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https://www.quantamagazine.org/stellar-disks-reveal-how-planets-get-made-20180521/ May 21, 2018

Stellar Disks Reveal How Planets Get MadeDetailed images of disks swirling around young stars show the details of how solar systems come tobe.

By Joshua Sokol

Over the past two and half centuries, scientists envisioning the origin of planetary systems(including our own) have focused on a specific scene: a spinning disk around a newborn star,sculpting planets out of gas and dust like clay on a potter’s wheel.

But as for testing the idea, by actually spotting an exoplanet coalesce from swirling matter? No luckyet. “Nowadays, everybody says planets form in protoplanetary disks,” said Ruobing Dong, anastrophysicist at the University of Arizona.“This sentence is, technically, a theoretical statement.”

Advances over the past few years suggest it won’t stay theoretical for long. Using second-generationinstruments mounted on giant ground-based telescopes, several teams have finally resolved theinner regions of a few protoplanetary disks, uncovering unexpected, enigmatic patterns.

The latest views came on April 11, when the European Southern Observatory released eight imagesof disks around young, sunlike stars, perhaps illustrating what our own solar system looked like inits infancy.

The images don’t show clear, unambiguous points of lights from planets. But these and othersystems do contain tantalizing — albeit indirect — hints that infant planets may be hiding within.Some disks are like a vinyl record, with rings and gaps that could have been carved out by youngworlds. In others, starlight illuminates both a top and bottom surface of the disk, forming a structurethat resembles a yo-yo.

If astronomers could find an embryonic planet in a place like this, the payoff would be far-reaching.Beyond just proving one of astronomy’s deepest-held ideas, the quantitative measurement of wherea planet is forming, and at what size, would immediately help differentiate between battling theoriesof how planets are born.

One account of planet formation, called core accretion, holds that planets form slowly, coalescingaround rocky cores, and in a region close to their stars. Another theory appeals to gravitationalinstabilities in the disk, suggesting giant planets can coalesce quickly, far away from their stars.Currently, these ideas can be tested against the distribution of current planets in our solar systemand extrasolar systems. But they’ve never been studied with the process still under way, beforeplanets have a chance to migrate and rearrange themselves.

That gives astronomers who study these systems a unifying, unfinished quest. Look at dim, distant,

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untidy disks. Hunt down baby planets. And at long last, after centuries of anticipation, begin tounravel the fundamental processes that shape countless worlds across the universe.

Direct DetectionWhen searching for planets in protoplanetary disks, it’s easy to convince yourself that you’re seeingthem. Astronomers who study these disks have already spotted multiple specks of light hiding inside.As recently as May 6, for example, an international team reported signs of a giant planet lurking in asystem called CS Cha. But for now these specks remain mere planetary candidates, not confirmedworlds.

“We’re at the very hairy edge of the technology,” said Katherine Follette, an astronomer at AmherstCollege. “In the case of planets embedded in disks, absolutely every single one of them is stilldebated heavily.”

This ambiguity is intimately tied to the same messy environments that would make these planetsspecial.

One instrument leading the search is SPHERE, mounted on the Very Large Telescope in Chile’sAtacama Desert, which obtained the eight recent protoplanetary disk images. Another, whichFollette works on, is the Gemini Planet Imager (GPI), a rival instrument on another Chileanmountain.

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S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)

The disk surrounding TW Hydrae has rings that could reveal hidden planets.

Both were designed to catch photons from planets around other stars, unlike most techniques forstudying exoplanets that rely on more indirect signatures. Both also produce data that’s easiest tointerpret when they’re trained on uncluttered, older solar systems where disks have already eroded.

These cameras need ways to peel faint pinpricks of light away from bright host stars, like finding afirefly sitting on the rim of a distant spotlight. They use adaptive optics, a technology that tracksfluctuations in the atmosphere and then warps its own optics in real time to compensate. Thiscancels out Earth’s roiling air, de-twinkling the night sky to achieve higher resolution. They also usecoronagraphs, which block out light from the star.

