Sharp pictures of planetary embryo show ultra-quick mode of planet formation

17. März 2016

Observations using the VLA radio telescope array in New Mexico show the innermost portion of a planetary birthplace around the young star HL Tauri in unprecedented detail. Clearly visible is a lump of dust with 3 to 8 times the mass of the Earth, which represents the ideal conditions for the formation of a planet: a planetary nursery with sufficient building material for a planet somewhere between the mass of our own Earth and that of Neptune. The presence of a lump points towards a solution for a fundamental problem of planet formation: how planets can form on the limited time scale available for such processes.

In-depth information: Sharp pictures of planetary embryo show ultra-quick mode of planet formation

The Atacama Large Millimeter/submillimeter Array (ALMA) observatory uses a technique known as interferometry to combine data from 66 high-precision radio antennas in just the right way so as to obtain a level of detail that would require a much larger, kilometer-size single-dish telescope. When ALMA was completed in 2014, the astronomers had already decided that one object to test ALMA's most powerful configuration, in which the ALMA antennae are spread around a maximal area 16 kilometers in diameter, would be the protoplanetary disk around the young star HL Tauri.

A bright disk of dust

The disk around HL Tauri has been studied for over a decade, and at millimeter wavelengths, is one of the brightest disks known, making it a promising target for the high-resolution ALMA configuration to show its capabilities. At such wavelengths, what astronomers observe is the thermal glow of dust particles with sizes between fractions of a micrometer and several millimeters.

HL Tauri is a very young star of the type called T Tauri stars, only between one and two million years old and, in the manner of very young stars, still contained within a cocoon of gas. The star is situated in the constellation Taurus (the Bull) at a distance of nearly 460 light-years from Earth.

The ALMA image of the disk around HL Tauri exceeded the astronomers' expectations considerably. They were exquisitely detailed, showing a complex set of rings dividing the disk into multiple sections. The image was by far the most detailed image of its kind, provoking enthusiastic responses from the astronomical community. The smallest details discernible in these images were between 3.5 and 10 astronomical units in size (1 astronomical unit corresponding to the mean distance between the Earth and the Sun).

Surprisingly mature planets?

While the details of the structure were amazing, the presence of the rings that are clearly visible in the image was something of a surprise. The most direct interpretation of the ring structure with its prominent gaps assumes the presence of fully developed planets that, having formed within the disk, now orbit within the remaining gas and dust, clearing their orbits by scooping up any material they encounter, and leaving the tell-tale circular gaps. An analogous phenomenon within our own Solar system are the divisions of Saturns rings: gaps created by small moons known as shepherd moons.

But the presence of fully formed planets within the gaps would be at odds with current models of planet formation: HL Tauri is a comparatively young star. At such a young age, such stars might have formed smaller planets in the innermost part of their protoplanetary disks, but they should not yet have had sufficient time to form planets at larger distances between 10 and 100 astronomical units (that is, 10 to 100 times the mean distance between the Earth and the Sun) that would correspond to the location of the observed gaps.

Observing HL Tauri with the VLA

The ALMA results led to a number of follow-up observations - including fruitless searches for the planets that were thought to reside in the disk gaps. Now, an observational campaign using the Karl G. Jansky Very Large Array (VLA) in New Mexico has taken the story one important step further. The campaign is an international endeavour, involving the Max Planck Institute for Astronomy (MPIA), the Universidad Nacional Autónoma de México (UNAM), the National Radio Astronomy Observatory (NRAO) which operates the Very Large Array, and the Spanish Consejo Superior de Investigaciones Científicas (CSIC). The project leaders are Carlos Carrasco González (UNAM) and Thomas Henning (MPIA).

The VLA is a large radio interferometer whose 27 antennas, distributed along a gigantic Y-shaped track, operate at wavelengths longer than those of ALMA. For the HL Tauri observations, data was taken on 10 different occasions between December 2014 and September 2015, mostly in the largest configuration of the VLA, with the outermost radio antennas placed at a distance of nearly 40 kilometers from each other. The total exposure time of these observations amounted to 45 hours. The observations were taken at a wavelength of 7 mm (Q band 39-47 GHz, and thus considerably longer than the 0.87, 1.3, and 2.9 mm of the ALMA observations).

These are by far the most sensitive and most detailed observations yet targeting the HL Tauri disk at this wavelength. Just as the ALMA observations did, the VLA observations showing the presence of dust particles within the HL Tauri disk.

A clear image of the innermost disk regions

The new VLA images yielded the clearest picture yet of the innermost parts of the disk. These innermost rings are interesting as the potential birthplace of planets similar to those of our Solar System - analogous to Earth, Neptune or even Jupiter. The fact that the new observations used longer radio waves meant that the researchers were able to see not just the surface of the dust distribution within the disk, but could receive light from deeper regions as well. (In physics parlance, the regions in question are "optically thick" for the shorter-wavelength ALMA observations, but "optically thin" for the VLA observations.) This allowed for an estimate of the amount of dust contained in this part of the disk.

The observations also allow for an estimate of relative sizes of the dust grains in question. The observations show that larger grains can be found closer to the center of the disk, as expected from measurements of older, more evolved disks. The largest grains appear to have already reached sizes of almost a centimeter in diameter. This makes these particles (commonly called pebbles) ideal candidates for being trapped in flow features, and to grow into larger bodies that will eventually combine to form planets.

