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In-depth description: First radio detection of lonely planet disk shows similarities between stars and planet-like objects

A new study of the lonely, planet-like object OTS44 has provided evidence that this object has formed in a similar way as ordinary stars and brown dwarfs – a surprising result that challenges current models of star and planet formation. The study by a group of astronomers, led by Amelia Bayo of the University of Valparaiso and involving several astronomers from the Max Planck Institute for Astronomy, used the ALMA observatory in Chile to detect dust from the disk surrounding OTS44.

From collapsing clouds to stars

Stars are formed when part of a giant cloud of gas collapses under its own gravity. But not every such collapse results in a star. The key criterion is one of mass: If the resulting object has sufficient mass, its gravity is strong enough to compress the central regions to such high densities, and heat them to such high temperatures, that nuclear fusion sets in, turning hydrogen nuclei (protons) into helium. The result is, by definition, a star: an object bound by its own gravity, with nuclear fusion in its core region, shining brightly as the energy liberated during the fusion processes is transported outwards.

Initially, the newly born star is surrounded by the remnants of the collapsed cloud. But in the natural course of collapsing, both the star and the cloud have begun to rotate at an appreciable rate. The rotation serves to flatten the material surrounding the young star, forming what is known as a protoplanetary disk of gas and dust. True to its name, this is where planets begin to form: The dust clumps to larger and larger grains and pebbles, increasing in size until, finally, the resulting objects are large enough to join together under the influence of its own gravity, forming solid planets thousands or even tens of thousands kilometers in diameter like our Earth, or collecting appreciable amounts of the surrounding gas to form gas giants, like Jupiter in our solar system.

If the object resulting from the collapse of the initial cloud has between 0.072 and 0.012 times the mass of the Sun – which corresponds to between 75 and 13 times the mass of Jupiter – what emerges is called a brown dwarf: a failed star, with some intermittent fusion reactions of deuterium (heavy hydrogen, consisting of one proton and one neutron) in the core regions, but no sustained, long-lasting phase of hydrogen fusion.

The strange case of OTS44

Can collapse produce even lighter objects, with similar masses as that of planets? A thorough analysis of the object OTS44, published in 2013 by a group of astronomers led by Viki Joergens from the Max Planck Institute for Astronomy (MPIA), presented strong evidence that this is indeed the case. OTS44 is a mere two million years old – in terms of stellar or planetary time-scales a newborn baby. The object has an estimated 12 Jupiter masses and is floating through space without a close companion. It is part of the Chamaeleon star forming region in the Southern constellation Chamaeleon, a little over 500 light-years from Earth, where numerous new stars are in the process of being born from collapsing clouds of gas and dust.

Just like a young star, OTS44 is surrounded by a disk of gas and dust, one of only four known low-mass objects (with about a dozen Jupiter masses or less) known to harbour a disk. Most conspicuously, OTS44 is still in the process of growing – that is, drawing material from the disk onto itself at a substantial rate. The disk itself is quite substantial; both this disk and the infalling material (accretion) are telltale signs of the standard mode of star formation – an indication that there is no fundamental difference between the formation of low-mass objects such as OTS44 and the formation of ordinary stars. OTS44 probably has the lowest mass of all objects where both a disk and infalling material have been detected.

Brown dwarf vs. planet-like object

We have so far avoided calling OTS44 either a brown dwarf or something else. In fact, nomenclature varies: Some astronomers call every object that has formed by direct collapse and is not a star a brown dwarf; by this criterion, only objects that form in disks around a central object can be planets. There is an alternative definition that hinges on the fact that an object like OTS44 does not have sufficient mass for a significant episode of deuterium fusion, and does not qualify as a brown dwarf on that account. We will compromise by referring to OTS44 as a planet-like object.

While the case of OTS44 shows that even planet-like objects can form by collapse, the details are anything but clear. For the formation of comparatively low-mass objects, be they very light stars, or brown dwarfs, or lonely planets, there are two main possibilities – but both are problematic in the case of OTS44. The first possibility is a direct collapse by a small isolated cloud. But going by our current knowledge, such a direct collapse should not be able to form such a planetary-mass object directly.

Much more likely is the alternative, namely that OTS44 could have formed as part of a larger collapsing cloud, when the collapsing regions fragmented, producing several objects, including OTS44, instead of a single larger body. But this does not mesh well with the observations. OTS44 is not now part of any multiple system. And even if we assume it was somehow ejected from such a system, OTS44 is still very young, and could not have moved far from its birth system – and that birth system would not have had time to dissolve completely into separate stars and/or brown dwarfs. But there is only a single object within 10,000 astronomical units (10,000 times the average Sun-Earth-distance) of OTS44, where the siblings of OTS44 could reasonably be expected, and there are no signs that this object was part of a collapsing, fragmenting cloud.

Tracking dust with ALMA

Clearly, there is more to be learned. That is what motivated a group of researchers led by Amelia Bayo (University of Valparaiso, Chile) to find out more about OTS44. The group includes a number of researchers from the Max Planck Institute for Astronomy (MPIA), as well as several former MPIA astronomers. Amelia Bayo was herself a postdoctoral researcher at MPIA before moving on to the University of Valparaiso, and in science, the international stations of an astronomer’s career often result in collaboration networks – in this case, a strategic collaboration between astronomers at the Universidad de Valparaiso in Chile and the MPIA's Planet and Star Formation Department led by Thomas Henning. The two institutions have an additional link: the Universidad de Valparaiso hosts an astronomical Max Planck Tandem Group, which commenced work in early 2017. With such tandem groups, the Max Planck Society fosters international cooperation with specific excellent research institutions.

In this particular case, the group gathered by Bayo for observing OTS44 included several members with the necessary skills and experience to make full use of the ALMA observatory: a constellation of 50 radio antennae for detecting millimeter and submillimeter radiation, operated by an international consortium and located in the Atacama desert in Chile.

The astronomers applied for ALMA time to observe the disk of OTS44 at millimeter wavelengths. Millimeter wavelengths are particularly suited to detect dust grains, which are present in protoplanetary disks (and account for one percent or more of the disk mass; these mass estimates are a subject of ongoing research). At least in the disks around more massive objects, these dust grains are the seeds of planet formation.

Dust mass and a surprisingly universal relation

For millimeter waves, the disk is optically thin, in other words: observations show the millimeter radiation from all the dust in the disk. (In an optically thick disk, we would only see radiation from the surface layers; the lower layers would be obscured by the upper layers.) This allowed the astronomers to estimate the total amount of dust in the disk – although the result still depends on the disk temperature. Temperature estimates for such disks, given the measured overall luminosity, give values between 5.5 Kelvin and 20 Kelvin for the OTS44 disk. This leads to estimates for the dust mass between 0.07 times the mass of the Earth (for the highest temperature estimate) and 0.64 Earth masses (for the lowest temperature).

These mass estimates confirm the similarity between stars and lower-mass objects: Systematic studies had shown earlier that for young stars and brown dwarfs, there is an approximate relationship between the mass of the central object and the mass of the dust in the surrounding disk. Inserting the data points for OTS44, the lonely planet-like object fits very well into the overall picture – indicating that the same overall mechanism is involved in all these cases, putting all central objects from about a hundredth to a few solar masses onto the same footing.

Dust grains of unusual size

Another interesting consequence stems from the fact that the disk is emitting significant amounts of millimeter radiation in the first place. This indicates the presence of certain amounts of grains of dust that are about a millimeter in size. Going by the current theories of planet formation, this is surprising: such larger dust grains should not have been able to form in a disk around such a low-mass object. In such a disk, the dust grains orbit the central mass like so many microscopic planets, following the laws first found by Johannes Kepler in the early 17th century. The gas of the disk, on the other hand, has internal pressure, which makes it rotation somewhat slower. The “head wind” felt by dust grains as they move through the slower gas should slow down the smaller grains, making them drift inwards before they finally fall onto the central object. There are arguments that these detrimental effects are particularly strong in lower-mass objects. From these calculations, it follows that the dust grains in the disk should have vanished when they were somewhat smaller – and should not have had the time to clump to form the observed millimetre-size grains.

Once the millimetre-size grains are there, the situation becomes less problematic – with their larger size, these grains do not feel the head wind as acutely as their smaller kin. But the presence of these larger grains poses a puzzle – and hints at the intriguing possibility that lonely planets might even be able to grow even larger dust grains, and may be even go as far as forming downright miniature moons, in their surrounding disks.

Similarities with young stars

All, in all, the new results make OTS44 look more and more similar to a young star, surrounded as it is by a disk, given the earlier evidence that it is still growing by incorporating material from that disk, and now with the new evidence that the ratio of the dust mass to the mass of the central object follows the same relation as for brown dwarfs and stars.

Evidently, the current models that preclude low-mass objects from forming in this particular way, via the collapse of a cloud of gas, are missing something. Observations like these new ones for OTS44 can be hoped to point us in the right direction for what that missing something might be, and thus towards a better understanding of the formation of low-mass objects in the universe.

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