For the first time, a nebulous region around an exoplanet has been clearly detected.
With a mass three times that of our Moon, it is quite possibly the disk in which this exoplanet, similar to Jupiter, will form a satellite system.
Protoplanetary system
Located about 400 light-years from Earth in the southern constellation Centaurus, PDS 70 is a young star of enormous interest. Surrounding the star, a prominent ring of dusty gas has been detected from which planets are forming. With the help of the large VLT telescope, two of these young planets, called PDS 70b and PDS 70c, had already been detected, a pair very similar to the pair of Jupiter and Saturn that dominates our solar system.
Both planets are located in the area between the great dust ring and the star itself, one of them very close to the star and the other (PDS 70c) further away.
The great circumstellar ring has a radius of about 50 times the Earth-Sun distance, while the planet PDS 70c is separated from its star by a distance 34 times greater than the Earth-Sun distance, that is, slightly greater than the separation. between the Sun and Neptune.
Around the exoplanet
A team coordinated by astronomer Myriam Benesty (from the universities of Chile and Grenoble, France) has observed the star PDS 70 with the large ALMA interferometer located on the Chajnantor plateau, at an altitude of 5000 meters in the middle of the Atacama desert.
Their observations, summarized in the image at the top of this article, show the highly patterned disk surrounding the star.
There is also a nebulous region accumulated in the immediate vicinity of the central star, possibly a more compact disk that cannot be seen very clearly in the observations.
Cloud of exoplanet PDS 70c (indicated with arrow) observed with ALMAALMA / ESO / NSF / NAOJ / Benisity et al.
In the empty region between the great ring and the star the position of the planet PDS 70c appears clearly. However, the smallest and closest 70b to the star is not well detected in these observations that only trace gas and dust and not solid objects like the planets themselves. What is most surprising about these fantastic observations is that at the position of planet 70c a nebulous mass appears, well centered on the exoplanet and well differentiated from the great ring that surrounds the star. The authors of the study have calculated that this cloud has a size similar to the Earth-Sun distance and a mass of only 3% of the Earth's mass, in round numbers: about three times the mass of our Moon.
Benesty and colleagues conclude, quite reasonably, that such a cloud around exoplanet 70c is a small disk in which a satellite system is forming.
The observations are not able to reveal if there is already a moon already formed (the data is not sensitive to compact objects), but the disk is very important in itself, since it confirms the theories of formation of planets and their satellites.
Exomoons
It is well established that planets form in dusty disks around young stars. Each planet, as it is being formed, burrows an annular hole as it accumulates material from the circumstellar disk and, in this way, it grows. It is thought that, during this process, the planet can also acquire its own circumplanetary disk. And this disk must contribute to the growth of the planet by regulating the amount of material that falls on it. Simultaneously, the gas and dust of the circumplanetary disk, through multiple collisions, can form progressively larger bodies, which must eventually lead to the birth of moons.
More than 4000 planets are known, but all of them have been detected in mature star systems where, so to speak, all the work is already done, and it is not possible to observe the mechanisms of their formation. Protoplanetary systems like the one in PDS 70 are much less frequent, but much more interesting to see the processes of planet and moon formation. Unfortunately these protoplanetary systems are not very massive and not very bright. It is extremely difficult to get to see details like the ones we observed in PDS 70 and, in particular, in PDS 70c.
However, the new observations leave the other minor planet, PDS 70b, which shows no signs of having a disk, shrouded in mystery.
We do not know if this planet has swallowed up all the circumplanetary material or if this material has already completely formed a satellite system that cannot be seen with these observations.
In any case, they are magnificent data that illustrate the virtuosity of ALMA in detecting objects miniscule in size and mass.
When the large ELT telescope becomes operational in Chile, towards the year 2027, it will be possible to obtain optical / infrared images of this star system, in which we will see its protoplanetary system with total clarity and, who knows, perhaps we will come to appreciate the incipient exomoons.
The work of Benisty and collaborators entitled "A Circumplanetary Disk Around PDS70 c" has been published a few days ago in the prestigious magazine The Astrophysical Journal Letters.
The manuscript can be consulted at this link.
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Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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