images of the LkCa 15 system obtained from complementary instruments on Maunakea. At Subaru, researchers used a new cutting-edge planet imaging instrument – the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system coupled to the CHARIS integral field spectrograph to obtain extremely sharp images at near-infrared wavelengths. The team also used Keck Observatory’s powerful adaptive optics system and Near-Infrared Camera (NIRC2) to obtain new sharp images at longer, thermal-infrared wavelengths where young planets emit more light.

The team also obtained a ‘before-and-after’ view of the system by accessing the Keck Observatory Archive (KOA) to find NIRC2 data taken for LkCa 15 from 2009 – over eight years before the most recent SCExAO/CHARIS and NIRC2 images. KOA is a publicly accessible repository of all the high-value data obtained at the Observatory and is operated by Keck Observatory in partnership with the NASA Exoplanet Science Institute (NExScI) at Caltech.

The combined data showed that most of the light surrounding LkCa 15 originates from an extended arc-like structure – the visible edge of another component of LkCa 15’s disk. This arc has the same brightness previously attributed to planets around LkCa 15. The nearly decade-old KOA data for LkCa 15 play a unique role in understanding this planetary system. When compared with new Keck Observatory and Subaru Telescope data, the KOA data also showed that light emitted from LkCa 15’s arc-like structure is static over the course of eight years.

“This is consistent with a fixed, disk-like structure. Without the KOA, we would not have been able to know this key fact,” said Currie.

“It’s great to see this new data from Keck and Subaru combined with data from the KOA,” said John O’Meara, chief scientist at Keck Observatory. “This result shows the importance of the KOA, and is a great demonstration of how new discoveries can be made with ‘old’ data.”

Source: W.M. Keck Observatory/Science News

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Direct Images of Disks Unravel Mystery of Planet Formation

Figure 1: Near-infrared (1.6 micron) images of AB Aur.

The top panels compare images taken by HiCIAO and CIAO. Both images have a field of view of 7.5" by 7.5". Top left: Image taken by HiCIAO using a coronagraphic occulting mask with a 0.3" diameter. Top right: Image taken by CIAO using a software mask with a 1.7” diameter. The bottom panels show close-up views of the inner part of AB Aur's disk. Both images have a field of view of 2.0” by 2.0". Bottom left: Image has a coronagraphic occulting mask with a 0.3" diameter.

Bottom right: Image includes labels of its prominent features. The central position (0, 0) refers to the location of the star. Ellipsoids in dashed lines show the outer and inner rings. The solid ellipsoid indicates the wide gap between the rings. The "+" shows where the star is. The filled circle is the center of the outer ring. For this object, 1" (one arc second) corresponds to 144 AU in real scale (144 times the distance between Earth and Sun). For high resolution versions of all of the above images, click anywhere on any of them. For a high resolution version of ONLY the bottom left image, click on the following links: image only or image with English label.

Figure 2: High-contrast near-infrared imaging of LkCa 15 disk.

Top: Image taken by HiCIAO. The central star is hidden by the dark brown area toward the center. The inner edge of the outer disk is visible. The white feature below the blocked out area is part of the disk illuminated by the central star. The opposite side of the disk is not readily visible. There is a gap between the inner boundary of the disk and the star, at a distance of about 50 AU (Astronomical Unit, the distance between the Earth and the Sun, about 150 million kilometers or 94 million miles. The middle figure is a sketch of the LkCa 15 star and its disk system. The yellow-colored region corresponds to the features in the HiCIAO image. Bottom: Neptune's orbit in the Solar System as a scale comparison.

A sketch of the star and its disk system with detailed explanations

Abstract

The fruits of the SEEDS (Strategic Explorations of Exoplanets and Disks with Subaru) Project, led by Motohide Tamura at NAOJ (National Astronomical Observatory of Japan), are accumulating. Composed of over 100 scientists and 25 institutions, the international consortium of researchers supporting the project has announced another set of stunning findings obtained with the recently commissioned Subaru instrument HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics), an upgraded version of its predecessor CIAO (Coronographic Imager with Adaptive Optics). Their initial announcement of a significant discovery came in December, 2009: an exoplanet candidate around a Sun-like star. Now they are announcing another remarkable discovery: direct and sharp images of the protoplanetary disks of two young stars that reveal how planets may have formed within them. No other telescopes, whether ground-based or in space, have ever penetrated so close to a central star, showing the details of its disk.

The Significance and Challenges of Research on Protoplanetary Disks

One key to understanding planetary formation lies in the protoplanetary disks of young stars, which provide the initial conditions for the development of planets. These flattened structures of dense gas and dust evolve as by-products of star formation and become the womb for the maturation of planets (Conventional Schematic Diagrams of the Evolution of a Protoplanetary Disk around a Sun-like Star). However, scientists have not determined the actual details of how planets originate and mature. The detection of over 500 exoplanets that orbit around stars outside of our solar system has heightened interest in disks as a source of planetary formation. Did planets grow from the collision of bodies of rock and/or ice (planetesimals) or from gravitational instability in their disks? How does the development of exoplanets help us explain how the Earth formed?

