Star disks reveal to us the secrets of the appearance of planets

Detailed images of disks spinning around young stars, tell the details of the appearance of the solar system

[ clickable ]

For the last two and a half centuries, scientists who have tried to imagine the emergence of planetary systems (including ours) have focused on a certain point: a disk rotating around a newly born star , in which planets are molded of gas and dust, like clay on a pottery wheel.

But what about the experimental verification of this idea, the real detection of exoplanets clumping together from pieces of swirling matter? So far unfortunate. "Today, everyone is talking about the fact that the planets are formed from protoplanetary disks," says Rubin Dong , an astrophysicist at the University of Arizona. “But this sentence, strictly speaking, is a theoretical statement.”

The achievements of recent years show that it will not be theoretical for long. Using the tools of the second generation, located on the giant ground-based telescopes, several teams were finally able to examine the internal portions of several protoplanetary disks, revealing unexpected and mysterious pictures.

The most recent observations took place on April 11, when the European Southern Observatory released eight images of disks surrounding young, sun-like stars, which may show us how our Solar System looked like in infancy.

The images do not see clear, unambiguous points of light that would mean planets. But in these and other systems there are still seductive, though indirect, hints that they can hide newborns of the planet. Some discs look like a vinyl record, with rings and gaps that could be cut out by the young worlds. In other disks, the star highlights the upper and lower surfaces, forming a structure resembling a yo-yo.

If astronomers could find an embryo of a planet in such a place, the gain would be significant. This would not just prove one of the deepest ideas of astronomy - the numerical measurement of the place where a planet is formed, and its size, would immediately help to reason the hostile theories of the formation of planets.

One opinion about the formation of planets is called the accretion of the nucleus, and says that the planets are forming slowly, accumulating around the rocky nuclei in regions close to their star. Another theory addresses the gravitational instabilities of a disk, suggesting that giant planets can clump quickly, and far from their star. Now these ideas can be tested on the distribution of known planets in our solar system and in extrasolar systems. But they were never studied with the example of the process underway, before the planets had a chance to migrate and reorganize their order.

This gives astronomers studying these systems, uniting their unfinished goal. Look at the dim, distant, uneven wheels. To hunt for young planets. Finally, after centuries of waiting, begin to uncover the fundamental processes that form worlds in the universe.

Direct detection

Looking for planets in protoplanetary disks it is easy to convince ourselves that we see them. Astronomers studying these discs have already noticed a lot of points of light hiding inside them. For example, on May 6, an international group reported signs of a giant planet hiding in the CS Chameleon system. But for now these points remain candidates for planets, and not confirmed worlds.


The CS Chameleon system hides what looks like a small companion - marked with a dotted circle. Special polarization filters (blue image) provide an opportunity to see the dust disk along with the hidden object.

“We are now at the forefront of technology,” said Catherine Follett , an astronomer at Amherst College. “Regarding the planets inside the disks, absolutely all such cases are the subject of heated debate.”

One of the leading search tools is SPHERE , mounted on the Very Large Telescope in Chile, in the Atacama Desert. He made eight recent photos of protoplanetary disks. Another one, the one Folett works on is the Gemini Planet Imager (GPI), a competing tool located on another mountain in Chile.


The disk surrounding TW Hydra has rings that can reveal to us the planets hidden there.

Both were designed to capture photons coming from planets orbiting other stars, unlike most other exoplanet research techniques that rely on indirect signs. Both provide data that is easiest to interpret after training on older and neat star systems, with discs that have already disappeared.

These cameras need the ability to separate the faint specks of light from bright stars - it is like detecting a firefly sitting on the edge of a distant searchlight. They use adaptive optics , technology that tracks the fluctuations of the atmosphere, and alters the optics as needed to compensate for distortion. This compensates for the influence of the Earth's agitated air, and the stars in the images stop flashing, giving a better resolution. They also use coronagraphs to cut off the light coming from the star.

In addition, these cameras that hunt the planets use another trick - differential imaging. For example, SPHERE simultaneously takes two photos through different polarized filters. Starlight does not have polarization, so the star in both pictures looks the same. It can be subtracted. But when the light scatters, it becomes polarized. This allows astronomers to isolate photons reflected from a disk or a planet.

Then the algorithms look for the remaining points of light. But in the case of planets in disks, the algorithm may confuse lumps and clouds with new worlds.


Concentric circles surrounding the young star HD 163296 , most likely, are created by planets with a mass approximately from Saturn, clearing entire areas of gas and dust.

Follett and colleagues spent the past few years trying to analyze these false signals. They also studied mysterious candidates for planets, including those that did not seem to move in orbits around their parent star in accordance with Kepler’s laws of motion , like all normal planets.

In the meantime, another path to the planets opens. Although SPHERE and GPI did not find an unequivocally confirmed emerging world, they managed to make the clearest available photos of protoplanetary disks.

When, finally, we saw these discs in detail, we discovered in them a whole zoo of strange features that may be associated with the formation of planets. “This has completely changed the whole picture,” said Konstantin Batygin , an astrophysicist at the California Institute of Technology. “There has been a revolution.”

The problem is in comparing these features with the supposed planets causing them. And it is quite difficult. “We speak of disks as signs of planets,” Folette said. “But if these are signs of planets, then we still do not know how to interpret them.”

Spiral cradles

Consider an amazing picture, first discovered in 2012 . At least in six protoplanetary disks something twists gas and dust in spirals, resembling shells, or sleeves of galaxies.


The protoplanetary disk surrounding the young star HL Tauri has many concentric rings. Astronomers believe that the planets in the process of formation cut out complex structures in it.

