Studies concerning the formation of planets is a relatively young field of astronomy and astrophysics. The main reason for this is related to the complexities of exoplanetary explorations: now we are able to detect exoplanets, but observations of their surroundings is an entirely different story. However, with the advancement of technology the spectral resolution and sensitivity of modern telescopes arrived to a point where it became possible to make reliable conclusions about circumstellar disks surrounding young stars, giving birth to new planets.
Several young planet candidates have been discovered during previous scientific studies. As observed, all of them were located inside large gaps of circumstellar disks surrounding their host stars. Scientists have proposed theories stating that these gaps originate as the newly formed planets aggregate their building materials from their nearest ‘territory’ via force of gravitational attraction. However, not a single protoplanet has been detected still embedded in the circumstellar disk from which it is gathering resources for its formation.
Now, in a paper published on arXiv.org this week, a team of astronomers reported a promising discovery. The team successfully confirmed that a candidate protoplanet orbiting around a young star HD100546 and first observed in 2013 is exactly the type of object they have been looking for: a newly forming gas giant planet still ‘immersed’ in the circumstellar disk.
The star HD100546 is located at a distance of approximately 97 pc and has a mass 2.4 times exceeding that of our Sun. It is also surrounded by a large (>300 au) disk of gas and dust. Earlier studies concluded that the star is actively accreting material from the innermost regions of the disk. Meanwhile, a gap located at approximate distance of 14 au from the star is an indication of the planet in its late stages of formation. Its embedded counterpart was detected much further from the star, orbiting at a distance of 53 astronomical units.
The discovery was made using NACO instrument installed at one of the 8.2 meter Utility Telescopes of the Very Large Telescope at the Paranal Observatory (ESO). The orbital time of the observed embedded protoplanet is approximately 250 years, and this would imply that the object is not very massive, quite young or both, the authors of the study say. They also note that there is a possibility that a planetary disk gap associated with this object exists, but its formation could be suppressed on the disk surface by local turbulent processes. Alternatively, some observational effects like a combination of disk flaring and inclination could complicate the detection of a disk gap in scattered light.
“Higher spatial resolution observations in the future, either in scattered light or with ALMA, can help us to search for clear gap signatures at the object’s location, which could then be used to put some constraints on the object’s mass”, the scientists say. According to their preliminary calculations, a gap generated by a relatively small exoplanet would not be resolved in the observed images. For example, assuming a protoplanet with a mass two times greater than the mass of Jupiter, its circumplanetary disk would be ~1.4 au in radius which is too small to observe with current equipment, the authors explain. Therefore high-precision follow-up observations would be very useful in order to improve current models of planetary formation, especially those which describe processes of protoplanets in their early stages.
A relatively large distance of a young gas giant planet still embedded in the circumstellar disk at ~53 au from its host star suggests that objects like this one can form at large separations. Massive gas giant planets have been detected at comparable orbital separations in earlier studies. However, it is not easy to explain the emergence of such an object at its current location using available theoretical models, the authors argue.
“In the classical core accretion model, the time required to build up a rocky core of several Earth masses in situ at the given distance from the star exceeds by far the age of the system. In the gravitational instability model, the disk has to be massive enough to locally fragment. The remaining mass available in the HD100556 circumstellar disk is cenrtainly not sufficient for fragmentation to occur”, the authors of the study explain.
The scientists say that the formation of this young embedded protoplanet can be explained using a recently suggested theory (Lambrechts and Johansen 2012) according to which the accretion of small (cm-sized) pebbles loosely coupled to the gas in the circumstellar disk could substantially increase the speed of rocky core formation even at large separation from the central star. Then HD100546 could become a perfect laboratory to empirically study alternative formation processes for gas giant planets, the researchers conclude.
Written by Alius Noreika