A planetary embryo

The new images show signs that, instead of being unexpectedly advanced and having already formed massive planets, the disk is still in the earliest stages of planet formation. This would indicate that the rings that are so prominently visible in the ALMA image have come about in a different way - through dynamical instabilities in the disk, for instance, that is: through the fact that certain flow patterns within such a disk will result in inhomogeneous structures.
Most intriguingly, the new observations indicate that the rings and their substructures might well be not consequences, but an integral part of the planet formation process. As the new images show, the rings in the innermost zone are not smooth. In particular, they feature one bright lump that appears to be caused by a dense concentration of dust, with a total mass between 3 and 8 times that of the Earth.

Dust grains within this lump appear to be somewhat larger than in the rest of the disk, and there is the tantalizing possibility that this is the first direct image of the earliest stages of planet formation. The lump would be a planetary embryo that already at this early stage contains sufficient material for a planet somewhere between the mass of our Earth and that of the planet Neptune.

Planet formation in disks

The general scenario of planet formation has been around for decades and more: a protoplanetary disk made of gas and dust, surrounding the nascent star; small dust grains in the disk sticking to other small dust grains, forming larger grains, followed by further accumulation until, at last, the resulting solid objects are sufficiently massive to attract each other via gravitational attraction, forming ever larger bodies (planetesimals) and, in the end, planets.

This scenario has long been known to suffer from a problem of timing. Over a time scale of ten million years or so, the intense radiation of the young star will drive away the gas and dust particles making up the protoplanetary disk, leaving only whatever larger objects have formed in the meantime.

But at least if one starts with a homogeneous disk, successive collisions between smaller precursor object are too infrequent and inefficient for sufficiently speedy growth, and more likely to result in fragmentation of existing objects than in accumulation and growth.

Under such conditions, it is impossible for planets, in particular for giant planets, to form sufficiently quickly: In a scenario with a homogeneous disk, the disk would have lost its dust to the star and be evaporated, and planet formation would have been cut short, too early for any planets to form.

The role of large-scale disk structure

Ten years ago, scientists in the planet and star formation theory group at the Max Planck Institute for Astronomy were able to show that planet formation can be sped up significantly if the dust is not spread evenly and homogenously throughout the disk, but instead has local concentrations. Such concentrations can above through local flow pattern in the gas disk, such as gigantic vortices and zonal flows - flow structures that self-organize in a rotating system, such as the jet stream, cyclones or anticyclones in the Earth's atmosphere.

A good analogy for what happens to the dust is a slowly flowing river with leaves, small twigs and other assorted debris drifting along. Depending on the flow pattern, debris will accumulate in particular locations, such as in vortices of the water flow. The higher concentrations of debris are clearly visible on the surface. Analogously, dust and pebbles are likely to accumulate in certain flow patterns of the surrounding gas, leading to specific locations in which the dust concentration is far above average.

Here is a scenario for how such additional larger-scale mechanisms could speed up planet formation: Instabilities involving either magnetic fields or thermal gradients within the protoplanetary disk can lead to the formation of large-scale structure in the form of rings and gaps. Dense and massive rings of pebbles could fragment into clumps and could eventually gravitationally collapse further accreting mass from their surroundings, growing in mass and, finally, forming planets.

This is exactly what the new observations show: The mass measurements demonstrate that the inner rings visible in the ALMA image are indeed sufficiently dense and massive, and the prominent lump visible in the new VLA images is the first image of fragmentation within rings in a protoplanetary disk. For the first time, astronomers are seeing this key process of planet formation in action.

The necessary material for forming planets is certainly present in the disk. The VLA observations show that the inner disk region contains dust with a total mass of between 10 and 50 Earth masses. When combined with the ALMA data, this yields an estimate of between 300 and 900 Earth masses (between one and three Jupiter masses, or between one and three thousandths of a solar mass) for the mass of the dust contained in the complete HL Tauri disk. For comparison: at the present time, our solar system contains about 60 Earth masses' worth of solid material, mostly in the shape of solid, terrestrial planets and in the cores of gas planets. Current estimates argue that originally, our Solar System's protoplanetary disk must have contained about 180 Earth masses' worth of solid material to allow for the formation of all its solid bodies sufficiently quickly before the protoplanetary disk had dissolved. As a caveat, one should note that such estimates are hampered by lack of knowledge about how efficient planet formation is in converting solids into planets.

Next steps with ALMA and the VLA

As the next step, the researchers will use both the ALMA data and the new VLA observations to create a detailed model of the disk, including the density profile of gas and dust and the structures observed, simulating how light from the central star is absorbed and re-emitted in the disk's varying regions, and yielding temperature estimates for the various regions of the disk (radiative transfer model).

The gas distribution will then allow researchers to decide between the various mechanisms that could have created the ring and led to its fragmentation. It will also allow for an estimate of the time scale for the clump to contract to one or more solid objects, notably to a cloud of planetesimals or even a finished planet.

Of particular interest are planned observations that will be able to show whether or not the bright lump is currently accreting gas and other matter from its surroundings - the key indication that the lump is indeed a steadily growing precursor that will eventually become a planet. Such observations would use the radiation emitted by specific molecules to show that energy is being liberated as gas accretes onto the protoplanet, and the shape of these molecule's spectral lines to show that there is indeed matter falling towards a central mass. From the accretion rate, it should even be possible to estimate a final mass for the newly-forming planet.

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