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And on top of that, these planet-hunting cameras employ yet another trick called differentialimaging. SPHERE, for example, takes two simultaneous pictures through different polarized filters.Starlight itself isn’t polarized, so the star looks the same in both versions. It can be subtracted away.But when light scatters, it gets polarized. This allows astronomers to accentuate the photons thathave bounced off a disk or a planet.

Algorithms then search for leftover points of light. But when looking for planets within disks, thealgorithms can confuse clumps and clouds for newborn worlds.

ESO, ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)

The concentric rings surrounding the young star HD 163296 are likely caused by Saturn-mass planets that clear outregions of gas and dust.

Follette and colleagues have spent the past few years trying to analyze these false signals. They’ve

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also studied puzzling planet candidates, including some that don’t seem to be orbiting their host starin accordance with Kepler’s laws of motion, as all planets would.

Meanwhile, there’s another path to planets unfolding in parallel. Although SPHERE and GPI haven’tunambiguously found a forming world, they have managed to take the sharpest-ever pictures ofprotoplanetary disks themselves.

Finally seen up close, these disks host a menagerie of strange features that may be linked to planetformation. “That has completely changed the game,” said Konstantin Batygin, an astrophysicist atthe California Institute of Technology. “It has been revolutionary.”

The problem lies in associating these features with the putative planets causing them. And that’s noteasy, either. “We talk about disks as signposts of planets,” said Follette. “But if they’re signposts ofplanets, they’re ones that we have no idea how to interpret yet.”

Spiral CradlesConsider a striking pattern first noticed in 2012. In at least half a dozen protoplanetary disks,something seems be winding gas and dust into seashell whorls like the arms of spiral galaxies.

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ALMA (ESO/NAOJ/NRAO)

The protoplanetary disk surrounding the young star HL Tauri has multiple concentric rings. Astronomers believethat newly formed planets are carving out the complex structure.

Astrophysicists have two main ideas to explain what’s making these spiral arms. Both borrow from adecades-old theory of galactic spirals. According to this idea, gas and dust spinning around anewborn star begin to pile up in a celestial traffic jam. Something has to trigger the initial snarl-up,however.

Astronomers have suggested that in stars surrounded by heavy disks — those that weigh at least aquarter as much as the star they orbit — gravitational instabilities can cause pileups of material intospiral arms. But researchers have found many spiral disks that appear to be far below this massthreshold, intimating that another mechanism may be at work.

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Perhaps a hidden puppeteer is to blame. In 2015, a team led by Dong, the Arizona astrophysicist,built simulations that showed how giant planets a little bigger than Jupiter could trigger spiralwhorls, too. The planet would sit right at the tip of one the arms and drag the spiral along as itorbited the star. If this is the case, every spiral is like a giant arrow pointing toward the field’sultimate quarry — a planet in the process of being born.

In 2016, Dong’s team found evidence that these spirals can be triggered by a massive body. In thiscase, the triggering object orbiting the star HD 100453 was a dwarf star, which is easier to spot thana planet. But it served as a proof of concept. “After that, people started believing more in themodel,” Dong said.

Finding an arm-tip planet itself would seal the deal, but astronomers are still waiting. In an recentpaper in The Astrophysical Journal Letters, a team led by Bin Ren, a researcher at Johns HopkinsUniversity, gathered and analyzed data from MWC 758’s spiral going back more than a decade.

NASA, ESA, ESO, M. Benisty et al. (University of Grenoble), R. Dong (Lawrence Berkeley National Laboratory), and Z.Zhu (Princeton University)

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The spiral arms surrounding the star MWC 758 could have been sculpted by a giant planet out by the tip of one ofthe arms.

Over this time, Ren’s analysis shows, the whorls may have rotated ever so slightly, at about sixtenths of a degree per year. This rotation would be expected from a giant planet out at the tip of anarm that orbits the star every 600 years or so, Ren said. But such a planet, if it exists, is still hiding.