The answers are difficult to obtain because disks are challenging to study. Both the small angular size of disks as well as the apparent dimness of a disk relative to its bright central star pose barriers to detailed observations of disks. In addition, available spatial resolution has only permitted the study of the outer envelope of a disk's structure. Finally, the scale size for observations is much larger than the familiar scale of our solar system, a scale that even the highest resolution telescopes have had difficulty in accessing so far.

Specialized instruments help scientists meet these challenges. A coronagraph facilitates observation of dim objects around a star by masking its extremely bright light while adaptive optics (AO) enhance spatial resolution by compensating for the blurring effects of the Earth’s atmosphere.

Subaru Telescope has been in the forefront of developing instruments designed for planet-hunting. In late 2009 Subaru Telescope replaced its earlier coronograph CIAO with HiCIAO that features not only a 188-element AO system and a stellar coronograph thats block out the central star's light but also various advanced techniques to enhance observation of the fine features inside a disk.

The Strategic Approach of the SEEDS Project

The SEEDS Project uses HiCIAO's planet-hunting technology to study exoplanets and their processes of formation. Begun in 2009 and led by Motohide Tamura of NAOJ, the project is one of the first large-scale undertakings approved by Subaru to implement a strategic, coherent approach to exploring the universe with its telescope. The consortium of project supporters has grown to include an international group of scientists and institutions as well as a variety of experts, including those in the areas of data analysis and high resolution imaging.

The SEEDS Project’s New Discoveries about Protoplanetary Disks

The SEEDS' Project has yielded new discoveries about protoplanetary disks that contribute to our understanding of how planets may form. They focus on observations of two young stars.

Details of the Disk of AB Aur

One of the primary targets was the very young star AB Aur in the constellation Auriga ("the Charioteer"). It is only about one million years old and 460 light years away from Earth. The research group has succeeded in directly imaging the fine details of AB Aur's disk. This is the first, finest, and sharpest image of a disk ever taken for this or any other objects.

Figure 1 shows the recent images of AB Aur taken by HiCIAO compared with the image taken by CIAO in 2004. The HiCIAO images display high spatial resolution and high contrast that reveals fine details of the disk’s inner structure that are on a scale similar to Neptune’s orbit in our solar system. Such precision of features near its central star contrasts with their masking in the earlier CIAO image.

The bottom two images in Figure 1 show close-ups of the inner disk, displaying the richness of its features. Double rings with some intricate bright and dark patterns are readily visible; they are tilted relative to each other, and strangely, their centers do not coincide with the position of the central star. A gap between these rings is rather strikingly void of material.

The disk's irregularities point to the presence of a giant planet that may sweep up material between the rings and cause irregularities in them with its gravitational force. Unfortunately this planet is not visible because disk material still covers it, extinguishing the planet's light.

A Gap in the Disk of the Star LkCa 15

The research group also targeted the star LkCa 15 for an examination of its disk. The star LkCa 15 is located toward the constellation Taurus, has a weight similar to the Sun's, and is several million years old. Its disk has been observed for some time. Although spectral energy distributions have indicated a gap in the disk, no direct imaging has clearly confirmed the presence of the gap-until now. The SEEDS group used HiCIAO for their observations, which succeeded in capturing a high-resolution image of the LkCa 15 disk.

Figure 2 shows an image of the disk of LKCa 15, which is masked in the dark area. The faint feature below the masked star is part of the disk illuminated by the central star. The opposite segment of the disk is not visible. The void between these features is the gap between the disk and the central star, which has a scale similar to Neptune's orbit in our solar system. The lack of material in the vicinity of the central star implies that a giant planet is sweeping up the disk's leftover materials that the central star did not swallow. Although the image might seem a bit blurry to readers, this is the first clear example of a truncated structure of a protoplanetary disk.

Implications for Planet Formation

These two results are the first to show features within disks where the planets are actually being born, and on a scale similar to that of our solar system. The direct imaging strongly indicates the existence of Jupiter-like giant planets that have affected the structure of the disks. Theorists were surprised to discover planets already formed within one million years. They had thought that giant planets such as Jupiter and Saturn in our solar system as well as giant exoplanets would take several tens of million years to form. The findings from the current research give them a tighter boundary condition for developing a theory of planetary formation. And the SEEDS Project will continue to search for and study exoplanets over the next five years, contributing even more to solving the mysteries of planet formation.

This result has been published in the Astrophysical Journal (volume 729, page 17, 2011 March 10 issue)

Institutions and Institutional Affiliations of Researchers

National Astronomical Observatory of Japan, The Graduate University for Advanced Studies (SOKENDAI), Max Planck Institute for Astronomy (Germany), Hokkaido University, Tohoku University, Ibaraki University, Saitama University, The University of Tokyo, Tokyo Institute of Technology, Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency, Kanagawa University, Nagoya University, Nagoya City University, Osaka University, Kobe University, The Open University of Japan, Princeton University, University of Hawaii, Jet Propulsion Laboratory, Academia Sinica Institute of Astronomy and Astrophysics ASIAA (Taiwan), University of Nice Sophia Antipolis (France), University of Hertfordshire (UK), Eureka Scientific and Goddard Space Flight Center, University of Washington, The College of Charleston.

Acknowledgment

This work is supported by a Grant-in-Aid for Specially Promoted Research from the Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT).