Artists' impression of the gas and dust disk around the planet-like object OTS44. First radio observations indicate that OTS44 has formed in the same way as a young star. Image: Johan Olofsson (U Valparaiso & MPIA)
First radio observations of the lonely, planet-like object OTS44 reveal a dusty protoplanetary disk that is very similar to disks around young stars. This is unexpected, given that models of star and planet formation predict that formation from a collapsing cloud, forming a central object with surrounding disk, should not be possible for such low-mass objects. Apparently, stars and planet-like objects are more similar than previously thought. The finding, by an international team led by Amelia Bayo and including several astronomers from the Max Planck Institute for Astronomy, has been published in Astrophysical Journal Letters.
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.
This detection yielded mass estimates for the dust contained in the
disk, which place OTS44 in a row with stars and brown dwarfs (that is,
failed stars with too little mass for sustained nuclear fusion): All
these objects, it seems, have rather similar properties, including a
similar ratio between the mass of dust in the disk and the mass of the
central object. The findings supplement earlier research that found
OTS44 is still growing by drawing matter from its disk onto itself –
another tell-tale similarity between the object and young stars.
Similarities with young stars
Taken together, this is compelling evidence that OTS44 formed in the
same way as stars and brown dwarfs, namely by the collapse of a cloud of
gas and dust. But going by current models of star and planet formation,
it should not be possible for an object as low-mass as OTS44 to form in
this way. An alternative way, the formation of multiple objects in one
go, with low-mass objects like OTS44 among them, is contradicted by the
observations, which show no such companion objects anywhere near OTS44.
The strength of the radiation received from the dust at millimetre
wavelength also suggests the presence of large, millimetre sized dust
grains. This, too, is surprising. Under the conditions in the disk of a
low-mass object, dust is not expected to clump together to reach this
size (or beyond). Instead, the OTS44 dust grains appear to be growing –
and might even be on the way of forming a mini-moon around the object;
another similarity with stars and their planetary systems.
Amelia Bayo (University of Valparaiso), who led this research effort,
says: “The more we know about OTS44, the greater its similarities with a
young star. But its mass is so low that theory tells us it cannot have
formed like a star!”
Thomas Henning of the Max Planck Institute for Astronomy adds: “It is
amazing how an observatory like ALMA allows us to see half an Earth
mass worth of dust orbiting an object with ten times the mass of Jupiter
at a distance of 500 light-years. But the new data also shows the limit
of our understanding. Clearly, there is still a lot to learn about the
formation of low-mass astronomical objects!”
Background information
The work described here has been published as A. Bayo et al., "First
millimeter detection of the disk around a young, isolated,
planetary-mass object" in the May 18, 2017 edition of the Astrophysical
Journal Letters.
Link to the article
The MPIA researchers involved are:
Viki Joergens, Yao Liu (also Purple Mountain Observatory, Nanjing,
China), Johan Oloffson (also Universidad de Valparaíso), Thomas Henning,
and Henrik Beuther in collaboration with Amelia Bayo (first author; Universidad de Valparaíso [UV]), Robert
Brauer (University of Kiel), Javier Arancibia (UV), Paola Pinilla
(University of Arizona), Sebastian Wolf, Jan Philipp Ruge (both
University of Kiel), Antonella Natta (Dublin Institute for Advanced
Studies and INAF-Osservatorio Astrofisico di Arcetri), Katharine G.
Johnson (University of Leeds), Mickael Bonnefoy (IPAG Grenoble), and
Gael Chauvin (IPAG Grenoble and Unidad Mixta Internacional
Franco-Chilena de Astronomía, Santiago).
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.
Science Contact
Prof. Dr. Thomas Henning
Director - Max Planck Institute for Astronomy; Professor at the University of Heidelberg
Phone:+49 6221 528-200
Email: henning@mpia-hd.mpg.de
Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg
Personal homepage - MPIA Heidelberg
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Markus Pössel
Public Information Officer
Phone:(+49|0) 6221 528-261
Email: pr@mpia.de