Astronomers have two main ideas for explaining what creates these spiral arms. Both are based on the old, several decades old, theory of galactic spirals. According to this idea, gas and dust revolve around a newborn star, and begin to accumulate in the likeness of a heavenly automobile congestion. But however, something must cause a primary disorder.

Astronomers have suggested that in stars surrounded by heavy disks — such as weighing at least a quarter of the star itself — gravitationally unstable areas can lead to material accumulation in the form of spiral arms. But researchers have discovered many spiral disks, whose mass is much less than this threshold, which means that some other mechanism should work here.

Perhaps you should blame the hidden puppeteer. In 2015, a team led by Dong, astrophysics from Arizona, created simulations that showed how gigantic planets, a little larger than Jupiter, could trigger spiral whirlpools. The planet will be right on the tip of one of the sleeves, and drag the spiral along, moving in orbit. In this case, each spiral will be a giant arrow pointing to the victim - the planet in the process of nucleation.

In 2016, the Dong team found evidence that these spirals can be generated by massive bodies. In this case, the star initiator of the star HD 100453 was a dwarf star, which is easier to see than the planet. And she became proof of the viability of the idea. “After that, people began to trust this model more,” said Dong.

Finding such a planet at the tip of the sleeve would close this case, but astronomers are still waiting for it. In a recent paper published in The Astrophysical Journal Letters, a team led by Bean Rehn , a researcher at Johns Hopkins University, collected and analyzed data on the MWC 758 spiral, collected over a decade.


The spiral arms surrounding the star MWC 758 could have been created by a giant planet located at the end of one of the arms.

Rena's analysis shows that during this time the curls could turn a little, about 6/10 degrees a year. Such a rotation corresponds to a giant planet located on the tip of a sleeve that makes a revolution every 600 years, Ren said. But such a planet, if it exists, is still hiding from us.

Of course, even if the spirals are uniquely connected to the planets, they will not lead us to all the newborn worlds. In simulations, only gas giants are powerful enough to create spiral patterns. Smaller worlds will have to be discovered in other ways. In addition, not all protoplanetary disks have spirals.

For example, not a single photo of SPHERE shows that there are no spiral arms on the disks formed around sun-like stars. As Henning Avenhaus of the Max Planck Astronomical Institute said, this may indicate that the appearance of spirals is more effective around more massive stars. But they and many other protoplanetary disks demonstrate something else, more promising: breaks.

Planets in the cracks

In the fall of 2014, astronomers, testing ALMA, a set of radio telescope plates in the Chilean Andes, decided to train it on the most massive protoplanetary disk that they could detect. When the final picture showing breaks and thick rings in the HL Taurus system was displayed, everyone froze.

“And the rest of the meeting time we spent talking about HL Taurus,” said Lucas Cieza , an astronomer at Diego Portales University. Studying the gaps, the gathered scientists argued about whether they were not created by the planets. Later, scientists from ALMA studied images of another nearby system, TW Hydra, on which the same gaps were visible even more in detail. But none of the systems could resolve the dispute about whether the breaks are caused by emerging planets, or anything else. “The controversy is still going on,” Chiesa said.


The 66 antennas of the ALMA Observatory explore the sky above them while on the Chaynantor plateau in the Chilean Andes.

Like spirals, planets and other effects can change the shape of a gas. For thousands and millions of years, a planet will cut a furrow in it. When moving in orbit, it will attract the disk material to itself, as well as disperse it from orbit , leaving an empty cutout.

This gravity engraving should be cumulative. To create a spiral, something more than Jupiter is needed - but worlds the size of Neptune or even as small as Earth can create noticeable cuts, said Jeffrey Fung , an astrophysicist at the University of California at Berkeley.

“All of these planets have the potential to create deep enough cuts so that we can easily see them with today's tools,” he said. What is important, these notches may be our only chance for the near future to study the formation of small planets that will be even harder to see in the disk than worlds the size of Jupiter.

What can create such cuts, if not the planets? The magnetic field of the disk can create regions with turbulence that accelerate material from areas that can become empty magnetic “dead zones”. Or, abrupt changes in chemical composition can create a gap similar to the work of the planet. For example, the star system’s snow line marks the boundary between a hot internal disk, where water exists in the form of steam, and an external disk, where water freezes into solid granules. Similar transitions occur in other substances, such as carbon monoxide and ammonia.

This confusion causes astronomers to look for the key to the answer. “At best, we will actually see the planet in a gap,” Fang said. Technically, the current technology could see not the planet itself, but a disk of material falling around the planet falling on it. If such a signal could be connected with a spiral or a break, it would help the observers to better compare with each other new worlds and the properties of the disks.

Wait, perhaps not for long. “The most interesting photographs, from those that I have seen, have not yet been published,” said Chieza, who refused to go into details. “In the coming months we can expect a lot of very interesting things to come out.”

Next-generation telescopes should also be able to help. The James Webb Space Telescope will be able to look inside the infrared disk and find the planets directly. Its launch has recently been postponed again, this time for 2020.


An extremely large telescope, which is now being built in Chile, will use lasers to create artificial "stars" in the upper atmosphere, which will allow researchers to remove the twinkling of the sky.

The problem of trapping the formation of planets is an “excellent scientific task” for 30-meter telescopes, said Bruce McIntosh of Stanford University, who leads the GPI team. Observatories of this size, such as, for example, an extremely large telescope , which is now being built in Chile, will be able to examine even smaller structures inside the protoplanetary disks.

And when this happens, the confirmation of the observation of the formation of planets will be a “breakthrough,” as Dong said. What was a mathematical fairy tale about the birth of worlds will be played out in real time, in real data. "And all this is connected with the fundamental question of our own origin."