Of course, even if spirals are conclusively linked to planets, they won’t lead the way to all newbornworlds. In simulations, only gas giant planets are hefty enough to draw spiral patterns. Smallerworlds would have to be discovered through other means. And not all protoplanetary disks hostspirals, either.

For example, none of the new SPHERE images of disks around sunlike stars have spiral arms. (Thatsuggests the spiral process, whatever it is, may be more efficient around more massive stars,said Henning Avenhaus of the Max Planck Institute for Astronomy in Heidelberg.) But they and manyother protoplanetary disks do show something else, something perhaps even more promising: gaps.

Planets in the CracksIn the fall of 2014, astronomers testing ALMA, a collection of radio dishes in the Chilean Andes,decided to train it on the most massive protoplanetary disk they could find. When the resultingpicture of empty gaps and thick rings in a system called HL Tauri was later displayed at an internalALMA meeting, it stopped the show.

“We just spent the rest of the meeting talking about HL Tau,” said Lucas Cieza, an astronomer atDiego Portales University in Chile. Looking at the gaps, the assembled scientists debated whetherthey were produced by planets. ALMA scientists later studied images of another, nearby systemcalled TW Hydrae, which show similar gaps in even higher detail. But neither system can settle theissue of whether the gaps are caused by planets or by something else. “The debate is still ongoing,”Cieza said.

Just like spirals, both planets and other effects can sculpt gaps. A planet would carve out a gap overthousands to millions of years. As it orbits, it would both pull disk material in toward itself as well asscatter it away from the planet’s orbit, leaving an empty groove.

This gravitational engraving would be cumulative. While it takes something bigger than Jupiter tomake a spiral, worlds the size of Neptune or even as small as Earth could create noticeable gaps,said Jeffrey Fung, an astrophysicist at the University of California, Berkeley.

“All of these planets have the potential to open deep enough gaps that we can easily see with today’sinstruments,” he said. Crucially, these gaps might be the only near-term chance to study theformation of small planets, which would be even more difficult than Jupiter-size worlds to spotdirectly in a disk.

What might be making these gaps if not planets? The magnetic field of a disk can lead to regions ofturbulence, sweeping material away from what become empty, magnetic “dead zones.” Or abruptchanges in chemistry can cause a gap that also mimics the action of a planet. A solar system’s snowline, for example, marks the boundary between the hot inner disk, where water exists as vapor, andthe outer disk, where water freezes into solid grains. Similar transitions occur for other compounds,like carbon monoxide and ammonia.

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The confusion leaves astronomers searching for an answer key. “The best-case scenario is weactually see a planet in a gap,” said Fung. Technically, current technology would not pick up a planetitself, but a smaller, circumplanetary disk of material falling onto one. If such a signal could belinked to a spiral or gap, it would help observers start translating back and forth between worlds anddisk features more generally.

The wait might not be too long. “The most exciting things that I have seen are not published,” saidCieza, who declined to comment on specifics. “We can expect a lot of very exciting things coming inthe next few months.”

Next-generation telescopes should also be able to help. The James Webb Space Telescope will beable to peer inside disks in infrared wavelengths and look directly for planets. Its launch hasrecently been delayed again, this time to 2020.

ESO/L. Calçada

The Extremely Large Telescope, currently under construction in Chile, will use lasers to create artificial stars high inthe atmosphere, allowing researchers to “de-twinkle” the sky.

And the challenge of catching planet formation in the act is “a beautiful science case” for 30-meter-class telescopes, said Bruce Macintosh of Stanford University, who leads the GPI team.Observatories that size, like the Extremely Large Telescope currently being constructed in Chile, willbe able to resolve even smaller structures inside protoplanetary disks.

Whenever it happens, confirmed cases of forming planets will be “groundbreaking,” Dong said. Whatused to be a mathematical bedtime story of the birth of worlds would be playing out in real time, inreal data. “It’s related to the fundamental question of where we come from.”

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This article was reprinted on Wired.com.